2001 - 14th Annual Bluegrass Laminitis Symposium Notes
New Developments Using the Four Point Concept
Written and presented January 2001 by R.F. (Ric) Redden, DVM
The strength of the four point trim concept continues to grow as I find new ways to apply the magic “pivot point” to the foot. This concept has the ability to transfer load and effect the tension in the suspensory/tendons of the equine digit, which appears to be directly related to the reperfusion of compromised growth centers. Reperfusing these vital growth centers greatly enhances the genetically programmed healing response of the foot.
Many people ask, “How does this method differ from traditional shoeing?” and “How do other versions of the four-point method differ?” The answers to these questions are as varied as the number of farriers using the four-point trim, but the basic formula remains the same.
Traditional shoeing is basically designed to protect the foot while offering little or no mechanical aid to enhance perfusion of the sole corium and laminae. It also fails to attenuate the tension of the deep digital flexor (DDF) tendon as it courses over the navicular bone.
Today’s definition of traditional shoeing can be traced back a couple of hundred years at best. Do you ever wonder, “What was considered ‘traditional’ before this?” Shoes worn by horses throughout Europe and Scandinavian countries hundreds of years ago are displayed at teaching institutions and museums, while thousands of others are in storage. Most differ greatly from the traditional shoe of today or even of the shoes that made their way into the first few books written on the subject.
I challenge those individuals who decry the four-point concept as not being “traditional”. This concept is actually so old that it is now new again. Hundreds of years ago, horses wore shoes designed to enhance the blood flow to the foot; and therefore, aid natural healing. These shoes were used well before present day shoeing standards were altered by more recent writings. It would be interesting to know when and how the farriers of Columbus’ era discovered the strong influence of placing the breakover well behind the apex of PIII. Obviously there were no x-ray machines at that time, so the only way to discover the location of the bone in relation to the shoe would have been to cut the foot and shoe down the sagital plane. I am certain that some who were inspired to move forward did just that.
How does the four-point concept work to enhance the healing environment of the foot? Apparently a very delicate equilibrium exists between the flexor tendons and suspensory structures of the foot. When in harmony, these groups act as a unit with a multi-complex function.
The laminae, especially the anterior two-thirds, are a direct antagonist of the DDF tendon. The extensor tendon that attaches to the most proximal prominence of PIII is also an antagonist of the DDF, but it is considered the weaker of the two. Furthermore, the coffin bone is supported within the capsule by the laminae, chondral collateral ligaments, impar ligament, the sole corium, sensitive frog and digital cushion.
The laminae along the posterior half of the foot are also attached to the flexible ungual cartilage. Apparently, this cartilage is a very forgiving structure that significantly prevents laminae sheering in most laminitis cases. The entire system rests in the sling of the DDF that is multi-functional itself.
For the sake of discussion, lets simplify the biomechanics of the digit and develop a meaningful and useful alertness for the load sensors and their caution lights. Lets assume a large majority of the horse’s weight is resting in the sling of the DDF. This weight extends through the tendon, down the cannon bone and across the proximal sesamoids. It makes a turn and continues down the back of the pastern, followed by another abrupt turn under the navicular bone where it attaches to the base or semi-lunar surface of the coffin bone. Remember that the bone is rigid. It is attached to the laminae and a very flexible, highly sensitive blood barrier, which are both attached to the semi-rigid hoof capsule.
As down load is placed upon the limb, each structure comes into play. If we see the tendon as if it were a large “rubber band” attached to a highly sensitive coil (the muscle) programmed for delicate suspension, we can imagine the weight of the horse passing through the fetlock and lower digits causing them to reach maximum flexion.
If the suspensory is cut while the limb is in full flexion, the fetlock will remain very close to the ground. The suspensory is the major support for the fetlock joint. However, if the superficial tendon is cut, the fetlock will come up but not reach its original height. If the DDF is cut, the fetlock angle returns to its original relationship with the pastern.
