Cam type femoroacetabular impingement - a cause of hip osteoarthritis and where to find it?

Introduction: Cam-type femoroacetabular impingement (FAI) morphology of the proximal femur is an abnormal contour of the head-neck junction, and it is associated with progression of primary osteoarthritis (OA). It can be measured objectively using plain radiographs. Patients undergoing total hip arthroplasty (THA) have an increased risk of developing OA in their contralateral hip.
Aim of the study: To investigate the frequency of Cam type FAI associated deformities in the contralateral asymptomatic hip of patients undergoing unilateral THA for primary OA compared with a healthy population using different x-ray views.
Materials and methods: A total of 164 patients were included (114– OA group; 50– control group). Patients in the OA group were included before undergoing THA and the controls received their x- ray on a control visit unrelated to hip. All patients (100% in both groups) received AP view; 112 in OA group (98.2%) and 45 in control group (90%) Dunn view; 46 in OA group (40.4%) and 29 in control group (58%) a lateral (LL) view. In both groups alpha angle, HN offset ratio and pistol grip deformity were 180 analysed using mediCAD 7.0 software. Cam deformity signs were defined as alpha angle >50 ̊ and/or HN offset ratio <0.17 and/or a positive pistol grip deformity.
Results: Groups were matched by age, gender and BMI. There were significant differences for mean alpha angles between OA and control group in all three x-ray views, highest results in AP (OA 59.75±10.38; control 51.84±8.28 p<0.001), lowest in Dunn (OA 53.49±7.76; control 45.98±6.55 p<0.001). HN offset ratio was significantly lower in OA group (0.153±0.051 LL; 0.151±0.043 Dunn) than in the control group (0.183±0.047 LL; 0.185±0.036 Dunn) with p<0.001. Pistol grip deformity was significantly more prevalent in OA group (25.8%) vs control group (2%) with p<0.001. In all views, odds ratio for the OA group to have a cam deformity was significantly higher than for controls.
Conclusions: Higher prevelance of Cam signs in the OA group shows that this morphological abnormality plays a role in the development of primary OA. Multiple x-ray imaging techniques, especially Dunn, are necessary for accurate diagnostics.