Effect of Pelvic Tilt on Radiographic Migration and Wear Measurements after Total Hip Arthroplasty

Tilt effects on radiographic measurements after total hip arthroplasty should be studied. A new computerized method for measurement, called EBRA, making three-dimensional calculations and intended to identify tilted radiographs was evaluated for its clinical usefulness.

In an experimental set-up repeated radiographs were taken with a human pelvis tilted gradually around its horizontal and vertical axes. Migration and wear were measured with the EBRA method and the results were compared with those made using standard methods.

Tilted radiographs were identified and excluded from analysis by the EBRA method, significantly reducing the maximum error of measurement. A systematic error of measurement was found in the presence of consecutively changing tilt in a single direction. Wear measurements were only slightly affected by pelvic tilt. A pelvic tilt can cause considerable errors in the measurement of cup migration-i.e. up to 8.2 mm. The EBRA method improves the accuracy of measurements by taking the effects of tilt into account.