This article addresses the problem of structural design with osteochondral grafts used for cartilage resurfacing.
Photooxidized cylindrical or mushroom-shaped grafts were surgically implanted in the weight bearing area of the medial and lateral femoral condyles of eight sheep (condyles: N=8/group). Both types of photooxidized grafts contained no viable chondrocytes at the time of implantation. Results were evaluated at 2 and 6 months after surgical implantation of the grafts. Qualitative and quantitative evaluation of the subchondral bone area was performed using plastic embedded sections of non-decalcified bone and cartilage specimens and placing emphasis on graft anchorage, cyst-like lesions at the base of the cartilage junction and at the base of the graft in the subchondral bone region. Cartilage morphology was studied qualitatively focusing on viability of the graft and adjacent host cartilage, while a score system was developed for semi-quantitative evaluation of the overall articular cartilage performance. The semiquantitative scores and histomorphometrical measurements were subjected to statistical analysis using a factorial analysis of variance (ANOVA-test).
The photooxidized mushroom-shaped grafts developed less fibrous tissue and cyst-like lesions in the subchondral bone area at 2 and 6 months compared to the cylindrical grafts. Areas of endochondral ossification and bone remodeling were noticeable in the mushroom structured grafts at 2 months, and also bone remodeling was more complete at 6 months than with the cylindrical grafts. Increased numbers of cells were seen in the basal remodeling zones of both graft types increased from the 2 months to the 6 months specimens, but mushroom structured grafts showed better results. In both graft types, however, the midzone of the cartilage matrix was still acellular at 6 months. Cells from the subchondral bone area started to penetrate the calcified cartilage zone and tide mark at 2 months and repopulated the old photooxidized cartilage matrix already at 6 months after implantation. Cartilage repopulation was dependent on a stable subchondral bone area in both types of grafts. Matrix degradation of the adjacent host cartilage was minimal at 2 and 6 months. At 6 months a junction between host and graft cartilage was already noticed in some of the mushroom-shaped grafts.
This study confirmed the importance of the subchondral bone area for osteochondral graft survival. In addition it demonstrated that the structure of the graft influences considerably the architecture of the subchondral bone, and with this the possibility for the repopulation of the old cartilage matrix including the junction between the host and graft cartilage matrix.