Abstract
Purpose
Accurate assessment of cup orientation on postoperative radiographs is essential for evaluating outcome after THA. However, accuracy is impeded by the deviation of the central X-ray beam in relation to the cup and the impossibility of measuring retroversion on standard pelvic radiographs.
Method
In an experimental trial, we built an artificial cup holder enabling the setting of different angles of anatomical anteversion and inclination. Twelve different cup orientations were investigated by three examiners. After comparing the two methods for radiographic measurement of the cup position developed by Lewinnek and Widmer, we showed how to differentiate between anteversion and retroversion in each cup position by using a second plane. To show the effect of the central beam offset on the cup, we X-rayed a defined cup position using a multidirectional central beam offset. According to Murray's definition of anteversion and inclination, we created a novel corrective procedure to balance measurement errors caused by deviation of the central beam.
Results
Measurement of the 12 different cup positions with the Lewinnek's method yielded a mean deviation of \(1.8{^{\circ }}\) (95 % CI 1.3–2.3) from the original cup anteversion. The respective deviation with the Widmer/Liaw's method was \(3.2{^{\circ }}\) (95 % CI 2.4–4.0). In each case, retroversion could be differentiated from anteversion with a second radiograph. Because of the multidirectional central beam offset ( \({\pm }5\) cm) from the acetabular cup in the cup holder ( \(25{^{\circ }}\) anteversion and \(36{^{\circ }}\) inclination), the mean absolute difference for anteversion was \(3.9{^{\circ }}\) (range \(-4.2{^{\circ }}\) to \(+4.0{^{\circ }})\) and \(0.7{^{\circ }}\) (range \(-1.2{^{\circ }}\) to \(+1.2{^{\circ }})\) for inclination. The application of our novel mathematical correction of the central beam offset reduced deviation to a mean difference of \(0.4{^{\circ }}\) for anteversion and \(0.3{^{\circ }}\) for inclination.
Conclusion
This novel calculation for central beam offset correction enables highly accurate measurement of the cup position.
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