SUMMARY
The source of a previously described radiolucent crescent in the flexor cortex of the distal sesamoid (navicular) bone on the palmaro45°proximal-palmarodistal oblique (Pa45°Pr-PaDio) clinical radiographic projection was investigated in 48 forelimb navicular bones from 24 Thoroughbreds by use of high-detail radiography and x-ray computed tomography (ct). Twenty-five of these bones also were evaluated, using microradiography and histologic examinations. Of these 25 bones, 5 had been labeled in vivo with fluorochrome markers. Tetrachrome-stained 100-µm-thick nondecalcified sections of these 5 bones were examined, using epifluorescence microscopy.
A reinforcement line of compacted cancellous bone, parallel and several millimeters deep to the flexor cortex in the region of the flexor central eminence, was visualized by ct in 42 of 48 navicular bones and by microradiography in 23 of 25 navicular bones investigated. Variable degrees of compaction were observed in the cancellous bone between the flexor cortex and the reinforcement line. High-detail skyline radiographic projections and reconstructed ct images indicated a crescent-shaped lucency within the flexor central eminence of the flexor cortex in the bones in which the reinforcement line was identified, but the cancellous bone between the reinforcement line and the flexor cortex had not been compacted. The radiolucent crescent seen in the flexor central eminence of the navicular bone on the Pa45°Pr-PaDiO projection was not caused by the concave defect or synovial fossa of the flexor central eminence overlying the flexor cortex, as was described. The crescent-shaped lucency within the navicular bone flexor central eminence identified on clinical radiographs was associated with remodeling of cancellous bone within the medullary cavity of the navicular bone. It is hypothesized that remodeling of the cancellous bone is secondary to biomechanical stresses and strains placed on the navicular bone, although the clinical relevance of this finding was not determined during the study.
