Correlation between radiographic measures of acetabular morphology with 3D femoral head coverage in patients with acetabular retroversion

Abstract

Background and purpose: Acetabular retroversion may result in anterior acetabular over-coverage and posterior deficiency. It is unclear how standard radiographic measures of retroversion relate to measurements from 3D models, generated from volumetric CT data. We sought to: (1) compare 2D radiographic measurements between patients with acetabular retroversion and normal control subjects, (2) compare 3D measurements of total and regional femoral head coverage between patients and controls, and (3) quantify relationships between radiographic measurements of acetabular retroversion to total and regional coverage of the femoral head.

Patients and methods: For 16 patients and 18 controls we measured the extrusion index, crossover ratio, acetabular angle, acetabular index, lateral center edge angle, and a new measurement termed the "posterior wall distance". 3D femoral coverage was determined from volumetric CT data using objectively defined acetabular rim projections, head-neck junctions, and 4 anatomic regions. For radiographic measurements, intra-observer and inter-observer reliabilities were evaluated and associations between 2D radiographic and 3D model-based measures were determined.

Results: Compared to control subjects, patients with acetabular retroversion had a negative posterior wall distance, increased extrusion index, and smaller lateral center edge angle. Differences in the acetabular index between groups approached statistical significance. The acetabular angle was similar between groups. Acetabular retroversion was associated with a slight but statistically significant increase in anterior acetabular coverage, especially in the anterolateral region. Retroverted hips had substantially less posterior coverage, especially in the posterolateral region.

Interpretation: We found that a number of 2D radiographic measures of acetabular morphology were correlated with 3D model-based measures of total and regional femoral head coverage. These correlations may be used to assist in the diagnosis of retroversion and for preoperative planning.

Figure 1.

A. Right hip: The acetabular index is the angle (β) between the line parallel with pelvic tilt (solid black line which marks the horizontal reference) and the black dashed line from the medial sourcil (sclerotic radiographic density corresponding to the acetabular roof) to the lateral sourcil (where the sourcil meets the lateral acetabular rim). The acetabular angle (ϕ) is the angle made by the solid black line and the white dashed line from the acetabular teardrop to the lateral sourcil. Left hip: The extrusion index is the amount of femoral head uncovered by the acetabulum (distance a) divided by the diameter of the femoral head (distance a + distance b). B. Right hip: The lateral center edge angle (θ) is formed by a line passing through the center of the femoral head perpendicular to the inferior aspect of obturator foramina (thick black line) and a line from the center of the femoral head to the lateral aspect of the congruent sourcil (medial to calcified labra and up-sloping sourcil and even with the posterior wall). Left hip: Crossover sign is positive on the left, demonstrated by the anterior wall (solid) crossing the posterior wall (dashed). The crossover ratio is the ratio of the distance from the lateral-most acetabular rim to the point of the crossover (distance a) divided by the acetabular diameter (the distance from the lateral acetabular rim to the teardrop, b). The posterior wall distance is the horizontal distance (distance c) measured from the center of the femoral head to the posterior wall. Distances are positive if the posterior wall is lateral to the head center and negative if medial to the head center.

Figure 2.

3D reconstruction of femur from CT image data from a control subject. Left: The femur head-neck junction was defined automatically (line at head-neck junction). The region of femoral head that was covered (blue) was determined by projecting the rim of the acetabulum to the femur (line representing boundary of covered region in blue). Right: Two planes were created at the center of the femoral head (white) to divide the head into four anatomical regions. A = anterior, P = posterior, M = medial, and L = lateral. Each region includes the portion of the head from the most superior aspect to the femoral head/neck junction inferiorly.

Figure 3.

Comparisons of coverage between controls (solid) and subjects with acetabular retroversion (hatched), overall and by regions. Error bars represent SD. TC: total coverage; Ant: anterior coverage; Post: posterior coverage; ALC: anterolateral coverage; AMC: anteromedial coverage; PLC: posterolateral coverage; PMC: posteromedial coverage.

Figure 4.

Scatter plot of posterior wall distance against posterior coverage. The solid bar represents regression line.