Spatial assessment of femoral neck bone density and microstructure in hip osteoarthritis

Abstract

Osteoarthritis (OA) is known to involve profound changes in bone density and microstructure near to, and even distal to, the joint. Critically, however, a full, spatial picture of these abnormalities has not been well documented in a quantitative fashion in hip OA. Here, micro-computed tomography (44.8 μm/voxel) and data-driven computational anatomy were used to generate 3-D maps of the distribution of bone density and microstructure in human femoral neck samples with early (6F/4M, mean age = 51.3 years), moderate (14F/8M, mean age = 60 years), and severe (16F/6M, mean age = 63.3 years) radiographic OA. With increasing severity of radiographic OA, there was decreased cortical bone mineral density (BMD) (p=0.003), increased cortical thickness (p=0.001), increased cortical porosity (p=0.0028), and increased cortical cross-sectional area (p=0.0012, due to an increase in periosteal radius (p=0.018)), with no differences detected in the total femoral neck or trabecular compartment measures. No OA-related region-specific differences were detected through Statistical Parametric Mapping, but there were trends towards decreased tissue mineral density (TMD) in the inferior femoral neck with increasing OA severity (0.050 < p ≤ 0.091), possibly due to osteophytes. Overall, the lack of differences in cortical TMD among radiographic OA groups indicated that the decrease in cortical BMD with increasing OA severity was largely due to the increased cortical porosity rather than decreased tissue mineralization. As porosity is inversely associated with stiffness and strength in cortical bone, increased porosity may offset the effect that increased cortical cross-sectional area would be expected to have on reducing stresses within the femoral neck. The use of high-resolution imaging and quantitative spatial assessment in this study provide insight into the heterogeneous and multi-faceted changes in density and microstructure in hip OA, which have implications for OA progression and fracture risk.

Keywords: Bone; Femur; Hip; Imaging; Micro-computed tomography; Microstructure; Osteoarthritis.

Graphical abstract

Fig. 1

A representative pre-operative patient MRI scan (part A) demarcates where the femoral head and neck tissue was excised during elective THA (part A, dotted cut plane 1) and then the femoral neck tissue was separated at the femoral head-neck junction (part A, dotted cut plane 2). The VOI (part A, white box) underwent micro-computed tomography scanning, resulting in a stack of grayscale images (part B). The cortical and trabecular compartments were then segmented in ImageJ (part C). A 3-D render of the entire femoral neck sample (part D) and the cortical (part E, top) and trabecular (part E, bottom) compartments are also shown.

Fig. 2

A representative μCT slice of the femoral neck where the initial contouring results (in red) of the automated segmentation script (left) necessitated manual correction (right) by a trained operator, thus all segmentation was performed in a semi-automated fashion.

Fig. 3

The canonical femoral neck template (A), defined as the average of all femoral neck shapes present, was ascribed a mesh (B, mean element side length = 0.98 mm) that was then morphed onto each individual femoral neck sample (C) (IA-FEMesh, The University of Iowa, Iowa City, IA). Cubic sub-regions (subL = 4 mm) centered around each node (D) were evaluated for each measure of bone density and microstructure using ImageJ, the results of which were mapped back onto the anatomic reference model (E) and visualized in ParaView (v5.9.0, Kitware Inc., New York, NY).

Fig. 4

Non-parametric ANOVA with Bonferonni correction revealed differences in cortical compartment measures Ct.BMD, Ct.Po, and Ct.CSA across early, moderate, and severe radiographic OA. In contrast, no differences were found in any total or trabecular measures. Pairwise differences from non-parametric Wilcoxon of each pair are noted.

Fig. 5

Schematic representations of the mean effective periosteal and endosteal radii of the femoral neck by radiographic OA groups. The dotted lines denote the periosteal and endosteal radii of the early radiographic OA group for reference. The grayscale shading reflects the difference in Ct.BMD across OA groups. Periosteal radius was different between early and severe OA groups (p=0.0174), while no difference was detected in endosteal radius.

Fig. 6

3-D spatial maps of median values of each measure of density and microstructure for early (column 1), moderate (column 2), and severe (column 3) radiographic OA groups. Spatial maps of F-values from SPM (column 4) for each measure show no significant regional effects for any measure (F_critical for α=0.05 specified for each measure). To identify trending regions in TMD, F_critical for α=0.1 is labeled on the color scale. Anatomical orientation and scale bar in the bottom left corner.