The association between Femoral Tilt and impingement-free range-of-motion in total hip arthroplasty
- Tobias Renkawitz†1Email author,
- Martin Haimerl†2,
- Lars Dohmen2,
- Sabine Gneiting2,
- Philipp Lechler1,
- Michael Woerner1,
- Hans-Robert Springorum1,
- Markus Weber1,
- Patrick Sussmann1,
- Ernst Sendtner1 and
- Joachim Grifka1
© Renkawitz et al.; licensee BioMed Central Ltd. 2012
Received: 2 September 2011
Accepted: 4 May 2012
Published: 4 May 2012
There is a complex interaction among acetabular component position and antetorsion of the femoral stem in determining the maximum, impingement-free prosthetic range-of-motion (ROM) in total hip arthroplasty (THA). By insertion into the femoral canal, stems of any geometry follow the natural anterior bow of the proximal femur, creating a sagittal Femoral Tilt (FT). We sought to study the incidence of FT as measured on postoperative computed tomography scans and its influence on impingement-free ROM in THA.
The incidence of the postoperative FT was evaluated on 40 computed tomography scans after cementless THA. With the help of a three-dimensional computer model of the hip, we then systematically analyzed the effects of FT on femoral antetorsion and its influence on calculations for a ROM maximized and impingement-free compliant stem/cup orientation.
The mean postoperative FT on CT scans was 5.7° ± 1.8°. In all tests, FT significantly influenced the antetorsion values. Re-calculating the compliant component positions according to the concept of combined anteversion with and without the influence of FT revealed that the zone of compliance could differ by more than 200%. For a 7° change in FT, the impingement-free cup position differed by 4° for inclination when the same antetorsion was used.
A range-of-motion optimized cup position in THA cannot be calculated based on antetorsion values alone. The FT has a significant impact on recommended cup positions within the concept of “femur first” or “combined anteversion”. Ignoring FT may pose an increased risk of impingement as well as dislocation.
To study the postoperative incidence of FT for cementless femoral stems on computed tomography scans.
Systematically analyze the effects of FT on femoral antetorsion by means of a three-dimensional computer model.
Re-calculate the zones of impingement-free compliant stem/cup orientation with and without the effect of FT.
For studying the incidence of FT after cementless THA, 40 postoperative computed tomography (CT) scans were analyzed by a single investigator (LP). This study was conducted after authorization by the Institutional Ethical Board (No. 06/100) and the Federal Office for Radiation Protection (Z5-22462/2-2007-008) and informed consent was obtained from all patients. Average patient age was 69 (±4.8) years and average body mass index (BMI) 26.4 (±3.7) kg/m2. Exclusion criteria were arthritis secondary to hip dysplasia, post-traumatic deformities of the pelvis, and - because a post-operative pelvic CT scan was required - age below 50 years at the time of surgery. All operations were done with the patient in lateral position through a modified Smith-Petersen (Micro-Hip®) approach  by two surgeons (TR, ES). Press-fit components and cement-free hydroxyapatite-coated stems (Pinnacle cup, Corail stem, DePuy, Warsaw, IN, USA) were used in all cases. The CT data sets (pelvis and femoral condyles) included 16 male and 24 female, 17 left and 23 right hips. None of the patients had a THA on both sides.
Alignment of stem implant and planning of landmarks
Creation of a geometric model to represent the orientation of the stem implant
Additional file 1: Dynamic model of the Femoral Tilt. FT is continuously increased from 0° to 10°. The resulting change of antetorsion is represented by the red line. (MP4 941 KB)
Comparison between femoral tilt and zones-of-compliance
As a last step, resulting zones-of-compliance were compared for two variations in iFT (2.1°, 9.3°). The same (effective) AT of 15° was used for this analysis, i.e. the antetorsion was adapted according to the variations in iFT. This allowed assessing differences in the zones-of-compliance when equal (effective) antetorsion values but different iFT values are given. In analogy to Widmer et al., the optimum cup position was determined as the point with the lowest inclination within the zone-of-compliance where a safety zone of approximately 1° was respected. All cup orientation angles were calculated in terms of the radiographic definition according to Murray .
Statistical analyses were performed using Microsoft Excel (Microsoft Inc, Redmond, WA, USA). Mean values, standard deviations, ranges, and confidence intervals with 95% confidence level were calculated. Statistical differences between the group of male and female patients were analyzed by a Student’s two sample t-tests assuming equal variances (significance level: 5%).
Incidence of Femoral Tilt as measured on 40 postoperative CT scans after cementless total hip arthroplasty
95% Confidence interval
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 = 37.3 [5, 8]. 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 4). 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 , there is only one study so far that has addressed the issue of sagittal femoral stem alignment . 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 , 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 . Last, our radiological and biomathematical analysis was conducted for only one type of non-modular cementless stem.
In summary, we have shown that there is a significant association between FT and an impingement-free ROM in THA. Additional parameters, such as neck-shaft angle, head-neck ratio, the design of the acetabular opening, and modular stems are additional parameters which influence impingement and ROM [16–18]. Therefore, our results can be used as an input for a next generation of computer-assisted navigation systems that couple FT and an individual three-dimensional impingement analysis to achieve patient specific, ROM optimized component orientation within the concept of femur first for THA .
The help of Ms. Herzog, Ms. Poitzsch (LP), Ms. Schmalzl, Mr. Schubert, Mr. Voelkel, and Ms. Wegner in this project is highly appreciated.
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