Characteristics of tissue growth into proplast and porous polyethylene implants in bone

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Abstract
One objective of this study was to determine why fibrous tissue, not bone, forms in internal pores of Proplast implants even though the material is labeled as having pore characteristics compatible with bone ingrowth. A second objective was to assess the suitability of the fibrous tissue in Proplast implants for supporting load‐bearing prostheses. Thirty cylindrical rods of Proplast were implanted through the lateral diaphyseal cortex of eight dogs for periods of 1 to 20 weeks. Porous high density polyethylene was used as a „control”︁ in the study since it is a low modulus polymer which does undergo bone ingrowth. Twenty rods of porous polyethylene (400 μm average pore size) were implanted in the experimental animals along with the Proplast. Postmortem specimens were prepared for microradiography and histological evaluation. Optical point‐counting techniques and mercury intrusion porosimetry were used to determine the pore characteristics of the materials. Microradiography and histological evaluation revealed bone growth throughout the porous polyethylene implants. In contrast, bone growth was only occasionally observed in surface pores of the Proplast. However, bone completely filled Proplast specimens which were teased to purposely enlarge the pores of the material prior to implantation. This observation, along with the pore analyses which revealed an average pore size of only 75 μm and interconnecting pores less than 50 μm in diameter, led to the conclusion that fibrous tissue, not bone, forms in Proplast specimens because of the small pore size of the material. Further, the results demonstrated that there was no collagen continuity of the fibrous tissue in the pores of the material with surrounding bone (e.g., no Sharpey' fibers) suggesting that the load‐bearing support which can be afforded by Proplast implants is limited by the incomplete bone ingrowth along the margins of the material and the tensile strength of Proplast.