Notice that cutting the DDF doesn’t affect the fetlock angle. Instead, it causes major changes within the hoof capsule. Apparently the suspensory is the major support for the fetlock while the DDF is the major support for the third digit. Radiographically, the coffin joint can be luxated as much as one-fourth inch depending on the unique characteristics of the digital alignment and the relationship of the hoof capsule to the ground.
After cutting the “rubber band,” the DDF tendon releases the down load that was once cradled in the sling. Therefore, the sling no longer exists. There is no longer direct pressure over the proximal sesamoids, the navicular bone, bursa and sole corium. PIII also floats forward because there is no structure left to hold it tightly against the articulation of PIII.
The impar ligament is a very strong attachment capable of supporting serious down load. When the DDF is cut, it also moves forward with the bone. The apex of PIII also tips upward as load is passed through the proximal digits to the articular surface of PIII. In absence of the support sling, the apex of PIII is pushed upward and forward. PII also moves in a posterior direction, transferring load to the caudal articular surface, navicular bone and impar ligament.
So, what has happened to the original pressure points?
The distal sesamoid acts as a pulley over a high-stress area to reduce friction and absorb energy. It also supports the coffin joint along the palmar surface. This hot spot is a high friction point, but the cutting of the “rubber band” has significantly diminished the amount if friction. Subsequently, the pain associated with this spot is immediately relieved. In addition, pathological lesions involving the tendon, bursa, bone or supporting ligaments are no longer tightly compressed against the opposing forces of the DDF tendon.
The palmar surface of PIII is concave with a very sharp solar margin along the anterior two-thirds of the bone. A student of mine once commented that, “it would make a terrific hatchet”. There is no doubt that this very thin, fragile border of PIII plays a role in its function. The question is, “How?”
When the sling is cut the bone tips upward, the action immediately releases down load against the very sensitive sole corium. Performing venograms on numerous sound feet with acceptable mass and “balance”, I found that lateral views revealed that the vascular complex of the sole measures one full centimeter in depth. The circumflex vessel is palmar to the surface of the bone and slightly outside the cutting edge of the bone. In some cases, the thin-soled horse may have only one centimeter of total distance between the bone and shoe.
The venogram reveals that the circumflex vessel is sandwiched between the wall and the bone. This serves a nice hiding place otherwise the cutting surface of PIII could cause irreversible damage. Suddenly relieving the down load against the delicate vascular supply not only relieves pain (the hot spot is no longer there), but it allows the vessels to perfuse (fill with blood) as never before. Observing the effects of literally hundreds of cases that were realigned immediately prior to a tenotomy, it is evident that perfusion results in sole proliferation.
Relieving this hot spot in acute laminitic cases is a major contributing factor for successful treatment. The bone acts as an anchor for the sensitive laminae, which basically remain functional as long as the basement membrane remains intact. When the function of the basement membrane is compromised, the desmosomes lose their adhesive strength and are pulled apart by the relentless pull of the DDF tendon. When the tendon is cut, this pulling force no longer exists and the desmosomes continue to remain intact.
Anatomically, the sensitive laminae are attached to the non-sensitive laminae, which are the inner zone of the hoof wall. Migrating abscesses can form puncture wounds, keratomas, white line disease, cause pathological lesions of the zone and subsequently cause pain due to the close association with the sensitive laminae.
This semi-rigid zone extends along the entire circumference of the hoof capsule. At the inner sole margin the laminae become modified and form the sole wall junction. These terminal laminae can be seen on the ground surface of a freshly trimmed foot and are erroneously called the white line. To be accurate, it is actually more yellow/yellow-brown than white.
Adjacent to this zone is the stratum medium, which comprises the bulk of the hoof wall. Its growth comes primarily from the secondary laminae. It is non-pigmented and appears white in color. Often it is erroneously referred to as the anatomical white line that was previously mentioned. It is the only white line on the exterior of the foot. This zone is continuous to the top of the coronary groove and is the primary site for white line disease.
When treating white line disease, one key ingredient is relieving the downward pull of the tendon against the bone, and