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The hoof pastern axis and its relevance
Sarah Logie AWCF
When discussing conformation or common lameness issues the expression hoof
pastern axis is commonly used by vets and farriers, but what are they referring to
and how do you see it?
The hoof pastern axis (HPA) on both front and hind limbs can be assessed when, an
imaginary line drawn through the centre of the phalanges is compared to the line of
the dorsal wall of the hoof capsule, when viewed from the lateral aspect (Adams,
2002).
When assessing for a correct HPA, the phalangeal axis should be parallel with the
dorsal surface of the pedal bone. The dorsal wall of the hoof capsule should be free
from flares/distortion, if it is going to be used as an accurate guide (Figure 1).
When assessing conformation in the forelimb it is accepted that the HPA should
correspond with the angle of the shoulder 48° to 55° (Figure 2), and that it will be
more upright in the hinds by 1° to 5° (Adams, 2002) (Ross & Dyson, 2011).
A straight HPA, means that the movement within the joints will be as efficient as
possible for the individual, and the soft tissue will not be placed under additional
strain.
A horse may have a straight HPA which is either sloping (less than 47° fore), normal
(48° to 55° fore), or upright (greater than 56° fore). This may be breed dependant, for
example in a thoroughbred is considered normally to have a more sloping HPA than
in a Warmblood. It is common for a horse with a sloping HPA to have a long pastern
and conversely an upright HPA is often related to a shorter pastern (Ross & Dyson,
2011).
It should be noted that the stance of the horse can affect the visual appearance of
the HPA as it can be altered by shifting the bodyweight, correct stance is when the
limb is perpendicular above the foot (Stewart, 2013).
When a straight HPA is not present it will be described as either ‘broken forward/
positive’ or ‘broken back/negative’ (Figure 3). When trimming a horses feet with a
less than ideal HPA, a farrier will aim to restore it to ‘straight’ although many factors
such as limited hoof growth may prevent that happening. Further correction may
have to occur through the application of a shoe.
In the horse with a straight HPA, the limb conformation will determine the height and
length of the animals’ natural stride. A horse with upright HPA will have a shorter
‘choppy’ stride and are recognised as being less comfortable to ride, those with a
sloping HPA and seen to have a longer smoother stride and are therefore more
comfortable for riding (Butler, 2004).
The importance of the HPA goes further than the riders comfort. The mechanical
strains placed upon the lower limb differ depending on the angle of the HPA. As a
result of these forces the shape of the hoof capsule is closely related to the angle of
the HPA. In the straight HPA with a ‘normal’ pastern length the there is an
equilibrium between the extensor and flexor forces within the hoof capsule.
The anatomy of the lower limb allows the fetlock to act as a huge shock absorber
and it is supported by very strong ligaments and the tendons. These structures have
finite elasticity and can be damaged through excessive sudden load or by repetitive
strain (figure 4)
In the horse with a straight HPA and a healthy functioning foot, every structure is
able to do as designed and absorb or transfer shock. The external structures are
able to absorb shock through compression (frog), expansion (wall) or flattening
(sole). The descending body weight is supported within the hoof capsule by the joints
and sensitive tissues, then ultimately cushioned by the digital cushion and
suspended by the attachment between the sensitive laminae and the hoof wall
(figure 5).
In the negative HPA more force is placed on the flexor tendons, which is transmitted
as pressure into the navicular region, the horse may try to alleviate this pressure by
changing its stance, so that its feet are in front of the perpendicular. The hoof
capsule is overloaded in the posterior portion and may crush as a result, this and the
change in stance can create a vicious pain cycle creating collapsed feet which are
slow to recover, if at all.
In the positive HPA the strain is placed more to the extensor region and although this
may not directly affect the horse much, the resulting tearing effect in the dorsal wall
of the hoof capsule can start to affect the hoof wall integrity. The lack of load on the
heels creates contraction and an atrophied frog. The muscles of the flexors may start
to shorten, which can produce a stance where the foot is behind perpendicular.
With a straight upright HPA the hoof capsule tends to be more tubular with a more
upright heel (possibly contracted) and often a smaller surface area than the
equivalent size of animal with a normal HPA. This smaller more upright foot is less
able to absorb concussive forces, due to the lesser amount of soft tissue in the
posterior third of the foot and the more upright walls expanding less than a normal
foot, the shock is transferred more suddenly to the bone column (Jackman, 2001). In
the upright HPA the forces tend to be more concussive than in a horse with a normal
HPA, the upright foot is loaded more suddenly than when the normal foot ‘rolls’ into
weight bearing. The angle of the pastern and fetlock means that the fetlock will drop
less reducing the load taken by its supportive soft tissue structures This leads the
animals with this conformation to be more prone to arthritic conditions, splints,
sidebone and other soft tissue damage such as corns.
In the horse with a straight sloping HPA the hoof takes more load through the back
third, so tends to be flatter in angle with a more developed frog and more rounded
bulbs of the heels. The foot tends to expand more and is therefore generally larger
than the equivalent horse with a normal HPA. In the sloping HPA the foot lands and
bears weight more gradually but the load is taken up by the soft tissue, primarily
within the hoof capsule and then by the suspensory apparatus within the fetlock.
