Abstract
Purpose:
In order to acquire good stability of an arthroplasty hip, the proper placement of the implants, which prevents impingement between the stem neck and the socket, is important. In general, the anteversion of the uncemented femoral stem depends on the relationship between the three-dimensional structure of the proximal femoral canal and the proximal stem geometry. The exact degree of the anteversion will be known just after broaching during the operation. If the stem anteversion could be forecasted, preoperative planning of the socket placement would be relatively easy. Furthermore, when a high degree of anteversion is forecasted, a special femoral stem to reduce it, such as a modular stem, could be prepared. However, we experienced that the preoperatively measured anteversion of the femoral neck using computer tomography (CT) was sometimes different from that of the stem measured during the operation. The purpose of this study was to investigate whether the preoperative measurement would be helpful to predict the stem anteversion by examining the relationship between the anteversion of the femoral neck and the stem.
Patients and methods:
A total of 57 primary THAs by one senior surgeon from April 2011 until March 2012 were carried out. Two THAs using a modular stem and one for the hip after previous proximal femoral osteotomy were excluded. The remaining 54 THAs were examined. The used uncemented stems were designed for proximal metaphyseal fixation. CT scans, including the distal femoral condyles as well as the hips, were carried out in all cases preoperatively. The anteversion of the femoral neck was measured as the angle of the maximum longitudinal line of the cross section of the femoral neck to the line connecting the posterior surfaces of both of the distal femoral condyles (Fig. 1). The femoral neck anteversion was measured at three levels (Fig. 1). The stem anteversion was measured just after the femoral broaching during the THA. The relationship between the anteversion angles of the femoral neck and of the stem was examined by using a regression analysis. The institutional review board approved this study.
Results:
The anteversion angles of the femoral neck varied widely when they were measured at all of the levels (Table 1). The anteversion angle of the femoral neck was not always identical to that of the stem. There were 32–46% of cases in which the difference between the stem anteversion and the femoral neck anteversion was within 5 degrees. There was a significant relationship between the anteversion of the stem and that of the femoral neck measured at all three levels (Fig. 2). When it was measured just below the femoral head, it was the closest to one, and the p-value was the lowest.
Discusssion and Conclusions:
The anteversion of the uncemented stem could be calculated by using the formula to show the relationship between the stem anteversion and the femoral neck antevesion measured preoperatively. The values appeared to be sufficiently correct for making clinical decisions, although a prospective study may be necessary to confirm this.
