Cartilage degeneration in post-collapse cases of osteonecrosis of the human femoral head: altered mechanical properties in tension, compression, and shear.

Osteonecrosis, or avascular necrosis, is a painful and debilitating condition characterized by progressive joint degeneration subsequent to collapse of necrotic regions of trabecular bone. A clear understanding of the mechanism of cartilage degeneration in osteonecrosis is critical to the development of treatment strategies aimed at sparing the femoral head. An analysis of 13 post-collapse osteonecrotic (ON) human femoral heads was performed relative to 24 non-ON controls to determine quantitatively the biomechanical and histological properties of post-collapse osteonecrotic cartilage. Cartilage mechanical properties were measured in tension, compression, and shear at different sites on the femoral head and correlated to histologic measures of cartilage degeneration using a semi-quantitative grading scale. Decreasing cartilage tensile strength correlated with histologic evidence of degeneration in the ON group; however, less correlation was noted with shear and compressive properties. After statistical correction for the more severe histologic degeneration in the ON group, the non-ON samples were found to exhibit significantly smaller loss angles during shear testing, while samples from the ON group were found to have greater tensile strength. Similarly, the ON group was found to exhibit significantly greater proteoglycan loss while the non-ON group showed significantly increased surface fibrillation. This study provides evidence that the changes occurring in post-collapse cases of osteonecrosis may involve mechanisms other than those typically attributed to osteoarthritic degeneration. One potential explanation is that decreased loading of cartilage overlying collapsed ON lesions leads to proteoglycan loss similar to that occurring with cartilage disuse. Unlike degenerative changes, some articular cartilage changes caused by disuse may be reversible. The presence of reversible changes would likely increase the chances of success of head sparing treatment strategies that rely on the viability and mechanical integrity of the articular cartilage.