Mohamed Attawia Robert Dainton, Allegheny Univ of Health Sciences Delaware Technology Park Ste 300
Robert Langer Cato Laurencin, MIT Allegheny Univ of Health Sciences
* Invited paper
SESSION O1: TISSUE ENGINEERING
Chair: Kathryn E. Uhrich
Wednesday Afternoon, December 3, 1997
Independence W (S)
1:30 PM *O1.1
COLLAGEN-GLYCOSAMINOGLYCAN COPOLYMERS FOR THE REGENERATION OF MUSCULOSKELETAL SOFT TISSUES. Myron Spector Department of Orthopedic Surgery, Brigham and Women's Hospital, Harvard Medical School, Boston, MA.
Absorbable synthetic and natural polymers are currently being investigated as matrices for the regeneration of certain tissues and organs in vitro and as implants to facilitate the regeneration of tissue in vivo. Previous reports in the literature have shown that collagen-glycosaminoglycan (GAG) copolymers with tissue-specific pore diameters and degradation rates, serving as analogs of extracellular matrix (ECM), favor the processes of regeneration in dermis and peripheral nerve. This work served as the basis for their implementation in the treatment of defects in articular cartilage, meniscus, ligament, and tendon. That the properties and function of these musculoskeletal tissues rely on an ECM architecture regulated by mechanical forces during its formation, prompted the development of collagen-GAG matrices to accomplish the regeneration of these tissues in vivo. That these tissues are completely or partially avascular and only sparsely populated with cells of low mitotic activity, indicates that cell-seeded matrices may be required as implants. Investigation of the interaction the parenchymal cells of these musculoskeletal tissues with collagen-GAG matrices in vitro revealed that the collagen type comprising the analog can regulate the phenotypic behavior of these musculoskeletal cells. A greater percentage of articular chondrocytes in a type II collagen-GAG matrix (Chondrocell, Geistlich Biomaterials, Wolhusen, Switzerland) retained their spherical chondrocyte morphology and the cells synthesized more GAG, compared to cells in analogs synthesized from type I collagen. Moreover, the type I collagen-GAG matrices, seeded with each of the cell types, underwent significant cell-mediated contraction in vitro, in contrast to the cell-seeded type II collagen analogs that displayed little change in shape. These findings provide the basis for the development of collagen-GAG matrices as implants to facilitate the regeneration of musculoskeletal soft tissues.
2:00 PM O1.2
THE BEHAVIOR OF BOVINE MENISCUS CELLS SEEDED IN TYPE I AND TYPE II COLLAGEN-GAG MATRICES. S.M. Mueller, H.A. Breinan, S. Shortkroff, T.O. Schneider, M. Spector, Dept of Orthopedic Surgery, Brigham and Women's Hospital, Harvard Medical School, Boston, MA.
Collagen-glycosaminoglycan (GAG) copolymers, serving as analogs of extracellular matrix, have been used successfully as implants to facilitate regeneration of dermis and peripheral nerve. Their applications for regeneration of certain musculoskeletal tissues (e.g. knee meniscus), that are avascular and sparsely populated with cells of low mitotic activity, may require prior seeding with parenchymal cells. The goal of this study was to investigate the behavior of meniscus cells seeded in type I and type II collagen-GAG matrices. The type I collagen-GAG matrix was produced using the freeze-drying method previously reported. The type II collagen-GAG material (Chondrocell) was obtained from Geistlich Biomaterials, Wolhusen, Switzerland. The matrices had comparable mean pore diameters (approximately 85m). Calf meniscus was enzymatically isolated, incubated to confluence, passaged four times, and then seeded at a density of 9E5 cells per matrix. After 1, 7, 14 and 21 days, the matrices were analyzed for DNA content and GAG production. Histology and immunohistochemistry were also performed. 58% of the cells seeded in type I and 42% seeded in type II matrices were retained after one day.Thereafter, DNA for the type II matrix showed a continuous increase to 7.7E5 cells/matrix by 21 days while the type I matrix DNA decreased by 40% after one week then increased to 6.9E5 cells/matrix after 21 days. GAG content of the type II matrix increased by 50% more than the type I matrix after 21 days. Histologically, cells were observed throughout the type II matrix, whereas the type I matrix was densely populated at the margins. At 21 days the meniscus cells in both the type I and II matrices displayed similar morphology, a round to elongated shape, and produced a distinct fibrous matrix that stained positive for type I collagen and for GAG. The behavior of meniscus cells is influenced by the collagen comprising the matrix in which they are seeded. The number of cells and higer GAG synthesis in the type II matrix commend it for future investigation of the regneeration of meniscus . SMM was supported in part by Ciba-Geigy Jubilaeumsstiftung and Schweizerische Akademie der medizinischen Wissenschaften (BK 232/96).
2:15 PM O1.3
3-D FIBER ARCHITECTURES FOR TISSUE ENGINEERING. Frank K. Ko, Drexel Univ, Dept of Materials Engineering and School of Biomedical Engineering, Sciences and Health Systems, Philadelphia, PA.
It is well established that biological tissues are hierarchical fibrous structures consisting of many levels of structural dimensions ranging from nanometer scale to millimeter scale. These fiber structures are organized into various levels of order and orientations, resulting in an interconnected 3-D fiber network. The development of 3-D scaffolds for tissue ingrowth and proliferation in the tissue engineering of orhtopedic implants can benefit from the creative use of the large family of 3-D fiber architectures created by advanced textile manufacturing technology. This paper introduces the recent progress in the engineering design and manufacturing technology of 3-D fiber architectures and their potential use in tissue engineering.
2:30 PM O1.4
MICROSTRUCTURAL ANALYSIS AND MECHANICAL PROPERTIES OF POLYMER SCAFFOLDS FOR TISSUE INGROWTH. Seth M. Toomay1, Hyun D. Kim1, Robert F. Calentini1,2, Nina R. Silverman3, and Brian W. Sheldon3, Brown University, Providence, RI. (1Department of Molecular Pharmacology, Physiology, and Biotechnology; 2Department of Orthopedics; 3Division of Engineering.)
Polymer scaffolds were prepared from both modified hyaluronic acid (HYAFF) and poly-lactic acid (PLA). The microstructure of these materials was characterized with optical microscopy, SEM, and TEM. Computer-based methods were developed to obtain void fraction, pore size, and surface area measurements from the images. Electron microscopy was particularly important for the characterization of composite scaffolds. The elastic modulus was determined as a function of the scaffold microstructure (i.e., pore size, void fraction, and second phase content). Several methods were then used to relate the modulus and other properties to the microstructural parameters. The results of this analysis will be presented in detail. Tissue ingrowth studies were also conducted with select scaffolds (including in vivo studies). The amount of ingrowth and polymer degradation were assayed at different time intervals, up to l 2 weeks. The observed fibrovascular tissue ingrowth with HYAFF was an order of magnitude lower than that observed for similar PLA scaffolds.
2:45 PM O1.5
SPATIAL CONTROL OF LIGAND PRESENTATION ON BIOMATERIAL SURFACES. Gillian Brown, Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA; Gargi Maheshwari, Douglas Lauffenburger, Linda Griffith, Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA.
Adhesion of many cell types to the extracellular matrix or to synthetic bioactive surfaces is mediated by integrin receptors. Integrin clustering is believed to be closely associated with focal contact formation and signaling, as assessed by the behavior of cells on surfaces presenting relatively uniform ligand distributions. Controlled clustering of integrins might be achieved by controlling the spatial distribution of adhesion ligands on bioamterial surfaces. Substrates have been prepared on which cell-surface interactions are controlled by modifying non-adhesive poly(ethylene oxide) (PEO) hydrogels with the minimal cell adhesion peptide sequence GRGDY (RGD). The peptide is tethered to the hydrogel surfaces via star PEO molecules, producing surfaces on which the ligands are presented to cells in domains of high concentration. The substrates are compared with others on which the RGD peptide is uniformly distributed. The effects of variation in RGD surface density on cell adhesion and migration are examined. Thus, we have fabricated surfaces which, because of their resistance to non-specific cell interactions and the control of specific interactions at the molecular level, can serve as a model for artificial matrix development and can be used for fundamental studies.
