• theequinedocumentalist

Haemodynamic Mechanism - The Key to Hoof Health

Updated: Jun 11

The ability to withstand concussive and load forces is vitally important in musculoskeletal health and the hoof is on the front line. At a walk the horse puts ½ of its body weight through a limb, 2.5 times its body weight at a gallop! (Weller 2019), it’s no wonder having an efficient force dampening structure on the front line can be the difference between soundness and catastrophic injury and studies have shown the link between these and hoof size, shape and balance (Kane et al 1999).

The ability to efficiently dissipate the forces of locomotion directly affects hoof morphology (Gunkelman and Hammer 2017), therefore having well conformed and utilised structures involved in this mechanism can dictate the longevity of functionality and hoof conformation. If these structures are weak the whole hoof is predisposed to exceeding its elastic modulus with inevitable collapse. The studies of Bowker have delved into the mechanisms of force dissipation and the micro-conformational differences between “Good” and “Bad” footed horses and found that although every horse has the same structures, their shape and composition play a fundamental role in functionality.

Fig.1 Authors illustration of the traditional theories of the haemodynamics of the hoof, (Bowker 1998).

Early Traditional theories express that the digital cushion was predominantly responsible for absorbing the concussive forces, however further studies have outlined the hoof as a hydraulic dampener, with further differing theories.

One of those theories state, displacement of the digital cushion by the ground reaction forces acting through the frog and frog stay, presses against the lateral cartilages and subsequently compresses the vascular structures (Fig.1 green arrows, compression theory). Another theory suggests the descension of the middle phalanx induces an outward displacement of the lateral cartilages, while also displacing the digital cushion (Fig.1 Black arrows, Depression theory), this theory is somewhat backed up Taylor et al (2005) which indicated that the function of the digital cushion was mainly to counteract this displacement of the middle phalanx and not to provide a pressure force.

Fig.1b Bowker suggests another theory, “When the foot hits the ground, the bars of the heels and pillars of the hoof wall force a small “shelf” of the cartilage outward, creating negative pressure in the digital cushion. Impact is thus transmitted to a complex venous network inside the cartilage, creating more negative energy, which draws blood up from the solar area of the hoof.”

Which ever theory one accepts and perhaps all are true, It is widely understood that the ideal anatomical arrangement and composition of the structures of the hoof are so, that they create an effective hydraulic dampening effect on the impact forces of locomotion.

In the perfect foot, it could be hypothesised that all three theories happen at the same time to create the most efficient shock absorption. However, the reality is that different hoof conformations, will utilise one or more of these mechanisms.

Fig.2 In the perfectly conformed, barefoot, all three theories work together for ultimate shock dispersion. The hoof has a good frog and digital cushion, thick lateral cartilages and well conformed bars of the hoof, all on the same plane, to utilise all the mechanisms.

Fig. 2b Differently conformed caudal hoof's will utilise different haemodynamic mechanisms and have different responses to being shod. Far left has a good system that utilises the majority of theories. When shod it is likely to cope better then the middle hoof that only has the ground reaction force working through the frog as a mechanism. Once shod it then has none of the systems working efficiently. Image courtesy of Paige Poss.

Fig.3 Schematic Diagram of the Hydraulic Damper effect of the haemodynamic system of the hoof. The hoof becomes a hydraulic cylinder filled with pressurised blood to produce a pressure difference, resulting in damping force. Due to the high pressures involved the structures of the hoof need to be strong and durable to cope with the forces. We can also see that the more vascular structures in the system the greater amount of blood that can be displaced and therefore the more efficient the dampening effect.

When the composition and position of the hoof structures are not ideal, their ability to absorb the same forces are altered. How strong and efficient their system is, dictates how much shock they are able to disperse, which dictates the hoofs shape. This also dictates their response to traditional peripheral shoeing, which inevitably removes the compression mechanism.

Fig.3b Authors illustration of Bowker’s good (Left) vs bad (Right) foot. Although both feet have the same structures their micro-conformational differences are what play a role in their durability.

The thickness and extent of the lateral cartilages plays a large role in the strength of a hoof, in the good foot you can see that cartilaginous tissue surrounds and fuses with the digital cushion and is thick enough to envelop the blood vessels. In the weak foot the cartilage is thin and less extensive with external vascular structures. Bowker also found the composition of the digital cushion to be different and Important in its elastic modulus, in the stronger foot the digital cushion had fibro-cartilage within it. However, new research (Hallock 2020) suggests that it is the arrangement of these fibres in an orderly or disordered fashion that dictates a strong or weak digital cushion.

Fig.4 Authors representation of Bowker’s strong hoof in a section at the level of the navicular.

Bowker (1998, 2003) found that strong feet had a digital cushion containing fibro-cartilage which had fibrous connections with the lateral cartilages, also the deep digital flexor tendon had ligamentous attachments to the lateral cartilages. Essentially, horses with thicker more fibro-cartilaginous structures and more vascular structures will dissipate more forces by their hydraulic mechanism. Faramarzi et al (2017) discovered that the different areas of the digital cushion in quarter horses had different compositions, attributed to the different functional role of that area, perhaps pointing at horses with more universal composition as not being developed to their function.

The “bad” foot in Fig.3b is likely to be a typical, flat, long toe, low heel hoof which becomes self-perpetuating due to its reduced ability to address these forces. As expressed by Bowker these feet have the impact and load bearing structures of the heels and bars under the distal phalanx rather than under the cartilages. This can result in inefficient use of the haemodynamic, hydraulic mechanisms due to the forces being transmitted to the bone instead of the structures designed for force dissipation.

Fig.5 Authors illustration of the force transference from underrun heels compared to more upright heels.

The hoof on the left is likely to have the presentation of weak cartilages (Fig.2), with decreased blood flow through the cartilages and/or with a fatty rather than fibrous digital cushion. Less energy will be dissipated, resulting in more energy being transmitted to bone and ligaments within the foot. Eventually elastic moduli will be exceeded, damage will be done and lameness may ensue.

Understanding the haemodynamic mechanisms role in force dissipation, you can see from Fig.5 how the inefficient dissipation, by disordered anatomy, in poor hoof conformation, can lead to the genesis of pathologies. The structures where loads are concentrated and the tissues which receive the loads are important in maintaining a healthy foot, because certain tissues are well adapted to energy dissipation and load support, whereas other tissues are not.

Also again considering how many of the haemodynamic mechanisms are being utilised by an individual foot, will dictate how significant the negation of the compression theory is when shoes raise the caudal structures from the ground.