Heterogeneity of bone microstructure in the femoral head in patients with osteoporosis: an ex vivo HR-pQCT study

Abstract

Introduction: Trabecular bone in the femoral head has a complicated and heterogeneous structure with few studies having analyzed heterogeneity in this structure quantitatively. We analyze trabecular bone microstructure in the femoral head with osteoporosis (OP) using high resolution peripheral quantitative CT (HR-pQCT) to investigate its regional characteristics.

Methods: Fifteen femoral heads extracted from female OP patients with femoral neck fracture (85 ± 7, 67-94 years) were scanned by HR-pQCT at 41 μm voxel size. The femoral head was segmented into 15 regions (3 longitudinal regions: superior, center, and inferior, and 5 axial subregions: center, medial, lateral, anterior, posterior). Of these 15 regions, five were excluded due to overlap with the fracture site, leaving a total of 10 regions of cancellous bone microstructures to be quantitatively assessed using the following parameters: bone volume fraction, trabecular thickness, number, separation, connectivity density, structure model index, and degree and orientation of anisotropy. These parameters were compared among each region.

Results: Trabecular bone at the center, superior, and supero-posterior regions of the femoral head had higher bone volume, trabecular number, thickness, narrower bone marrow spaces, higher connectivity and anisotropy, and more plate-like structure. This plate-like structure ran supero-inferiorly and antero-posteriorly at the superior and center regions. Bone volume at the anterior, posterior, and medial regions was almost half of the central and superior regions.

Conclusion: Significant heterogeneity of the trabecular bone microstructure in the OP femoral head was showed quantitatively in this study. These data offer new insight into bone microstructural anatomy and may prove to provide useful information on clinical medicine such as hip surgeries.

Keywords: Bone microstructure; Femoral head; Femoral neck fracture; HR-pQCT; Osteoporosis.

Fig. 1

3D image of the femoral head of 76 y.o. female patient with femoral neck fracture. Ex vivo HR-pQCT allows high resolution images (pixel size 41 um, isotropic) of large bone specimens (within 126 mm in diameter, 150 mm length) to be obtained.

Fig. 2

A–C, Multi-planar reconstructed images of the femoral head (A–C). Orientations of all femoral head specimens were adjusted by anatomical landmarks: lining up the principal compressive trabecular band (arrows) supero-inferiorly in the coronal and sagittal views (A, B), and facing the fovea capitis femoris (arrow) medially in the axial view (C). The femoral head was divided into 3 regions (Sup., Cen., and Inf.) (A, B), and then each region was sub-divided into 5 regions (Cen., Med., Lat., Ant., and Post.) (C). D–E, Overlaid images of CT axial view and ROIs (D) and 3D images of ROIs (E). Fracture regions were excluded with a 2.5 mm margin (D). As a result, 5 regions (Cen-Lat, Inf-Cen, Inf -Lat, Inf -Ant, Inf -Post) were not ultimately studied. Cancellous bone microstructure was measured in the 10 remaining regions (Sup-Cen, Sup-Med, Sup-Lat, Sup-Ant, Sup-Post, Cen-Cen, Cen-Med, Cen-Ant, Cen-Post, and Inf-Med) (E).

Fig. 2

A–C, Multi-planar reconstructed images of the femoral head (A–C). Orientations of all femoral head specimens were adjusted by anatomical landmarks: lining up the principal compressive trabecular band (arrows) supero-inferiorly in the coronal and sagittal views (A, B), and facing the fovea capitis femoris (arrow) medially in the axial view (C). The femoral head was divided into 3 regions (Sup., Cen., and Inf.) (A, B), and then each region was sub-divided into 5 regions (Cen., Med., Lat., Ant., and Post.) (C). D–E, Overlaid images of CT axial view and ROIs (D) and 3D images of ROIs (E). Fracture regions were excluded with a 2.5 mm margin (D). As a result, 5 regions (Cen-Lat, Inf-Cen, Inf -Lat, Inf -Ant, Inf -Post) were not ultimately studied. Cancellous bone microstructure was measured in the 10 remaining regions (Sup-Cen, Sup-Med, Sup-Lat, Sup-Ant, Sup-Post, Cen-Cen, Cen-Med, Cen-Ant, Cen-Post, and Inf-Med) (E).

Fig. 3

A–B, Anisotropy was calculated by the MIL method. Degree of anisotropy was defined as the length of the primary axis / tertiary axis of the MIL ellipsoid (DA=a/c) (A). High DA indicates high anisotropy. Orientation of anisotropy was defined as the absolute angle between the MIL axis, z-axis (theta) and y axis (phi) in this study (B). a-theta=0 means the bones are running supero-inferiorly, a-theta=90 indicates a horizontal orientation, a-phi=0 means the bones are running antero-posteriorly, and a-phi=90 means they run medio-laterally. In plate-like bones, b-theta and b-phi also define the orientation of the plate structure.

Fig. 4

A–C, Regional distribution of BV/TV. BV/TV was higher in Cen-Cen, Sup-Cen, and Sup-Post regions. These regions would be weight bearing sites during standing and hip flex-extension movements. BV/TV at Cen-Ant and Cen-Post regions, where fixation of the screw for correcting a hip fracture may occasionally be improperly inserted, was almost half of Cen-Cen region.

Fig. 5

A–D, Features of trabecular bone structure at the Sup-Cen region (A and B: its orientation, C: AP view, D: Lat. view). Plate-like bone structure is predominantly observed in this region (SMI is average 1.59) and tend to run supero-inferiorly and antero-posteriorly (DA is 2.36, a-theta was 4.6, and b-phi was 9.3) (C, D). Weight bearing may indicate the need for the supero-inferior plate-like sturdy structure in this region. In addition, daily hip flex-extension movements (walk, sit down, stand up, etc.) might explain its antero-posterior orientation.

Fig. 6

A–B, 3D images at the Cen-Cen region of 2 cases (BV/TV: 32.3 and 13.6 %). Lower BV/TV was related to lower Tb.Th, Tb.N, and Conn.D, and higher Tb.Sp, SMI (more rod-like structure), and DA. Due to OP progression and consistent with loss of bone connectivity, the plate-like structures became more rod-like. An increase in anisotropy may be explained by the loss of horizontal bone connectivity while vertical bone, which is more influence by weight bearing, remains relatively stable.