3:00 PM O1.6
CELLULAR REACTION TO SYNTHETIC AND NATURAL POLYMERIC TUBE IMPLANTS USED FOR PERIPHERAL NERVE REGENERATION. Lila J. Chamberlain, Myron Spector, Hu-Ping Hsu, and Ioannis V. Yannas, Massachusetts Institute of Technology, Dept of Mechanical Engineering, Cambridge, MA; Brigham and Women's Hospital, Dept of Orthopedic Research, Boston, MA.
Clinical and experimental models for the treatment and study of peripheral nerve gap injuries involve insertion of the severed nerve stumps within tubes. The objective of this study was to compare the cellular reaction to silicone (synthetic polymer, nondegradable) and collagen (natural polymer, biodegradable) tubes. The sciatic nerve of adult Sprague-Dawley rats was transected at mid-thigh and a 10mm gap generated. The severed nerve stumps were inserted into either a silicone (Dow-Corning), porous collagen (Integra LifeSciences), or non-porous collagen tube (Integra LifeSciences). At six weeks post-operative, the sciatic nerves, including the implants, were removed and prepared for light and electron microscopy. There were no notable accumulations of polymorphonuclear neutrophils, lymphocytes or other acute inflammatory cells that would indicate an adverse reaction to the materials comprising the devices. Macrophages were identified on the surfaces of both the collagen and silicone tubes, occasionally engulfing particles that appeared to be collagen tube remnants. Contractile cells (myofibroblasts) formed a concentric capsule around the outside of the silicone tubes in 100% of the animals (n=3). The myofibroblasts were identified using immunohistochemical stoning for a-smooth muscle actin and were verified using TEM. Inside the silicone tube, a similar concentric capsule of myofibroblasts was observed in every instance (n=7). The collagen tubes, in contrast. contained myofibroblasts on the inside tube surface in only 1 of 7 tubes and no myofibroblasts were found on the outside tube surfaces (n=7). In addition, the myofibroblasts, when present on the collagen tube surface, did not form a cell-continuous system. There were no differences in the cellular reactions to the porous and non-porous collagen tubes. Long-term clinical data show chronic nerve compression at the lesion site following repay with a silicone tube implant. We hypothesize that this constriction within the tube is a result of the formation of a myofibroblastic capsule, and that the use of alternative biomaterials for the fabrication of tubular implants could potentially eliminate this adverse effect of the tube repair technique.
SESSION O2: RESORBABLES AND IMPLANTS
Chair: Frank K. Ko
Wednesday Afternoon, December 3, 1997
Independence W (S)
3:45 PM O2.1
CHARACTERIZATION OF MEDIATOR EXPRESSION INDUCED BY THE DEGRADATION PRODUCTS OF BIODEGRADABLE POLYMERS WITH ORTHOPAEDIC APPLICATION, J.J. Nicholson , M.A. Attawia, S.J. Kelly, C.T. Laurencin, The Department of Orthopaedic Surgery, Allegheny University of the Health Sciences and The Department of Chemical Engineering, Drexel University, Philadelphia, PA.
It has been proposed that following the macrophage non-specific phagocytosis of polymer wear debris, such as poly(ethylene) and poly(methylmethacrylate) (PMMA), the activated macrophages release osteolytic cytokines such as Interleukin 1(IL-1), Interleukin 6 (IL-6) and Tumor Necrosis factor (TNF), which stimulate the neighboring osteoblasts to release the bone resorption mediator Prostaglandin E2 (PGE2). We recently developed a filter co-culture system which allows macrophages and osteoblasts to grow, interact, and modulate each other's mediator expression within a single system, yet are physically separated by a porous membrane; thus, simulating the in vivo interaction which exists at the local implant-osseous microenvironment. We utilized this system to examine the biocompatibility of orthopaedic polymers which our laboratory synthesizes. Our laboratory has developed three novel classes of biodegradable poly(anhydride-co-imides) which have been proposed as alternatives to current orthopaedic materials: poly[pyromellitylimidoalanine-co-1, 6-bis(carboxyphenoxy)-hexane] 30% PMA-ala: 70% CPH), poly[bis(carboxyphenoxy))propane-co-sebacic acid] (30% CPP: 70% SA), and poly(pyromellitylimidoalanine-co-sebacic acid) (10% PMA-ala: 90% SA). These poly(anhydride-co-imides) have been shown to support osteoblast cell growth and phenotypic expression in vitro . These polymers degrade through surface erosion into their monomers; there, it is necessary to examine the bone resorption potential of the degradation products of each of the poly(anhydride-co-imides). Utilizing the filter co-culture system, the mediator profile (IL1, IL6, TNF and PGE2) of each of the poly(anhydride-co-imide) monomers were compared to the mediator profile elicited by the degradation products of poly(lactide-co-glycolide) (PLAGA), clinically the most widely used degradable orthopaedic polymer, in addition to PMMA, a polymer which is known to elicit the release of bone resorption mediators. Our preliminary results indicate that the monomers from the poly(anhydride-co-imides) and the PLAGA elicit significantly lower amounts of osteolytic mediators than PMMA. This study will prove helpful in determining which monomers are safe to use as components of future biodegradable orthopaedic polymers.
4:00 PM O2.2
THERMAL AND DYNAMIC PROPERTIES OF POLY(DL-LACTIC ACID) AND POLY(DL-LACTIC--GLYCOLIC ACID). Meng Deng, Kathryn Uhrich, Rutgers Univ., Dept. of Chemistry, Piscataway, NJ.
Biodegradable polymers are being used as pins and fixation devices in orthopedic implants. In this research, thermal and dynamic properties of poly(DL-lactic acid) (PLA) and poly(DL-lactic-co-glycolic acid) (PLGA) were investigated. Five kinds of amorphous polymeric materials (PLA 100%, PLGA 85:15, PLGA 75:25, PLGA 65:35, PLGA 50:50) were evaluated. The glass transition temperatures (Tg), thermal decomposition temperatures (Td) and relaxation enthalpy (H) of the materials were determined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It was found that copolymerization of lactic acid with glycolic acid decreased the Tg of PLA with PLGA 50:50 having lowest Tg. Similar results were obtained for H. However, Td's were not significantly affected. For dynamic mechanical analysis, the raw polymers were compression-molded into sheets with thickness of 1.5 mm at temperatures 40C higher than their Tg . The dynamic mechanical properties of the polymers were deterrnined under three point bending load conditions. Effects of temperature on storage modulus (E') and tan were determined at a frequency of 1 Hz. The Tg's obtained from dynarnic mechanical analysis (DMA) were similar to the values obtained from DSC studies. Finally, the molecular weight of the polymers were determined using gel permission chromatography (GPC) and the correlation between molecular weight and thermal and dynamic mechanical properties were examined.
4:15 PM O2.3
FIBER REINFORCEMENT OF BIODEGRADABLE FRACTURE FIXATION DEVICES. Jae-Won Choe, Donald L Wise, Northeastern Univ, Dept of Chemical Engineering, Boston, MA; Charles M Lyons, Edgardo J Mantilla, Ali J Satvat, Yankai Zhang, Cambridge Scientific, Inc, Boston, MA.
Biodegradable polymer composites are being developed to replace stainless steel and titanium alloys as the material of choice in fracture fixation devices. Fiber reinforcement has been proposed as a means of strengthening the polymer composite. Initial experiments have shown that the fibers delaminate from the polymer matrix when a stress is applied, resulting in little or no significant reinforcement. This problem may be eliminated by chemically bonding the fiber reinforcements to the polymer matrix. We are currently investigating the possibility of cross-linking the fibers to the polymer by irradiation. Experimental procedures include irradiation, SEM, and tensile tests. Initial results have been promising, but additional tests are required before any conclusions can be made. The results of our experiments will be the focus of this presentation.
4:30 PM O2.4
DESIGN, FABRICATION, AND ANLAYSIS OF A HYBRID METAL/POLYMERIC COMPOSITE FEMORAL HIP STEM FOR TOTAL HIP ARTHROPLASTY. Joseph D. Trentacosta, E.I. DuPont de Nemours Co, Wilmington, DE; Michele Marcolongo, Drexel University, Philadelphia, PA; Robert S. Hastings, DePuy Inc, Warsaw, IN.
