The intricate microchannel structure of the human femoral head

Abstract

Purpose: The presence of a subchondral channel-like microstructure that provides a direct link between articular cartilage (AC) and deeper trabecular bone has been confirmed in a handful of classic literature. Referred to as "defects” or "breaks", they are not yet three-dimensionally visualized or characterized, despite their potential importance in understanding osteoarthritis (OA) progression/initiation. Here, we demonstrate the intricate 3D microstructure of the subchondral bone (SCB) channels in healthy human femoral head, and their region-dependent pattern. The microchannels are quantified in terms of average number (Ch.N), thickness (Ch.Th), as well as their correlation to the subchondral bone plate, and cartilage thicknesses. Methods: Human femurs from the anatomical gift program of the Medizinische Hochschule Hannover (MHH) were kindly provided to us. They were independently graded by three orthopaedic surgeons according to the Outerbridge classification system, and subsequently, five healthy female joints were selected. 44 measuring points were systematically defined on each sample. They were the intersections of two sets of geometrical features that were drawn on the femoral heads: 1) Twelve concentric lines, each separated by a 30°-angle interval, starting from the center (C) of the femoral head and ending on the neck junction. 2) four parallel parasagittal planes, which divided the arc between the C and the neck junctions into four equally-spaced regions. The location of each set of measuring points was standardized using a template grid, taking into account the normalized size of each femoral head. Cylindrical specimens (n=220, D=2.00 mm) were then drilled out from each sample and scanned using a high-definition micro-CT system (μCT 50, SCANCO Medical AG, Switzerland; voxel size=1.2 μm, source voltage=90 kVp, intensity=88 μA). The SCB channels were segmented and analyzed using appropriate evaluation scripts and threshold setting. 5-m thick sagittal sections of hip joint were cut and processed for toluidine blue and hematoxylin and eosin (H&E) staining. Sections were also incubated with primary antibodies against CD31 (ab28364, Abcam, 1:100), von Willebrand Factor (vWF, Dako, 1:200), calcitonin receptor (ab11042, Abcam, 1:75), and collagen type I (ab34710, Abcam, 1:200), and then processed for immunohistochemical staining using standard protocols. Statistical evaluation was carried out using SPSS package (IBM SPSS, version 25.0). Results: The data were categorized into three groups based on the contact area of the femoral head with the acetabulum: load-bearing region (LBR), non-load-bearing region (NLBR), and partial contact (PC). The microchannels showed a strong region-dependent architecture. They were abundant and small (Ch.N = 34.2 ± 11.3 1/mm; Ch.Th = 26 ± 16 μm) on the LBR, perpendicularly connecting SCB's surface to deeper trabecular spacing, while scarce and relatively thick (Ch.N = 3.7 ± 1.3 1/mm; Ch.Th = 168 ± 82 μm) on the NLBR of the femoral head. The area where the femoral head and acetabulum were only partially in contact (PC) showed a combination of thick and small channels (Ch.N = 19.6 ± 7.7 1/mm; Ch.Th = 75 ± 53 μm). The mean AC thickness was positively correlated (Pearson’s r = 0.43) with the number of microchannels (LBR: 696 ± 209 μm, PC: 575 ± 162 μm, NLBR: 315 ± 124 μm). H&E staining of the subchondral zone showed that articular cartilage was occasionally extended through the calcified cartilage and marrow spaces. Likewise, immunohistochemistry results demonstrated positive expression for vWF inside microchannels, a specific marker for endothelial cells. Conclusions: Our results demonstrate that an intricate microchannel structure exists in the subchondral bone of the human femoral head. The number, width, and distribution pattern of the channels is strongly region-dependent. The positive correlation of microchannel numbers with AC thickness in healthy femurs shows that they might play a dynamic role in the nutrition of AC.