The Mechanical Formula
Written and presented August 2014 by R.F. (Ric) Redden, DVM
The term “mechanics” is a fairly new term the author uses to describe how the foot deals with energy and the forces at play. Large volumes of clinical evidence and observation of the very potent effects the deep digital flexor tendon (DDFT) has on the foot indicate it is the key to understanding the mechanics of the foot. The DDFT is the distal end of the large flexor muscle group that originates at the back (caudal side) of the forearm and is the source of energy for the tendon. The function of all muscles is to contract and relax. Contraction can be voluntary or involuntary, and apparently the degree and frequency of involuntary contraction has a variety of effects on different horses and even feet on the same horse.
The DDFT passes through the back of the knee over the sesamoids at the fetlock, along the back of the pastern, over the navicular bone and attaches with an incredibly strong, fan-shaped anchor on the bottom and caudal aspect of the coffin bone. The tendon is quite strong and about the size of an index finger in most horses, however it can only stretch about 1% of its overall length. Therefore we need to think of it as a cable rather than a big rubber band.
The ill effects of injury or disease to various areas of the foot invariably create a pain response and can greatly reduce circulation to an already weakened area. In turn, the action of the DDFT, which is still enacting the same force on an area that is now weak, can cause further pain, soft tissue and bone damage. Our job as veterinarians and farriers is to first recognize the hot spots, assess the overall quality of the horn, sole and depth of cushion mass and develop a solution to address the mechanics required to reverse the forces at play (the DDFT) and give the damaged tissue time to heal.
When we speak of increasing or decreasing mechanics we are referring to altering the distance between origin and insertion of the DDFT. Increasing mechanics means decreasing this distance, which in turn reduces the level of tension the tendon asserts on the coffin bone and associated structures. Decreasing mechanics does the opposite. Using sheer mechanics, we can reduce tension, compression and shearing force on several key components once we have working knowledge of how these structures work in harmony when maintaining optimum health.
Suspension and Support
The basic suspension system of the foot has three main components: laminae, coffin bone and DDFT. Visualize the laminae as Velcro or microsprings that attach the coffin bone and cartilage in the heel to the hoof wall. These springs basically suspend the bone within the capsule on three sides while the caudal aspect of the bone is suspended via the DDFT. The springs anchor firmly to the wall. Load passes through the digit, then the tendon and laminae. Apparently they become equally engaged as they allow the bony column to move distally in all conceivable planes.
Working in tandem with the suspension system are the support components. The coffin bone rests on the sole corium, a blood-rich velvet cushion that houses the nutrient supply of the palmar zone of the bone and growth centers of the sole. When the highly flexible solar corium is compressed, circulation in this area is immediately impaired. This occurs naturally with every step, however when the foot has adequate mass (horn protection) and all components are in sync, natural recall of these structures exists. If any one component becomes weak, injured or diseased a cascading series of events occurs that affects adjacent components. Simply put, we can’t have pathology in only one specific area. If one component fails, others begin to fail under the brunt of excessive load, tension and shear.
By measuring soft tissue parameters on lateral radiographs we can visualize the repositioning of the coffin bone within the capsule simply by alluding to the position of the foot and load. Add the information gained from venograms and we have a live research model that offers a far more reliable means of tracking the influence of the DDFT than computerized models and/or cadaver limbs placed in load simulations. The data obtained by observing and recording the response of mechanical enhancement greatly helps us better understand the intricate interconnections of each component and how we can use this knowledge to enhance the healing environment. We can only observe the response to mechanics in the live horse, therefore the value of this information is well above all other data extrapolated from inert or cadaver models.
Using mechanics to enhance the healing environment
There are many ways to enhance the healing environment by using mechanics to shift internal load from a failing component to a healthier one. However manipulating the mechanics of the foot requires a reference or starting point. Palmar angle (PA), sole depth, digital alignment and tendon surface angles (TSA) are important parameters that are greatly influenced by the degree of tension exhibited by the DDFT under full load. These soft tissue parameters can help us better understand the level of tension present and design a specific shoe that best shifts internal load and tension to healthier areas of the foot.
Wedges (pads or shoes), squared toes, reverse shoes, rockered toes and backed up shoes are all used in various ways to increase PA and/or decrease the energy needed for or in conjunction with breakover. All are beneficial to some degree but provide minimal mechanics as they only influence PA by a few degrees at best, if at all. This is great when only minor tension adjustment is needed to offset load on a failing component, however in many cases this simply is not enough to obtain the desired results. When static PA has not been increased, tension is not significantly altered until load is decreased as the horse takes a step. This is beneficial when the horse is moving, but has very little medical benefits when standing still for hours on end. The author prefers to use the rocker technique to accomplish a wider range of mechanical enhancement that remains in effect even when the horse is standing still.
A great misconception exists among vets, farriers and trainers worldwide concerning the large scale of mechanics that exist. Knowing where, when and how to apply mechanics requires knowledge of the formula, accomplished farrier skills and ability to interpret radiographs, identify landmarks and design, fabricate and attach the shoes. Many of these requirements are certainly not part of everyday shoeing practices even at the top level and therefore can pose a problem for all who try to use the rocker technique armed only with good farrier skills and years of good experience.
The mechanical formula puts therapeutic shoeing and equine podiatry into a new and exciting category all of its own. The great news is that everyone can get there given a strong desire to learn and eagerness for new challenges and skill development.