Microporous calcium phosphate ceramics as tissue engineering scaffolds for the repair of osteochondral defects: Biomechanical results

Abstract

This work investigated the suitability of microporous β-tricalcium phosphate (TCP) scaffolds pre-seeded with autologous chondrocytes for treatment of osteochondral defects in a large animal model. Microporous β-TCP cylinders (Ø 7mm; length 25mm) were seeded with autologous chondrocytes and cultured for 4weeks in vitro. Only the upper end of the cylinder was seeded with chondrocytes. Chondrocytes formed a multilayer on the top. The implants were then implanted in defects (diameter 7mm) created in the left medial femoral condyle of ovine knees. The implants were covered with synovial membrane from the superior recess of the same joint. For the right knees, an empty defect with the same dimensions served as control. Twenty-eight sheep were split into 6-, 12-, 26- and 52week groups of seven animals. Indentation tests with a spherical (Ø 3mm) indenter were used to determine the biomechanical properties of regenerated tissue. A software-based limit switch was implemented to ensure a maximal penetration depth of 200μm and maximal load of 1.5N. The achieved load, the absorbed energy and the contact stiffness were measured. Newly formed cartilage was assessed with the International Cartilage Repair Society Visual Assessment Scale (ICRS score) and histomorphometric analysis. Results were analysed statistically using the t-test, Mann–Whitney U-test and Wilcoxon test. Statistical significance was set at p<0.05. After 6weeks of implantation, the transplanted area tolerated an indentation load of 0.05±0.20N. This value increased to 0.10±0.06N after 12weeks, to 0.27±0.18N after 26weeks, and 0.27±0.11N after 52weeks. The increase in the tolerated load was highly significant (p<0.0001), but the final value was not significantly different from that of intact cartilage (0.30±0.12N). Similarly, the increase in contact stiffness from 0.87±0.29Nmm−1 after 6weeks to 3.14±0.86Nmm−1 after 52weeks was highly significant (p<0.0001). The absorbed energy increased significantly (p=0.02) from 0.74×10−6±0.38×10−6Nm after 6weeks to 2.83×10−6±1.35×10−6Nm after 52weeks. At 52weeks, the International Cartilage Repair Society (ICRS) scores for the central area of the transplanted area and untreated defects were comparable. In contrast, the score for the area from the edge to the centre of the transplanted area was significantly higher (p=0.001) than the score for the unfilled defects. A biomechanically stable cartilage was built outside the centre of defect. After 52weeks, all but one empty control defect were covered by bone and a very thin layer of cartilage (ICRS 7 points). The empty hole could still be demonstrated beneath the bone. The histomorphometric evaluation revealed that 81.0±10.6% of TCP was resorbed after 52weeks. The increase in TCP resorption and replacement by spongy bone during the observation period was highly significant (p<0.0001). In this sheep trial, the mechanical properties of microporous TCP scaffolds seeded with transplanted autologous chondrocytes were similar to those of natural cartilage after 52weeks of implantation. However, the central area of the implants had a lower ICRS score than healthy cartilage. Microporous TCP was almost fully resorbed at 52weeks and replaced by bone.