ASSESSMENT OF ACETABULAR COVERAGE IN BORDERLINE ACETABULAR DYSPLASIA: ARE PLAIN RADIOGRAPHIC PARAMETERS ACCURATELY ESTIMATES OF THREE-DIMENSIONAL COVERAGE?

INTRODUCTION: Assessment of anterior acetabular coverage is commonly done with measurement of the anterior center edge angle (ACEA) or anterior wall index (AWI). This is particularly important in cases of borderline acetabular dysplasia where it may influence treatment decisions. However, the ACEA and AWI has been poorly validated. PURPOSE: The purpose of the current study was to investigate the correlation between plain radiographic measurements and three-dimensional femoral head coverage on low-dose CT in borderline acetabular dysplasia. METHODS: Seventy consecutive hips with borderline acetabular dysplasia (LCEA 20-25°) were included in the current study. Radiographic evaluation was performed prospectively including LCEA, acetabular inclination, and AWI on AP pelvis radiographs, and ACEA on false profile radiographs. The mean LCEA was 22.1±1.4°, while the mean acetabular inclination was 10.3±3.3. All patients underwent low-dose pelvic CT assessment for preoperative planning. The radial acetabular coverage was calculated according to the standardized clock-face position [measured at 12:00 (lateral), 1:00, 2:00, 3:00 (anterior), and 4:00] as described by Larson et al. Statistical analysis determined the correlation between ACEA and radial coverage. RESULTS: The mean ACEA in the group was 25.3±5.8° (range 10.1-43.9), with 16% having ACEA≤20° and 50% having ACEA≤25°. The mean radial coverages were 63.5%±1.7 (12:00), 60.7%±2.2 (1:00), 50.8%±3.2 (2:00), 37.0%±3.3 (3:00), and 27.9%±3.1 (4:00). The ACEA had poor correlation with radial coverage at all positions from 12:00 to 4:00 (range –0.068-0.173). The AWI had moderate correlation with radial coverage at 3:00 (PCC 0.499) and 4:00 (PCC 0.573). Comparing hips with an ACEA <20° versus >20°, there was no difference between the mean radial acetabular coverage at any position 12:00-4:00 (p=0.18-0.95). Comparing hips with an ACEA <25° versus >25°, there was no difference between the mean radial acetabular coverage at any position 12:00-4:00 (p=0.12-0.71). No significant difference in AWI was present between subgroups with normal and deficient radial coverage from 12:00 to 4:00 (p=0.09-0.72). DISCUSSION: The current study demonstrates poor correlation of the ACEA measurement with true anterior femoral head coverage as evaluated at clock-face positions from 12:00 to 4:00. The AWI demonstrated moderate correlation for 3:00-4:00 coverage but fails to differentiate hips with normal and deficient coverage. In the setting of borderline acetabular dysplasia, anterior and anterolateral femoral coverage should be assessed via low-dose CT rather than ACEA or AWI.