An advanced 3D analysis of femoral anteversion angle
- Femoral rotational alignment is expressed as femoral anteversion angle (FAVA). It is defined as the angle between the femoral neck axis (FNA) and posterior condylar line (PCL) in the axial plane. FAVA affects both hip and knee biomechanics and has important clinical implications in treatment of femoral fractures, derotational osteotomies and total hip arthroplasty design. In this study five different FAVA measurement methods were studied using advanced 3D imaging and compared to Murphy’s method.
- CT scans of 102 dry femur specimens were taken with subsequent density-based segmentation using the Mimics® software package to obtain 3D images. Murphy’s method and five different 3D measurement methods, to determine FAVA, were applied and compared.
Murphy’s method was performed to the contours of the femoral 3D models by calculating the angle between the line formed by the femoral head center and the center of the base of the femoral neck and the condylar axis.
In the first 3D measurement method, a best fit sphere was plotted on the femoral head. Next, an elliptical least-square fitting approach was applied to find the cross-sectional center of the femoral neck. The FNA connects those two center points. In method 2, a second sphere with the same center point as in method 1 was created to intersect the femoral neck by increasing the radius of the first sphere with 25%. Subsequently, a best fit arc and its center at intersection of the femoral neck were determined. The line connecting the centers of sphere and arc is called FNA. Method 3 is the same as method 2, except that the radius of the best fit sphere was increased with 40%. In method 4, the complete femoral neck surface was used, and the center of mass was calculated. The line defined by this point and the center of the best fit sphere through femoral head was used as FNA. To assess the robustness of method 4, it was repeated
5) with another selection of femoral neck surface points (same observer, 1 week later). This simulates a surgical navigation, defining femoral head and neck surface by means of a spatially tracked pointer device.
- The mean FAVA (and SD) for the 102 specimens measured with Murphy’s method, 3D methods 1, 2, 3, 4 and 5 were respectively 8.12° (7.30°), 9.93° (8.24°), 9.53° (7.87°), 10.46° (7.83°), 13.21° (8.60°) and 8.21° (7.64°). A paired student t-test showed significant differences except for method 1 and 2 (p=0.193). Those results differ from the reference FAVA range of 10-25° as found in literature. The presence of a CAM deformity resulted in an increased femoral anteversion, independent of the used measurement technique. In addition, the presence of a CAM deformity tends to underestimate the measured FAVA in method 2, as compared to method 3. Strong correlations between the different methods were observed above 7.5° of anteversion (Murphy’s method). Pearson correlation coefficients between Murphy’s method and method 2 decreased in the presence of a CAM deformity.
- Advanced 3D morphology analysis of FAVA revealed significant differences between methodologies and questions the validity of the current reference data.
More research is needed to investigate the increase in FAVA in hips with a CAM deformity. We propose method 3 (sphere +40%) to measure the FAVA, since the presence of a CAM deformity does not influence the results as compared with method 2.
In addition, advanced 3D imaging is considered more robust than surgical navigation-based methods. The definition of FNA by the center of mass of the femoral neck surface is not a solid method to apply in surgical navigation and is too difficult to standardize in clinical settings.
These findings provide new insights into femoral anteversion and its variability and may have applications in the development of new image-based diagnostic strategies, hip implant design and positioning.