Discussion

Singer, E.R., Garcia-Nolen, T., Stover, S.M.
Aims: The aim of this in vitro study was to investigate metacarpophalangeal joint movement out of the sagittal plane, in particular, collateromotion of the proximal phalanx (P1) relative to the third metacarpal bone (McIII). Methods: Six equine cadaveric unilateral forelimbs, from mid-radius distally, were instrumented with reflective bone reference markers in McIII and P1 for measurement of bone positions during axial loading (1800 - 10,500 N). The limb preparations were axially loaded in a materials testing system to a load of 10,500 N. Bone reference markers, recorded by video, were digitised and analysed using kinematic analysis software to determine collateromotion of P1 relative to McIII during extension of the metacarpophalangeal joint. Results: Fetlock extension, axial rotation and lateromotion of P1 relative to McIII increased with increasing load. As measured from baseline angles at stance (1800 N), loading to 10,500 N induced maximum fetlock extension of 38.4degree plusminus 5.5degrees (mean plusminus s.d.), axial rotation of 4.5degrees plusminus 2.7degrees and lateromotion of 1.8degrees plusminus 1.8degree. The relationship between axial rotation and load is linear, whereas the degree of lateromotion plateaus after the fetlock has extended approximately 20degrees from baseline. Conclusions and practical significance: Movement out of the sagittal plane occurs during extension of the MCP joint with loading to a force equivalent to galloping. As loading reaches a maximum, P1 appears to undergo external rotation and lateromotion relative to McIII likely resulting in asymmetric alignment between the sagittal ridge of McIII and the sagittal groove of P1. The observed collateromotion and rotation of P1 in this study is the first concrete evidence to support the biomechanical theory of sagittal fractures of P1 which suggests that: compressive and torsional forces imposed on the sagittal groove by the sagittal ridge of McIII influence the development of sagittal fractures of P1. Acknowledgements: Jen Symonds for assistance with digitisation.