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

Keywords: computed tomography; cone beam; fan beam; fatigue injury; fissure; magnetic resonance imaging; stress fracture.

Figure 1

Locations for image analysis and locations observed with fissures on image analysis. The third metacarpal/tarsal parasagittal groove and the proximal phalanx sagittal groove were divided into dorsal, middle, and palmar/plantar locations by the dorsal and palmar/plantar borders of the collateral fossae ((A); dotted lines). Locations with fissures observed on CT and MR images were tracked for the collection of bone specimens ((B,C); marks)).

Figure 2

Fissures at the palmar aspect of the medial and lateral parasagittal grooves. Lateral is to the right. CBCT (A–D) and FBCT (E–H) dorsal and transverse orientation. Images (A,E) are diagrams showing the normal appearance of dorsal sections, without fissures in the parasagittal grooves. Images (C,G) are magnifications of the selected area on images (B,F). The medial parasagittal groove fissure appeared as striated hypoattenuating lines on the CBCT (arrow; (C,D)) and very subtle subchondral outline irregularity on the FBCT (arrow; (G)). The lateral parasagittal groove fissure appeared as hypoattenuating linear defects on both CBCT and FBCT (arrowhead; (C,D,G,H)). Macroscopic appearance of medial and lateral parasagittal groove fissures with articular cartilage defects ((I); arrow and arrowhead). Microscopic appearance of fissures at the medial (J) and lateral (K) parasagittal grooves. Microcracks in the calcified cartilage and subchondral bone plate appeared as oblique striated linear lesions and some were coalescing (arrowhead; (J,K)). Calcified cartilage cleft (arrow). Scale bar = 1 mm.

Figure 3

Fissures at the dorsal and middle aspects of the sagittal groove. Lateral is to the right. CBCT (A–F) and FBCT (G–L) dorsal and transverse orientation. Images (A,G) are diagrams showing the normal appearance of dorsal sections, without fissures in the sagittal groove. Images (C,I) are magnifications of the selected area on images (B,H). Striated hypoattenuating lines on CBCT and subchondral outline irregularity on FBCT at the dorsal aspect of sagittal groove (arrowheads; (C,I)). Images (E,K) are magnifications of the selected area on images (D,J). Striated hypoattenuating lines in the subchondral bone on CBCT and FBCT at the middle aspect of sagittal groove (arrow; (E,K)). A subtle, hypoattenuating line was seen in the transverse orientation (arrow; (F,L)). Macroscopic appearance of fissures at the dorsal and middle aspects of sagittal groove with subchondral discolouration (arrowheads and arrows; (M)). Artefacts of light reflection (*). Microscopic appearance of fissures at the dorsal (N) and middle (O) aspects of sagittal groove. Microcracks in the calcified cartilage and subchondral bone plate appeared as oblique striated linear lesions and some were coalescing (arrowhead; (N,O)). Tidemark incongruence (arrow, (N,O)). Interface between articular hyaline cartilage and calcified cartilage (dotted lines, (N,O)). Scale bar = 1 mm.

Figure 4

Parasagittal grooves and sagittal groove fissures on MR images. Lateral is to the right. Parasagittal groove fissures appeared as hypointense or hyperintense defects (arrow) on T1W 3D (A) and PD SE (B) transverse plane (the same lesions as Figure 2). Sagittal groove fissures appeared as subchondral bone irregularity (arrowhead) on the dorsal plane (C) and a hyperintense defect (arrowhead) on the sagittal plane (D) of the T1W 3D sequence. Macroscopic appearance of the fissure (E). Microscopic appearance of the sagittal groove fissure (F). Microcracks in the calcified cartilage appeared as coalesced oblique striated linear lesion (arrowhead). Tidemark incongruence (arrow) and variation in calcified cartilage depth (*). Scale bar = 1 mm.