Necropsy survey of metacarpal fusion in the horse.

This research examined the prevalence and sites of metacarpal fusion in horses, finding that it was common and could be related to age, occupation, and specific anatomical features. Metacarpal fusion could potentially be a functional adaptation to various loading conditions, not just the result of trauma.

Metacarpal Fusion Overview

• The study surveyed metacarpal fusions in horses, which refers to the merging or joining of metacarpal bones in the horse’s leg.
• There were 100 horses included in the study, their ages ranged from term fetus to 35 years old. They found that 78% of horses aged two or older had two or more metacarpal fusions.
• This suggests that metacarpal fusion is quite common in horses, particularly mature ones.

Fusion Sites and Patterns

• The fusion was more commonly found between the second and third metacarpal bones.
• The fusions were not random; they often appeared in pairs and were bilateral, meaning they occurred symmetrically on both sides of the horse.
• Also, two distinct carpometacarpal joint configurations were noted; one where there’s no measurable articulation between certain bones, and another with measurable articulation.
• The latter, Rooney-Prickett type-B, was observed in most of the cases – about 98.5%.

Correlation with Age, Occupation and Proximal Projection

• Medial metacarpal fusion, or fusion located towards the midline of the body, was found to be positively correlated with factors such as age, occupation, and the proportion of the proximal projection of the carpometacarpal distal joint surface taken by the second metacarpal bone.
• Meanwhile, lateral metacarpal fusion, or fusion located towards the side of the body, was correlated with age and the proportion of the proximal projection of the carpometacarpal distal joint surface taken by the fourth metacarpal bone.
• Horses in performance careers (racing, training, or show ring occupations) tended to develop the first two fusions earlier than horses involved in different occupations.

Concept of Functional Adaptation

• It was also hypothesized that many instances of metacarpal fusion may not be the result of isolated traumatic events, but rather due to functional adaptation to increased or changed loading conditions.
• In other words, the horse’s body could be adapting over time to the physical stress of its work or environment by fusing the metacarpal bones together for more structural integrity and support.