The Effects of Porosity on in Vitro Degradation of Polylactic Acid–Polyglycolic Acid Implants Used in Repair of Articular Cartilage

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Abstract
The objective of this study was to examine the effects of porosity on in vitro degradation characteristics of an osteochondral biodegradable implant made of a 50:50 copolymer of polylactic acid–polyglycolic acid. Ninety specimens were produced to represent three porosities: approximately 0% (low porosity), 33% (medium porosity), and 75% (high porosity). These specimens were allowed to degrade in phosphate-buffered saline at 37°C for 8 weeks or until essentially complete degradation was achieved. Degradation of the specimens was examined using creep indentation mechanical testing, pH measurements, mass loss quantification, molecular weight loss measurements, gross morphology, and scanning electron microscopy. The results indicate that specimen porosity has important implications insofar as implant structural properties are concerned. In general, low-porosity implants underwent significantly larger losses in molecular weight than medium- or high-porosity specimens. Although no significant decreases in mass loss were found for any group for up to 4 weeks, between 4 and 6 weeks a significant drop in mass was observed in all groups. This drop was most severe in the low-porosity group. Mechanical testing showed that high- and low-porosity specimens were two to three times stiffer (Young's modulus and shear modulus) than medium-porosity specimens at 0 weeks. Measurement of pH demonstrated that with decreasing porosity, the implant pH values became significantly more acidic. It thus appears that low-porosity specimens undergo faster degradation than either medium- or high-porosity specimens. As a result, low porosity may contribute to shortening of the functional life of the implant. This may be due to enhanced autocatalysis in those implants, which are unable to evacuate acidic degradation by-products.