Metacarpal geometry changes during Thoroughbred race training are compatible with sagittal-plane cantilever bending.

REASONS FOR PERFORMING STUDY: Bending of the equine metacarpal bones during locomotion is poorly understood. Cantilever bending, in particular, may influence the loading of the metacarpal bones and surrounding structures in unique ways. HYPOTHESIS: We hypothesised that increased amounts of sagittal-plane cantilever bending may govern changes to the shape of the metacarpal bones of Thoroughbred racehorses during training. We hypothesised that this type of bending would require a linear change to occur in the combined second moment of area of the bones for sagittal-plane bending (I) during race training. METHODS: Six Thoroughbred racehorses were used, who had all completed at least 4 years of race training at a commercial stable. The approximate change in I that had occurred during race training was computed from radiographic measurements at the start and end of training using a simple model of bone shape. RESULTS: A significant (P < 0.001), approximately linear pattern of change in I was observed in each horse, with the maximum change occurring proximally and the minimum change occurring distally. CONCLUSIONS: The pattern of change in I was compatible with the hypothesis that sagittal-plane cantilever bending governed changes to the shape of the metacarpal bones during race training.

Determination of mechanical loading components of the equine metacarpus from measurements of strain during walking.

REASONS FOR PERFORMING STUDY: The mechanical environment of the distal limb is thought to be involved in the pathogenesis of many injuries, but has not yet been thoroughly described. OBJECTIVES: To determine the forces and moments experienced by the metacarpus in vivo during walking and also to assess the effect of some simplifying assumptions used in analysis. METHODS: Strains from 8 gauges adhered to the left metacarpus of one horse were recorded in vivo during walking. Two different models - one based upon the mechanical theory of beams and shafts and, the other, based upon a finite element analysis (FEA) - were used to determine the external loads applied at the ends of the bone. RESULTS: Five orthogonal force and moment components were resolved by the analysis. In addition, 2 orthogonal bending moments were calculated near mid-shaft. Axial force was found to be the major loading component and displayed a bi-modal pattern during the stance phase of the stride. The shaft model of the bone showed good agreement with the FEA model, despite making many simplifying assumptions. CONCLUSIONS: A 3-dimensional loading scenario was observed in the metacarpus, with axial force being the major component. POTENTIAL RELEVANCE: These results provide an opportunity to validate mathematical (computer) models of the limb. The data may also assist in the formulation of hypotheses regarding the pathogenesis of injuries to the distal limb.

Surface strains around the midshaft of the third metacarpal bone during turning.

REASONS FOR PERFORMING STUDY: Bone strains quantify skeletal effects of specific exercise and hence assist in designing training programmes to avoid bone injury. OBJECTIVE: To test whether compressive strains increase on the lateral surface of the inside third metacarpal bone (McIII) and the medial surface of the outside McIII in a turn. METHODS: Rosette strain gauges on dorsal, medial and lateral surfaces of the midshaft of the left McIII in 2 Thoroughbred geldings were recorded simultaneously during turning at the walk on a bitumen surface. RESULTS: Medial surface: Compression peaks were larger in the outside limb. Tension peaks were larger in the inside limb and in a tighter turn. On the lateral surface compression and tension peaks were larger on the inside limb, which showed the largest recorded strains (compression of -1400 microstrains). Dorsal compression strains were larger on the outside limb and on a larger circle. Tensile strains were similar in both directions and larger on a larger circle. CONCLUSIONS: Compressive strains increased on the lateral surface of the inside McIII and medial surface of the outside McIII in a turn. POTENTIAL RELEVANCE: Slow-speed turning exercise may be sufficient to maintain bone mechanical characteristics in the inside limb lateral McIII cortex. Further work is needed to confirm these findings and to determine whether faster gaits and/or tighter turns are sufficient to cause bone modelling levels of strain in the medial and lateral McIII cortex.