CORR Insights®: Is There an Association Between Borderline-to-mild Dysplasia and Hip Osteoarthritis? Analysis of CT Osteoabsorptiometry

Where Are We Now?

Hip osteoarthritis (OA) is a common ailment that causes pain, decreased mobility, and restrictions on activities of daily living [1]. It currently affects approximately 8% of the US population, and its medical, social, and economic consequences both in the United States and worldwide are likely to increase in the years to come [1]. While we have good information about the prevalence of hip OA [1, 3, 7, 8], its etiology remains unclear [1], making it more difficult for surgeons to determine whether and when nonarthroplasty procedures like periacetabular osteotomy or arthroscopy are likely to be helpful, especially among patients with borderline or mildly dysplastic hips. Clinical studies have shown that early hip arthritis may be associated with unfavorable distribution of contact stress over the weight bearing area in patients with hip dysplasia, primary hip arthritis, and avascular necrosis of the femoral head [5]. Microscopic analyses have associated stress distribution with cartilage and subchondral bone deterioration [6].

In the current study, Irie and colleagues [4] used CT absorptiometry to evaluate subchondral bone density (as a surrogate for cumulative stress and loading abnormalities), and found important differences in severity and patterns of loading based on the severity of dysplasia. They divided the population into four groups according to the extent of dysplasia from mild to severe. In all groups, the highest density was found in the centrolateral region of the acetabulum where contact stress is highest. However, in hips with severe dysplasia, the region of higher density remained on the lateral side, but gradually spread medially as the severity of the dysplasia decreased.

With these findings, Irie and colleagues [4] have provided a link to previous studies [2, 5], which showed that the weight bearing area extends medially with increasing coverage of the femoral head by the acetabulum.

Where Do We Need To Go?

A collaboration between experts in medicine, biophysics, and biomechanics could potentially provide a pathway to better treatment options that are customized to each individual patient. But in order to provide better treatment and preventive measures for patients with hip dysplasia, we need to clarify the mechanisms that lead to hip arthritis. The current evidence suggests that in patients with hip dysplasia, the problem is one of abnormal loading (a smaller weight-bearing area results in higher contact stresses in those areas, resulting in cartilage breakdown). However, questions remain: How should we measure hip stress? Should classifying hip dysplasia be based on hip stress distribution? Beyond these questions, we also need to determine whether the majority of patients with primary hip arthritis have unfavorable stress distribution.

How Do We Get There?

To further validate the connection between abnormal loading and hip dysplasia, researchers should conduct clinical studies that examine stress distribution based on mathematical models that capture the relevant features of abnormal loading. We can estimate contact stress by using previously developed mathematical models and some geometrical parameters based on our patients’ radiographs. Center-edge angle is the most important parameter for calculating contact stress, but mathematical models have shown that in the grey zone, between 15° and 25°, other geometrical parameters such as the lateral extent of the greater trochanter, the pelvis width and height, the inter-hip distance, and the size of femoral head should also be considered when calculating stress.