Healing of osteochondral osteotomies after fixation with a hydroxyapatite-buffered polylactide. A histomorphometric and radiographic study in rabbits

The tissue response of subchondral bone to a biodegradable fixation device manufactured in the shape of a screw and made of polylactide with a hydroxyapatite buffer were implanted through the articular surface of the intercondylar portion of the distal rabbit femur. One screw was implanted per animal. The screws had a core diameter of 3.2 mm and an outer diameter of 4.5 mm. At insertion, the implants were cut flush with the articular surface. After follow-up times of 8 and 16 weeks, the specimens were examined radiographically and histomorphometrically. The intact contralateral femur served as a control for comparison. Only minimal signs of degradation of the polymer could be seen in the histologic specimens. These implant degradation sites were commonly areas of new bone formation adjacent to the screw implant. A brim of repair tissue was formed at the entrance and exit of the implant channel. The width of the repair tissue from the tissue-implant boundary towards the center of the entrance hole varied greatly between the specimens, from 80 to 750 microm. In most specimens this bridging tissue consisted of newly formed bone and undifferentiated mesenchymal tissue. Degenerative chondrocyte clustering occurred in the pre-existing cartilage within a 400 microm wide zone from the tissue-implant interface into the recipient tissues. Some new-bone formation was seen to envelop the implant in all specimens, but the fractional osteoid formation surface of the trabeculae was only significantly higher in the screw-implanted 16-week specimens, when compared to the non-operated contralateral controls. Although the bony osteotomy was invariably healed in all specimens with good implant integration, the quality and quantity of the reparative tissue of the articular cartilage near the screw hole was variable. This study showed that large polylactide implants, which are buffered with hydroxyapatite show benign tissue responses and good implant osteointegration when implanted in bone. They may be suitable for fixation of small bone fractures. However, insertion through intra-articular surfaces may require further improvement of the implant material to avoid the degenerative repair processes seen in this study.