Validation of Imageless Total Hip Navigation

Abstract

Introduction Optimal acetabular component orientation in total hip arthroplasty is a complex three dimensional problem with failure leading to increased wear and instability [1–6]. Although the exact frequency of acetabular component mal-position and the quantitative linkage to hip re-operation is uncertain, it is clear that at least some re-operations could be avoided through more reliable acetabular component positioning at the time of surgery. Extremes of component mal-position are associated with an increased risk of dislocation and loosening. In Lewinnek’s investigation, the acetabular cup »safe zone« was radiographically identified as 15 degrees of anteversion and 40 degrees of opening angle in the performance of routine hip arthroplasty. The risk of dislocation increased from 1.5% to 6.1% if the cup was placed outside of the two degree of freedom, described »safe zone« [7]. The tolerance associated with optimal cup positioning was thought to be similar for both anteversion and opening angle at +/– 10 degrees. Computed tomography studies of post-operative cup insertions have shown that a large percentage of cases have an unacceptable positioning when depending on freehand or conventional mechanical instrumentation [8, 9]. According to a recent European investigation of total hip arthroplasty cups positioned using manual instrumentation and evaluated using CT, it was found that only 27/105 (26%) fell within Lewinnek’s safe zone [10]. Computer-assisted orthopaedic surgery has been recently defined as the ability to utilize sophisticated computer algorithms to allow the surgeon to determine three dimensional placement of total hip acetabular implants in situ. Computer-assisted navigation for acetabular cup placement requires a registration that defines the anterior pelvic plane. McKibbin et al. first defined the anterior pelvic plane as a plane connecting the ventral surfaces of the anterior superior iliac spines and the pubic tubercles of the pubic rami [11]. Basically, a cadaver pelvis was placed »table down« with these points contacting the table. Inclination and anteversion of the acetabulum were then measured in relation to this plane. From the beginning, computed tomography was the most accurate and reliable imaging modality to define these three dimensional relationships and has a proven precision of about one millimeter or one degree [12–15]. Other methods have been sought due to the amount of resources and time required to utilize computed tomography for navigation. One promising alternative is imageless registration where simple anatomical referencing can be done at the time of the operation [16–21]. However, the laboratory validation of this clinical application is lacking. This study compared the precision, repeatability and reproducibility of these methods against known metrological and computed tomography standards.