Sonography in total hip arthroplasty

Purpose: The distance between the anterior surface of the neck of the prosthetic stem and the anterior joint capsule, the "capsular distance", is increased in total hip arthroplasty (THA) with synovitis. We evaluated the potential of ultrasonography (US) in measuring the "capsular distance" in THA hips one year after insertion. Materials and Methods: We compared the measurements of the capsular distance using a ruler with those performed with US. A plastic pelvis and femur model with a prosthetic hip and paper tape to simulate the joint capsule were used. We also evaluated the intra- and interobserver agreements between 3 examiners of the US measurements of the anterior capsular distance in 22 patients with THA. The effect of experience in such type of examination was estimated. Results: There was a high correlation when measuring the anterior capsular distance in the prosthetic hip model with a ruler as compared with US. The interobserver agreement in the US measurements was good and became better after examiners gained experience in this procedure. The intraobserver agreement was always better than the interobserver agreement and also improved with increasing numbers of examinations. Conclusion: Ultrasonography is a reliable method to measure the anterior capsular distance in THA, especially if performed by an experienced examiner.