In an effort to extend the life of a total hip arthroplasty procedure by reducing ``stress shielding'' of bone tissue by the femoral component, we have designed, fabricated, and analyzed a hybrid metal/polymeric composite femoral hip prosthesis. In this system, the proximal region consists of porous-coated titanium alloy (Ti6A14V), while the distal region is fabricated from a composite material composed of graphite fibers and polyetherketoneketone (PEKK) polymeric matrix. The composite section was designed to have a variable modulus along the length of the component, matched to the modulus of the metal alloy in the longitudinal direction in the proximal region of the composite and then reduced to that of cortical bone tissue toward the distal region of the composite stem. The goal of the design was to provide enough initial stability to allow fixation by porous ingrowth, but enough flexibility to enhance the stress transfer between the implant and bone. To select the composite matrix, extensive testing was performed on several polymers. We found that PEKK was mechanically stable under repeated cyclical loading in a lipid environment (compared to polysulfone, which was not). The graphite fiber/PEKK composite stems were filament wound and molded to near net shape. Further testing on the composite structure showed the material to be biocompatible according the to ISO guidelines. The hybrid stems were mechanically cycled in a lipid environment for 10 million cycles under 5-7 times body weight using an analog femur model and survived without event. Subsequent to fatigue, the devices were loaded to failure in a modified Semlitsch test and compared to devices which had not been fatigued. Negligible mechanical degradation in the residual strength of the stems was measured as a result of the fatigue. Pre-clinical testing is complete for this device and we are evaluating the device clinically.
4:45 PM O2.5
PERFORMANCE OF HYBRID METAL/COMPOSITE SR-71 FEMORAL COMPONENT FOR HIP JOINT ARTHROPLASTY. Wei Fu, Sherrill B. Biggers Jr., Dept of Mechanical Engineering, Clemson, SC; Chaodi Li, Robert A. Latour Jr., Clemson Univ, Dept of Bioengineering, Clemson, SC; Michele S. Marcolongo, DePuy-DuPont Orthopaedics, Newark, DE.
Stress shielding induced bone resorption in the proximal femur and thigh pain due to high implant stem stiffness are still serious problems limiting the success of femoral components for hip joint arthroplasty. Due to their wide tailorability, fiber reinforced polymer (FRP) composite materials are very attractive for femoral component design as a means to address these problems. Implants fabricated completely from FRP composites, however, may not possess sufficient fatigue strength in the neck portion of the prosthesis. A new hybrid femoral component (SR-71AE, DePuy-DuPont Orthopaedics) was designed which incorporates a titanium alloy neck and proximal body with a carbon fiber/ polyetherketoneketone composite stem. This design provides high component fatigue strength with low stem bending stiffness. The performance of this new design was evaluated using a 3-D finite element model with empirical calibration. An anatomically accurate 3-D computer model of a femur was developed from transaxial CT-scans. The actual femur was then strain gaged and subjected to structural analysis under axial compression, in-plane bending, and out-of-plane bending. The experimental data were used to calibrate the computer model. The computer model was then utilized to evaluate the performance of the new SR-71AE hybrid component and to compare it with a titanium alloy StabilityAE femoral component and the intact femur. Loading consisted of the application of both joint and muscle loads to simulate conditions of the toe-off phase of gait and stair-climb/chair-rise. The SR-71AE component generally yielded both higher stress levels in the proximal femur and lower stress levels around the distal stem compared to the all metal StabilityAE stem, as intended. These results indicate that by using proper design strategies, composite materials can be utilized in femoral component design to obtain superior implant mechanical performance compared to completely metal-based components.
SESSION O3: BONE GRAFTS AND BONE CEMENT
Chair: David Pienkowski
Thursday Morning, December 4, 1997
Independence W (S)
8:30 AM *O3.1
BONE REGENERATION USING BONE MORPHOGENETIC PROTEIN AND A DELIVERY SYSTEM. Jeffrey O. Hollinger and Shelley R. Winn, Oregon Health Sciences University, Portland, OR.
The therapeutic effectiveness of bone morphogenetic protein (BMP) for bone regeneration is well known. It is accepted that a delivery system will enhance the clinical effectiveness. There are key properties required in a delivery system for BMP that may be fulfilled by either contemporary materials or materials in development. Therefore, this presentation will highlight selected concepts consistant with the physiologic criteria of delivery systems, BMP, and bone regeneration. We will present some data with delivery systems and BMP in characterized animal wound models. Basic fundamentals must be addressed in delivery systems designed for BMPs. Engineering the delivery system must account for the controlled release of BMP, sequenced removal of the delivery system, and cell attachment. Moreover, clinical convenience must not be overlooked. Delivery systems for BMPs that have enjoyed pre-clinical and clinical roles include collagens, calcium-phosphates, and bidegradable polymers. Therapeutic effectiveness of these compositions has been applied to regenerate bone in craniomandibular, spinal, and appendicular locales. We will review some of our experience in these areas.
9:00 AM O3.2
IN VITRO AND IN VIVO EVALUATIONS OF A NEW SYNTHETIC BONE GRAFT MATERIAL. D.P.Mukherjee, M. Pearson, S. Rogers, P. Menon, Department of Orthopaedic Surgery, LSUMC-Shreveport, Shreveport, LA; A.B. Chausmer, V.A. Medical Center, Shreveport, LA.
The objective of this study was to develop a synthetic bone graft material in a paste form which can be delivered to the defect site. A mixture of N,O carboxymethyl chitosan (NOCC) and hydroxyapatite (HA) was first optimized from bench top studies. Viscosity, setting time, compressive modulus and push out resistance (through cadaver bone holes) were evaluated for pastes of HA:NOCC ratios 1:1,2:1,3:1,4:1,5:1 and 10:1, made from a 5% aqueous solution of NOCC. Based on these parameters a formulation of 4:1 ratio was selected. On completion of these bench top evaluations an animal study was initiated. A defect of 2 mm diameter was created in the femur of male Lewis rats. A total of 27 rats (3/group/sacrifice interval) were tested at sacrifice intervals of 2,4 & 6 weeks. In the experimental group (I) the hole in the left femur was filled with NOCC and the hole in the right femur was filled with the NOCC and HA paste. In the control group (II) the hole in the left femur was filled with bone chips harvested from the right femur of the same animal which served as the empty control. Group(III) was sham operated control where only skin was opened followed by suturing. On sacrifice, the bone density in the defect area was measured by Quantitative Computer Tomography (QCT) and Dual Energy Xray Absorptiometry (DEXA). The attachment of bone ingrowth into the holes was measured by a mechanical push out test. The undecalcified (150 micron) and decalcified (5 micron) histological sections were stained by toluidine blue and examined under light microscopy. The preliminary results of the in vivo evaluations showed that the NOCC/HA paste was osteoconducive, demonstrating bone in growth into the defect area.
9:15 AM O3.3
DEVELOPMENT OF A BIODEGRADABLE FLEXIBLE MATRIX AND A BIODEGRADABLE PUTTY FOR BONE REPAIR APPLICATIONS. Mark D. Borden1,2, Mohamed A. Attawia1, and Cato T. Laurencin1,2,3, 1Department of Orthopaedic Surgery, Allegheny University of the Health Sciences, Philadelphia, PA. 2School of Biomedical Engineering, Science, and Health Systems, and 3Department of Chemical Engineering, Drexel University, Philadelphia, PA.
One of the major disadvantages of currently available orthopaedic materials is their lack of flexibility and inability to be custom fit to the implant site. Synthetic bone graft onlays and inlays come in a manufactured form that causes surgeons to fit the surgical site to the implant. Bone graft onlays are used to reinforce existing bone, provide added support to the trauma site, and to aid in fusing comminuted bone, while bone graft inlays are used to replace missing or diseased bone. Based on our gel, solvent cast, and sintered microsphere methods, we have created two types of customizable bone graft composites. A flexible onlay was created by using a porous poly(lactide-co-glycolide) [PLAGA]/hydroxyapatite [HA] composite. Cast in a 3 mm sheet, this rigid material can be made temporarily flexible by heating the onlay past the glass transition temperature of PLAGA (Tg=67.4C) component. Once past this temperature, the onlay can be shaped to give a custom fit of the surgical site. As the polymer component cools, the onlay hardens providing rigid fixation while maintaining the shape of the implant site. This study examined the mechanical and structural properties of the onlay before and after implant shaping. Using HA coated PLAGA microspheres isolated from the gel microsphere method, a moldable CaSO4 composite was also created. The addition of water to a 1:1 mixture of calcium sulfate hemihydrate and HA coated microspheres allowed for the formation of a moldable putty. Once molded to the desired shape, the composite dough set via the conversion of calcium sulfate hemihydrate to calcium sulfate, and hardened into its final implant form. The implant was characterized by mechanical testing and evaluated in an in vivo femoral defect model in rats.
