Abstract
Summary Statement
The circle theorem is a simple and effective measurement tool for estimating acetabular version after total hip arthroplasty
Introduction
Position of the acetabular cup is a major factor in the range of motion and risk of dislocation after total hip arthroplasty. However, there is no well established technique for accurately and easily estimating acetabular cup version intraoperatively or postoperatively. The objective of this study was to evaluate a recently proposed method for measuring acetabular cup version on a single plain radiograph of the hip, which is based on one of the circle theorems in basic geometry.
Patients & Methods
Radiographic version is defined as the angle between the cup face plane and a plane perpendicular to the body coronal plane. Using this definition, a metal hemispheric cup was placed in a pelvic sawbone model at a series of known angles of radiographic version (based on direct goniometer measurement). Cup inclination, pelvic tilt, and pelvic rotation were held constant for all version angles. A single antero-posterior hip radiograph was then obtained and reviewed for each version angle. The acetabular cup version was next estimated by using a compass and protractor in accordance with the circle theorem. Statistical analysis was performed utilizing Student's t-test with an alpha=0.05
Results
20 known angles of version were evaluated: 11 anteverted angles, 7 retroverted angles, and 2 neutral angles. Mean difference between the circle theorem estimate and the true version was 0.90 degrees (range −2 to 3). There was no statistically significant difference between the circle theorem's estimates and the true version (p=0.84). Similarly, there was no significant difference between the anteverted estimates (mean difference 0.91) and the retroverted estimates (mean difference 0.86)(p=0.95).
Discussion/Conclusion
Methods of measuring component position are essential for evaluating surgical technique, monitoring cup stability, and maximizing patient outcomes. Radiographic version of an acetabular cup can be estimated by using the circle theorem. This theorem can provide a quick, easy, and accurate estimate of version with the use of simple instruments (compass and protractor) and readily available plain radiographs.
