In this study, the prevalence of primary OA of the hip was 16.1% based on the diagnostic criteria of Crowe’s classification type 1,16 Sharp’s angle < 45°,17 and a CE angle > 25°.18 Nakamura et al. reported in 1989 that primary OA was detected in 13 cases (0.9%) out of 2,000 consecutive cases diagnosed with hip OA24. The diagnostic criteria included the absence of femoral head deformity, CE angle > 19°, Sharp’s angle < 45°, and acetabular roof obliquity < 15° 25. This low prevalence could be influenced by the patient population, which was collected from a specific outpatient clinic for the treatment of acetabular dysplasia. Hoagland et al. evaluated 200 consecutive Japanese patients in Japan and 199 consecutive white American patients in the USA, all of whom were admitted for hip surgery26. They reported in 1985 that the prevalence of primary OA was 18% in Japanese patients and 90% in white American patients26. Recently, primary OA of the hip was reported to be increasing in Japan8. The Japanese Arthroplasty Register reported the percentage of primary OA patients referred for primary total hip arthroplasty was 16.3% in 2013, 21.5% in 2015 and 26.6% in 2017 8.
In 2010, Jingushi et al. conducted a multi-institutional examination of patients with hip OA who were newly admitted to the orthopedic outpatient clinic in Japan. They reported the prevalence of primary OA was 9% (44 out of 485 hips) 4. Thus, the prevalence of primary OA of the hip has been reported variously in Japan. The prevalence of primary OA might differ depending on the differences in diagnostic criteria and patient populations. In this study, the prevalence was 16.1% but if the hips in group L are excluded from the diagnosis of primary OA, the prevalence dropped to 7.5%, similar to the data reported by Jingushi et al.4
Femoral head lateralization in our study was recognized in approximately half of all primary OA cases (53.3%). Hartofilakidis et al. reported that 80% (218/272) of primary hip OA had eccentric,1 and eccentric type (or superior migration), which included superolateral or superomedial migration. Nakamura et al. reported that eight hips (62%) demonstrated superolateral type.14 In our study, femoral head lateralization was defined as the distance between the ilioischial line and the medial aspect of the femoral head ≥ 10 mm. The prevalence of femoral head lateralization in primary OA in our study was similar to the result reported by Nakamura et al.14 and less than Hartofilakidis et al., which may suggest the possibility that a racial difference might influence the prevalence of femoral head lateralization.
The etiology of femoral head lateralization in patients with primary OA is uncertain. Nakamura et al. reported that the superolateral type of primary OA develops from the subset of normal hips with a greater degree of acetabular roof obliquity.14 Our radiographic evaluation demonstrated that CE angle, ADR, and AHI of group L were significantly less than those of group N, and the FNA of group L was significantly larger than that of group N. Furthermore, the radiographic evaluation in the patients with unilateral primary OA demonstrated that the FNA of the affected side was slightly larger than that of the unaffected side, and the ADR of the affected side was significantly less than that of the unaffected side. CE angle and AHI are reduced by femoral head lateralization. Regarding ADR, dysplastic hips usually have a smaller ADR than normal hips and femoral head lateralization often occurs in dysplastic hips 27, 28. Thus, primary OA with femoral head lateralization might be a boundary condition between primary and dysplastic OA. Regarding FNA, Pauwels and Maquet reported that larger femoral neck-shaft angles might induce a laterally-directed joint reaction force to potentiate hip instability 27, 29. Therefore, a larger FNA might have the potential role of femoral head lateralization in primary OA. Eventually, the smaller ADR (shallow acetabulum) and larger FNA (coxa valgus) might induce hip joint instability and correlate with the development of OA associated with femoral head lateralization.
The effect of femoral head lateralization on the clinical course in primary OA is unclear in this cross-sectional study. In dysplastic hips, femoral head lateralization correlates strongly with the development of hip OA 1, 28. Harris et al. propose that femoral head lateralization induces greater hip joint pressure to maintain stabilization of the joint 28. Hartofilakidis et al. reported that hips with concentric idiopathic OA underwent THA on average 10 years after symptom onset; by contrast, eccentric hips underwent THA on average four years after symptom onset.1 Thus, femoral head lateralization might accelerate the osteoarthritic change of the hip and advance the timing of THA. Therefore, femoral head lateralization might strongly correlate to the instability of the hip and lead to the development and progression of OA.
Our study has several limitations. First, the radiographic review was based only on AP radiographs. However, we consider that it is essential for physicians to acquire common and reliable radiographic views as well as parameters for plain radiographic assessment that can serve as a foundation for accurate diagnosis, disease classification, and surgical decision-making. Second, osteophytes of the proximal femur and acetabulum are variable, such that measurement errors in radiographic factors could occur. However, the data were reviewed by two experienced orthopedic surgeons, and measurements for cases with complex imaging findings were made following discussion. Third, the lateralization of the femoral head was determined by the distance between the ilioischial line and the medial aspect of the femoral head. The distance of 10 mm should be considered a general reference number as opposed to a strict parameter, as magnification errors and variability in patient size can influence this measurement. Fourth, FNA might be influenced by the rotation of the lower extremity 30. However, FNA was measured using AP pelvic radiographs performed with the lower limb in internal rotation, thus placing the patella in the frontal plane. Furthermore, FNAV measured by CT in both groups was approximately 15° and there was no significant difference between the two groups. These relatively small values of FNAV were considered to be negligible in terms of the measurement of FNA. Fifth, the stages of OA varied, and we did not distinguish among them. There is a possibility that femoral head lateralization can progress over time in accordance with the stage of OA.