The equine hoof is very well specialised for many functions,
such as shock absorption, propulsion and protection. This essay will discuss a
few of the structural adaptations of the equine hoof and how they contribute to
its function.

 

The hoof wall has three divisions, the stratum
externum, the stratum medium and the stratum internum (Pollitt, 2004). The stratum externum is a thin layer covering the
outside of the hoof wall. Its functions include waterproofing (Bowker, 2003 A)
as well as inhibiting dehydration (Kasapi and Gosline, 1997). 

 

The stratum medium is composed of thin, hollow tubules and is the main
system of load support in the foot (Bowker, 2003 A). Tubules are formed from
coronet basal cells, which then undergo keratinisation. The tubules are
continual from the coronet to the ground surface (Pollitt, 2004). There are fibres that wind around the tubule and
alternate direction between each layer, forming concentric lamellae (Bertram
and Gosline, 1987, Kasapi and Gosline,
1997). Keratinocytes are generated from the
holes in the tubules, these mature to form the intertubular horn (Pollitt, 2004). The intertubular horn is
formed at right angles to the tubular horn which provides “mechanically stable,
fiber-reinforced composite” that has strength in all directions (Pollitt,
2004).

 

One function of the stratum medium is crack propagation. The
tubules are able to cause the pathway of the crack to be deviated, and redirect
it away from sensitive structures of the foot (Kasapi and Gosline, 1998). It is argued the tubules
reinforce the hoof wall along the longitudinal axis which provides resistance
and leads the crack through a more tortuous route, while at the same time
absorbing and dissipating fracture energy (Kasapi and Gosline, 1997).  The hollow centre of the tubules play an important
role in crack propagation. In the middle regions of the hoof the fracture path
is redirected across hollow tubules and then along intertubular material, which
stops the cracks progressing up the hoof wall (Kasapi and Gosline, 1997). The intertubular horn is
also important in crack propagation as it has more stiffness and fracture
toughness than the tubular horn (Lancaster,
Bowker and Mauer, 2013). This function is especially important because
the hoof wall cannot be remodelled, as only the coronet and proximal lamellae
have active cell proliferation
(Pollitt, 2004), therefore will not be able to repair small
fractures. 

 

The stratum medium is required to
resist compressive loads. The
density of the tubules changes through the stratum medium (Pollitt, 2004),
producing a radial pattern of increased tubule density, leading to a
lower density at the dermal edge and higher density at the hoof wall. This
consistent around the circumference of the hoof (Lancaster, Bowker and Mauer, 2013). The density gradient aids smooth
energy transfer, from high tubule density to low tubule density (Pollitt,
2004). The intertubular horn is also able to shift within the stratum medium
when there are inner wall stresses and strains. This reduces the magnitude of
the forces acting on one area of the hoof (Lancaster, Bowker and Mauer, 2013).

 

The stratum internum is made of
keratinized lamellae and non-keratinized secondary lamellae (Bowker, 2003 A).
There is a connection between the dermis of the corium, near the distal
phalanx, and the epidermis near the wall horn. The basement membrane is a
structural boundary between this connection, which is mainly composed of
extracellular matrix (Jansová et al., 2015). It is woven to create primary and
secondary epidermal lamellae that interdigitate with the dermal laminae, this
provides a highly rigid and strong connection (Jansová et al., 2015). The
basement membrane is important for coordinating biological processes between
the two tissues (Pollitt, 1994) by
organising the cytoskeleton of the epidermal cells and influences the exchange
of nutrients and molecules (Abrahamson, 1986). The lamina densa is the middle
layer of the basement membrane, this consists of extensions and anchoring
fibrils which are made of collagen VII (Pollitt, 1994). The anchoring fibrils
hook onto collagen I of the connective tissue fibrils in the lamellar corium
forming a vital attachment between the dermis and epidermis (Pollitt, 1994).
There is a high density of lamina densa extensions and anchoring fibrils on the
tip of the secondary epidermal lamellae, this gives a larger surface area for
attachments (Pollitt, 1994). Therefore, there is a strong attachment between
the basement membrane and the connective tissue which is especially important
in horses as they are ungulates, so all of their weight baring is on one digit.
There are also electron dense plaques called hemidesmosomes which contain a series
of proteins which can connect basal cells to the basement membrane (Pollitt,
2007). It is vital that that there is a firm attachment as it important in the
weight bearing hoof lamellae. The primary epidermal lamellae (PEL) can reduce
tension forces in the inner hoof as “there is approximately 600 PEL that
suspend the distal phalanx and each PEL has roughly 100 secondary epidermal
lamellae” (Bowker, 2003 A). Therefore, there is a large surface area for
attachments, so tension forces are reduced

