Measuring cup alignment from radiographs after THR

Abstract

After total hip replacement, standard AP radiographs are typically used to evaluate implant alignment, particularly the alignment of the acetabular cup. While cup version can only be assessed qualitatively, the abduction (inclination) of the cup can be measured directly on a given radiograph with a high degree of repeatability, even with simple tools. For this reason, radiographic measurements of cup orientation are a part of standard THR follow-up procedure. In most cases, the X-ray measurements are sufficiently accurate to either confirm that the implants are placed within a desired range or to detect serious misalignment. Unfortunately, measurements from standard radiographs can suffer from inaccuracies due to different reasons. While the order of these errors does not typically affect the qualitative assessment, they become apparent when the radiographic measurements are expected to yield accurate quantitative measurements. One such example is when a cup alignment achieved using surgical navigation is inconsistent with the postoperative radiographic measurements. Surgical navigation systems claim an angular accuracy on the order of one to two degrees, and the radiographic measurements of cup abduction can be performed manually with the repeatability of the same order. It may therefore be difficult for surgeons to understand the much larger discrepancies that can be observed between the two measurements. Several different sources may contribute to this outcome - most notably using a different anatomic reference, different definitions of cup orientation, and ignoring the effects of X-ray source position and variations in pelvic flexion. In the following, we will try to explain some methodological differences in the two measurement approaches and the sources of inaccuracy in radiographic measurements. © 2007 Springer Medizin Verlag Heidelberg.