Analysis of version in the acetabular cup.

To determine the amount of anteversion or retroversion of the acetabular component of the implanted total hip prosthesis, two anteroposterior radiographs of the hip are obtained, with the contralateral hip flexed to compensate for the possible existing flexion contracture. The X-ray beam is centered on the implanted total hip in one radiograph, and moved away from it toward the contralateral hip in the second radiograph. If the cup is anteverted, the opening will seem wider in the second radiograph. To calculate the angle, the location of the center of the X-ray beam on the X-ray plate must be know. The center of the X-ray beam can be marked on the radiograph by putting a metalic cross on the patient, over the centering cross of the X-ray light source. If the distance of the signature of the X-ray's center beam is less than 8 mm for the center of the cup on the X-ray film, the cup version can be calculated from the arcsin of the shortest to the largest diameter of the cup. If the central ray's signature is farther away, correction is necessary for this calculation. Also, the variable parameters, e.g., cup size, and magnification rate, should be considered in the calculations. The anteroposterior radiographs of the implanted total hip, obtained with the central beam being marked on the X-ray plate, not only are useful for evaluation of the implant but also can be used to calculate the version angle with an accuracy of +/-2 degrees. The necessary calculation is tabulated for cups with an outside diameter of 44-56 mm.