Is the damage of cartilage a global or localized phenomenon in hip dysplasia, measured by dGEMRIC?

BACKGROUND: The mechanism of damage in osteoarthritis is believed to be multifactorial where mechanical and biological factors are important in its initiation and progression. Hip dysplasia is a classic model of increased mechanical loading on cartilage attributable to insufficient acetabular coverage that leads to osteoarthritis. If the damage is all attributable to direct mechanical damage then one initially would expect only local, not global changes. QUESTIONS/PURPOSES: We hypothesize that in hip dysplasia although the elevated cumulative contact stresses are localized, the damage to cartilage is biologically mediated, therefore, biochemical changes will be global.
METHODS: Thirty-two patients with symptomatic hip dysplasia were scanned using a 1.5-T MRI scanner. We used a high-resolution three-dimensional dGEMRIC technique to characterize the distribution of cartilage damage in dysplastic hips. High-resolution isotropic acquisition was reformatted around the femoral neck axis and the dGEMRIC index was calculated separately for femoral and acetabular cartilages. Joint space widths also were evaluated in each reformatted slice. Each hip was characterized by the presence or absence of joint migration and by Tönnis grade.
RESULTS: The global dGEMRIC index correlated with the dGEMRIC indices of individual regions with the highest correlations occurring in the anterosuperior to posterosuperior regions. The corresponding correlations for joint space width were uniformly lower, suggesting that tissue loss is a more local phenomenon. Higher Tönnis grades and hips with joint migration were associated with lower dGEMRIC indices.
CONCLUSIONS: The dGEMRIC index shows a global decrease, whereas tissue loss is more localized. This suggests that hip osteoarthritis in acetabular dysplasia is a biologically mediated event that affects the entire joint.