The microchannel structure of the subchondral bone in healthy human femoral head

Introduction:

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. They are quantified in terms of average number (Ch.N), thickness (Ch.Th), as well as their correlation to the 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. 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 = 88µA). The SCB channels were segmented using appropriate evaluation script and threshold setting. 5-m thick sagittal sections of hip joint were cut and processed for hematoxylin and eosin (H&E), von Willebrand Factor (vWF), and collagen type I staining using standard protocols.

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 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. At the PC area, a combination of thick and small channels (Ch.N = 19.6 ± 7.7 1/mm; Ch.Th = 75 ± 53 µm) were observed. The mean AC thickness was positively correlated (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 SCB showed that AC was occasionally extended through the CC and marrow spaces. Likewise, immunohistochemistry results demonstrated positive expression for vWF inside microchannels, a specific marker for endothelial cells.

Discussion:

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 are 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.