Quantitative comparison of cortical bone thickness using correspondence-based shape modeling in patients with cam femoroacetabular impingement

Abstract

The proximal femur is abnormally shaped in patients with cam-type femoroacetabular impingement (FAI). Impingement may elicit bone remodeling at the proximal femur, causing increases in cortical bone thickness. We used correspondence-based shape modeling to quantify and compare cortical thickness between cam patients and controls for the location of the cam lesion and the proximal femur. Computed tomography images were segmented for 45 controls and 28 cam-type FAI patients. The segmentations were input to a correspondence-based shape model to identify the region of the cam lesion. Median cortical thickness data over the region of the cam lesion and the proximal femur were compared between mixed-gender and gender-specific groups. Median [interquartile range] thickness was significantly greater in FAI patients than controls in the cam lesion (1.47 [0.64] vs. 1.13 [0.22] mm, respectively; p < 0.001) and proximal femur (1.28 [0.30] vs. 0.97 [0.22] mm, respectively; p < 0.001). Maximum thickness in the region of the cam lesion was more anterior and less lateral (p < 0.001) in FAI patients. Male FAI patients had increased thickness compared to male controls in the cam lesion (1.47 [0.72] vs. 1.10 [0.19] mm, respectively; p < 0.001) and proximal femur (1.25 [0.29] vs. 0.94 [0.17] mm, respectively; p < 0.001). Thickness was not significantly different between male and female controls.

Clinical significance: Studies of non-pathologic cadavers have provided guidelines regarding safe surgical resection depth for FAI patients. However, our results suggest impingement induces cortical thickening in cam patients, which may strengthen the proximal femur. Thus, these previously established guidelines may be too conservative. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1743-1753, 2017.

Keywords: FAI and morphology; bone; disease process; statistics.

Figure 1

Flowchart for modeling pipeline. Subject specific shapes of the inner and outer boundaries of the cortex were segmented and reconstructed from computed tomography scans; cortical thickness was determined. Surfaces were aligned and rasterized in preparation for analysis. Cutting planes were transformed to each shape and used to limit locations of correspondence points. The correspondence model was generated by placement of particles on each rasterized surface. Mean shapes were calculated from average locations of the correspondence particles. Cortical thickness obtained from each subject was then mapped to the mean shapes of each group using the correspondence model to display variation.

Figure 2

Contours representing the 2D projection of femur surfaces in the axial and coronal planes represent variation of plus (blue) and minus (orange) two standard deviations of each PCA mode that corresponded to a significant difference between the cam and control shapes. Modes represent variation in the curvature of the femoral head–neck junction and greater trochanter.

Figure 3

The correspondence model was used to determine a linear discrimination of the variation between the mean cam and mean control shapes in shape space (bottom row) which was normalized from −1 to +1. Standard deviations for each are shown in parentheses and mapped above the mean shapes. Each subject shape was then mapped to this linear representation of shape variabilities. Five subject specific shapes from three cam patients and two controls are shown with their mapping value at the appropriate location.

Figure 4

Anterior view of fringe plots showing differences in anatomy and mean cortical thickness for cam and control groups. (A) Mean shapes for the control (left) and cam (middle) groups shown with the shape difference mapped onto the mean control femur (right). (B) The region of the head–neck junction with greater than 1.5 mm of difference in the anatomy of the outer cortex was identified as the region of the cam lesion (right); thickness was evaluated for this region on the mean control (left) and cam (middle) shapes. (C) Mean cortical thickness of the control (left) and cam (middle) groups with the difference in mean thickness between groups (right). The greatest difference in thickness corresponded with the region of the cam lesion (right, inset). Note: The fringe plot in the inset has been re-scaled.

Figure 5

Percentile thickness representing the 10th, 25th, 50th, 75th, and 90th percentile of thickness for each of the vertices on the mean control (top) and cam (bottom) femur (median thickness is the 50th percentile). Increased cortical thickness was evident overall and in the region of the cam lesion on the mean cam femur.

Figure 6

Vectorial representation of the location of maximum cortical bone thickness plotted relative to the mean shape of the control group. Solid vectors represent mean location for the cam (blue) and control (red) groups. Dashed vectors represent one standard deviation of angular variation in each view.