J. BIOMED. MATER. RES. SYMPOSIUM

No. 6, pp. 221-225 (1975)

Composite Implants for Orthopedic Applications: In Vivo Evaluation of Candidate Resins M A R C L. JACOBS* and J O N A T H A N BLACK, Department of Orthopedic Surgery, University of Pennsylvania Medical School, Philadelphia, Pennsylvania Summary Three resins which include poly (methyl methacrylate). surgical Simplex P, and ultrahigh molecular weight polyethylene were selected to be evaluated as candidate systems for a polymer based composite for hard tissue prosthesis. Characterization of the mechanical behavior of these polymers in different environments including in vitro and in vivo storage was accomplished. As a result, conclusions were made as to which material maintained the least amount of mechanical variance as influenced by environmental effects. In vivo studies of implanted materials provided for a study of soft tissue response to each material. Conclusions were then developed as to the varying degrees of tissue reaction initiated by each material, and which resin generated the greatest tissue response with respect to the others.

INTRODUCTION The main objective of this project [ l ] was to select a polymer resin which would be appropriate for use as a matrix material in a polymer based composite for hard tissue prosthesis. Possible filling materials for this composite which include glass, quartz, ceramics, and graphite fibers have been discussed thoroughly with respect to mechanical properties and biocompatibility [2], [3]. Therefore, it was felt that characterization of candidate polymer systems in terms of their mechanical behavior in different environments would be a preliminary step toward achieving the objective. This is especially relevant when considering that the limiting factors for the overall strength of a composite (with a particular type of filling material) is determined by polymer matrix strength and interfacial bonding between polymer and filler. Environmental tests were chosen to evaluate the mechanical behavior of each polymer as influenced by in vitro and in vivo effects. Tests were developed to assess acute soft tissue response to the selected polymers. From this, conclusions could be made as to which material initiated the least inflammatory response. Three resins were chosen for this study; poly (methyl methacrylate) (PMMA), surgical simplex P (manufactured by North Hill Plastics Ltd. of London, England and distributed in the U.S. by Howmedica Inc. * Present Address: Extracorporeal Medical Specialties, Inc., King of Prussia, Pennsylvania. LL I

0 1975 by John Wiley & Sons, lnc.

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Rutherford, N.J.), and ultra-high molecular weight (UHMW) polyethylene (Hifax 1900, manufactured and supplied by Hercules Powder Co., Wilmington, Del.) Each material has been used extensively in or with devices for hard tissue prosthesis. PMMA has been used in dentistry for anterior restorations, composite restorations, and as a denture base material. Simplex P, which is a radiopaque methyl methacrylate styrene copolymer, was developed as a luting material for use with orthopedic prosthesis. UHMW polyethylene has been used almost exclusively as a bearing surface in orthopedic devices such as the total hip and total knee prosthesis. A review of mechanical data published for each material [4], [5] reveals that these polymers have sufficient strength to be reinforced with high strength filling materials to approach the mechanical properties of hard tissues [6]. METHOD

Testing of the candidate polymers was divided into five groups of environmental storage conditions. Two specimen designs within each group permitted compression and tensile testing. The five environmental test groups consisted of the following: 1 ) Specimens which were mechanically tested within 24 hr of fabrication (“as fabricated” group), 2) Specimens stored under laboratory conditions at room temperature for 120 days (“laboratory storage” group), 3) Specimens stored at 37°C and approximately 97% humidity for 120 days (“humidity storage”), 4) Specimens stored at 37°C in normal saline for 120 days (“saline storage”), and 5 ) Specimens implanted in the sacrospinalis muscle of 4.0 Kg white New Zealand rabbits for 120 days (implant study). The five test conditions can be divided into three categories. “As fabricated” and “laboratory storage” conditions represented the normal physical states of each material. “Humidity storage” and “saline storage” conditions depicted in vitro tests, and the implant study an in vivo test. I n vivo evaluation of the test materials was based on a successful short term soft tissue implant model [7]. After the 120 day test period, rabbits were sacrificed and implants were recovered for compression and tensile testing. Tissue surrounding each implant was prepared histologically for microscopic tissue pathology. I n this way tissue response to each material could be determined. Tension and compression testing was performed on an Instron machine at constant strain rate. Variables calculated from tensile data included fracture stress, and stress and modulus at 1%, 2%, and 3% total strain. Compression data was used to calculate ultimate yield stress, and stress, modulus, and strain energy per unit volume at 1%, 2%, and 3% total strain. These parameters were chosen to provide an accurate

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analysis of the characteristic behavior of each material in tension and compression. Statistical analysis of the data at the normal biological confidence level of 95% provided for the characterization of mechanical behavior of each material with respect t o environmental effects; the most critical comparisons being between in vitro and in vivo tests versus the normal physical state.