 

In the horse hoof loads must be dissipated rapidly to reduce the
potential damage to bone and connective tissue (Bowker et al., 1998).
This is achieved by the many internal structures of the hoof including the
digital cushion, the frog and lateral cartilages.  The digital cushion lies between the lateral
cartilages but above the frog and epidermal bars (Gunkelman and Hammer, 2017).
It consists of a network of collagen and elastic fibre bundles, which contain
proteoglycans, and small areas of adipose tissues (Bowker, 2003 B). However,
above the age of 5 the internal structure begins to change. The collagen
bundles and the lateral cartilage begin to form fibrocartilage, this mainly
consists of proteoglycans which is crucial in energy dissipation (Bowker, 2003
B).

 

Internal structures of the hoof have many functions. It has been
thought that they aid in a blood pumping mechanism that encourages venous blood
return from the digit to the leg (Gunkelman and Hammer, 2017). There is also
evidence to show that the internal structures have a role in absorbing and
dissipating energy during locomotion and stance (Bowker et al., 1998).
This is supported by Dyhre-Poulsen et al., 1994 who showed that the
frequency and amplitude of forces in the proximal phalanx were much lower than
the forces in the hoof wall, therefore the laminar attachments and the distal
structures must have a role in reducing these forces. However, the function of
the frog during locomotion is not completely known as when it has been
surgically removed the horse’s ability to trot and canter is not diminished
(Bowker et al., 1998).

 

There are various theories about how the internal structures can
absorb such high forces. The pressure theory states that upon impact the sole
and the frog compress the digital cushion. This applies pressure to the lateral
cartilage on the distal phalanx, pushing the hoof wall outwards (Dyhre-Poulsen et
al., 1994). Lungwitz, 1884 had conducted experiments and concluded that the
horses whose feet had no frog pressure, made “unsatisfactory angle with the
ground” and had “upright heels”, which therefore supported the pressure theory.
In contrast to this in 1989 Colles suggested the Lungwitz has only assumed this
was caused by frog pressure, when there were several factors to consider.
Colles also stated that frog pressure changes can result in “increased
expansion, or contraction or may have virtually no effect”. This shows that
there is not enough evidence to support the pressure theory

 

The depression theory states that forces are transmitted though
laminar attachments in the hoof wall. These forces are redirected as the middle
phalanx in lowered, this caused the hoof walls and lateral cartilage to be
pushed outwards (Bowker et al., 1998). Both theories state that blood is
pumped from the foot at impact. This is supported by experiments from Ratliff,
Shindell and DeBowes which showed that there is said to be a change in venous
pressure during locomotion (Ratliff, Shindell and DeBowes, 1985).

 

On the other had neither theory explains the negative pressures
observed within the digital cushions during stance and locomotion (Bowker, 2003
B). the hydraulic mechanism involved the lateral cartilage, vasculature and
digital cushion all working within the neuromodulation of the microvasculature.
At impact blood is forced through microvasculature. This provides a resistance
to the flow of blood in microvasculature, therefore it reduced the amount of
high impact forced that are transferred to the bones and ligaments. The
negative pressure change is caused by the refilling of vessels before the next
fall (Bowker, 2003 B). This system is more effective in horses with thicker
lateral cartilage. This is because they have more small vessels. A
fibrocartilaginous digital cushion means that more energy is dissipated as
there are more proteoglycans. Elastic tissue in the digital cushion is used as
a spring to return the tissues of the foot to their original position (Bowker,
2003 B).

