Femoral epiphyseal cartilage matrix changes at predilection sites of equine osteochondrosis: Quantitative MRI, second‐harmonic microscopy, and histological findings

Abstract

Osteochondrosis is an ischemic chondronecrosis of epiphyseal growth cartilage that results in focal failure of endochondral ossification and osteochondritis dissecans at specific sites in the epiphyses of humans and animals, including horses. The upstream events leading to the focal ischemia remain unknown. The epiphyseal growth cartilage matrix is composed of proteoglycan and collagen macromolecules and encases its vascular tree in canals. The matrix undergoes major dynamic changes in early life that could weaken it biomechanically and predispose it to focal trauma and vascular failure. Subregions in neonatal foal femoral epiphyses (n ¼ 10 osteochondrosis predisposed; n ¼ 6 control) were assessed 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. Growth cartilage T1r and T2 relaxation times were significantly increased (p < 0.002) within the proximal third of the trochlea, a site predisposed to osteochondrosis, when compared with other regions. However, this was observed in both control and osteochondrosis predisposed specimens. 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 osteochondrosis. However, additional investigations are now required to confirm this. 3T qMRI will be useful for future non-invasive longitudinal studies to track the osteochondrosis disease trajectory in animals and humans.

Administration of enrofloxacin during late pregnancy failed to induce lesions in the resulting newborn foals

“…The degenerating cartilage canal vessels and occasional adjacent areas of necrotic cartilage are typical findings, and are interpreted as either normal loss of cartilage canals that occurs during development, or early osteochondrosis lesions. Interestingly, the fissures or cartilage canals seen were most frequently seen in the distal scapula, and not in the typical joints where osteochondrosis lesions develop, or where lesions were seen in treated foals [3,[14][15][16]. Superficial and deep erosions were only seen in the foals treated with enrofloxacin post-partum.…”

Ultrasonographic screening for subclinical osteochondrosis of the femoral trochlea in foals (28–166 days old): a prospective farm study

“…There has recently been a plethora of experimental imaging investigations with a goal to detect early osteochondrosis lesions in foals and pigs including CT (both clinical [9] and micro-CT [10]) and MRI (quantitative [11][12][13] and susceptibility-weighted imaging [14][15][16][17]). However, although advancing knowledge of early lesions and their morphology, these methods are not widely available or economically practical for screening purposes at the farm.…”

Characterization of cellular and matrix alterations in the early pathogenesis of osteochondritis dissecans in pigs using second harmonic generation and two‐photon excitation fluorescence microscopy

“…For a clinical application of these results, future studies should correlate SHG with MRI and test whether the findings of changes in the matrix integrity can be translated and detected using an in vivo imaging method. Chondronecrosis was shown previously to be visible using MRI, and areas in cartilage with reduced SHG intensity were associated with increased T2 relaxation time . If translation is possible, a longitudinal study could test whether change in matrix integrity is a predictor of progression to OCD.…”

“…On the other hand, due to the cross‐sectional nature of this study, one cannot exclude the possibility that the differences in the SHG intensity between normal and necrotic matrix existed prior to ischemic necrosis. A recent study demonstrated that the SHG intensity in normal equine growth cartilage varies in different regions, and it was speculated that the risk of vascular failure could be higher in areas with weaker SHG intensity. However, this observation fails to explain why the matrix changes in the current study were focal and confined to the necrotic areas.…”

“…In recent studies from our research group, second harmonic generation (SHG) and two photon‐excitation fluorescence (TPEF) microscopy were used to characterize normal growth cartilage and cartilage canals . These and previous studies have shown that SHG is highly sensitive and specific to collagen fibrils and that the morphology of cells could be visualized in 3D using TPEF. The techniques allow thick sections to be examined without staining and are therefore less prone to artifacts compared to conventional histology or electron microscopy.…”