EBRA: a method to measure migration of acetabular components.

In orthopedics there is a demand for determining migration of hip sockets by evaluation of standard radiographs. In this case problems are caused mainly by changing pelvis positions on the X-ray table at successive exposures. A method (EBRA) is described that evaluates standard AP radiographs without requiring additional means at exposure (e.g. ball markers). Simulating the spatial situation it computes parameters of longitudinal and transverse migration of prosthetic cup and femoral head. A comparability algorithm using a grid of transverse and longitudinal tangents of the pelvis contour divides serial radiographs into sets of comparable ones. Comparability of serial radiographs takes place if the distances of corresponding grid lines do not transcend a given limit L. Migration is measured only between comparable radiographs. Different studies are described concerning the interdependence of pelvis rotations and changes of the grid lines, the degree of pelvis rotations appearing in practice, the choice of the limit L, the properties of the comparability algorithm and the accuracy of EBRA. The 95% confidence limits for EBRA results are 1.0 mm for longitudinal and 0.8 mm for transverse migration.