9:30 AM O3.4
DEBONDING OF PMMA/METAL AND PMMA/BONE INTERFACES IN CEMENTED JOINT REPLACEMENTS: ADHESION AND FATIGUE BEHAVIOR. Reiner H. Dauskardt, Kevin L. Ohashi, and N. Christine Nguyen, Dept. of Materials Science and Engineering, Stanford University, Stanford, CA.
The performance of cemented femoral components is strongly influenced by the interfacial adhesion and resistance to time- or cycle-dependent debonding of the resulting PMMA/prosthetic and PMMA/bone interfaces. We report on novel interface fracture methods which allow a reproducible and quantitative measure of adhesion in clinically relevant interface systems. Adhesion, expressed in terms of the macroscopic fracture energy, is shown to increase with initial debond extension (R-curve behavior) and to be strongly dependent on interface morphology in the absence of chemical bonding across the interface. More importantly, however, we demonstrate that progressive debonding of these interfaces may be significantly exacerbated by fatigue processes associated with physiological loading. Such debonding occurs subcritically and may lead to subsequent loosening and failure of these cemented systems. The fatigue debonding process and its dependence on salient variables including interface morphology, cement layer thickness, and fatigue loading parameters is discussed. Issues of crack path selection involving the competition of weak microstructural paths and mechanical driving forces are explored. Engineering strategies to promote adhesion to bone involving novel bone preparation methods are considered. In addition, since the fracture resistance of these interfaces largely involves crack surface interactions behind the crack tip (e.g. frictional sliding and pullout of rough asperities), fatigue loading results in wear processes which produce particulate wear debris. Evidence of such mechanisms are obtained from fractographic examination and analysis of the wear debris collected from the environmental chamber. Implications for long term reliability and life prediction procedures for prosthetic-PMMA interfaces are discussed.
9:45 AM O3.5
STRUCTURE-PROPERTY CORRELATIONS OF FUMARATE BASED POLYESTERS FOR USE IN BIORESORBABLE BONE CEMENT COMPOSITES. Gregory B. Kharas1, Gretchen A. Caywood2, Marina Kemetsky1, James Simantirakis1, Kimberly C. Beinlich1, Ann Marie T. Rizzo1, and Matthew J. Baugh1, 1 DePaul University, Chemistry Department, Chicago, IL; 2 DynaGEn Inc., Cambridge, MA.
Fumarate based polyesters were prepared by the transesterification polycondensation of diethyl fumarate and diols, ()-1,2-propanediol, (S)-(+)-1,2-propanediol, 2- methyl 1,3-propanediol and 2,2-dimethyl-1,3-propanediol. Different polyester microstructures were observed by 1H and 13C NMR spectroscopy when the reaction was conducted in the presence of p-toluenesulfonic acid monohydrate or metal containing catalysts, aluminum trichloride, titanium tetrachloride, titanium tetrabutoxide, and zinc chloride. The extent of formation of branched structures associated with hydroxyl end groups addition to the unsaturated polyester double bonds depended on the acidity of the catalyst. The bone cement composites were prepared by mixing the fumarate polyesters with an inorganic filler, CaSO4 2H2O, N-vinyl pyrrolidone and a radical initiator, benzoyl peroxide at ambient temperatures. The compressive strength and hydrolytic stability of the cement compositions was correlated with the structure of the polyesters.
10:00 AM O3.6
DEVELOPMENT OF A BIODEGRADABLE INJECTABLE BONE CEMENT. Triantafillos J. Fillos, Neha P. Reshamwala, Donald L. Wise, Northeastern University, Center for Biotechnology, Boston, MA; Yung-Yueh Hsu, Charles M. Lyons, Debra J. Trantolo, Joseph D. Gresser, Cambridge Scientific Inc., Boston, MA.
We have developed an injectable, biodegradable bone cement that will provide immediate structural support for fractured bones and allow for normal bone healing and remodeling. The injectable cement is composed of a three part formulation. The first part consists of the unsaturated polyester poly(propylene fumarate), calcium gluconate which provides the cement with porosity, and hydroxyapetite which acts as a osteoconductive material. The second part consists of the cross linking agent vinyl pyrrolidone, ethanol which acts as an accelerator, and peanut oil. The third part consists of the initiator benzoyl peroxide. When the three parts are mixed, PPF gets crosslinked and produces a hard cement pellet. Degradation of the bone cement occurs via hydrolysis of the ester bonds and produces linear monomers, fumaric acid, and propylene glycol, all of which can be eliminated by the body. Our studies of this injectable bone cement include in vitro determination of the degradation of the bone cement over a four week period. The parameters tested in this analysis included, weight loss, hardness, porosity, and compressive strength changes. Scanning electron microscopic analysis of the degrading bone cement was also done over the four week period. Elemental analysis was performed to determine the nature of the cross links including cross link length, density, and the number of cross links per mole, as well as finding the proportion of vinyl pyrrolidone incorporated into cross linked PPF. In vivo tests were performed in rats. Bone cement was incorporated into the legs of the rats. Histological analysis was done on the tissue surrounding the site of injection to determine the extent of inflammation and tissue damage. Our presentation will focus on the results and conclusions gathered from our investigation. The results of all our analysis have been promising thus far. It is hoped that this injectable cement will be used in the future by orthopedists as an easy yet effective support in curing fractured bones.
SESSION O4: DRUG DELIVERY
Chair: Harry A. McKellop
Thursday Morning, December 4, 1997
Independence W (S)
10:45 AM O4.1
BIOCOMPATIBLE POLY(MMA-HEMA) COPOLYMERIC HYDROGEL MICROSPHERES AND THEIR USE IN ORTHOPAEDICS. M. Sivakumar and K. Panduranga Rao, Biomaterials Division, Central Leather Research Institute, Tamal Nadu, INDIA.
Initially Poly(methyl methacrylate) [PMMA] core microspheres were prepared by free radical initiation technique. On these core microspheres 2-hydroxyethyl methacrylate (HEMA) was polymerized by swelling PMMA microspheres with the HEMA monomer using ammonium persulphate. Cross-linking monomer such as ethyleneglycol dimethacrylate has also been included along with HEMA for polymerization. By this technique it was possible to obtain core shell type microspheres. The core is hard PMMA microspheres having hydrophilic poly HEMA shell coat on it. These microspheres are highly hydrophilic as compared to PMMA microspheres. The size of the hydrogel microspheres doubled when swollen in benzoyl alcohol. These microspheres were characterized by various techniques such as optical and scanning electron microscopy, FT-IR, thermogravimetric analysis and differential scanning calorimetry. The particle size range of the microspheres (150-250 m) were analyzed using Malvern master sizer/E particle size analyzer. Since, the microspheres can be swollen in water and organic solvents, both hydrophilic and hydrophobic drugs can be incorporated in the microspheres. Investigations are in progress to incorporate antinflammetry and antibacterial drugs with a view to use them in orthopedic applications.
11:00 AM O4.2
AND RELEASE OF VANCOMYCIN AND GENTAMICIN FROM AN INJECTABLE ABSORBABLE GEL-FORMING MATRIX FOR THE TREATMENT OF OSTEOMYELITIS. Joel T. Corbett, James E. Jerome, Jacqueline Allan, William Kelly, Jonathan Kline, Harold Farris, Linda Fulton, R. Larry Dooley, Shalaby w. Shalaby*, Poly-Med, Inc. Pendleton, SC.
