Theory of Two Loads
I have struggled with what forces are involved in the hoof and how they changed with different palmar angles and varying degrees of deep digital flexor tension (DDF). So to aid in my understanding I consider two extreme examples to help describe my simplified idea of two major loads within the hoof capsule. First I will describe tendon load (TL) and the extreme example to be used is a high grade club. Next we will discuss bone load (BL) or ram load with the extreme example of a post ddf tenotomy laminitis case.
I think we can all agree that there is a significant pull from the DDF in club foot cases. Lets consider the action of the DDF. As weight is applied to the limb or the DDF muscle contracts the pulling force is transferred to the coffin bone via the semilunar crest at the DDF tendon insertion. This pulls the coffin bone around its articulation with the distal end of the second phanlanx (P2) and the DDF tendon also is pressed against the flexor surface of the navicular bone. Extraction forces are apparent at the horn-lamellar interdigitation and compression forces on the solar corium directly beneath the apex of the coffin bone. Club feet are affected by a shortened musculotendonous unit via increased neurologic stimulation of the flexor muscle. This tranfers load to the apex of the coffin bone and the horn-lamellar interface at the toe. So for simplicity sake consider two lengths of rope both attached above carpus and at the semilunar crest of coffin bone. The shorter length will transfer more load to the apex than the longer when weight is applied to the limb.
Figure 1 short rope/high pa/club
Figure 2 Longer Rope/low pa/slam dunk
These forces and the changes implied are noted on radiograph's of club feet, as a remodeled tip of coffin bone, a small bump midway down on the face of P3 and often smaller,and a less dense navicular bone. These changes follow Wolfes law of bone remodels along lines of tension and compression. Now consider the external characteristics of this extreme example: Atrophied frog, deep central sulcus, wider growth rings at heel than toe, bulging or flat sole at and around apex of frog. These characteristics are created by the excessive DDF tension which allows for an unbalanced load distribution between tendon load and bone load. This excessive TL prevents loading and stimulation of the palmar portion of the hoof and leaves the frog and heel suspended in the air.
Figure 3 bone remodeling on tip of coffin bone
The second load to consider is bone or ram load (BL). This is the weight that is transferred through the bony column directly to the ground. If no DDF was present then all load is distributed through this manner and forces are increased in the heel region. Consider the case of a post deep digital flexor tenotomy when all TL has been negated due to severing of the DDF tendon. All weight and forces are concentrated in the heel region and has more of a table leg distribution of forces. I feel that many of the crushed heel, low to negative palmar angle hooves have a similar situation. Just as the club foot is born with shortened musculotendinous unit the low Palmar angle/crushed heel or slam dunk foot may have a longer than ideal musculotendinous unit allowing a greater bone load that will allow more weight or load through the bony column to the palmar/plantar aspect. I think it is possible to create a negative palmar angle and crushed heels with poor mechanics in many of our everyday shoeing practice that could possibly take a normal healthy foot with good sole depth and palmar angle to thin soles and negative palmar angle, however many are destined for that path from a very early age due to conformation. It is impossible to take a club foot caused by shortened musculotendinous unit and create a negative palmar angle and the same may be true for the slam dunk foot as many will revert back to crushed and under run heels once orthotic devices have been applied to increase hoof quality, sole depth and aid in treatment of lameness.
Consider a heel sore horse that is landing toe first, this is evidence to me that the horse can use the tendon to transfer load to the front of the foot to unload the painful buttress, digital cushion and many related soft tissue structures. Many horses compensate quite well by transferring load to the front of the foot via DDF with initial heel soreness but it is not long until the extra workload by the tendon creates inflammation within the tendon itself and many of soft tissues and ligaments associated with the palmar/plantar aspect and fatiguing the flexor muscle group. This is when a trip to the vet usually occurs as they are now unable to effectively transfer load to a non painful region and show obvious signs of lameness. The increased load transferred to the front by the toe first landing and often long digital breakover in these cases decreases blood supply to vital growth centers and adds to the further compromise of hoof and sole quantity and quality. Radiographs would show very thin soles below wings of coffin bone, low to negative palmar angle, a very low tendon surface angle, as scallop of bone remodeling in palmar/plantar aspect of solar margin of coffin bone and upright pasterns. External characteristics noted are: Wider growth rings at toe than heel, flat and thin soles, 2-3 sets of nail holes, wide robust frog, and under run heels.
