There are several theories that account for equine foot function, most refer to the function of the digital cushion-lateral cartilage anatomy. Close examination of the digital cushion and the relationship it holds to the lateral cartilage anatomy, and surrounding tissue calls into question current theory. The two most widely accepted theories on foot function are the depression theory, pressure theory, and more recently the haemodynamic theory. What follows is a review of those theories, and the introduction of a new theory on foot function.

The depression theory holds that pastern movement into the digital cushion during the impact phase of the stride causes the digital cushion to force the cartilages of the foot outward, aiding in circulation and energy management.

The pressure theory utilizes ground (solar) contact, with the frog stay pushing upwards into the digital cushion forcing the lateral cartilages to move outward. Both theories speculate that the digital cushion and the vasculature that accompanies it play a role in energy management, with the digital cushion absorbing the energy. 1

The haemodynamic theory attempts to define haemodynamic function of the digital cushion have also suggested that during ground impact, the outward expansion of the cartilages of the foot occurs through the bars’ contact with the axial projections of the cartilage, and the downward movement of the bony column into the digital cushion. When this occurs, it is hypothesized that venous blood within the vessels of the palmar aspect of the foot is forced into the micro venous vasculature within the vascular channels of the ungular cartilage of the foot. Hydraulic resistance to flow through the micro vasculature dissipates the high energy. It is thus hypothesized that foot haemodynamic action accounts for the negative pressure recorded at mid stance, stating that the negative pressure would allow for refilling of the vasculature before next foot fall.1,2 It is further hypothesized that the negative pressure is the result of rapid outward movement of the cartilages of the foot. 2

Research into those structures that join with the cartilages of the foot, and the digital cushion provide evidence that may contradict the pressure, depression, and haemodynamic theories and support several aspects of a fourth theory, the Suspension Theory of Hoof Dynamics(TM).

Examination of those structures that may work in concert with the cartilages and digital cushion is necessary to formulate a working hypothesis for foot function. We need to look to areas that may have otherwise been over looked in previous attempts to understand foot function.

The coronary band and its attachment are very poorly defined, when compared to those of the ligaments, cartilage, and digital cushion of the foot. Its attachment to the ungular cartilages and extensor process could prove to be a vital piece of the puzzle in the search to define proper foot function. The coronary band (Pulvinus coronae) lies in the coronary groove immediately distal to the periople corium, proximal to the parietal surface of the distal phalanx, and abaxial of the ungular cartilages of the foot.

In vitro studies of the coronary band suggest that its relationship to the ligaments of the foot and cartilages of the foot may play a significant role in haemodynamic flow.3

The Suspension Theory of Hoof Dynamics(TM) hypothesis that during the ground impact phase, the pastern begins to descend, causing the lateral cartilages of the foot to move outward. This occurs as a result of ligament, fibrous and fascia attachment influences, and displacement caused by the second phalanx, as opposed to digital cushion displacement. The pressure exerted on the vasculature of the foot by the displacement of the cartilages by the distal palmar movement of P2, and the resistance provided by the coronary band and its attachment restrict venous blood flow.

The Suspension Theory of Hoof Dynamics(TM) further hypothesizes that just prior to mid stance, the pastern begins to ascend, this releasing venous blood now under pressure. This rapid exchange of blood under pressure from the ungular cartilage, and coronary vasculature to the proper palmar digital vein would result in a negative pressure in the foot. This action would presumably cause rejection of both the pressure, and depression theories, as well as dispel the concept that hoof expansion was responsible for finding negative pressure within the digital cushion at mid stance. The suspension theory redefines haemodynamic function, to include haemodynamic response.

The amount of resistance that the venous blood meets during the stance phase would depend upon several factors including, health of internal arch apparatus (defined as all foot structures, less hoof capsule), pastern movement, and amount of force. The greater the force, the greater the pastern movement, the greater the resistance the coronary band , ligaments, and cartilages would need to provide. The amount of pressure within the foot during the impact and stance phase will be in direct ratio to pastern movement, and the resistance to expansion provided by the cartilage, coronary band, and hoof capsule. It then becomes the amount of pressure, and the health of hoof capsule, connective tissue, ungular cartilages, and digital cushion that will determine haemodynamic response and energy utilization. All directional movement of the ungular cartilages, coupled with distal palmar movement of P2 would result in a variable restriction of blood flow proximally from the foot. It is likely that medial-lateral and proximal-distal movement of the palmar axial projection of the lateral cartilages would be influential in the timing, and the ratio of force to pressure occurring during the impact and stance phases of the stride.4 It can easily be understood why the coronary band has been overlooked as an important component in energy management, with the coronary band being commonly viewed as elastic in nature. 5,6,7

Anatomical evidence further supports our hypothesis of a functional internal arch apparatus, where all structures work in concert to regulate haemodynamic flow, haemodynamic response, and energy management. This hypothesis would seem to negate the simplistic belief that the frog’s primary function is to pump blood, or to act as a vehicle for the necessary displacement of the digital cushion, as outlined in the pressure, and depression theories. The STHD defines the angle of the bar/wall, and the relationship they hold to the palmar axial projections of the ungular cartilages as the primary instigators of pastern movement upon impact, this would explain why performance horses are capable of dealing with the energies created at speed, with less than healthy frogs. Injury appears to occur more often in the foot with poor conformation of heels, than in those that have unhealthy frogs, although unhealthy frogs often accompany poor heel conformation. Whereas shoeing will support the depression, pressure and haemodynamic theories, it will not support the STHD. The depression, pressure, haemodynamic theories require only expansion and contraction of the palmar aspect of the foot, where the suspension theory requires three dimensional distortion of the cartilages and palmar aspect of the Teambuilding.

1. Bowker RM, New Theory may help avoid Navicular, News Release, March 1999, Mich. State

2. Dyhre-Poulsen P, Smedgaard HH, Roed J, et al: Equine hoof function investigated by pressure
transducers inside the hoof and accelerometers mounted on the first phalanx, Equine Vet J 26:362, 1994

3. La Pierre KC, Lord RA, et al: Unpublished data. Coronary Band Functional Anatomy: a biomechanical study, 2006

4. Denoix JM, The Equine Distal Limb, An Atlas of Clinical Anatomy and Comparative Imaging, ed 4th, 2005, London, Manson Publishing Ltd.

5. Butler D, Butler KD, The Principles of Horseshoeing, 3rd ed, pg 219, Doug Butler Enterprises, Co. 2004

6. Dollar AW, The elastic tissues of the foot, In: A handbook of horse shoeing, New York: Jenkins
Veterinary Publisher & Bookseller, 1898;15-16

7. Egerbacher M, Helmreich H, et al, Digital cushions in horses comprise coarse connective tissue, myxoid tissue, and cartilage but only little unilocular fat tissue, Anat, Histol, Embryol, Vol.34, 2:112, 2005

KC La Pierre, RJF is the Co-Founder of the Institute of Applied Equine Podiatry, Inc. A professional Farrier for over two decades, a researcher, and educator. KC La Pierre is a proud member of the Guild of Professional Farriers and a traditional Journeyman Blacksmith. Recongnized by many as one of the foremost Applied Equine Podiatrist in the world today. His lectures and teachings have been presented throughout the U.S. and Abroad. His innovative approach to teaching the art of Applied Equine Podiatry has met with rave reviews the world over.