Osteomyelitis and its treatment remains a major therapeutic challenge. Current therapy includes debridement and systemic as well as local antibiotic treatment This research examines the use of absorbable liquid gel-forming polyester copolymers that deliver gentamicin and vancomycin directly at the site of infection. An absorbable cation exchange resin was also investigated to determine its effect on modulating the release of the two antibiotics The vancomycin loaded gel-former was implanted near an incised area of a healthy tibia in goats. Drug levels were determined in the blood at various time periods and in the bone at the time of euthanasia. Vancomycin loaded gel-former was also examined in vitro to determine the drug release profiles in an environmental chamber that simulates the In vitro situation Gentamicin was loaded into the same gel matrix and examined in vitro only. In vitro samples for the vancomycin were analyzed by a modified USP HPLC method. Vancomycin in vitro and all gentamicin samples were analyzed externally by fluorescence polarization immunoassay (FPIA). Release profiles of both antibiotics were determined from the in vitro data. These indicate that vancomycin had a much higher release than an analogous gentamicin system, and the cation exchanger regulates the release profile. In vitro release results demonstrate the presence of detectable vancomycin levels at four weeks post-operation about the tibial administration site. Placebo and active formulations do not cause site inflammation beyond the initial traumatic reaction to the dose form. If fully realized, an injectable absorbable carrier system for osteomyelitis will be advantageous over current treatment regimes in both cost, efficacy and patient comfort.
11:15 AM O4.3
BIODEGRADABLE POLYPHOSPHAZENE/POLY(LACTIDE-CO-GLYCOLIDE) BLENDS FOR CONTROLLED DRUG DELIVERY. Archel M.A. Ambrosio1 and Cato T. Laurencin1,2, 1Department of Orthopaedic Surgery, Allegheny University of Health Sciences, Philadelphia, PA; 2Department of Chemical Engineering, Drexel University, Philadelphia, PA.
Previously, polyphosphazene/poly(lactide-co-glycolide) blends were developed in our laboratory for potential biomedical applications. In that study, the blends were found to be miscible and degraded at a rate intermediate of those of the corresponding parent polymers. In the present study, the release of p-nitroaniline from the blends was examined as a model for the controlled release of drugs and other bioactive agents. Using the mutual solvent approach, 50:50 poly(lactide-co glycolide) PLAGA) was blended with poly[(50 p-methylphenoxy)(50 ethyl glycinato)phosphazene] (PPHOS-EG50) in a 50:50 ratio and impregnated with p-nitroaniline at various concentrations. The release of p-nitroaniline from these blends in pH 7.4 phosphate buffer at 37C was then determined spectrophotometrically at designated time points.
11:30 AM O4.4
POLYMER BONDED CALCIUM PHOSPHATE COATINGS FOR TITANIUM IMPLANTS. Shantha Sarangapani, ICET Inc., Norwood, MA; Paul Calvert, Arizona Materials Labs, Tucson, AZ; David Gage, Karen Lein ( ICET, Inc.), Suzanne Maxian, Pennsylvania College of Podiatric Medicine, PA.
Biomimetic coating processes have been used by several groups to grow thin calcium phosphate layers on titanium implants. As developed by other workers the implant material is generally exposed to supersaturated phosphate solutions for times upto a week to form coatings a few microns thick. We have found that pretreatment of implant material with phosphated surface modifiers and certain biocompatible polymers allows films to reach thicknesses of 20-30 microns. Subsequent post treatment with the same polymer increases the strength and adhesion of the coating. The talk will present the mechanical properties, invitro bone tissue growth pattern, alkaline phosphatase activities, and calcium release characteristics of these coatings in comparison with a commercial plasma sprayed coating. Such biomimetic thin coatings offer a highly resorbable interface for dense bone growth and allow incorporation of growth factors.
SESSION O5: POLYETHYLENE
Chair: Michele S. Marcolongo
Thursday Afternoon, December 4, 1997
Independence W (S)
1:15 PM *O5.1
BALANCING WEAR PERFORMANCE, OXIDATION RESISTANCE AND PHYSICAL PROPERTIES OF CROSS LINKED UHMWPE. Harry McKellop, Fu-Wen Shen, Orthopaedic Hospital/USC, Los Angeles, CA; William DiMaio, DePuy DuPont Orthopaedics, Newark, DE.
In Total Hip Arthroplasty, the long term performance of the ultra high molecular weight polyethylene bearing is determined by its resistance to wear and degradation. Improvement in these properties could strongly reduce the severity of the tissue reaction to wear debris (chronic inflammation and bone resorption), extending the clinical lifespan of joint replacements. This study describes several radiation crosslinked UHMW polyethylenes having very high resistance to wear and to oxidative degradation. Crosslinking of UHMW polyethylene is accomplished by generating free radicals in the polymer, which may react in several ways. They may recombine, effecting no change in the polymer, they may form crosslinks, which tends to improve the wear resistance, or they may react with absorbed gases such as oxygen, degrading the polyethylene and reducing the molecular weight, especially at or near the surface. Additionally, free radicals can remain trapped in the crystalline regions of the polymer for years, eventually reacting with oxygen that diffuses in from the exposed surface In this study, UHMW polyethylene was crosslinked to improve its wear resistance by exposure to gamma radiation doses ranging from 3.3 to 50 Mrads, and then heated above its melt temperature to cause the unreacted radicals to form additional crosslinks, thereby stabilizing the polyethylene against long-term oxidation. These processes were done both in the presence and absence of oxygen, and the effects on the resistance to wear and oxidation were measured.
1:45 PM O5.2
THE EFFECTS OF OXIDATION AND STERILIZATION ON THE STRUCTURE AND PROPERTIES OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE. M. Goldman, R. Gronsky, University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA; G.G. Long, National Institute of Standards and Technology, Gaithersburg, MD; D. Baker and L. Pruitt, Department of Mechanical Engineering, University of California, Berkeley, CA.
Ultra high molecular weight polyethylene (UHMWPE) remains the material of choice to substitute for articulating cartilage in total joint replacement surgeries. However, long-term degradation often leads to accelerated wear of the polymer in vivo resulting in the generation of polyethylene debris particles which in turn lead to osteolysis. It is believed that an oxidation degradation mechanism induced by gamma radiation sterilization is responsible for the continuous degradation and embrittlement of UHMWPE. This paper addresses the issue of how oxidation changes the microstructure, morphology and fatigue properties of the polymer. To accelerate oxidation, UHMWPE is aged in hydrogen peroxide after sterilization. Differential scanning calorimetry, density gradient column, transmission electron microscopy and small-angle x-ray scattering are used to characterize the structure while fatigue tests are conducted in both tension and compression. Oxidation leads to dramatic changes in both the structure and properties of UHMWPE including a decrease in melting temperature, an increase in heat of fusion, an increase in density, a tortuosity in lamellae, an increase in crack propagation and a tendency to microcrack. An oxidation mechanism is proposed in which oxygen is incorporated into the amorphous phase of the UHMWPE, leading to the development of strains and the breaking of tie molecules in the polymer. This leads to the observed microcracks and the embrittlement of the material.
2:00 PM O5.3
ANALYSIS OF UHMWPE ACETABULAR COMPONENTS FOLLOWING LONG TERM IMPLANTATION. Hallie E. Placko, Marcus L. Scott, Jack E. Lemons, University of Alabama at Birmingham, Department of Biomedical Engineering, Birmingham, AL.
The effects of various constituting, manufacturing, and sterilizing procedures on the shelf aging behavior of ultra-high molecular weight polyethylene (UHMWPE) has been well documented. In addition, several accelerated in vitro aging techniques have been developed to study the progression of degradation, primarily oxidation, of these materials on the shelf. However, little information has been gathered concerning the long term aging processes of UHMWPE total joint components in vivo. We have hypothesized that retrieved implants with implantation times greater than 10 years will exhibit a lower degree of degradation as compared with shelf aged components. In this study, 10 UHMWPE acetabular components of various designs with implantation times greater than ten years (range = 10 - 24 years; mean = 14.2 years) were examined for possible degradation, including the volumetric wear relationship to the material degradation (oxidation). Volumetric wear was estimated using a coordinate measuring machine and a sphere fitting algorithm. Core samples were taken from worn and unworn regions of each component and sectioned into discs, approximately 200 microns thick. Material characteristics such as microstructural profile (white banding), oxidation index and crystallinity are being measured for each component using Fourier Transform infrared spectroscopy (FTIR), creating a degradation map throughout the thicknesses of the acetabular component cups. In general, volumetric wear increased with cup diameter, with the 22 mm diameter cup (implantation time = 23 years) showing as much as 748 cubic mm of wear while the 28 mm cup (implantation time = 11 years) showed more than 1900 cubic mm of wear. Preliminary FTIR results show a trend toward lower oxidation levels than those seen in shelf aged specimens and shorter term in vivo measurements as reported in the literature. Ongoing research will emphasize the effects of the in vivo environment on the performance, wear, and aging processes of these UHMWPE components.
