From “The Horse”
|AAEP Convention 2005: The Science of Lameness|
|February 17 2006 Article # 6501
Both horse owners and veterinarians spend a lot of time observing horses for lameness, but not all observers perceive lameness the same. Sometimes this is because of a less than clear understanding of equine biomechanics. Researchers such as Florian Buchner, DMV, PhD, an equine orthopedic surgeon at the University of Vienna, are seeking to better explain how the horse moves when sound and lame. During the 2005 American Association of Equine Practitioners Convention held Dec. 3-7 in Seattle, Wash., he discussed lame forelimb and hind limb movements and orthopedic shoeing.
“If loading (supporting limb lameness) or moving (swinging limb lameness) of the limbs causes pain, horses change limb timing, stride pattern, and limb joint movement pattern in all limbs to cope with the pain,” he said.
Some of his observations of lame horses are as follows:
- Stride duration decreases, and there is an increased stride frequency.
- Stance duration is longer, symmetrically (for both lame and sound sides).
- There is a marked decrease in the length of the suspension phase.
- The kinematic pattern of limb movement reveals two different spring-like systems. The distal (lower) spring consists of the fetlock and the coffin joint and represents, by its passive nature, the loading pattern of the limb. The proximal (upper) spring consists of the shoulder/elbow joints or the tarsal/stifle and hip joint complex. The proximal spring can adapt actively to lameness by increasing flexion during midstance in the lame limb, which smooths out the loading of the painful limb. The distal joints indicate reduced loading in the lame limb by decreased midstance excursions ( i.e., reduced hyperextension of the fetlock joint and reduced flexion of the coffin joint).
- A lame limb is loaded less than its sound counterparts, but the sound limbs are not overloaded (to the point of failure). There is greater loading of the contralateral (on the other side) limb, but no change in the diagonal limb, and even a small decrease in the ipsilateral (on the same side) limb.
- Fetlock hyperextension during midstance decreases with increasing lameness. This decrease is about 8° for a 2/5 forelimb lameness. In the sound contralateral forelimb, a compensatory increase in hyperextension can be seen; however, the change is smaller (about 2°) than the reduction in the lame limb.
- With increasing lameness, maximal coffin joint flexion during the stance phase in the lame limb is reduced, whereas maximal joint flexion in the contralateral limb is increased.
- Carpal or tarsal joint lameness is characterized by reduced carpal/tarsal flexion during the swing phase.
- There is a significant reduction or absence of the suspension phase after the lame stance. This stance phase of the lame limb is not shorter than stance duration in the same limb without lameness. During this painful stance phase, the sound limb is earlier and further protracted (brought forward), compensating for the missing flight phase (suspension phase) after the lame stance.
Lower Limb Dynamics and Orthopedic Shoeing
Buchner’s next presentation discussed the forces on inner lower limb structures. Since these forces can only be directly measured with invasive techniques, calculation is often used to estimate them in living horses in a process called inverse dynamics. This analysis uses limb position (from video motion analysis) and force plate data to compute forces on inner structures, effectively working backward from outer measured observations to estimate inner forces.
This type of analysis has provided information on tendon and bone stress in sound and lame horses, and has estimated the effects of different types of shoes on horses with some types of lameness. Below are some of those findings:
- Horses with navicular disease exhibit higher pressure of the deep digital flexor tendon (DDFT) on the compromised navicular bone during the stance phase, not less as one might think. Why? The horse must use the DDFT to raise his sore heels, resulting in counterproductive pressure on the navicular bone.
- Six-degree heel wedges decrease DDFT tension by about 27%, peak forces on the coffin joint by 22%, distal check ligament forces by about 15%, and forces on the navicular bone by about 24% in sound horses . However, they increase suspensory ligament and superficial digital flexor tendon forces by about 11%, so they would not be recommended for tendonitis cases.
- In laminitic horses, rotation of the coffin bone decreases the force of the DDFT.
- In vitro testing of the forces necessary to tear equine tendons showed that distal check ligament loads at the walk reach 40% of the ultimate failure load. The DDFT acts in a much more secure range, reaching only 20% of its failure load at the trot.
- The suspensory ligament experiences the highest strain of all tendons, reaching about 50% of the failure force at the normal trot. The tendons are most stressed during landing from a jump; this stress has been estimated at nearly 80% of the failure force. Calculated force in the SDFT even exceeded failure force values measured in vitro. “Therefore, care must be taken to compare in vivo loading with the in vitro failure forces measured in different horses,” Buchner cautioned.
- Experimental desmotomy of the distal check ligament caused no changes in limb or tendon loading, but reduced forces on the coffin joint.
- In horses with induced superficial digital flexor tendonitis, both fetlock and coffin joint moments are reduced in the lame limb as a result of a reduced vertical load and the respective decreased fetlock hyperextension.
- Egg bar shoes did not show any difference in coffin or fetlock joint forces or in any tendon force value in some studies, which evaluated horses trotting on a firm rubber surface. However, Buchner suggested that the extended heels of this shoe might keep the heels from sinking into soft ground and thus, from hyperextending the coffin joint and increasing the forces in the DDFT.
“This example illustrates the limitations of studies on selected loading scenarios; whereas general effects in standardized situations can be described quite accurately, the multiple, diverse, and sometimes extreme loadings during equine exercise remain only estimates,” he advised.
- “A positive effect of easy breakover on equine distal limb load, if present at all, cannot be based on a reduced maximal load of any tendon,” he reported.
“The take-home message is that inverse dynamics allow for in vivo calculation of internal forces without invasive procedures. We can look into the limb!” concluded Buchner.