HOOF-pastern axis (HPA) refers to the arrangement of the equine digits, which are located below the fetlock. It is divided into pastern axis and hoof axis.

A line is taken from the dorsal aspect of the pastern and continues down the dorsal surface of the hoof, there is another line taken from the centre of the fetlock joint that dissects the pastern and coffin joint. If these lines are parallel, the correct HPA angulation occurs (Fig 1).

Digit alignment is largely determined by genetics passed on by the dam and sire, however what we observe is the resulting phenotype. This is the combination of genetics and environment with the latter being an area of debate between equine professionals and enthusiasts alike. Common environmental factors are hoof care, nourishment, lameness, body weight, discipline, and laterality.

HPA is observed in both the anterior (front) and lateral (side) view of the equine limb, the average toe angle is between 45° and 50° for the forelimb. this can be mistaken for ideal angles.

It is not always possible or advisable to achieve these angles. Most discussed is the forelimb HPA due to the level of forelimb lameness. This article will concentrate on the forelimb for simplicity.

HOOF-PASTERN AXIS

When accessing the anterior view, the most noticeable deviation is an offset HPA (Fig 2).

When accessing the lateral view, a broken forward HPA (steeper foot than pastern) or broken back HPA (lower foot than pastern) may be present.

Offset HPA – can create strain on the fetlock, pastern and coffin joints, unaddressed medio-lateral imbalance causes uneven pressure on the joint capsule and collateral ligaments, suspensory branch desmitis may present as a result of increased strain.

Broken back HPA – a low hoof angle can cause a negative palmar angle (Fig 3) due to coffin joint extension, this may strain the coffin joint and deep digital flexor tendon. While also causing uneven pressure on the digital cushion and hydraulic cushion of the foot (pressurised blood contained below the plexuses that aid the dissipation of energy beneath the coffin bone during ground contact).

Long toes will encourage the heels to grow forward and lower. This reduces caudal support, compounding the issue. The horse may adopt a toe first landing that does not employ the shock absorbing structures correctly while delaying breakover and causing friction of the navicular area.

Broken forward HPA – often happens in young horses when the bone grows faster than the tendons. A high hoof angle can accompany rotation or flexion of the coffin joint. This increases strain on the suspensory ligament and extension tendon. It usually promotes a heel first landing. Pedal osteitis and or coffin joint inflammation may develop.

Asymmetrical HPA - may be more evident in certain breeds of horses, where a horse will exert more weight on one hoof lowering the heels while the opposing hoof atrophies slightly. This can be due to habitual static loading of one limb while cross-tied (Fig 4). Laterality, clinical or sub clinical lameness may cause this imbalance. An excessively high hoof angle is referred to as a club foot.

Juvenile HPA hoof care – a growth plate is the region in a long bone between the epiphysis (head or neck) and diaphysis (shaft) where growth in length occurs, these growth plates close in the digits at approximately six months of age. Afterwards it is not possible to influence bone growth bellow the fetlock.

Juvenile orthopaedic hoof care – the application of orthotics, and/or hoof alignment by a farrier to influence the bones longitude growth. In more severe cases, surgical intervention may be necessary by a veterinary surgeon, it is always recommended to have foals examined regularly for deviations. With rapid development in foals, bi-monthly hoof care is often advised to limit or address angular limb deformities.

Adult HPA hoof care – the hoof capsule is an extremely strong yet pliable structure, this partially allows the dissipation of concussion, however it leaves it open to distortion. Most adult horses have experienced capsular distortion at some stage.

Farriers continually adjust length and balance of the hoof capsule to correct HPA and mitigate the effects of any deviation, reducing limb stress to help maintain soundness.

Long toes increase the stress on the navicular region and deep digital flexor tendon while predisposing the horse to stumbling. Riders and trainers may prefer an increase in the toe angle to reduce these stresses, be that as it may over reducing the toe length can increase stress on the suspensory ligament and extensor tendons. Over shortening the toe can create a broken forward HPA also marginally reducing stride length.

The hoof wall is naturally thickest at the toe, this helps with wear and is an intrinsic part of healthy hoofs. Continually thinning of the toe wall could lead to weaker feet. This may not manifest on a strong-footed horse, however on weak-footed horses may present problems.

The heels are an area of stress due to the nature of capsular loading, horse shoe attachment perforates the quarters of the foot with nails, the toe remains as an area of strength, removing this strength may present problems in certain horses (Fig 5).

With that said, farriers do not advocate long toes but wish to inform on the mechanics involved in shortening the toe, this can complicate matters when presented with a case that is both long toed and weak footed!

BALANCE

Horse owner knowledge and awareness is crucial while understanding ideals are not always obtainable. Farriers must juggle both theoretical and practical experience for successful outcomes.

Often we use the word balance, as we strive to establish equilibrium of the limb. Traditionally this may have been subjective, whereas now the frequency of digital radiographs (Fig 6), performed by veterinary surgeons, and their instant availability, contributes to advanced equine performance management. Shoeing prescriptions and support provided by veterinary surgeons to farriers helps greatly in tailoring specific hoof-care plans to correct and/or maintain the HPA.

If concerned about your horse’s HPA, consult both your veterinary surgeon and farrier, as diagnostics will undoubtedly lead to better results.