2:15 PM O5.4
IMPLANTATION OF 7BE RADIOACTIVE TRACER: A NEW METHOD FOR WEAR MEASUREMENTS OF POLYMERS. T. Sauvage, O. Lacroix, and G. Blondiaux, CERI-CNRS, Orleans, FRANCE.
The long-term performance of most joint replacements is limited primarily by polymer wear causing necrosis and osteolysis. A new method is proposed to determine the wear resistance of polymers used in orthopedic. This method is based upon the principle of ion implantation by recoiling, applied to 7Be radioactive ions generated by a 3He particles beam. The radioactive ions are implanted into the near surface (a few micrometers) without degradations of material chemical or mechanical properties. Two detection modes can be performed : remaining radioactivity detection of implanted piece or radioactivity detection into tissue. In case of in vivo experiments, the radioactive dose delivered to the animal is negligible. Three kinds of polymers (mylar, bioploymer and polypropylen) have been activated and for the first time radioactive ions depth distribution has been determined. The method sensivities are a few hundred nanometers for detection mode of remaining activity and one microgram for detection mode into tissue.
2:45 PM *O5.5
THE MICROANATOMY OF UHMWPE FROM RESIN TO RUIN: A REVIEW. D. Pienkowski1, R.J. Jacob2, and H. Kaufer3, 1Center for Biomedical Engineering and Division of Orthopaedic Surgery, 2Department of Microbiology and Immunology, 3Division of Orthopaedic Surgery, University of Kentucky, Lexington, KY.
Abstract not available
3:15 PM O5.6
CHEMICAL MODIFICATIONS IN UHMWPE PROSTETHIC COMPONENTS INDUCED BY STERILIZATION. Luigi Costa and Maria Paola Luda, Torino Univ., IFM Chemistry Dept, ITALY; Elena Maria Brach del Prever, Torino Univ., I#176# Clinica Ortopedica, CTO, ITALY.
Wearing and delamination are the major drawbacks in using UHMWPE for prosthetic components: The life expectancy of the prosthetic components is 10-12 years and the need for a re-implant causes humane sufferance and severe limitations of prosthesis application in young people. Despite the great deal of work on this subject the reason for wearing and delamination are still not well understood. However, sterilization with rays, which is widely used for prosthetic implant, is a potential candidate for effecting wearing and delamination: it is well known in fact that macromolecular chains undergo chemical modification under rays irradiation. Two main factors control the extensions of these modifications: the radical formation due to the interaction energy/matter which is related to the absorbed dose and the radical oxidation which is controlled by the oxygen solubility and by its diffusion in the material. In truth, oxidation of UHMWPE leads to the decreasing of the molecular weight and consequently reduces the physical properties of the material. Calculations of the depth profile of the absorbed dose and of the oxygen concentration are in good agreement with the distribution of oxidized products (namely hydroperoxides and acids) which have been experimentally found. Oxidation is quite limited on the surface because of the low radical concentration and on the depth because of the oxygen starvation. However, it is more significant at 800-900 m depth. The storing environment and sterilization conditions (atmosphere, dose and rate of absorbed dose) are of paramount importance in determining the oxidation level in ready-to-use prosthetic implants.
3:30 PM O5.7
EFFECT OF CROSSLINKING ON THE WEAR BEHAVIOR OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE USED IN TOTAL JOINT REPLACEMENTS. Orhun K. Muratoglu, Daniel O. O'Connor, Charles R. Bragdon, Murali Jasty, William H. Harris, Massachusetts General Hospital, Orthopaedic Biomechanics Lab, Boston, MA.
The major problem in total joint replacements remains to be the wear debris induced peri-prosthetic osteolysis (mostly from UHMWPE components) that leads to aseptic loosening. Recently, cross-linking was shown to reduce the adhesive wear of UHMWPE under bovine serum lubrication. In this study, we investigated the properties of radiation cross-linked and subsequently melted UHMWPE at varying cross-link densities. Bi-directional pin-on-disk (POD) testing up to 2 million cycles with a Paul physiologic loading curve was used to determine the wear behavior as a function of cross-link density. The mechanical properties, thermal properties, density and surface oxidation was also determined. The cross-link density and molecular weight between cross-links was determined using volume swelling ratio (in hot xylene). While the yield strength, ultimate tensile strength, density, and crystallinity decreased with increasing radiation dose, the wear resistance of the polymer increased dramatically. The wear rate of the cross-linked polymer followed a transition from high wear to negligibly low wear. The transition occurred at around absorbed radiation dose of 150 kGy, above which the pins tested on the POD did not show any sign of wear. When the wear rate was correlated with molecular weight between cross-links, a linear relationship was found. Based on this observation, a micromolecular mechanism for the improvement of wear in cross-linked UHMWPEis will be discussed.
3:45 PM O5.8
SELF-REINFORCED COMPOSITE POLYETHYLENE. Spiro Megremis, Jeremy Gilbert, Northwestern Univ, Div of Biological Materials, Chicago, IL; Steven Duray, Bosworth Inc, Skokie, IL.
Ultra-high-molecular-weight polyethylene (UHMWPE) was first used as a bearing material in hip joint arthroplasty in the early 1960's. However, an increased emphasis on the reduction of UHMWPE wear particles has created a demand for an improved UHMWPE. An example of such an improved UHMWPE material may be a composite in which both the fiber and the matrix consist of UHMWPE, or self-reinforced composite polyethylene (SRC-PE). The unidirectional SRC-PE composites used in this study were produced by way of a thermomechanical processing method in which heat and pressure were applied to aligned UHMWPE fibers (Spectra 900, Allied Signal) for a controlled period of time; this results in preferential melting of the surfaces of the fibers which bonds the fibers together and forms the matrix of the composite. Time, temperature, and pressure were varied in order to investigate the effects of these parameters on the consolidation of the composites. After fabrication, SRC-PE specimens were sectioned, etched, gold coated, and then viewed using SEM for the purpose of observing differences in the microstructure as the processing parameters were changed. Furthermore, tension, compression, and three point bending tests were performed to ascertain mechanical properties of a unidirectional form of the SRC-PE composite. As expected, these properties proved to be highly dependent on loading and orientation. Tensile tests revealed average yield strengths of 500 MPa along with moduli in the range of 10 GPa. Compression tests (in the fiber direction) resulted in average yield strengths of 33 MPa and moduli of 1.7 GPa. In bending, the SRC-PE exhibited yield strengths of about 100 MPa and moduli of around 9 GPa. Currently, tests are being performed to evaluate the wear performance of SRC-PE and to further characterize its mechanical properties at different processing parameters.
SESSION O6: IN-ROOM POSTER SESSION
Thursday Afternoon, December 4, 1997
Independence W (S)
CHARACTERIZATION OF DOUBLE LAYERED HA COATING BY DIPPING-THERMAL DECOMPOSITION METHOD (1). Ping Zhou, Masataka Ohgaki, Satoshi nakamura, Masaru Akao, Division of Inorganic Materials, Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo, JAPAN.
