Pelvic Incidence and Femoroacetabular Impingement - A Novel Relationship

Abstract

Pelvic Incidence (PI) is a fixed anatomic parameter that plays an integral role in the sagittal balance of the spine and in maintaining stable posture while expending a minimum of energy - conus of economy. While PI has been studied in relation to the spine, PI has yet to be studied as it relates to disorders of the hip. We propose that abnormal PI is associated with femoroacetabular impingement (FAI). We performed a retrospective analysis of 50 consecutive patients (60 hips) who underwent hip arthroscopy for labral tears secondary to FAI. There were 31 women and 19 men. The average age was 33.1±8.7. All patient's underwent hip CT evaluation as well as plain radiographs. PI, acetabular version (AV) at a 3-o'clock position, alpha- angle, center-edge angle (CEA) and acetabular index (AI) were all measured. The PI was measured using scout lateral radiographs from the CT scan. Cam Impingement was defined as a bump on the head neck junction with an alpha-angle > 55°. Alpha angles were measured on all radiographs available as well as the oblique femoral head neck CT reformatted views and the largest number was used. Pincer Impingement was defined as global overcoverage with an AI < 0° or a CEA > 39° or as focal overcoverage/retroversion with an AV < 14°. The AI and the CEA were measured on AP x-rays. The AV was measured on the available CT imaging. Our cohort was compared to a historical control: Vialle et al. JBJS 2005. Vialle et al. measured spinopelvic measurements including PI on 300 asymptomatic patients (without spine or hip complaints). Student t-tests were used to compare all groups. Our mean PI was 50.8±11.3, less than the asymptomatic historical control (n=300) with a mean PI of 55.0±10.6 (p=0.01). Patients with global overcoverage (n=24) had a mean PI of 47.3±8.9, less than those without global overcoverage (n=36) with a mean PI of 53.2±12.3 (p=0.02). Patients with retroversion (n=23) had a mean PI of 45.6±9.9, less than those without retroversion (n=37) with a mean PI of 54.1±11.1 (p=0.003). Patients with cam impingement (n=39) had a mean PI of 51.1±9.8 compared to those without cam impingement with a mean PI of 49.6±14.0 (p=0.7). Pelvic incidence has been well described in the spine literature and has been shown to play an important role in spine pathology. The investigation of PI as it relates to the native hip is novel. Patients with labral tears due to FAI, particularly those with pincer impingement, have a smaller PI than an asymptomatic population. PI is the sum of sacral slope and pelvic tilt. Sacral slope and pelvic tilt change with position while PI is constant. With a smaller PI, the amount of change in sacral slope and pelvic tilt is limited. Thus, with a smaller pelvic incidence there is less range of motion between the lumbar spine and the pelvis around the hip joints. This restriction of movement and subsequent biomechanical adaptations may affect hip development, FAI, and resultant labral tears.