How should the acetabular angle of Sharp be measured on a pelvic radiograph?

Four experienced orthopaedic surgeons measured the acetabular angle of Sharp (AA) in 66 hips by using two methods. In the classic method (AA-c), the lateral margin of the acetabular roof was used as the landmark. In the modified method (AA-m), the lateral margin of the sourcil (subchondral bony condensation in the acetabular roof) was used as the landmark. Mean AA-c and AA-m measurements were 44.8 degrees and 48.4 degrees, respectively. Mean intraobserver variations of AA-c and AA-m were 1.8 degrees and 2.1 degrees, respectively. Mean interobserver variations of AA-c and AA-m were 2.1 degrees and 2.6 degrees, respectively. Means obtained using AA-c of normal and defective acetabular roofs were similar, but the difference between AA-m means of different acetabular roof types was significant. The authors conclude that both AA-c and AA-m were reliable measurement methods, but clinical use of AA-m would probably produce better understanding of the radiographic hip anatomy in dysplastic hips.