Hydroxyapatite (HA) has been widely used in medicine because of its favorable biocompatibility. However, it has disadvantages such as difficulty in molding or shaping and low mechanical properties. A new modified thermal decomposition method is described for preparing a double layered coating on titanium plates which includes an initial perovskite (CaTiO3) layer followed by a hydroxyapatite (HA) layer on top. Pure CaC03 powder was decomposed at 1200C for 3 h to yield CaO. The CaO powder was added to a 2-ethylhexanoic acid solution and then heated to 120C until all the CaO powder was dissolved. N-butanol was added to the resulting gel and stirred to form a transparent solution. Bis(2-ethylhexyl) hydrogen phosphate was added to the transparent solution, and then the HA coating solution was made up. The CaTiO3 coating solution was prepared similar to a method that was used for HA, except that Ti tetraisopropoxide was added instead of the bis(2-ethylhexyl) hydrogen phosphate used for the HA solution. The characterization of the coating was studied by X-ray diffractometry, infrared spectroscopy, SEM and chemical analysis. Said examination indicated that the double layer consisted of carbonate HA and CaTiO3 and the thickness of the layer was 2-4 m. The coating was performed on the inner surfaces of 50-200 m sized pores and was also consistent in the smallest of the pores, even those of 50 m. Our results indicated that the coating layer was homogenious and thin. In addition, both layers consisted of Ca which enhanced both stability and neutralization of Ti toxicity by the addition of Ca ions. The consistency of coating into even the smallest pore size suggested that our method promises to have widespread clinical applications in artificial implants because of increased surface contact area.
SYNTHESIS AND CHARACTERIZATION OF FUNCTIONAL MICROSPHERES FOR ORTHOPAEDIC APPLICATIONS. M. Sivakumar and K. Panduranga Rao, Biomaterials Division, Central Leather Research Institute, Madras, Tamil Nadu, INDIA.
Poly(methyl methacrylate)[PMMA] as an adhesive in arthroplasty applications is well known. Some investigators have also attempted to use PMMA beads for hard tissue repair and regeneration. In the present paper, attempts were made to prepare microspheres of PMMA having carboxylic functionality by synthesizing PMMA using chain transfer agent, thioglycolic acid. By this process the molecular weight of PMMA can be reduced considerably compared to PMMA prepared conventionally. PMMA microspheres were prepared by using solvent evaporation technique and they were fully characterized. The microspheres were analyzed for particle size distribution and found to be 15 - 20 microns. The molecular weight of microspheres was found to be 1.3x104 daltons by using Gel Permeation Charmatography. The presence of carboxylic groups in the microspheres was conformed using C-13 NMR. Equilibrium swelling experiments of the microspheres were carried out and it was found that the microspheres are quite hydrophilic compared to conventional PMMA microspheres. The SEM pictures of the microspheres indicated that they are spherical and porous. Experiments are in progress for the incorporation of gentamicin and ibuprofen in the microspheres and their in-vitro release profile studies. It is aimed to use these microspheres in orthopaedics particularly in the repair and regeneration of bone. It is also planned to incorporate growth factors into the spheres to make them osteoconductive in addition to their osteoconductive property.
AN ALTERNATIVE POLYMER-CERAMIC COMPOSITE FOR BONE AUGMENTATION. Lin Song, Anna Gutowska, Beth L. Armstrong, Allison A. Campbell, Pacific Northwest National Laboratory, Richland, WA.
Currently, large bone defects are repaired using materials composed of allografts or autografts. Unfortunately, available graft supply is limited and the concern about infectious disease has lead to the demand for synthetic bone tissue replacement. Ceramic materials are used to restore bone to form and function by serving as a substitute for natural tissue which is lost to disease, accident, or surgery. Recent interest in this area has focused on the use of the polymer-ceramic composites of either tricalcium phosphate (TCP) or hydroxyapatite (HAP) combined with thermoplastics, collagen and other polymeric materials used as bone scaffolds, fillers, and reconstructive agents. In this study, novel polymer ceramic composites based on stimuli-sensitive polymers were investigated. The polymer solution was mixed with HAP and/or TCP at room or an elevated temperature resulting in a liquid or a paste like suspension. When exposed to body temperature or pH, the paste like suspension changed into a solid polymer/ceramic composite with the ceramic component entrapped effectively within the polymer gel matrix. The gelation and rheological properties of the composites were investigated as a function of temperature, pH, polymer concentration and calcium phosphate solids loading. Their thermomechanical properties and dissolution kinetics were also studied.
THE EFFECT OF PHYSIOLOGICAL LOADS ON THE TEXTURE EVOLUTION OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE. C. Klapperich, D. Crane, K. Komvopoulos and L. Pruitt, University of California, Department of Mechanical Engineering, Berkeley, CA.
Ultra-high-molecular-weight polyethylene (UHMWPE) continues to be the material of choice for replacement of damaged cartilage in total joint replacement surgeries. However, the life of an orthopedic component is limited by the generation of polyethylene debris particles which in turn lead to implant loosening and chronic pain associated with osteolysis. Reduction of wear debris is critical to extending the life of joint replacements, and basic understanding of the fundamental wear mechanisms is essential to achieving this goal. It is believed that the accumulation of plastic deformation coupled with texture development of the polyethylene crystal lamellae are the prime reasons for the formation of wear debris by delamination wear. This paper addresses the issue of structural evolution in UHMWPE due to the effects of physiological contact stresses. Pin-on-disk wear tests were conducted with polished CoCr pins and UHMWPE flat disks lubricated with bovine serum at a range typical of physiological contact pressures and sliding speeds. The coefficient of friction was monitored continuously during testing and wear rate calculations were performed using surface profilometry measurements of worn disk surfaces. Transmission electron microscopy was used to determine the polymer morphology as a function of contact pressure. It was observed that even low contact pressures and short sliding distances result in lamellae alignment parallel to the sliding direction. Subsurface microstructure analysis was performed by observing thin sections at different depths below the articulating surface in order to develop a spatial mapping of texture evolution as a function of contact stress. This technique may provide a useful means for characterizing the effectiveness of plasma surface modifications of UHMWE for improved wear resistance.
CA ADSORPTION AND APATITE FORMATION ON CHEMICALLY MODIFIED SILK FABRIC. Yasushi Tamada, Tsutomu Furuzono, Masuhiro Tsukada, National Institute of Sericultural and Entomological Science, Tsukuba, JAPAN; Tetsushi Taguchi, Akio Kishida, Mitsuru Akashi, Kagoshima Univ, Dept of Applied Chemistry and Chemical Engineering, Kagoshima, JAPAN.
Silk material is expected to have a good biocompatibility because silk have been used as a suture for long time. Further the mechanical strength of silk fiber is similar to that of tendon. We have embarked on study attempting to develop a novel artificial tendon or ligament by the use of the silk material. Although the material for an artificial tendon/ligament have need of a good bone-binding property, it seems that the silk material itself do not have such a function. In order to endow the silk material with the bone-binding function, we have designed to modify the silk material chemically. Since the main component of bone is hydroxyapatite, it would be an important point to form hydroxyapatite effectively in/on the silk material. We expect anionic groups introduced in the silk material could accelerate the formation of hydroxyapatite on it due to increasing the adsorption of Ca ion which is the main molecule in the hydroxyapatite crystal. Chemical compounds used in this study are phthalic anhydride, succinic anhydride, sodium p-styrene sulfonate, methacryoiloxyethylphosphate, and the modification with these compounds result in incorporating caboxylate, sulfonate, and phosphate groups into the silk material, respectively. Ca adsorption in simulated body solution was dramatically increased on the modified silk fabrics, especially 10 times more amount of Ca adsorbed on the silk fabric incorporated phosphate groups in comparison with the original one. On the contrary, no significant effect on Ca adsorption to the silk material incorporated sulfonate groups was observed. In this paper, we will present the chemical modifications of silk fabric, and Ca adsorption and hydroxyapatite formation on the modified silk fabrics.
BIOLOGICAL EXAMINATION OF BONE INGROWTH AND BONDING USING AN INNOVATIVE DIPPING-THERMAL DECOMPOSITION TECHNIQUE (2). Ping Zhou1, Masashi Hosonuma2, Shigeru Yoshioka, Tsutoma Kamikura4, Hiroshi Mitsu5, 1. Division of Inorganic Materials, Institute for Medical and Dental Engineering, Tokyo Medical and Dental University,Tokyo, JAPAN; 2. Research & Development Division, Permelec Electrode Inc., Kanagawa, JAPAN; 3. Yoshioka Inc., Kanagawa, JAPAN; 4. Platon Japan Inc., Tokyo, JAPAN; 5. Dept. of Orthopedics, Mitsui Memorial Hospital, Tokyo, JAPAN.
