Temporomandibular joint motion is affected by dental wear and the presence of malocclusions, but the type and amount of motion at the equine TMJ have not been quantified accurately. Optical motion capture systems are now available that have sufficient accuracy and precision to measure 3-dimensional motion of the TMJ. A prerequisite to evaluating 3-dimensional motion is the establishment of coordinate systems in the segments proximal and distal to the joint. Grood and Suntay1 developed a system that provides a simple geometric description of the 3-dimensional rotational and translational motion between rigid bodies. Placing markers on key anatomic locations creates segmental joint coordinate systems, such that the axes of the coordinate systems are aligned with anatomic axes of the body segments.
The location of 1 coordinate system relative to another is described either by Euler angles (or Cardan angles) or by the concept of a helical axis. Euler angles are defined as a set of 3 finite rotations assumed to take place in sequence to achieve the final orientation from a reference orientation.2 Displacement of a segment from 1 position to another can also be represented as a rotation about and a translation along a particular axis in space called the helical axis or screw axis.3
In the study reported here, motion of the mandible relative to the skull will be defined by the following 3 successive rotations: pitch, roll, and yaw,4 where pitch is a rotation about the transverse horizontal axis, roll is a rotation about the longitudinal axis, and yaw is a rotation about the vertical axis. Fredricson and Drevemo5 used the same coordinate system to describe equine hoof motion. A complete description of 3-dimensional motion also requires the measurement of translations along 3 axes, which will be described in the rostrocaudal, dorsoventral, and mediolateral directions.
Human patients with TMJ disorders are clearly separated from asymptomatic subjects by significant differences in the range of mandibular movements, including mouth opening, right and left lateral movements, and protrusive movement of the mandible.6 Analysis of the human masticatory cycle in the frontal and sagittal planes has shown that TMJ abnormalities are associated with significantly slower chewing cycles, in which the jaws are closed more slowly but opened more rapidly.7 Furthermore, it is usually possible to use these data to determine whether the left or right joint was compromised. In horses, dental floating has been shown to increase TMJ mobility in the rostrocaudal direction but specific dental abnormalities associated with reduced mobility were not identified.8 Rostrocaudal mobility of the mandible was assessed by use of a ruler to measure the distance between the rostral aspect of the first upper incisor teeth and the rostral aspect of the first lower incisor teeth when the head was held with the mandible parallel to the ground and with the atlanto-occipital joint fully flexed. There does not appear to have been any accurate measurements of the pattern or amount of motion at the equine TMJ during chewing. The objectives of the study reported here were to develop a method of measuring the normal, 3-dimensional motion of the equine TMJ during chewing with 6 degrees of freedom (3 translations and 3 rotations) by use of Euler angles as the rotational position coordinates and to describe the 3-dimensional movements of the mandible in terms of a virtual marker created midway between the left and right rami in 4 horses chewing sweet feed.
Falcon infrared camera, Motion Analysis Corp, Santa Rosa, Calif.
Motion Analysis Corp, Santa Rosa, Calif.
Omolene 200, Purina Mills, St Louis, Mo.
MATLAB, The Mathworks, Natick, Mass.
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