Radiographic features associated with differing impinging hip morphologies with special attention to coxa profunda

Abstract

Background: Combined with clinical examination and MRI, radiographs have been mainstays in the management femoroacetabular impingement (FAI). Because hip morphology often portends intraoperative damage, radiographic features should inform surgical management.

Questions/purposes: We determined (1) the radiographic features of the various hip morphologies; (2) the prevalence of radiographic coxa profunda in each group; (3) the radiographic differences between hips with and without coxa profunda; and (4) its sensitivity and specificity as a measure of global acetabular overcoverage.

Methods: We reviewed preoperative radiographs and operative notes of 144 hips that underwent surgical dislocation and correction for FAI between August 2002 and February 2011. Hips were divided into four FAI subtypes by radiographic analysis (cam, global overcoverage, retroversion, and combined) and three subtypes (cam, pincer, or combined) by intraarticular pathology. Standard radiographic measurements were performed, and we introduce a novel measurement that assesses femoral head coverage.

Results: We found differences in median Angle of Sharp, femoral head-neck angle, and median roof length (and its subset) among the FAI morphologies. The prevalence of radiographic coxa profunda was 48% in cam hips, 85% in global overcoverage hips, 66% in retroverted hips, and 32% in combined hips. The sensitivity and specificity of radiographic coxa profunda as a measure of global overcoverage was 75% (95% CI, 0.62-0.85) and 62% (95% CI, 0.51-0.73), respectively.

Conclusions: We found major differences in radiographic measurements between FAI morphologies. Radiographic coxa profunda was poorly specific for global overcoverage. Measurement of roof length and ratio should be used to determine the morphology of the impinging hip.

Level of evidence: Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Fig. 1

This AP view of a left hip demonstrates the measurement of acetabular roof distance. The total acetabular roof length was measured using the sum of two distances, measured parallel to a line drawn between the femoral head centers, Roof 1 (the distance line from the lateral edge of the sourcil until it intersects with the sourcil more medially) and Roof 2 (defined as the distance between the termination of the previous line and the ilioischial line of Kohler).

Fig. 2

Logistic regression analysis using generalized additive models showing an increased likelihood of pincer impingement (based on intraoperative findings) with increasing Roof 1/Roof 2 ratio but no apparent cutoff associated with a high probability of pincer morphology.

Fig. 3

Logistic regression analysis using generalized additive models showing an increased likelihood of pincer impingement (based on intraoperative findings) with increasing Roof 2 ratio (total roof/roof 2) but no apparent cutoff associated with a high probability of pincer morphology.