There are numerous approaches to coating HA. However, tests suggest that our new method is a promising development. Many biological experiments though are required to positively prove its advantages. Important factors, like good biocompatability and stability, are crucial in body application. Both biocompatability and stability were studied in animal tests. Biocompatability was examined in canines from 2-32 weeks. Two types of Ti plates were put onto femurs-one with coated HA using our new thermal decomposition technique and the other was the plain Ti control. Micropores were punched into the Ti plates in sizes 50-300 m. Bonding strength was examined on the femurs and tibias of rabbits at two and four weeks. Similarly both types of Ti plates were used. Detaching tests were performed and tensile strength between the HA layer and bone were measured. Bone ingrowth was prevalent in the HA coated plates before 8 weeks, however, after 8 weeks the differences in ingrowth between the two types of plates were minimal. The results indicate that HA could stimulate early ingrowth; growth being evident in the pores as well as on flat surfaces. The results suggested that our coating method had advantages in quality..i.e., stability and biocompatability - which consequently recommends not only its applicability but efficacy. Tensile strength of the detaching test indicated that at the initial 2 week stage there was no bonding evident on the plain Ti plate, however, the HA coated plate had 1.22Mpa. After 4 weeks, the plain Ti control still did not exhibit good mechanical bonding; tensile strength at 4 weeks was Ti(0.35) and HA(2.50). The results of the biological tests proved that our coating technique promises to have significant applications for use on artificial implants.
REINFORCEMENT OF POLYMER MATRICES WITH CERAMIC PARTICLES FOR BIOMEDICAL APPLICATIONS. Jiazhong Luo1, John J. Lannutti 1 and Robert R. Seghi2, 1Department of Materials Science & Engineering;2College of Dentistry, The Ohio State University, Columbus, OH.
Ceramic particles provide many advantages in polymer reinforcement. Improvements in modulus and toughness can accompany a wider range of biologically suited chemical compositions. Interpenetration of the particle by the matrix provides for levels of stress transfer apparently greater than that possible using silane coupling agents. This transfer enhances the mechanical properties needed for strength retention by either resorbable or non-resorbable polymer matrices. Comparisons to human enamel, our targeted biological analog, show that conformance to its wear properties could be achieved by engineering the interface between the particle and the matrix. Net particle porosity was found to be the dominating factor. Wear patterns mimicked that of human enamel tested under the same experimental conditions. The porous nature of these particles should promote cellular digestion and offers possibilities for two-phase polymer composites (intra-particle vs. extra-particle) systems and thus better-controlled localized release/tissue response.
THE EFFECT OF UNIAXIAL DRAWING ON THE MORPHOLOGY OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE. Hsing-Chia Wang+, C. Sung++, A. Crugnola@ and J. Hamilton*, Center for Advanced Materials, University of Massachusetts at Lowell, Department of Chemistry+, Plastics@ and Chemical Engineering++, Lowell, MA; Johnson and Johnson Professional*, Raynham, MA.
Ultra high molecular weight polyethylene (UHMWPE) has been used in total hip and knee prosthetic applications since the early sixties. However, very little research has been done on the relationship between wear properties and microstructure of this polymer. Understanding the morphological changes that take place in the deformation of UHMWPE may help in understanding the mechanisms iinvolvedin its wear. We report the microstructures observation of lamella fibrils by transmission electron microscopy (TFM). These lamella structures start from nucleation center. There is no preferred orientation in the bulk. The thickness of lamella around 250 from measurement by TEM and wide angle X-ray diffraction data. The lamella structures can be oriented by drawing. Two dimensional observation of a drawn film by TEM reveals that the lamella structures oriented preferentially along the drawing direction with a reduction in tortuosity. The thickness of lamella was reduced to 130. X -ray diffraction study showed a broadening of the diffraction peaks after drawing. This suggests that the stretch process decreased the perfection of the crystalline domain. A novel technique has also been developed and utilized to image swelling behavior of the samples before and after drawing.
DEVELOPMENT OF A RESORBABLE INTERNAL FIXATION DEVICE. Patrick Hook, Triantafillos Fillos, Yankai Zhang, Donald Wise, Center for Biotechnology, Northeastern Univ., Boston, MA; Joseph Gresser, Debra Trantolo, Charles Lyons, Yung-Yueh Hsu, Cambridge Scientific, Inc., Boston, MA.
The debate continues between the use of a metal fixation device versus the use of a resorbable fixation device in the stabilization of bone fractures. There are distinct disadvantages/advantages of both the metal internal fixation device versus the resorbable (PLGA) fixation device. The disadvantages of a PLGA fixation device can be overcome more easily than its rigid counterpart. It is possible that these disadvantages may be corrected through experimental manipulation and that a PLGA-based internal resorbable fixation device will be widely accepted for use in a clinical setting. It is our proposal to use a buffered PLGA rod that maintains structural integrity in order to provide adequate support throughout the healing process. It will also exhibit a loss of strength that will act inversely proportional as the strength of the bone increases. This will enable the bone to increase strength without relying totally upon the fixation device for support. The buffer will aid in the suppression of the rapid pH decline during the hydrolytic degradation process and eliminate any inflammation response. Through the preceding modifications, a buffered, resorbable internal fixation device should be attainable that will provide an acceptable amount of strength for stabilization without the local development of an inflammatory response.
AN INVESTIGATION OF THE INTERACTION OF AN ELECTROLESS SILVER PLATING SOLUTION WITH A BIOMEDICAL POLYURETHANE. J.E. Gray, K. Griffiths, and P.R. Norton, Interface Science Western and the Department of Chemistry, The University of Western Ontario, London, Ontario, CANADA.
There is a great deal of interest in the surface modification of biomedical polymers to decrease the rate of device associated infections without altering properties which affect their function. One possibility is to coat the material with an antibacterial agent such as silver. The interaction of an electroless silver plating solution with a polyurethane surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS) and Rutherford Backscattering Spectrometry (RBS). The RAIR spectra show a frequency shift of the polyurethane C-N and N-H vibrational bands. The XPS data, which show the presence of silver in more than one oxidation state, suggest that an interaction occurs between silver and the urethane groups at the surface through bonding to nitrogen, and that metallic silver is also present at the surface. RBS implies that the silver is present in the form of clusters, and scanning electron microscopy (SEM) and atomic force microscopy (AFM) are being used to image the physical state of the deposited silver.
IMPLICATIONS OF PROCESSING CONDITIONS ON MORPHOLOGY AND WEAR BEHAVIOR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE. Anuj Bellare, Department of Orthopedic Surgery, Brigham & Women's Hospital, Harvard Medical School, Boston, MA; Myron Spector, Department of Orthopedic Surgery, Brigham & Women's Hospital, Harvard Medical School, Boston, MA; Robert E. Cohen, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA.
Ultra-high molecular weight polyethylene (PE) is used as a bearing material which articulates against a metal or ceramic counterface in total joint replacement (TJR) prostheses. It is well known that the presence of wear particles of PE generated during articulation leads to biological complications (referred to as osteolysis) and loosening of the device. Wear of PE must be controlled to increase the longevity of TJR prostheses. PE components of TJR prostheses are usually machined from ram-extruded rod stock and compression molded sheets. While a large number of such components have been successfully used in TJR prostheses, processing conditions have not yet been optimized for wear performance of PE. In this study, we have induced a variety of processing histories on commercial GUR 1050 PE rod stock. Crystallization conditions have been varied to produce sheets with varying amounts of crystallinity (or density) and crystallite thicknesses. Since commercial high molecular weight PE resins are known to have a wide distribution of molecular weights, the spatial organization of PE macromolecules of different molecular weights is expected to vary with crystallization conditions. The morphology and the above-mentioned spatial organization of macromolecules are expected to alter the macroscopic properties, including wear behavior, of PE. We have characterized the morphology induced be each processing method using differential scanning calorimetry, small-angle x-ray scattering and low-voltage high resolution scanning electron microscopy. ASTM standard tensile tests have been performed to determine variations in macroscopic properties with processing conditions. The implications of these processing conditions on wear performance of PE components in TJR prostheses will be discussed.