Longitudinal associations between dorsal carpal and third metacarpal bone morphology in Thoroughbred foals during growth.

Overview

• This study investigates how the shape of certain forelimb bones in Thoroughbred foals changes together during growth.
• It focuses on the relationship between the angles of the dorsal carpal bone and structural characteristics of the third metacarpal bone (MC3) as foals age up to 14 months.

Background

• The equine forelimb bones, specifically the carpal bones and the third metacarpal (MC3), undergo changes in shape and structure during early growth.
• These morphological adaptations are influenced by factors related to both natural development and mechanical loading (forces due to movement and exercise).
• Understanding how these bones change together over time is important because it may help identify early indicators of bone injuries before horses begin training.

Objectives

• To examine the longitudinal (over time) associations between the angular morphology of the dorsal carpal bone and various structural parameters of the MC3 bone in Thoroughbred foals.
• Specifically, to quantify how changes in dorsal carpal angles relate to changes in MC3 bone length, cortical width, and a radiographic index indicative of structural adaptation.

Methods

• Subjects: 20 Thoroughbred foals monitored from within two weeks old until 14 months of age.
• Imaging: Radiographs of the carpal bones (dorsopalmar view) and third metacarpal bones (lateromedial view) taken every two months.
• Measurements:
  • Two angular parameters measured on the dorsal carpal bone.
  • Three parameters measured on MC3: dorsal cortical width (thickness of bone cortex on the dorsal side), total bone length, and a radiographic index representing relative thickening and structural adaptation at the dorsal midshaft.
• Analysis: Linear mixed-effects models were used to analyze data with consideration for repeated measures per foal (random effect: foal identity) and fixed effects including foal age and limb side (left/right).
• Software: ImageJ was used to perform the image measurements from radiographs.

Results

• Both dorsal carpal angular measurements showed positive associations with all three MC3 parameters:
• MC3 length increased with increased dorsal carpal angles.
• Dorsal cortical width of MC3 increased alongside dorsal carpal angular changes.
• The radiographic index, indicative of structural adaptation linked to mechanical loading, was positively associated with dorsal carpal angles.
• The strongest statistical association was between the proximal dorsocranial inclination angle of the dorsal carpal and the radiographic index of the MC3 (a 2.55° increase in angle per unit increase in the radiographic index; highly significant with p < 0.001).

Conclusions

• There are significant longitudinal correlations between the angulation of the dorsal carpal bone and structural changes in the MC3 during early growth in foals.
• This pattern is consistent with the idea that these bones adapt structurally in response to mechanical loading during development.
• Because the study is observational, it cannot definitively determine whether changes are caused by growth alone, mechanical forces from exercise, or a combination of both.
• The results enhance understanding of bone development and may contribute to identifying morphological markers predictive of bone injury risk before the onset of heavy training in racehorses.