Abstract
Introduction
Optimal orientation of the acetabular cup is vital issue not only for primary but revision total hip arthroplasty (THA). Especially in revision THA, malorientation of the cup is likely to occur because anatomical landmark around acetabular rim often disappeared by the osteolytic bony destruction or the process of cup removal. As a consequence, higher dislocation rate and accelerated wear of bearing surface compared with primary THA, which affect the outcome of revision THA, are concerned. On the other hand, computer aided navigation system has been developed in recent years because of substantial errors of manual technique in cup placement even with experienced surgeon. The purpose of this study was to evaluate the accuracy of the cup orientation in revision cementless THA using CT based navigation system.
Materials and Methods
Thirteen patients who underwent revision cementless THA with CT based navigation system (Stryker Japan) were employed for this study. The average age at surgery was 64 years (range, 45–78 years, 3 men and 11 women). Primary surgery was cementless THA in 4 and BHA in 9 hips. Disorder which led to revision THA was loosening of the cup, massive retroacetabular osteolysis, and severe proximal migration of bipolar outer head. In most cases, acetabular rim was not conserved. After removal of the cup or outer head, we revised acetabular components with cementless hemispherical TriAD cups (Stryker Japan) using direct lateral approach in lateral decubitus position. For all the patients, post-operative CT scans were performed and the cup inclination and anteversion angle were measured using 3D image-processing software (Stryker, Japan). The difference between the intra-operative target angle and the angle measured from the post-operative CT image were calculated.
Results
The average cup orientation measured by postoperative CT was 39.6±3.8° (range, 34–46°) in inclination and 20.5±5.0° (range, 17–29°) in anteversion. The accuracy (calculated as a mean of the absolute difference between intra-operative target angle and post-operative CT angle) of inclination and anteversion angle were 2.0 ± 1.8° (range, 0–5°) and 2.3±2.2° (range, 0–5°), respectively. The accuracy was within 5° in all cases and there was no postoperative dislocation.
Discussion
Our study showed that CT based navigation system provided accurate orientation of the acetabular component even in revision cementless THA as well as in primary THA. Although the basic process of the navigation system in revision THA is same as primary THA, several pitfalls exist. Metal artifact from preexisting hardware such as screws, cup, and head-neck of the stem makes it difficult to do preoperative planning, intraoperative point matching, and surface registration. We have to pay maximum attention to avoid including metal artifact especially around acetabular rim when editing surface of the pelvis at preoperative planning, and also avoid pointing the area around acetabular rim when doing surface registration intraoperatively.
