FEMOROACETABULAR IMPINGEMENT AND THE CLASSIFICATION OF CAM DEFORMITY: NORMAL, BORDERLINE AND ABNORMAL VALUES IN THE ASYMPTOMATIC POPULATION

Abstract

Introduction: Femoroacetabular impingement (FAI) is an important cause of hip pain in young adults and a precursor to osteoarthritis. Morphological abnormality of either the acetabulum, proximal femur, or both, may result in FAI. The majority of patients however, have a cam deformity of the proximal femur. From a research perspective, FAI is an example of how subtle morphological abnormality results in a predictable pattern of cartilage damage, and thereby offers great potential as a model to study early degenerative disease and for clinical trials of joint preserving treatments. Accurate classification of the morphology of the hip is essential for this further study.

The aim of this study was to define normal, borderline, and abnormal parameters for the morphology of the proximal femur, in the context of the cam deformity, by studying asymptomatic individuals with normal clinical examination and no osteoarthritis from the general population.

Methods: 157 individuals (79 male, 78 female, mean age 46 years) with no previous history of hip problems were recruited. The participants were the spouses/partners of patients involved in a cohort study of osteoarthritis and FAI. All participants underwent clinical (interview, examination, and hip scores) and radiological assessment (standardised AP Pelvic and cross-table lateral radiographs of each hip). Radiographs were scored for the presence of osteoarthritis, and the morphology of the proximal femur was analysed. The alpha angle, anterior offset ratio, presence of a cam ‘bump’, synovial herniation pit were recorded in each hip.

Results: 21 subjects were excluded because they either had positive clinical features or radiological evidence of osteoarthritis (equivalent to a Kellgren and Lawrence grade of 2). From the remaining 136 subjects, with essentially ‘normal hips from the general population’, mean values for the alpha angle and anterior offset ratio were generated. Borderline and Abnormal values are suggested. Gender differences were noted with higher mean alpha angles and lower offset ratios occurring in men.

Discussion: Although it has limitations, standardised plain radiography remains the cheapest and most convenient way to screen an individual for the presence of a cam deformity. Despite the recent interest in FAI, our knowledge of what is normal in the general population, as assessed using appropriate radiographic techniques, is modest. The ranges provided by this study will facilitate the accurate classification of subjects with FAI, thereby providing guidance for surgeons treating such patients, and also enable the generation of refined cohorts for the study of the natural history of subtle morphological abnormalities of the hip, and for enrolment in clinical trials.