Abstract
Introduction
The value of collared stems for uncemented implants remains controversial. Some comparative studies have demonstrated advantages of collared stems regarding the potential for subsidence. Other studies with longer follow-up have shown no adverse effect of the use of a collar regarding the femoral component survivorship. To date, the adequate size of the collar with regards to the anatomy of the proximal femur has never been studied.
The goal of this study was to assess whether the size of the collar needs to be adjusted according to the size of the femoral component used, and according to the use of a standard or a lateralized component.
Materials and Method
102 CT of normal femurs have been divided into 2 groups of 51 femurs each. Each group has been analysed by 2 independant surgeons.
Each CT view passed through the axis of the proximal diaphysis and the center of the femoral head. The scale was 100%. Templates of femoral components have been set in order to reproduce the center of rotation and an optimal filling of the proximal femoral canal. Sizes of the femoral components as well as the need for standard or lateralized implants have been recorded. In order to determine the ideal size of the collar, the distance between the medial edge of the prothesis and the medial edge of the femur (so-called P-C distance) at the level of the neck cut (calcar) has been measured.
Results
The inter-observer concordance for the selection of the implant type (i.e. standard or lateralized), size, and P-C distance measurement was satisfactory (kappa 0.7). 56% of the selected implants were standard. The mean size was 5 (1 to 10). The mean P-C distance was 9.9mm (5 to 16mm). It was 8.8mm for standard implants and 11.3mm for lateralized implants, with significant difference (p<0.0001).
The size of the selected implant was significantly related to the P-C distance (r=0.27; p<0.005).
Conclusion
These results suggest that the size of the collar should increase with larger sizes, and that the use of a longer collar with lateralized implants should be advocated.
