Three-Dimensional Imaging and Histopathological Features of Third Metacarpal/Tarsal Parasagittal Groove and Proximal Phalanx Sagittal Groove Fissures in Thoroughbred Horses

SIMPLE SUMMARY: Fractures of the third metacarpal/tarsal parasagittal groove and proximal phalanx sagittal groove are common in racehorses. It is important to detect precursor pathologies including fissures to prevent the propagation to fracture. This study aims to identify the imaging features and compare the diagnosis of fissures on cone-beam (CB) computed tomography (CT), fan-beam (FB) CT, and low-field magnetic resonance imaging (MRI) to histopathology associated with fissures. Fissures were characterised on CBCT and FBCT as hypoattenuating linear defects, striated hypoattenuating lines, or subchondral irregularity. Fissures were characterised on MRI as subchondral hypo-/hyperintense defects. The diagnostic sensitivity was highest in CBCT, followed by FBCT and MRI, while specificity was highest in MRI, followed by FBCT and CBCT. Fissures identified on CT were associated with histopathology of subchondral bone sclerosis, microcracks, and collapse. In conclusion, all modalities were able to identify fissures with sensitivity higher in CT and specificity higher in MRI. CT-identified fissures were associated with histopathological indications of fatigue bone injuries. Imaging features and histopathological features of fissures characterised in this study may help clinical identification and image interpretation of fissures in horses. ABSTRACT: Fissure in the third metacarpal/tarsal parasagittal groove and proximal phalanx sagittal groove is a potential prodromal pathology of fracture; therefore, early identification and characterisation of fissures using non-invasive imaging could be of clinical value. Thirty-three equine cadaver limbs underwent standing cone-beam (CB) computed tomography (CT), fan-beam (FB) CT, low-field magnetic resonance imaging (MRI), and macro/histo-pathological examination. Imaging diagnoses of fissures were compared to microscopic examination. Imaging features of fissures were described. Histopathological findings were scored and compared between locations with and without fissures on CT. Microscopic examination identified 114/291 locations with fissures. The diagnostic sensitivity and specificity were 88.5% and 61.3% for CBCT, 84.1% and 72.3% for FBCT, and 43.6% and 85.2% for MRI. Four types of imaging features of fissures were characterised on CT: (1) CBCT/FBCT hypoattenuating linear defects, (2) CBCT/FBCT striated hypoattenuated lines, (3) CBCT/FBCT subchondral irregularity, and (4) CBCT striated hypoattenuating lines and FBCT subchondral irregularity. Fissures on MRI appeared as subchondral bone hypo-/hyperintense defects. Microscopic scores of subchondral bone sclerosis, microcracks, and collapse were significantly higher in locations with CT-identified fissures. All imaging modalities were able to identify fissures. Fissures identified on CT were associated with histopathology of fatigue injuries.