Évaluation par IRM 3T, par échographie et par microscopie du cartilage épiphysaire de poulains de 0-6 mois d’âge : mieux comprendre et diagnostiquer l’ostéochondrose

Osteochondrosis (OC) is an ischemic chondronecrosis of epiphyseal growth cartilage that results in focal failure of endochondral ossification and in the development of osteochondritis dissecans (OCD) at specific sites in the epiphyses of humans and animals, including horses. Extensive OC lesions at the lateral ridge of the equine trochlea have a poor outcome with surgical debridement and can be career ending. The pathophysiology of the disease is partly understood, but the upstream events leading to the focal ischemia remain unknown. The epiphyseal growth cartilage matrix undergoes major dynamic changes in early life that could weaken it biomechanically and predispose it to focal trauma and vascular failure. There is a clinical need to develop and validate non-invasive imaging techniques for screening and monitoring of subclinical lesions, as their early detection holds the promise to improve outcome. Subregions in neonatal foal femoral epiphyses (n=10 osteochondrosis predisposed; n=6 control) were assessed for vascularity using 3T Susceptibility-weighted imaging (3T SWI) and ultrasonography and for proteoglycan and collagen structure/content employing 3T quantitative MRI (3T qMRI: T1r and T2 maps). Site-matched validations were made with histology, immunohistochemistry and second harmonic microscopy. There was no vascular and cartilage matrix difference between control and OC specimens, but a striking difference was seen in vascular pattern and cartilage matrix structure and composition at the OC-predilected site in the lateral ridge of the trochlea in all specimens compared with others sites in the trochlear ridge. This site was less ossified with more perichondrial vessels not yet bridging with the subchondral bone. Furthermore, the mean vascular density was significantly higher at this site, as well as T1r and T2 relaxation times (p<0,002), suggesting matrix content and structure changes. Microscopic evaluation of this region revealed an expansive area with low proteoglycan content and a hypertrophic-like appearance on second-harmonic microscopy. We speculate that this matrix structure and composition, though physiological, may weaken the epiphyseal growth cartilage biomechanically in focal regions and could enhance the risk of vascular failure with trauma leading to OC. However, additional investigations are required to confirm this hypothesis. As ultrasonographic measurements strongly correlated with the histological measurements and with the site-matched SWI images, v and to translate our findings to the clinic, we indertook an ultrasonographic examination of the femoral trochleae of a cohort of foals in a field situation on a stud farm (n=46, 27-166 days old) was performed and radiographs were taken concurrently and at one year. Subclinical osteochondrosis lesions, characterised by semicircular indentations in the ossification front (indirect evidence of focal failure of ossification) were detected in six foals (28–145 days old), both with radiography and ultrasonography, but post-mortem validation of lesions was not possible. While 3T SWI MRI and 3T qMRI pave the way for non-destructive longitudinal studies to improve understanding of OC in all species, ultrasonographic examination of the femoral trochlea in young foals is a promising, practical tool for screening subclinical osteochondrosis and monitoring and managing lesions at important clinical sites.

Keyphrases: Foal, Epiphyseal growth cartilage, Vascularity, Proteoglycan, Collagen, Quantitative MRI, Ultrasonography, Osteochondrosis