Hock Joint Mechanics: Bluegrass Laminitis Symposium

“One of the most frequent sites of lameness is the hock joint,” said Hilary Clayton, BVMS, PhD, MRCVS, Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University (MSU), in her presentation “A New Look at the Hock Joint” at the 2003 Bluegrass Laminitis Symposium in Louisville, Ky. “Various shoeing modifications are used with the objective of modifying hock motion and/or force transmission across the joint; however, it is not possible to evaluate the effect of these treatments in the absence of reliable data describing normal and pathological function of the joint.” Lateral view of the hock joint

Clayton presented the results of several MSU hock studies, beginning with a description of normal hock motion. The hock, she explained, is a very complex joint with several smaller joints between its many bones. The majority of its motion occurs at the tarsocrural joint (between the tibia and the talus in the upper joint of the hock. Aside from flexion and extension when viewed from the side, which is the primary motion, the hock also is capable of sliding and small amounts of lateral and rotational motion. “The distal tarsal joints are thought to undergo small amounts of movement during normal locomotion, and these movements may be important in the etiology of hock lameness, such as bone spavin,” she said.

Describing Normal Hock Function

Clayton described the forces on the hock, including a large amount of torque on the caudal (rearward) aspect of the joint during the stance phase. “This torque controls the rate of hock flexion in early stance and causes the hock to extend in late stance,” she explained. “We should never be surprised by curb, as it shows damage from this torque on the point of the hock. Front (dorsal) view of the hock joint

“The mechanical function of the hock in early stance is to absorb concussion, and it seems to play a role in dampening impact shock,” she added. Later in the stance phase, she noted, the hip, hock, and fetlock all provide propulsion. In late stance, the hock extends more than the stifle, which extends the peroneus tertius tendon (that attaches to the distal end of the femur on the front and the proximal end of the cannon bone, and the fourth tarsal or hock bone); in early swing, the rebound from this tension flexes the hock in a motion that can be likened to a catapult.

During the stance phase (when the hoof is in contact with the ground), Clayton explained, the hock flexes by about 10-15 degrees. “Often the flexion cycle has two peaks, and it varies between horses whether peak flexion occurs before or after midstance,” she said. “The hock is maximally extended around the time of lift-off; often slightly before lift-off in horses with straight hock conformation, and slightly after lift-off in horses with more angulated hock conformation.”

She added that this variability in the movement pattern of the hock is unusual in that most of the horse’s joints show similar patterns of flexion and extension between horses. “In the swing phase, the hock flexes through about 40 degrees, and the angle-time graph is more consistent between horses,” she added.

Further Measuring Hock Motion

Since many of the hock joint’s bones are too small to allow accurate individual measurement, researchers at MSU have evaluated the overall motion of the hock by measuring movements of the cannon bone relative to the tibia.

“The 3-D studies showed that, during the stance phase, the hock flexed through an average of 11 degrees, which was similar to the value found in the two-dimensional studies,” she reported. “In addition, the cannon bone was abducted (rotated away from the midline) through three degrees and internally rotated through 1.5 degrees. At the same time the cannon bone moved forward, laterally (away from the midline), and distally relative to (away from) the tibia.

“Although most of the hock motion occurs at the tarsocrural joint, we were interested to determine how much motion occurred at the more distal joints, since these are the site of bone spavin, and it seems likely that the motion patterns at these joints are related to the development of spavin,” she continued. “There is evidence of both rotational and sliding movements at the distal hock joints. During the stance phase, the cannon bone rotates internally and slides forward and laterally at the distal joints, and during the swing phase, the cannon bone slides forward and laterally at the distal joints.”

These findings indicate that, in addition to the hock’s functions in raising and lowering the hoof during the swing phase, it also plays a role in avoiding interference. “The distal (lower) limb swings outward as the hock joint flexes,” she said. “This outward rotation helps avoid interference between hind limbs and forelimbs as the hind hoof swings forward. For every degree of flexion, there is a certain amount of abduction (movement away from the midline).”

Hock-Lame Horses

Another MSU study evaluated the mechanical effects of synovitis of the distal jock joints, which could precede bone spavin. Clayton reported that horses with grade 1-2 hock lameness showed a decrease in hock flexion of about 2.5 degrees, which is about 20% of the total stance phase flexion. They also exhibited decreased forward sliding of the cannon bone relative to the tibia during the stance phase, which “might result in repetitive loading on a focal area of articular cartilage, which may lead to the development of osteoarthritis,” she suggested.

She added that these horses also had decreased weight bearing on the lame limb, and the affected hock absorbed less concussion in early stance. Additionally, there was less weight bearing in the diagonal front limb, but no increase in weight bearing of the other diagonal and no evidence of compensation by other joints in the lame hind limb. “Thus, the horses were moving with less vertical motion in the gait, or a flatter gait, not pushing as hard against the ground,” she explained. “This is very important because horses become lame when the pain gets to a certain point. Before that, subclinical lameness may take the form of a deterioration in gait quality, so the flatter gait is the initial method of compensation.”

Clayton briefly discussed an MSU study presented at the 2002 American Association of Equine Practitioners on the effects of Corta-Flx, an equine joint supplement, on hock lameness. The full report of the study can be found here.

Shoeing for the Hocks

“Rotation and sliding movements at the distal hock joints are normal parts of hock motion,” Clayton said. “Shoeing methods that prevent the hoof sliding forward at impact may increase the longitudinal deceleration forces. Transmission of these forces through the hock joint may increase the sliding motion at the distal joints.

"An obvious project for the future is to evaluate the mechanics of hocks that ‘wobble’ during weight-bearing and the effects of shoeing modifications in correcting the wobble without imposing further abnormal stresses on the joint," she added.

When asked by an attendee if the MSU team had any studies planned on hoof balance, she answered that this area was “wide open for study. Lots of people have done isolated studies, but we need a more comprehensive plan. We also need more money than is needed for individual studies.

“There is still a considerable amount to be learned about hock function and compensation for lameness,” she concluded. “When these are better understood, we will be in a position to explore the effects of shoeing on hock function in normal horses and in horses with hock joint pathologies."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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