Composite resorbable polymer/hydroxylapatite composite screws for fixation of osteochondral osteotomies

The aim of this study was to evaluate biomechanically the healing of an osteochondral fragment created in the distal sheep femur in response to fixation with a resorbable composite screw made of polylactide and hydroxylapatite. Pure poly(L-lactide) screws were used for comparison. At follow-up times of 4 or 8 weeks, specimens were examined with standard radiography, biomechanics, and histology. The intact contralateral femur served as a control. Only minimal signs of polymer degradation were seen in the histologic specimens. At 8 weeks, most osteotomies had healed completely and there was no difference in compressive strength and elastic modulus of cylindrical cores between the two types of biodegradable implants used. The width of the repair tissue at the tissue-implant interface was 250+/-50 micro m representing a clear transition zone of newly formed trabecular bone separating the implant from the surrounding plexiform bone. We conclude that relatively large polylactide implants, blended with hydroxyapatite, are capable of fixing an osteochondral fragment in an animal model. Biomechanical data assessing the quality of the bone formed at the osteotomy sites were found to be equivalent when compared to the control poly(L-lactide) implants of similar design and size. In addition, hydroxylapatite composite implants showed benign tissue responses and good implant osteointegration. Results suggest that hydroxylapatite composite screw implants can be used for similar indications as pure poly(L-lactide) implants in current clinical use.