# Histological and Histopathological Features of the Third Metacarpal/Tarsal Parasagittal Groove and Proximal Phalanx Sagittal Groove in Thoroughbred Horses with Racing History

## TL;DR

This study examines bone tissue damage in specific areas of racehorse legs, finding that fissures and certain anatomical regions show more severe damage, which could help understand how fractures develop.

## Contribution

The study provides new histopathological insights into the P1 sagittal groove in racehorses, highlighting fissure-related damage and regional differences.

## Key findings

- Histopathology in fissure locations was more severe in both the MC3/MT3 and P1 grooves.
- SCB/TB damage was most severe in the palmar/plantar MC3/MT3 and the dorsal P1 sagittal groove.
- Bone fatigue injury patterns were linked to fissure locations and specific anatomical regions.

## Abstract

Information about the histological features of the proximal phalanx (P1) sagittal groove in racehorses is limited compared to the third metacarpal/metatarsal parasagittal groove. Fractures of the P1 sagittal groove are common in racehorses; therefore, it is important to improve the understanding of the histopathology of the P1 sagittal groove. This study aims to describe histopathological features of the third metacarpal/metatarsal parasagittal groove and the proximal phalanx sagittal groove in Thoroughbred racehorses. For third metacarpal/metatarsal parasagittal grooves, hyaline cartilage (HC), calcified cartilage (CC), and subchondral plate and trabecular bone (SCB/TB) histopathology including SCB collapse were most severe in locations with fissures (observed in a previous study) and the palmar/plantar aspect of the parasagittal groove. For P1 sagittal grooves, SCB/TB histopathology was most severe in locations with fissures, and SCB collapse was most severe in the dorsal and middle aspects of sagittal groove. In conclusion, histopathological features of bone fatigue injury were related to fissure locations, the palmar/plantar third metacarpal/metatarsal parasagittal groove, and the dorsal P1 sagittal groove. Histopathological findings in the P1 sagittal groove may be useful for investigating the pathogenesis of P1 sagittal groove fractures.

Information regarding the histopathology of the proximal phalanx (P1) sagittal groove in racehorses is limited. Twenty-nine cadaver limbs from nine Thoroughbred racehorses in racing/race-training underwent histological examination. Histological specimens of the third metacarpal/metatarsal (MC3/MT3) parasagittal grooves and P1 sagittal grooves were graded for histopathological findings in hyaline cartilage (HC), calcified cartilage (CC), and subchondral plate and trabecular bone (SCB/TB) regions. Histopathological grades were compared between (1) fissure and non-fissure locations observed in a previous study and (2) dorsal, middle, and palmar/plantar aspects. (1) HC, CC, and SCB/TB grades were more severe in fissure than non-fissure locations in the MC3/MT3 parasagittal groove (p < 0.001). SCB/TB grades were more severe in fissure than non-fissure locations in the P1 sagittal groove (p < 0.001). (2) HC, CC, and SCB/TB grades including SCB collapse were more severe in the palmar/plantar than the middle aspect of the MC3/MT3 parasagittal groove (p < 0.001). SCB/TB grades including SCB collapse were more severe in the dorsal and middle than the palmar/plantar aspect of the P1 sagittal groove (p < 0.001). Histopathology in the SCB/TB region including bone fatigue injury was related to fissure locations, the palmar/plantar MC3/MT3 parasagittal groove, and the dorsal P1 sagittal groove.