 

The equine hoof has a variety of structural adaptations, some of whose
functions are still under debate. This shows the extent of the complexity and
how specialised the hoof structures are.

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Bowker, R. M. (2003) A. The growth and adaptive capabilities of the hoof
wall and sole: functional changes in response to stress. Proceedings of the
49th Annual Convention of the American Association of Equine Practitioners, New
Orleans, Louisiana, USA, 21-25 November 2003, (January 2003), 146–168.

 

Jansová, M. et al.
(2015) ‘A finite element model of an equine hoof’, Journal of Equine
Veterinary Science, 35(1), pp. 60–69. doi: 10.1016/j.jevs.2014.11.008.

 

 Pollitt, C. C. (2004)
‘Anatomy and physiology of the inner hoof wall’, Clinical Techniques in
Equine Practice, 3(1), pp. 3–21. doi: 10.1053/j.ctep.2004.07.001.

 

Davies, H. M. and Philip, C. (2007) General Definitions and Structure,
Equine Podiatry. Elsevier Inc. doi: 10.1016/B978-0-7216-0383-4.50005-0.

 

POLLITT, C. C. (1994) ‘The basement membrane at the equine hoof dermal
epidermal junction’, Equine Veterinary Journal, 26(5), pp. 399–407. doi:
10.1111/j.2042-3306.1994.tb04410.x.

 

 

Abrahamson, D.R. (1986) Recent
studies on the structure and pathology of basement membranes. J. Pathol. 149,
257-278.

 

Pollitt, C. C. (2007) Microscopic
Anatomy and Physiology of the Hoof Secondary Epidermal, Equine Podiatry.
Elsevier Inc. doi: 10.1016/B978-0-7216-0383-4.50010-4.

 

Bowker, R. M. et al.
(1998) ‘Functional anatomy of the cartilage of the distal phalanx and digital
cushion in the equine foot and a hemodynamic flow hypothesis of energy
dissipation’, American Journal of Veterinary Research, 59(8), pp.
961–968.

 

DYHRE?POULSEN, P. et
al. (1994) ‘Equine hoof function investigated by pressure transducers
inside the hoof and accelerometers mounted on the first phalanx’, Equine
Veterinary Journal, 26(5), pp. 362–366. doi:
10.1111/j.2042-3306.1994.tb04404.x.

 

Colles, C. M. (1989)
‘relationship of frog pressure to heel expansion.pdf’, Equine Veterinary
Journal, 21(1), pp. 13–16.

 

Gunkelman, M. A. and
Hammer, C. J. (2017) ‘A Preliminary Study Examining the Digital Cushion in
Horses’, Journal of Equine Veterinary Science. Elsevier Ltd, 56, pp.
6–8. doi: 10.1016/j.jevs.2017.03.006.

 

Bowker, R. M. (2003) B.
‘Contrasting structural morphologies of “good” and “bad” footed horses’, Proc.
Am. Ass. equine Practnrs, 49.

 

LANCASTER, L. S., BOWKER,
R. M. and MAUER, W. A. (2013) ‘Equine Hoof Wall Tubule Density and Morphology’,
Journal of Veterinary Medical Science, 75(6), pp. 773–778. doi:
10.1292/jvms.12-0399.

 

Kasapi, M. A. and
Gosline, J. M. (1998) ‘Exploring the possible functions of equine hoof wall
tubules.’, Equine veterinary journal. Supplement. United States, (26),
pp. 10–14.

 

Bertram, J. E. and
Gosline, J. M. (1987) ‘Functional design of horse hoof keratin: the modulation
of mechanical properties through hydration effects.’, The Journal of
experimental biology, 130, pp. 121–136.

 

Kasapi, M. A. and
Gosline, J. M. (1997) ‘Design complexity and fracture control in the equine
hoof wall’, The Journal of experimental biology, 200(Pt 11), pp.
1639–1659.