Femoral acetabular impingement: the sex- and age-linked distribution of alpha angles in 146 patients of service age without pre-existing osteoarthritic or other hip pathology.

The use of the alpha angle to help the diagnosis of Femoral Acetabular Impingement (FAI) is common. However, there is currently no standard value available across an asymptomatic pre-arthritic population. We present the first large cohort of Computerised Tomography (CT) based alpha angles in patients with no history of hip pathology, including intra- and interobserver validation. We carried out a retrospective analysis of 73 consecutive individuals (146 hip joints) with ages ranging from 18 to 39 years. The age range 18-39 represents 82.4% of those currently serving in the UK Armed Forces. The cohort was drawn from those patients who had received a CT scan in the Lothian Region between 1 Jan 2011 and 31 Dec 2011 due to abdominal pathology. These patients had their electronic patient record checked to rule out any hip-related problems. The alpha angle of Nötzli was measured on the axial view bilaterally. The mean value for the 18-39 age range was found to be 51.89 degree for the left hip and 52.53 degree for the right. Femoral alpha angle is a reproducible measurement for assessing the femoral neck. However, there is wide variability in the alpha angle for patients, irrespective of the presence of symptoms. Our results would suggest that the alpha angle alone should not be used to diagnose FAI in service personnel, as even large angles may be normal.