Subchondral focal osteopenia associated with proximal sesamoid bone fracture in Thoroughbred racehorses

Abstract

Background Proximal sesamoid bone (PSB) fracture is the most common fatal injury in Thoroughbred (TB) racehorses in the United States. Epidemiological and pathological evidence indicates PSB fracture is likely the acute culmination of a chronic stress‐related process. However, the aetiopathogenesis of PSB fracture is poorly understood. Objective To characterise bone abnormalities that precede PSB fracture. Study design Two retrospective case‐control groups of PSBs from TB racehorses with, and without, unilateral biaxial PSB fracture. Methods Proximal sesamoid bones were harvested post‐mortem from TB racehorses subjected to euthanasia for unilateral biaxial PSB fracture (cases) or causes unrelated to PSB fracture (controls) while racing or training. The fractured medial PSB (FX‐PSB) and contralateral intact medial PSB (CLI‐PSB) from racehorses that sustained PSB fracture, and an intact medial PSB (CTRL‐PSB) from racehorses that did not have a PSB fracture were collected as case and control specimens. Study 1 distributions of morphological features were compared among case and control groups using visual examination, photographs, radiographs and histology of whole PSBs and serial sagittal sections (10 FX‐PSB, 10 CLI‐PSB and 10 CTRL‐PSB). Study 2 local bone volume fraction and mineral densities were compared among case and control PSBs using microcomputed tomography (9 FX‐PSB, 9 CLI‐PSB and 9 CTRL‐PSB). Results A focal subchondral lesion characterised by colocalised focal discoloration, radiolucency, osteopenia, low tissue mineral density and a surrounding region of dense cancellous bone was identified in most case horses but not in controls. This subchondral lesion was found in a slightly abaxial mid‐body location and was bilaterally present in most case horses. Main limitations The post‐mortem samples may not represent the spectrum of abnormalities that occur throughout the development of the subchondral lesion. Lateral PSBs were not examined, so their contribution to biaxial PSB fracture pathogenesis is unknown. Conclusion Abaxial subchondral lesions are consistent with pre‐existing injury and likely associated with PSB fracture.

Sudden death caused by spinal cord injury associated with vertebral fractures and fetlock failure in a Thoroughbred racehorse

“…6, 7) likely predisposed to complete fracture and catastrophic injury of the fetlock. 13 A comparable osteopenic lesion was present in the intact medial PSB of the left forelimb (Figs. 5, 7), which is consistent with bilateral repetitive overuse injury in racehorses.…”

“…5, 7), which is consistent with bilateral repetitive overuse injury in racehorses. 13 The metacarpophalangeal joint is a high motion joint with small cross-sectional area, and little soft tissue covering. When ground reaction and muscle forces are taken into consideration, this joint undergoes 5-7 times its body weight in stress.…”

“…Compressive forces are suspected to cause the microdamage on the subchondral surface; tensile forces may result in focal microdamage accumulation on the palmar surface. 3,13 The sclerosis associated with the osteopenic lesion may be an adaptation to increasing intensity of training and racing and/or serve to buttress weakness associated with the osteopenic lesion. Clinically, affected horses are difficult to detect.…”

“…Clinical signs of the subchondral bone disease may not be apparent until the intra-articular environment is involved, 8 which is rare in catastrophic PSB fracture. 13 Further, the osteopenic lesion is difficult to detect in affected horses. Lesions are located most frequently in the subchondral bone of the abaxial portion of the articular surface of the medial PSB.…”

“…Lesions are located most frequently in the subchondral bone of the abaxial portion of the articular surface of the medial PSB. 13 Lesion detection by 2-dimensional planar imaging modalities (radiography and scintigraphy) is difficult given the superimposition of dense bone tissues on the small lesions. Volumetric imaging modalities (computed tomography, CT; magnetic resonance imaging, MRI) may improve localization of lesions.…”