Composition and morphological features of the third interosseous muscle in normal and affected by flexural deformities newborn foals

Purpose of the work. Previous studies in foals affected by flexural deformities have shown that in severe cases the third interosseous muscle (TIOM) is one of the affected muscle-tendon structures with smaller weight, volume, cross-sectional area (CSA) and shorter TIOM length/bone ratio in comparison to non-affected newborn foal limbs1. The aim of this study was to investigate the morphological features of TIOM (total volume, % of connective and muscle tissue) in normal (control) and affected foals by comparing anatomical and histological images with magnetic resonance (MR) imaging acquisitions and subsequent 3D reconstruction. Materials and used methods. The third interosseous muscle from 16 forelimbs of newborn foals (5 normal and 11 affected by different degrees of deformity: mild, moderate and severe) were dissected, fixed in 10% buffered formalin and analyzed for morphometric measurements.MR images of TIOMs were then acquired, and samples subsequently processed and stained with Masson’s and Gomori’s trichrome at five standardized CSA levels for histological study. These CSAs were identified based on normal newborn (1-12 days old) average tendon lengths1, and according to previous morphological studies on adult horses2. Histological stains were used to identify the muscle versus the connective tissue at each level. Anatomical and histological images were used to guide the MR 3D reconstruction of the total and manually segmented muscle tissue volumes within the TIOM using dedicated software for image processing (Osirix©). Outcomes. Transverse sections of TIOMs from affected animals showed areas of intensely pigmented muscle tissue. The TIOM volume, recorded in foals severely affected by flexural deformities with MR 3D reconstruction, proved to be lower in comparison to those of normal and pathological foals with mild deformities (p=0.05). The content (in %) of muscle tissue, measured with MR in CSA of normal and mildly affected foals, ranged from 16.6 to 26.3% whereas in moderately and severely affected foals it ranged from 21.4 up to 39.3%. Foals affected by flexural deformities had less connective tissue (p=0.05) and tended to have higher muscle content within the TIOM compared to normal foals (p=0,0534). The amount of muscular tissue increased with severity of deformity. Conclusions. Third interosseous muscles in foals affected by flexural deformity had been shown to be shorter in length and volume compared to controls1. The advantage of MR 3D reconstruction for TIOM total and muscle volume measurements was to allow a more quantitative determination of the muscle tissue along the entire length of the third interosseous muscle and not only on the basis of limited number of sections as in previous studies3. Assuming muscle fibers of the TIOM have an active function due to their capacity of imparting active control over the metacarpophalangeal joint, they may play an important role in the pathogenesis of flexural deformity. The high content of mu246 scle tissue in the most severely affected foals suggests these young animals could still be at an immature fetal developmental stage in which the muscular component has not yet undergone any involution4. This exuberance of muscle tissue within the TIOM in combination with a lower connective tissue content suggests a decreased intrauterine motility with lack of collagen and muscle-tendon unit growth. Further studies will be necessary to elucidate and confirm these findings.