RESULTS Mechanical The effects of different storage environments on the mechanical behavior of Simplex P and PMMA in tension and compression were very similar. In many instances the mechanical parameters computed for PMMA stored in a particular test state were not statistically dissimilar to the same parameters generated from Simplex P data. It was therefore concluded that although Simplex P contained a methyl methacrylate styrene copolymer, the methyl methacrylate monomer common to both materials was the regulating factor in mechanical behavior. A comparison of data computed for samples for PMMA and Simplex P stored in vitro and in vivo with “laboratory storage” samples showed that a significant decrease in mechanical properties was caused by these wet environments. Tensile stress at different strain intervals and modulus had decreased approximately 20% and 25% respectively. Compression data demonstrated that in vitro tests induced the most dramatic degradation in mechanical properties as compared to “laboratory storage” samples; stress and modulus values at different strains decreasing by about 30%. This phenomenon was probably initiated by absorption of water into the polymer with a following plasticizing effect developing [8], [9]. Implantation of compression specimens also caused a degradation in compression properties with respect t o “laboratory storage” samples, but the decrease was not as great as that caused by in vitro conditions. Values of compression stress and modulus for implant specimens maintain a level approximately 10% higher than identical in vitro data. A question arises here as t o whether this implant behavior was initiated by absorption of substances peculiar t o this environment which caused a plasticizing effect in PMMA and Simplex P, or if sterilization effects altered these materials in such a fashion t o cause a difference between in vivo and in vitro samples. Strain energy per unit volume computed from compression samples of PMMA and Simplex P revealed an important trait. It was demonstrated that in vivo storage of these materials resulted in a decrease of strain energy by at least 25% over various total strains as compared to the strain energy per unit volume t o cause an identical amount of deforma-

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tion in “laboratory storage” specimens. This is a pertinent point to consider when designing a polymer based composite which must absorb energy generated by loading forces. What is expected t o occur in the normal physical state of the material may not necessarily be the case in vivo. The mechanical properties of polyethylene, although significantly lower than P M M A and Simplex P, did not exhibit dramatic responses to the environmental tests. There was no statistical dissimilarity between mechanical data accumulated for “laboratory storage” specimens and in vitro samples. However, statistical dissimilarity did exist between implant and “laboratory storage” specimens. This behavior was most prominent in tensile and compression stress data which shows that implant storage has increased the stress necessary t o deform polyethylene over various total strains. Implant stress values were approximately 15% higher than those for “laboratory storage” samples at an equivalent strain interval. The mechanism for this behavior raises similar questions as was the case in discussing the mechanical behavior of P M M A and Simplex P . Perhaps substances particular t o the in vivo environment were absorbed by polyethylene with a resultant antiplasticizing effect occurring. On the other hand, sterilization of polyethylene at temperatures close to its melting point would tend t o perfect the crystalline structure of the material and therefore make it stronger. Because all polyethylene compression samples were sterilized previous to storage in the test conditions, one would expect that the former principle merits further investigation. Histology

A capsule composed of dense fibrous tissue surrounded all implants. This fibrous membrane served to isolate the implant from normal muscle tissue. Qualitative measurements of capsule thickness, and the number of cells per unit area in the capsule were made. Pathological study of cells present in these capsules was also performed. Compared to P M M A and Simplex P samples, membranes surrounding polyethylene were the most inflammatory in nature. The overall capsule was thicker and number of cells per unit area larger than found in capsules surrounding P M M A and Simplex P samples. Large numbers of macrophage cells containing vacuoles populated the entire membrane surrounding polyethylene. These cells appeared t o be linked with the degradation of fibrous structure in the membrane. Further studies would be appropriate t o determine the nature of these vacuoles, i.e., storage or inclusion. A large number of cells, a small percentage of them being the inflammatory type, were present directly at the capsule-implant interface. Although polyethylene seemed to generate the greatest inflammatory

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response of all test materials, its capsule was composed mainly of fibroblasts while inflammatory cells maintained only a small portion of the cellular population. N o giant cells were observed in any membrane. Any final judgement on the tissue tolerance to polyethylene should not be made until questions such as the nature of the macrophages in the membrane can be answered. References [ I ] M. L . Jacobs, “Evaluation of Three Polymer Resins for Use in Polymer Based Composites for Hard Tissue Prosthesis,” M.S. Thesis, University of Pennsylvania, Philadelphia, Pa. (1974). [2] S. Musikant, J . Biomed. Muter. Res. Symp. No. 1. 225 (1971). [3] D. F. Williams, Biomed. Eng. 6 , 260 (1971). [4] W . J. Roff, Handbook of Common Polymers, Press, Cleveland, Ohio, 1971. [5] R. Treharne, “Creep and Recovery of Polymethyl Methacrylate Cements”, M.S. Thesis, University of Pennsylvania, Philadelphia, Pa. (1974). [6] F. G . Evans, Mechanical Properties of Bone, Charles C. Thomas Co., Springfield, I l l . , 1973. [7] P. G..Laing, J . Bone Joint Surg., 40A, 853 (1958). [8] M. Braden, Prosth. Dent., 14, 307 (1964). [9] R. I . Bowen, J . Amer. Denf.Ass., 69,481 (1964).

Composite implants for orthopedic applications: in vivo evaluation of candidate resins.

Three resins which include poly (methyl methacrylate), surgical Simplex P, and ultrahigh molecular weight polyethylene were selected to be evaluated a...
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