IN-DEPTH: HIND LEG FROM THE PELVIS DOWN


understanding of the functional anatomy and the normal kinematics is hence crucial to reveal biome- chanical stress or dysfunction. The knowledge about forces occurring at articular joint surfaces during different stages of joint movement may pro- vide insight into the etiology of common orthopedic issues.21 From a biomechanical point of view, joint stabili-


zation through perfect neuromuscular synchroniza- tion is crucial to maintain persistent physical integrity of the body. Studies in dogs have clearly demonstrated, that the epaxial musculature glob- ally stabilizes the spinal articulations against the external and internal limb forces acting on the pel- vis and trunk during walking and trotting.35 Pre- suming optimal muscle activation at the right time and frequency in movement, joints “float in the soft tissue” and shearing forces within the joint36 are assumed to be less than compared to melting ice cubes.37 The high incidence of pathological findings in ar-


ticular joint cartilages without a history of lameness in equine athletes seems to underline the capacity and importance of muscle stabilization during move- ment. In human sports medicine, optimization of neuromuscular joint stabilization is already consid- ered as a state-of-the-art procedure in orthopedic case management. Rehabilitation programs and prevention measures are routinely accompanied by muscle building and proprioceptive training and should be reconsidered in equine sports medicine and rehabilitation. This seems particularly important for the stabili-


zation of the equine lumbosacral and sacroiliac junc- tion. While poor performance in sport horses is often related to lameness issues, biomechanical dys- function of pelvic and spinal structures are still un- derestimated as possible causes, or contributing factors, for performance reduction or pain.3 This is probably partially because the pelvis and axial skel- eton are not easily accessible to thorough diagnostic examination and their clinical relevance is still not fully understood. Biomechanical concepts such as the “bow-and-


string” of the equine back try to explain the highly complex biomechanical features of the spine.38 Yet they fail to represent the full complexity, nor describe the interactions between spine, pelvis and hindlimb biomechanics.27 Longstanding hindlimb lameness may lead to de- velopment of visible pelvic asymmetry due to asym- metric muscular and ligamentous loading or pelvic bone remodeling.27 Such asymmetries may also lead to changes in lumbosacral flexibility and rota- tional motion of the pelvis. Fluoroscopic exploration of three-dimensional mo-


tions of the pelvis and the lumbar spine in walking and trotting dogs clearly demonstrated the tightly linked pelvic and lumbar intervertebral movement patterns.35 The ranges of motion occurring at the lumbosacral junction were significantly higher com-


pared to the other lumbar intervertebral motion segments,39 indicating the importance of the lumbo- sacral area for force transmission of the hind limbs to the axial skeleton. An increased understanding of the structural and functional components and biomechanics of the equine axial skeleton is also inevitable3 to better understand their relationship to poor performance and hindlimb lameness issues. The use of a kinematic approach to sacroiliac dys- function causing pain, borrowed from the human neuromotor control model, has advanced the knowl- edge of the equine sacroiliac joint and its kinemat- ics.6 Increasing muscular control of movement in trot was shown to lead to smaller ranges of motion between the pelvis and sacrum compared to walk.40 It is postulated that loss of the neuromotor control of the musculature surrounding the sacroiliac joint will lead to sacroiliac joint dysfunction and initiate pathological processes as in humans6 since bony morphological changes are commonly seen in equine specimens. Pathological articular changes may be a response to altered forces transmitted to the joint, either through joint movement limitations or in- creased mobility due to lack of stabilization. So despite the sacroiliac joint being a structurally strong joint, it seems susceptible to altered move- ment patterns and injury6 in the horse and “al- though motion in the sacroiliac joint may be overall limited, proper functioning of this joint is crucial to unhampered locomotion.”41 Considerable left-right asymmetry in pelvic mo-


tion, as well as pelvic bone asymmetry, is thought to be a common feature in horses suspected of having chronic sacroiliac dysfunction or injury,27 even though it is also seen in horses without any history of pelvic lameness.29 It seems evident though, that difficulty stabilizing the hindlimb on the affected side while lifting the non-affected leg is a consistent feature of patients with sacroiliac joint dysfunction and can be used as a diagnostic aid to the equine practitioner. Furthermore, with the thoracolum- bar fascia blending mediocaudally via the supraspi- nous ligament into the dorsal sacroiliac ligaments,29 ultrasonographically detectable left-to-right dorsal sacroiliac ligament asymmetry can also support the practitioner’s diagnosis. Vertical displacement asymmetry of the pelvis has


been demonstrated in induced hind-limb length dif- ferences.42 The pelvic downward movement was increased on the side of the shorter limb during stance, whereas increased pelvic upward movement during and after stance of the longer limb was mea- sured.42 Similar to human studies, limb length dif- ferences and movement asymmetry is associated with ground reaction force asymmetry, increasing the weight supported with the shorter limb as well as the pushoff forces of the longer limb. Especially when being ridden, a horse must be


able to move its legs closer to the center of gravity and lower its neck in order to achieve a posture of a convex back curvature. This permits the horse to


AAEP PROCEEDINGS  Vol. 65  2019 329


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