The natural history of focal cartilage defects is not entirely understood, but defect size is believed to be a critical factor in the success of surgical repair procedures. Clinical repair algorithms have identified 2 cm2 as the threshold area between marrow-stimulation techniques and cartilage restoration techniques, although there is little evidence to support this threshold. Studies examining this threshold size have used circular defects, while narrow, elongated defects are more clinically relevant. Furthermore, our clinical experience suggests that two defects of approximately equal size, but oriented differently, will exhibit different repair outcomes.
We first sought to identify how cartilage defect area, location, and orientation influence subchondral bone contact within oval-shaped defects. We used cylindrical punches to create bilateral oval defects with areas between 0.73 cm2 and 2.88 cm2 on the femoral condyles of twelve bovine knees. Four defect groups were examined, each comprised of defects in one of two orientations, anterior-posterior or medial-lateral, on the medial or the lateral femoral condyle. We statically loaded fully-extended joints in a uniaxial testing system, while thin-film sensors recorded joint contact, and we calculated the area within the defect demonstrating subchondral bone contact. We then performed a three-way analysis of variance (ANOVA) and determined that defect area, location, and orientation each had a significant effect on subchondral bone contact, and significant interactions were found between defect area and both location and orientation. We also determined that each defect group exhibited a different area threshold where significant contact first occurred. The findings of this study challenge current clinical algorithms that use defect area alone to dictate one cartilage surgery over another.
Cartilage defect size, location, orientation may impact the treatment of focal cartilage defects, but there are no methods to accurately measure these quantities intra-operatively. To address these limitations, we developed an image-free surgical navigation system capable of accurately measuring defect area, shape, orientation, and condyle curvature intra-operatively and arthroscopically. Hardware was also developed to provide a less-invasive means of attaching optical reference frames to bone.
We developed test methods to evaluate the accuracy and repeatability of the manual measurements and the navigation system. Seven volunteers measured the area and orientation of various shapes and the curvature of multiple surfaces, first using a common arthroscopic probe tip as a reference length, and then using the navigation system. Measurement error of the navigation system was smaller than that of manual measurements for all tasks and was less variable as well. The mean error for navigated area measurements was -0.36 ± 0.37 cm2, compared to 1.13 ± 1.50 cm2 for manual measurements. The mean error for navigated radius of curvature measurements was 0.48 ± 6.07 mm, compared to 18.00 ± 42.72 mm for manual measurements. The mean error for navigated measurements of orientation was 2.69° ± 2.76°.
This navigation system will be a valuable research tool to elucidate how various defect-specific factors influence cartilage repair success. In the future, the system will allow surgeons to accurately characterize cartilage defects, with the goal of ultimately improving clinical outcomes.