Third metacarpal bone laterality asymmetry and midshaft dimensions in Thoroughbred racehorses.

The study aims to understand the relationship between the length of a racehorse’s third metacarpal bones (MC3), commonly known as shin bones, and the thickness of their dorsal cortices or outer layer of bone. This research also explores if there’s a correlation between the size differences of the two MC3 bones in individual horses and variation in the thickness of the dorsal cortex.

Objective and Study Sample

• The study is based on Thoroughbred racehorses that maintain similar maximum running speeds. The objective is to determine if horses with longer MC3 bones also have thicker dorsal cortices.
• Additionally, the research investigates whether horses showcasing larger differences in the length of their right and left MC3 bones have significant variations in the thickness of the dorsal cortex in both limbs.
• A group of 40 Thoroughbred racehorses, aged between 2 to 6 years and in active racing training at two stables, were selected for the study.

Methods and Measurements

• Standardized procedures were used to measure the bone length and dorsal cortical width from two sets of radiographs for each horse’s left and right metacarpus.

Results

• Results indicated a linear relationship between dorsal cortex thickness and bone length for lengths between 25 and 30 cm for both the right (R2 = 0.30, P = 0.0003) and the left MC3 bones (R2 = 0.23, P = 0.002).
• In essence, longer bones were found to have thicker dorsal cortices.
• However, when comparing the thickness of the dorsal cortex and bone length within individual horses, the study found no consistent relationship (R2 = 0.008, P = 0.58).

Conclusions

• The study concluded that racehorses with longer MC3 bones are likely subjected to a greater dorsopalmar bending moment at the mid-shaft, resulting in a thicker dorsal cortex.
• However, the lack of a relationship between midshaft thickness and bone length in individual horses suggests that direct mechanical effects of body structure and environment are less critical than the horse’s skill level or the degree of their lateral movements, developed prior to exposure to fast exercise.
• The research also suggests that horses with longer right MC3 bones might better control the load on the right MC3 during fast exercise.