Correlation between bone apposition of the acetabular rim and morphological indicators of femoroacetabular impingement in the japanese asymptomatic population

Abstract

Purpose: Acetabular rim ossification has been generally accepted as an osteophyte, which appeared in advanced osteoarthritis of the hip. On the contrary, it is suggested that bone apposition of the acetabular rim was induced by femoroacetabular impingement (FAI), abnormal abutment of the femoral neck against the acetabulum. FAI has been well described as a pathologic process that can lead to early osteoarthritic changes. To date, the association between the bone apposition of the acetabular rim and FAI is still unknown. The purpose of this study is to evaluate the association between the presence of bone apposition of the acetabular rim and morphological indicators of FAI based on computed tomography (CT) examination. Methods: We reviewed the CT scans of 245 consecutive subjects (490 hips, age: 21–83 years) who underwent abdominal and pelvic CT from September, 2010 to June, 2011. These subjects were mainly examined for abdominal disorders. We confirmed that no subject had any symptoms for the hip joint by the questionnaire survey. We reviewed them to evaluate the presence of bone apposition of the acetabular rim and the morphological abnormalities of the femoral head and acetabulum. The multi-detector CT adjusted for the anterior and lateral inclination of the acetabulum was obtained. On these images, 3 radial sections, antero-, mid-, and postero-superior, were determined at 30 degrees interval. Each section passed through the midpoint of the acetabulum and crossed vertically to the acetabular margin. Bone apposition of the acetabular rim was diagnosed by the recess sign, which is characterized as a gap between the native acetabular rim and the extension of the osseous apposition. We measured following three signs as indicators for FAI: a angle, center edge angle (CE angle), and acetabular index (AI). At the time of measurement, the position of the pelvis was standardized. We defined an anterior pelvic plane involving the anterior superior iliac spines and the top of the pubis as a reference plane (coronal section). Mann–Whitney U-test was used for statistical analysis. Results: Bone apposition was observed in 156 hips (32%) in total of the three parts. The prevalence at each part, anterior, middle, and posterior was 3.4%, 13.6%, and 24.6% respectively. When we divided the subjects into the younger group (<60 years old) and the elderly group (S60years old), there was significant difference at the prevalence of bone apposition between two groups (younger: 18.3%, elderly: 44.6%; p < 0.01). In terms of the a angle and CE angle, there was no significant difference between the subjects with and without bone apposition (a angle: 45.8 7.3 vs 43.4 8.4 , CE angle: 32.6 5.5 vs 31.7 8.1 , respectively). Interestingly, there was different trend in AI between the younger and elderly groups. The younger group showed no significant difference between the subjects with and without bone appositionwith the AI of 3.1 3.7 vs 4.3 5.2 , whereas the elderly group showed significant difference with the AI of 4.4 2.1 for with the bone apposition group vs 2.9 3.4 for without bone apposition group, p < 0.05. Conclusions: Two different types of FAI have been described, one is cam impingement, which is caused by small offset of femoral head neck junction, and the other is pincer impingement that is attributed by the acetabular over-coverage. Pincer impingement was thought to be associated with ossification of the acetabular rim. It was reported that the bone apposition in FAI initiated from subperiosteal bone formation at the rim, which was distinct from osteophyte formation in OA, which is a result of endochondral ossification from articular cartilage. In the present study, we showed that the prevalence of bone apposition was 32%. It was observed more frequently at the posterior part of the acetabulum and in the elderly subjects of asymptomatic Japanese population. Moreover, statistical analysis revealed significantly larger AI, means shallow joint, in the subjects with bone apposition in the elderly population. These results indicate that the bone apposition of the acetabular rim may have degenerative characteristics without clinical symptoms and might be associated with the instability of the hip joint.