Measurement of the Acetabular Cup Anteversion on Simulated Radiographs

Widmer (J Arthroplasty 2004;19:387) reported a protractor for measuring the anteversion of acetabular cups on radiographs but with limited precision. We intended to improve its precision by trigonometric mathematics. We measured the anteversion of the acetabular cups on 336 simulated radiographs using aforementioned 2 methods. The anteversion measured by Widmer’s protractor ranged from 7° to 41° (mean ± SD = 28.0° ± 9.8°), and our methods, 5° to 51° (27.7° ± 13.2°). The mean ± SD of error by Widmer’s protractor was 5.2 ± 2.5°, and our protractor, 0.8° ± 0.8° (Student t test, P < .0001). The interobserver study showed the difference between measurements less than 2° for each method. Therefore, the smaller error of our method than that of Widmer implicated a potentially precise measurement of the anteversion (level of evidence: diagnostic study, level II).