Normal three-dimensional behaviour of the metacarpophalangeal joint and the effect of uneven foot bearing.

The purpose of this study was to quantify small amplitude rotational movements in the metacarpophalangeal joint (MPJ) of the horse and to measure the influence of asymmetric placement of the foot on these articular angles. Trihedrons, supporting kinematic markers defining a local frame, were screwed into the third metacarpal bone and the proximal phalanx of 4 isolated forelimbs. The limbs were loaded in a press, and the lateral or medial aspects of the foot were raised alternately by a 12 degrees wedge. The use of the joint coordinate system permitted the simultaneous and continuous computing of the 3 angles of rotation of the joint. During neutral loading, the extension of the MPJ (38.4 +/- 8.7 degrees) was associated with lateral axial rotation of the proximal phalanx (1.8 +/- 0.9 degrees). Addition of a lateral wedge induced medial rotation (-0.9 +/- 0.2 degree) and abduction (2.1 +/- 0.4 degrees) of the proximal phalanx. The opposite phenomenon was observed with a medial wedge. These quantitative results confirmed the combination of axial rotation and widening of the articular space on the opposite side to the raised part of the foot that had earlier been observed in semi-quantitative studies. Despite the high congruence of this joint, this study demonstrated the significant influence of uneven bearing of the foot on the three-dimensional (3-D) behaviour of the MPJ. Even though the amplitude of these movements remained small, their biomechanical effects should be considered to improve our understanding of MPJ injuries and to rationalise exercise management and corrective shoeing in the lame horse.