Figure 4Low Pa bone remodeling/low ddft tension
For further understanding let us consider treatment of these two scenarios and why they are successful l in increasing soundness and quality of hoof mass. For the club foot syndrome, lower grades that are not surgical candidates, increasing palmar angle and lengthening the heel base will allow more BL and less TL. Decreasing the TL will decrease the amount of load being transferred to the toe and allow more bone or ram load to push into the heels. The easiest and most successful approach I have found, is using rocker shoe mechanics. The heels are trimmed to the widest part of the frog parallel to the wings of the coffin bone and toe is trimmed perpendicular to the frog axis at a low rocker toe style angle.
Figure 5 Grade 3 club
Figure 6 Grade 3 club with Rocker Rail
The trim will vary based on such parameters as palmar angle, sole depth and digital breakover but the basic approach will stay the same. The next step is to determine what shoe to shape to fit our specific needs. In general low grade clubs will do fine in a rockered flat shoe as higher grade clubs may require starting with a wedged shoe that has greater mechanical potential. Consider a flat shoe that is rockered can alter pa 2-4 degrees and a 5 degree rail shoe is starting with 5 degrees, so any added rocker will increase potential to alter palmar angle. So the question to be answered is how much PA increase do I need to create less tendon load and more bone load? Low grade clubs require less than higher grades. This approach will allow more ram or bone load, more heel loading that will result in less atrophy of the frog, decreasing depth of the central sulcus, increased sole depth below the tip of coffin bone and more even toe to heel growth patterns. With less TL comes less H/L zone extraction force and less solar corium compression.
Now consider a case of acute lamintis with extensive H/L detachment and venogram shows decreased perfusion at the coronary waterfall, compromised vasculature down face of the coffin bone, tip of coffin bone has displaced 3 mm below the circumflex artery, and terminal papillae are horizontal versus being in normal orientation with the face of p3. This gives us a picture of severely compromised dorsal portion, including the horn-lamellar attachment and solar corium below the tip of coffin bone. A DDF tenotomy may be indicated in many cases such as this. This will completely unload the forces of the DDF and allow all weight to be transferred down through the bony column into the palmar/plantar region of the foot and unloading much of the compromised areas in dorsal aspect. This can be shown by post tenotomy radiographs and venograms. This release and increased load now through the bony column to heels will often push the coffin bone up closer to its original placement prior to laminitis episode and displacement and radiographs will show measurable decrease in distal h/l zone and increase in sole depth just from the unloading that occurs from complete release of DDF.
Figure 7 laminitis with rotation
Figure 8 Post tendon cutting and derotation shoeing
Figure 9Acute laminitis venogram
In the above drawings (Figure 7 and 8) shows the pull of the tendon with detached bone to horn attachments and a post tenotomy with derotational shoeing. Without a healthy lamellar attachment there is no antagonistic force to counteract the pull of the ddft (TL) and the coffin bone rotates around its articulation compressing solar corium at the tip of the coffin bone. Figure 9 shows an acute laminitis case in which the bone is compressing the blood supply at the tip of coffin bone due to lamellar detachment. You can see the tip of coffin bone below the circumflex artery. This area is heavily loaded secondary to the TL and loss of the bone to horn attachment. The image on the right is of the same horse 2 weeks after derotational shoeing and deep digital flexor tenotomy. The tenotomy negates all TL and its forces applied to the damaged areas (lamellar zone, sole under tip of p3) and heavily loads the palmar/plantar aspect of the hoof through BL only. Note the restructuring of the blood vessels under and around the tip of the coffin bone in this short 2 week period.
I have always considered that anytime we raise the palmar angle via wedges or rocker shoe mechanics that we increased the load on the heels but it really wasn't clear why until considering these two loads. These examples are two extreme versions and most feet will fall somewhere in between. When a healthy balance between TL and BL exist we find good feet that are easy to maintain with adequate sole depth and a positive palmar angle but when loads sway more to one side of spectrum to overloaded portions become unhealthy and need our assistance in balancing the load via a well designed protocol based on and monitored by serial podiatry style radiographs and venograms.