Differences in presence of radiographic features between two regions of the hip in patients with hip osteoarthritis

Purpose: It is unknown how radiographic features are distributed over the superior-lateral (SL) or central-axial (CA) region of the hip joint in patients with hip osteoarthritis (OA). Knowledge on the distribution of radiographic features over the SL and CA region of the hip may give more insight into underlying pathological mechanisms. Underlying mechanisms might be found in biomechanical factors (obesity, muscle weakness) or systemic inflammation (erythrocyte sedimentation rate (ESR) and c-reactive protein (CRP)). In this study we determined the difference in distribution of radiographic features over the SL and CA region of the hip in patients with hip OA and we compared demographic and clinical characteristics between patients with different distributions of radiographic features of hip OA. Methods: Hip OA patients (N = 78) from the Amsterdam Osteoarthritis (AMS-OA) cohort provided data. The SL and CA region of the hip were examined on weight-bearing standard anterior posterior and ‘false profile view’ radiographs. The following radiographic features were examined: joint space narrowing (JSN), osteophytes, sclerosis and cysts. Patients were grouped according to the location of the highest frequency of radiographic features into SL- or CA-hip OA. If the radiographic features were equally distributed over the two regions the patients had unclassified (UN) hip OA. The following clinical characteristics were assessed: body mass index (BMI), inflammatory markers (ESR and CRP), isokinetic strength of the quadriceps and hamstrings, performance based physical function (timed get-up-and-go test (GUG-test) and stair climbing test) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire. Differences in frequency of radiographic features between the regions were tested with a Fisher's exact test, differences in clinical characteristics were tested with analysis of variance (ANOVA) and chi-square test. A P-value <0.05 was considered statistical significant. Results: The mean age was 62.8 (SD = 10.2) years and a total of 59% of the patients were women. Most hips had a KL grade of 1 (doubtful) or 2 (moderate) (37.9% and 43.2% respectively). The majority of patients (89.7%) had bilateral hip OA. JSN was the most frequent feature (>80% in both the SL and CA region). Significant differences between the SL and CA regions were found for osteophytes, sclerosis and cysts in the left hip (P = 0.012, P = 0.013 and P<0.001 respectively) and for the right hip (P = 0.001, P = 0.017 and P<0.001 respectively). For both hips the highest frequency of these features was found in the SL region of the hip. The majority of the hips had SL-hip OA (left 47.9%, right 56.0%) or showed an equal distribution of radiographic features over the two regions (unclassified (UN)) (left 37.0%, right 29.3%). In only a small part of the hips the radiographic features were most abundant in the CA region (left 15.1%, right 14.7%). The majority of the patients with bilateral hip OA had an equal indication for the left and right hip (either SL-SL, CA-CA or UN-UN) (62.9%). No significant differences in clinical characteristics were found between patients with SL-, CA- or UN hip OA. Conclusions: JSN is the most prominent radiographic feature and is equally present in the SL and CA region. Therefore, JSN did not contribute to regionalization. The finding that osteophytes, sclerosis and cysts are more present in the SL region indicates that mechanical loading contributes to the OA process in the SL region, since these features are associated with stress redistribution. However, no differences in clinical characteristics between patients with different distributions of radiographic features are found. Alternative radiographic classification systems using more patients with predominant radiographic features in the CA region should be used in further studies.