The association between the sagittal femoral stem alignment and the resulting femoral head centre in total hip arthroplasty

Abstract

Adequate stem alignment is essential for the success of Total Hip Arthroplasty (THA) to avoid dislocation and impingement. One factor that has not been sufficiently investigated so far is the stem tilting in the sagittal plane, which has an influence on the position of the centre of the femoral head and thus also on prosthesis torsion. We aimed to evaluate sagittal stem position using 3D-CTs in patients with THA and to develop a mathematical-geometrical model to simulate the functional correlation between sagittal stem tilting and the influence on functional anteversion. Thirty patients with THA underwent a CT-scan. By 3D-reconstruction of the CT-data, femoral-/prosthesis-axis, torsion and sagittal tilt were determined. In accordance with the position of the femoral and prosthesis axes, the rotatory (rAV) (surgically adjusted) and functional (depending on sagittal tilt) anteversion (fAV) was measured. A three dimentionalcoordinate transformation was also performed using the Euler-angles to derive a mathematical-geometrical correlation between sagittal stem tilting and corresponding influence on anteversion. The mean rAV was 8°(-11.6 -26°), the fAV 18°( 6.2 -37°), and the difference 10°(8.8 -18°). The mean degree of stem tilting was 5.2°(0.7 -9°) anterior towards the femoral axis. The individually measured parameters are reflected in the mathematical-geometrical model. Depending on the extent of the sagittal deviation, a clear influence on the torsion emerges. For example, a stem implanted at a 15°a nteverted angle with a sagittal tilt by two degrees towards anterior results in a fAV of 20°. A clear association between the sagittal stem alignment and the impact on the fAV was demonstrated. Hence, the rotatory anteversion intended by the surgeon may be functionally significantly different. This might pose an increased risk of dislocation or impingement. The sagittal tilt of the prosthesis should therefore be considered in the context of impingement and dislocation diagnosis. In this respect, we recommend a 3D-analysis of stem alignment.

Influencing factors for the increased stem version compared to the native femur in cementless total hip arthroplasty

“…Sagittal stem alignment was calculated from the angle between the stem axis and the proximal femoral axis in the sagittal plane ( Fig. 3a) [23]. When the stem axis was anteriorly tilted compared to the proximal femoral axis, sagittal stem alignment was defined as having an anterior tilt.…”

Variations in sagittal and coronal stem tilt and their impact on prosthetic impingement in total hip arthroplasty

“…Regarding the sagittal stem tilt, the present results suggest that the anterior bowing of proximal femur did not vary widely according to patient background characteristics, including sex, gender, and diagnosis. Previous computer simulation studies assumed a sagittal stem tilt of 0°–3° , and previous studies investigating sagittal stem tilt reported a mean sagittal tilt of 5.2° based on the proximal femoral shaft axis and 5.7° based on the mechanical axis . However, as these previous studies evaluated the European population , and considering the possibility of racial differences in anterior bowing of the femur , it may be necessary to investigate the average sagittal femoral tilt of each race in separate simulation studies.…”

“…Previous computer simulation studies assumed a sagittal stem tilt of 0°-3° (6-9), and previous studies investigating sagittal stem tilt reported a mean sagittal tilt of 5.2° based on the proximal femoral shaft axis (13) and 5.7° based on the mechanical axis (14). However, as these previous studies evaluated the European population (13,14), and considering the possibility of racial differences in anterior bowing of the femur (18), it may be necessary to investigate the average sagittal femoral tilt of each race in separate simulation studies. Regarding the coronal stem tilt, multiple regression analysis revealed a significant correlation between lower coronal tilt and older age in the anatomical coordinate system.…”

“…Most studies have focused on the optimization of the positioning of the cup and the stem anteversion because of the wide variation in femoral anteversion . However, there are few studies focusing on the variations in sagittal and coronal stem tilt, which might influence the risk of prosthetic impingement . Among several reference femoral coordinate systems, we used the femoral retrocondylar coordinate system which used the retrocondylar plane as a coronal plane and the line passing through the trochanteric fossa and the intercondylar notch as a vertical reference line because this can be reconstructed even after THA .…”

Computer and Template Assisted Orthopedic Surgery

“…It relies on a multipoint fixation with cortical contact at the medial calcar, the lateral rim of the resected neck, as well as the distal appendix in the posterior lateral subtrochanteric region. Müller et al (2011) and described the effect of implant tilt and implant rotation on the reconstruction of the center of rotation in hip reconstruction, respectively. This final position determines the position of the implants in the femoral neck, probably changing offset and leg length as well as the center of rotation.…”