LEARNING CURVE AND INFLUENCING FACTORS OF NAVIGATED TOTAL HIP ARTHROPLASTY

Abstract

Navigation during the positioning of the acetabular component in total hip replacement is a promising tool to improve the prosthetic alignment. Correct placement of the cup will reduce the risk of mechanical complications such as dislocations and impingement. All navigation systems, be they CT or infra-red based, require exact determination of the symphysis and both anterior superior iliac spines, the landmarks of the patient’s pelvis. The accuracy of the intraoperative palpation of these landmarks influences the outcome of the cup-angulation more than any other factor.

Our experience in over 700 infra-red based navigated total hip replacements since 2002, shows a wide variation of acetabular cup anteversion. This study was intended to prove a correlation between the subcutaneous fat thickness and infra-red based measurements of the pelvis.

The navigation system (PiGalileo) used in this study is infra-red based, using the symphysis and both anterior superior iliac spines as reference points.

To determine the influence of the surgeons’ experience in palpating the landmarks on the outcome of the position of the acetabular cup, two series of 10 consecutive THRs were performed by a single surgeon. The first series was performed after the navigation had been introduced into the routine of our total hip replacements and the initial learning curve had passed. The second series was initiated to prove a correlation between the patient’s soft tissue cover and acetabular cup anteversion. The subcutaneous tissue overlying the landmarks was measured preoperatively by ultrasound. The computer calculated anteversion was corrected by a factor based on the clinical experience of the surgeon. In both series coronal tilt and cup anteversion were evaluated via post-operative CT-scans. Thus determined, the position of the cup was compared to the intraoperative measurements of the navigation system.

All acetabular cup angles were kept within the required limits. In the first series, the mean difference of the measurements of the coronal tilt and anteversion were 3.8° and 7.2° respectively. In the second series, the mean difference of the anteversion was improved by 2°. There was no change affecting the coronal tilt. In both series, the operating time was increased by 9 minutes compared to conventional THRs.

Precise landmark acquisition is essential in order to profit from navigation in total hip replacement and obtain a cup angulation far superior to conventional placement. The correlating factor of subcutaneous fat and cup anteversion has yet to be determined.