Our CT based analysis of post-operative Femoral Tilt (FT) in cementless femoral stems revealed a considerable variability and maximum FT values up to 10.2°. We found no significant difference between male and female patients. Although our analysis in 40 patients is limited by numbers, it confirms that variations of the FT between 1.7° and 10.2° occur in clinical practice.
For analysing the effects of FT on femoral antetorsion and an impingement-free compliant stem/cup position, we used a three-dimensional hip joint model including CAD files of actual implants with specific geometries. Previous studies used generic implant models represented by spheres and cones. Within our computer model, we compared initial (iAT) versus effective antetorsion (AT) values. From a biomechanical perspective, the definition of iAT is based on a rotational approach to determine the implant orientation, whereas the definition of AT is a measurement which uses projections to an axial plane. Thus, basically a rotation-based definition of stem orientation (iAT) was compared with projection measurements (AT). From a clinical perspective, both definitions are useful. iAT reflects a direct rotation around the shaft axis of the stem. Thus, it corresponds to the rotational alignment of the stem in the proximal part of the femur, which e.g. can be modified by using modular implants. Instead, AT refers to the overall rotational alignment around a cranial-caudal axis. Our results show considerable alteration of femoral antetorsion with variation of FT, even when the same rotational alignment at the proximal part of the femur (iAT) is applied. For a neck-shaft angle of 135° this effect is almost 1-to-1, i.e. an increment of 1° in the FT angle induces a decrement of 1° in antetorsion (Additional file 1
According to the results by Widmer at al., cup inclination, cup anteversion, and stem antetorsion determine an optimized, impingement-free ROM and are highly interdependent. Widmer determined a linear relationship between cup anteversion and stem antetorsion which has been summarized within the so-called “Widmer formula”: Cup anteversion
0.7 x Stem antetorsion
]. With the help of a 3D computer model of the hip, we were able to analyze these dependencies in terms of complementary component orientations with mating of the femoral head in the cup without impingement of the two throughout all body positions under the influence of FT. Our results clearly demonstrate that the size of this so-called “zone-of-compliance” can differ by more than 200% when clinical FT values are applied (Figure ). As an example, the optimum cup position according to Widmer changes from 35° radiographic inclination to 39° inclination when a FT is increased by 7° and the same (effective) antetorsion is used. These findings are very important clinically, particularly for surgeons following the concept of “combined anteversion” or “femur first” in THA. A ROM-optimized cup position cannot be calculated based on antetorsion values only. Thus, if the surgeon were to position the cup in relation to the femoral stem antetorsion, the influence of FT has to be considered as well.
Apart from our previous work on this topic [14
], there is only one study so far that has addressed the issue of sagittal femoral stem alignment [15
]. This analysis with another cementless stem type was based on a different coordinate system, i.e. proximal femoral axis instead of mechanical axis which was used as a reference for neutral alignment of the leg in our study. In this study by Mueller, sagittal tilt was calculated as the deviation between the shaft axis of the proximal femur and the stem. The anterior bow of the femur, i.e. deviation between the proximal or stem shaft axis and mechanical axis of the femur, was neglected and only a comparison between the pre- and postoperative situation was performed. Additionally, antetorsion was defined according to a connecting line between the center of the femoral head and the proximal shaft axis in this study. Thus, these calculations by Mueller did not directly represent the orientation of the stem neck axis. Based on this definition, the effect of sagittal tilt on antetorsion did not only depend on the sagittal tilt of the stem but also on the rotation point, which was used for the comparison between the neutral reference position and the final orientation of the stem. In particular, the translational difference of the point at the top of the stem shaft axis then influences the antetorsion calculation. Such translations may be relevant for addressing bone-to-bone impingements since the position of the stem in the femoral canal may influence this. The definition of antetorsion in our study was directly based on the orientation of the stem neck axis, because the analysis was directed to the determination of ROM according to implant-to-implant impingement. Because of these differences, the effect of sagittal tilt on antetorsion was approximately 2-to-1 to 3-to-1 (for 131° neck-shaft angle) according to Mueller [15
], i.e. 1° change in sagittal tilt changes the (effective) antetorsion by 2°–3°, whereas the relationship was approximately 1-to-1 (for 135° neck-shaft angle) and a bit lower (for 125° neck-shaft angle) in our study. Therefore, the results of our analysis and study by Mueller cannot be directly compared.
Our study has certain limitations. First, we manually superposed the implant models onto the CT images instead of directly defining the axes. Based on our experience, the alignment of the implants was more reproducible than the direct axis determinations as the implants can be aligned very clearly with the implants. Usually, the variation of implant alignment between different observers was in the order ≤1°. However, this was not evaluated in detail. Second, we considered only prosthetic impingement in a specific ROM. We did not assess functionality and clinical symptoms of impingement in our group of patients. Third, in addition to the influence of femoral tilt and stem antetorsion on post-operative ROM, stem tilting in the frontal plane (varus/valgus angle) influences the relationship between the shaft axis and the femoral coordinate system which defines the reference for assessing ROM [17
]. Last, our radiological and biomathematical analysis was conducted for only one type of non-modular cementless stem.