Tonnis Angle and Acetabular Retroversion Measurements in Asymptomatic Hips Are Predictive of Future Hip Pain: A Retrospective, Prognostic Clinical Study

BACKGROUND This study evaluated the prevalence of radiographic abnormalities potentially indicative of femoroacetabular impingement on AP pelvic radiographs in asymptomatic adolescents and young adults and aimed to determine whether the abnormalities were predictive of future hip pain. METHODS AP pelvis images from scoliosis radiographs were obtained from patients 12 to 25 years of age free of any clinical hip/lower extremity symptoms between January 2006 and September 2009. The following radiographic abnormalities were collected: lateral center-edge angle of Wiberg >40° or 10°, acetabular retroversion (crossover sign with a posterior wall sign), acetabular overcoverage (crossover sign without a posterior wall sign), and anterior offset alpha angle, calculated using alpha angle of Notzli >50°. Patients were retrospectively followed (average 3.11 years) to identify those who subsequently developed hip pain. RESULTS Of the 233 patients (466 hips) who were asymptomatic at the time of radiographic evaluation, at least one radiographic abnormality was present in 60% (281/466) of the hips. Within that group of hips (n = 281), 69% (195/281) of hips demonstrated a single abnormality, whereas 31% (86/281) of hips were associated with multiple abnormalities. Among all hips (n = 466), a lateral center-edge angle 40° was the most common radiographic abnormality, present in 27% (127/466) of hips. Anterior offset alpha angle and acetabular overcoverage were the most common abnormalities to present together, found in 5% (25/466) of hips. In the multivariable model, a decreasing Tonnis angle (hazard ratio per 1-degree decrease: 1.25, 95% confidence interval, 1.10-1.42, P = 0.0006) and the presence of acetabular retroversion (hazard ratio: 3.55, 95% confidence interval, 1.15-10.95, P = 0.0272) were predictive of the development of future hip pain. CONCLUSIONS Our study demonstrates a high prevalence of radiographic abnormalities indicative of femoroacetabular impingement in asymptomatic adolescents and young adults. A decrease in Tonnis angle and the presence of acetabular retroversion were predictive of future hip pain.