The equine foot and upper-body relationships
Medial-lateral imbalance is when the hoof capsule on a particular limb has uneven heel heights, again this will cause a horse to change its orthopaedic balance and the weight forces within the hooves; it also causes changes in the upper body of the horse. The upper body changes can be similar to changes seen in hi heel lo heel cases, simply because of the muscles acting on the limb in movement and suspension are the same. However there are changes and pain points in the body remote from these muscles. These invariably affect the horse’s performance. Again it is often to encounter mid thoracic back pain. Because of the biomechanics of the horse there are often neck (cervical) and poll muscle tension and pain patterns in upper body problems.
These can occur consistently over the third cervical vertebra. How many times have you heard about ‘inability to break or bend over the third vertebra’ Granted in many cases a chiropractic adjustment is needed to resolve this problem but in a sizeably majority of cases (95% or more) the primary problem is elsewhere. If the problem is a medial lateral imbalance, and this is not resolved properly, then the pain patterns will persist and performance will suffer.
It is worth mentioning that the horse shows different levels of pain tolerance and I have seen some horse perform well despite having imbalances. On the other hand I know a variety of horses that will consistently play up and show resistance towards the end of their shoeing interval when imbalances can just be starting to show.
Medial-lateral imbalance can occur on good shaped hooves and it is one of the problems that can occur when the hoof is not trimmed correctly. This means that the person trimming the foot has misread the hoof balance in accordance with the limb conformation and trimmed one branch of the hoof lower then the other. It can also be caused if the horse is placing its weight unevenly. To simulate this imbalance you can place a wedge under one branch of the shoe (between the shoe and the ground) and have the horse stand on it, and observe the vertical flexion of the hoof capsule. This type of vertical flexion is what is going on inside the hoof capsule to the distal phalanx and its relationship with distal sesamoid bone and impar ligament – collateral ligaments; it also places overwhelming pressure on the collateral distal cartilages coronet band and digital cushion. The other areas of strain are the suspensory apparatus and tendons of the limb as it adjusts to the twisting of the limb.
The twisting occurs because now the bearing surface of the foot is uneven, so when the hoof first strikes the ground the hoof will roll to the low side but in mid stride with the limb weight loaded the hoof and limb will twist towards the high side, then in the flight stage it unwinds and wing in the other direction releasing the tension in the limb. This aliment places a lot of stress on the hoof capsule and passive stay apparatus (PSA) of the upper body. This is one of the principles behind relieving back pain in the horse by
releasing the passive stay apparatus, a unique treatment option in the horse that has been thus far overlooked.
The above understanding of medial-lateral imbalance is mostly common knowledge but what is little known about is the stress placed on the hoof capsule and the internal structures of the foot. The main areas are the digital cushion/ distal cartilages/ bars of the foot/ laminae/ horn tubule and the vascular flow to the foot. It is these soft internal structures that take most of the pressure and begin to break down as the cells that make up the tissue do not receive correct vascular flow and are slow to regenerate or do not obtain their maximum strength to support the upper body.
The pathology of the foot will show that the hoof capsule starts to move around the circumference of distal phalanx and form an asymmetrical alignment on the limb. This misalignment places disproportionate stress on the sensitive laminae as it tries to maintain its alignment on distal phalanx and its hold on the inner wall of the hoof capsule. You will also find that the digital cushion will change shape and texture and the bar on the low side will also lie over and place pressure on the laminae opposite. This is due to the ongoing, uneven pressure and in long term extreme cases bone structure will be changed.
Darrall Clifford hoofcare specialist explains his approach to medial-lateral balance;
“In the past I would have wedged the low side without thinking as most farriers would do today as this makes the hoof look near normal. But to today I understand more of the structural and functional podiatry problems of the foot, and if we just wedge the low side with out taking into account that the high side is more than likely to be misaligned, due to orthopaedic weight changes then all I am doing is increasing the weight forces on the horn tubule and internal structures on the low side. First consider if the high side is at its correct height or can it be lowered as this will lower the stress placed on the low side and allow for better vascular flow to the back half of the foot. Remember that it won’t be just the low side under pressure but the entire back half of the foot and with the uneven weight distribution to the dorsal wall the relationship between distal phalanx, the hoof capsule and the upper body of the horse will also be under stress.”