Maturation of the medial femoral condyle epiphyseal growth plate and osteochondral junction

Abstract

Purpose: The osteochondral junction region has an important role in osteoarthritis and many early pathophysiological features (e.g. chondrocyte hypertrophy) of the disease recapitulate development events. Femorotibial osteoarthritis arises most commonly in the medial compartment in humans and horses. Furthermore, juvenile osteochondrosis (JOCD) also develops in this region, and if not treated successfully, the persisting incongruency can lead to secondary osteoarthritis. The maturation of the epiphyseal growth cartilage and the osteochondral junction of the medial femoral condyle (MFC) of the distal femur is poorly described in the literature. Horses are also increasingly recognized to be appropriate large animal models for cartilage repair investigations and information on maturation of the osteochondral junction is lacking. Our goal was to describe the normal development of the equine MFC osteochondral junction from birth to 2 years old. Methods: Distal femurs were collected from Thoroughbred horses (n=61, 0-2 years old), presented for post-mortem examination for reasons unrelated to this study. Radiographs and tomodensitometry were performed to screen for healthy specimens without focal osteochondral junction lesions. Micro-tomodensitometry (μ-CT), histology and immunohistochemistry were performed on the selected MFCs (n=29). μ-CT: A region of interest (ROI) was selected for study in a weight bearing area of the MFC that also incorporated developmental disease (JOCD and equine cysts/osteochondrosis) prone sites. Cartilage thickness (Cth) and trabecular bone volume were measured. 3D images were also constructed and the osteochondral junction morphology was assessed. Histology: MFC epiphyseal growth plate structure was studied on hematoxylin phloxine and saffron (HPS) stained sections. Collagen structure (picrosirius red) and proteoglycan content (Safranin O fast green) were also evaluated. Chondrocyte lacunae diameters were measured in predefined regions below the articular surface and near the osteochondral junction as an indicator of chondrocyte hypertrophy. Collagen X (Col X) immunohistochemical staining was also employed as a marker of endochondral ossification. Results: μ-CT: The Cthranged from 3.8mm in the youngest specimens to 1.2mm at 2 years of age. The Cthdecreased from birth to 7 months and remained constant thereafter (regression into 2 segments, p<0.0001). From birth to 6 months of age, the ossification front was porous on 3D images (fig 1.a) and later had a smooth appearance (fig 1. d). The trabecular bone volume did not change with age and was significantly higher at the periphery of the condyle, near the cartilage, compared to the center (p<0.0001). Histology: The structure of the epiphyseal growth cartilage differed from that reported for the physis as there was no columnar organization of the chondrocytes. The safranin O fast green and picrosirius stains showed a stratification of the extracellular matrix content that varied with age. The osteochondral unit was immature up to 6 months of age: Cartilage canals containing blood vessels crossed the ossification front (fig 1.b) and the chondrocytes were undergoing hypertrophy near the cartilage-bone junction. The chondrocyte lacunar volume increased 14-fold beside the osteochondral junction in the youngest specimens (fig 1.c). Expression of Col X was evident in the extracellular matrix surrounding hypertrophic chondrocytes in site matched specimens in the juvenile specimens alone. At six months of age, the absence of chondrocyte hypertrophy and Col X immunostaining indicated the conclusion of endochondral ossification. This coincided with the smallest Cth, similar to mature articular cartilage and the appearance of a narrow band of compact bone indicating formation of a subchondral cortical bone plate (fig 1.e). Conclusions: The equine MFC osteochondral junction is structurally immature and undergoes endochondral ossification up to 6 months of age. At this time, the histological structure of a subchondral cortical bone plate emerges. Cartilage canals and hypertrophic chondrocytes are no longer present and the cartilage thickness is stable and consistent with a mature articular cartilage. Although we have explored the structural and matrix maturation events, additional studies are now required to assess the biomechanical maturation of the region. New information is provided on MFC osteochondral junction development that may help towards the understanding of the remodeling events occurring in osteoarthritic joints.