These animals are more prone to damaging the collateral ligaments within the lower
joints, and also the suspensory ligament. The flexor tendons which are forced round
a sharper fulcrum at the fetlock, become more prone to damage and create pressure
within the joint (Thompson, 1993). Care is required to ensure that these horses do
not develop a negative HPA, growth at the toe of the hoof is likely to create
excessive leverage and overloading within the posterior third of the hoof. This
pressure in turn is likely to crush the juvenile horn at the heels, exacerbating the
condition. A negative HPA also further changes the angle of insertion for the deep
digital flexor tendon (DDFT) at the coffin joint, increasing the chance of damage.
Excessive shoe length in an attempt to support the soft tissue at the fetlock can
crush the heels of the foot further, making the condition worse.
In the horse with a negative HPA the aim of the farrier is to restore this axis without
weakening the hoof capsule and causing further problems. The authors chosen route
is to place the base (the shoe) where the foot should be and at an angle that the
hoof capsule should be at (figure 6), this changes the loading on the horn producing
structures of the foot and will therefore improve circulation and change the angle the
horn is produced at. The hoof capsule can be dressed to straight horn and further
growth controlled, without removing excessive horn and weakening the hoof wall. It
is also noted that the horse’s stance should improve when shod in this manner. By
placing the base where the foot should be the limb is instantly removed of the
excessive strain and leverage created by a ‘long toe low heel’ hoof and by controlling
the existing horn (preventing leverage) and encouraging growth at the correct angle
the hoof capsule will gradually change to a stronger healthier shape. It should be
noted that the frog and digital cushion within the hoof capsule can over time be
overloaded and crushed beyond a natural recovery and artificial methods to regain a
straight HPA may always be required.
A positive HPA may be the result of a poor hoof care and restored within one trim, or
may be a result of stance and soft tissue pain that prevents the horse loading its
heels comfortably. This lack of loading manifests itself as further heel growth and the
removal of this growth just increases the discomfort. The farriers’ role is to ascertain
the cause and to try return the HPA to straight if possible (Curtis, 2006). If discomfort
is the cause of the poor HPA then the foot can be trimmed to ideal and then
artificially returned to an angle which is comfortable for the animal, this allows the
foot to function more normally as the heels are maintained to where they should be.
If in a younger animal where it is preferable to try and return the limb to ‘normal’,
veterinary assistance with pain relief and appropriate physiotherapy can allow
gradual reduction in height of the heels.
When there is a differential between a pair of feet, then the aim should be to treat
each foot and limb individually and not to force symmetry upon the limbs, as this will
compromise one or both of the feet. It must be noted that dramatic changes in the
hoof capsule will transfer load to other structures in the limb, for example, over
dressing the heels will transfer load to the DDFT and this will stress the inferior
check ligament potentially causing lameness.
In the horse with a straight HPA – regardless of its angle- the role of the farrier is to
support the hoof and the limb without trying to change the angle of the hoof capsule
as integral strength will be lost and problems more like to occur. Correct farriery can
only address any differentials, it cannot change the conformation of the animal, and
the farrier’s goal is to support it within its weaknesses.
Bibliography
Adams, O., 2002. Adams lameness in horses. 5th ed. Baltimore: Lippincott Williams &Wilkins.
Butler, D., Butler. J., 2004. The Principles of Horseshoeing (P3). 3 ed. Colorado: Doug Butler
Enterprises, Inc..
Curtis, S., 2006. Corrective Farriery. Volume 2 ed. s.l.:Newmarket Farrier Consultancy.
Jackman, B., 2001. Common lameness in the cutting and reining horse. s.l., s.n.
Ross, M. & Dyson, S., 2011. Diagnosis and management of lameness in the horse. 2nd ed. St. Louis:
Elsevier Saunders.
Stewart, J., 2013. Understanding the horse's feet. 1st ed. Marlborough: The crowood press ltd.
Thompson, K. Cheung, TK., Silverman, M., 1993. The effect of toe angle on tendon, ligament and
hoof wall strains in vitro. Journal of equine veterinary science, 13(11), pp. 651-654.
Legend for Hoof pastern axis (Equine Health) article
Figure 1: Correct HPA, the phalangeal axis should be parallel with the dorsal surface
of the pedal bone. This can be assessed by viewing the dorsal wall of the undistorted
hoof capsule.
Figure 2: The HPA should correspond with the angle of the shoulder in the forelimb.
Figure 3: When a straight HPA is not present it will be described as either ‘broken
forward or positive’ (on left) or ‘broken back or negative’ (on right), these HPA’s
place extra stress on different parts of the distal phalangeal joint - circled.
Figure 4: The supporting tissue of the fetlock joint, and the pastern. Extensor
tendons (green), the flexor tendons (blue), interosseous ligaments (red) and the
suspensory ligament (purple). These structures absorb shock and prevent
hyperextension of the fetlock but are subject to huge stress and are therefore prone
to injury particularly when the HPA is negative.
Figure 5: The descending body weight (black arrows) is supported within the hoof
capsule and cushioned by the digital cushion (orange) and suspended by the
attachment between the sensitive laminae and the hoof wall (red and brown arrows).
Figure 6: shoeing to correct a negative HPA without weakening the hoof wall the
base (shoe) can be used to realign the HPA when the break over point of the shoe to
the coronary band is used as the reference point.
