Side bone

SIDEBONE

COLLATERAL DAMAGE

Sidebone is the name given to the condition that results in ossification of the collateral cartilages of the foot. It is a remarkably common finding in our domesticated horses, and a diagnosis of the condition strikes fear into the heart of many horse owners. Yet, according to our vets, a diagnosis of sidebone is not necessarily something to panic about. According to these vets, a finding of sidebone is often purely incidental and is usually not the root cause of the lameness itself.

Here we look at the pathology of sidebone and its significance in our equine companions.

THE COLLATERAL CARTILAGES

The collateral cartilages are paired structures on the medial (inside) and outside (lateral) parts of the horse’s foot. These cartilages are located just above the heel region and

Heavy repetitive landing on the front feet can cause sidebone.

extend forwards from the heel bulb for about one-third to half of the total distance from heel to toe. The collateral cartilages sit with about two-thirds of their mass within the hoof capsule and one-third projecting above. They are attached to the pedal bone, its ligaments and the short and long pastern bone. You can palpate these cartilages above the coronary band in the heel region as a piece of slightly malleable tissue. Coincidentally, they are often used as landmarks for nerve blocks by vets.

These cartilages, along with the digital cushion, are considered part of the shock-absorbing mechanism of the horse’s foot. The theory suggests that with weight bearing, the forces from the ground are pushed up through the frog putting pressure on the digital cushion and causing the collateral cartilages to move outwards. This movement results in two separate effects. Firstly, it helps dissipate the forces generated by movement by transferring the force to the soft tissue. Secondly, the movement of the cartilages with each stride assists with blood flow through the nearby blood vessels. This has led to many describing the hooves as the ‘additional hearts’ of the horse.

SIDEBONE

Sidebone is described as the mineralisation of the collateral cartilages of the hoof, transforming the collateral cartilages into more rigid and less flexible bone-like tissue. If the collateral cartilages have become mineralised in this way, the changes are often palpable. A swelling also commonly develops just above the hoof capsule. Sidebone is more commonly seen in the front feet than the hind feet due to the increased weight borne by the front legs.

However, as noted above, the reality is that the condition has minimal clinical significance in most cases. Lameness due to sidebone is usually due to a complicating feature,

Sidebone Sidebone

such as the growing sidebones pressing on adjacent sensitive hoof structures, rather than the sidebone itself.

PREDISPOSING FACTORS

The following factors predispose horses to developing sidebone: • Heavy repetitive landing on the front feet, such as is experienced in showjumping. • Poor foot balance, especially if this is a chronic issue. • Being a heavier horse, such as a Percheron or Shire. • Abnormal limb conformation can cause abnormal loading patterns. • Direct trauma to the collateral cartilages.

DIAGNOSING SIDEBONE

PHYSICAL EXAMINATION Physical examination will usually reveal hardened collateral cartilages on palpation and the characteristic swelling above the hoof capsule. NERVE BLOCKS Nerve blocks of the heel region will numb any pain associated with the collateral cartilage but will also desensitise numerous other structures, including the seat of corn, frog, digital cushion, navicular bone, navicular bursa and part of the coffin joint. This nerve block can therefore confirm the area causing the lameness but won’t provide a definitive structure as the cause of lameness.

X-RAYS X-Rays are often taken once the region of lameness has been identified. It is generally best practice to scan both front legs to allow comparison of the two sides. However, in cases of sidebone, X-Rays can create confusion, as many completely normal horses have mineralisation of these cartilages (sidebone) with no clinical issues.

NOTE: Mineralisation can often occur in more than one place in the collateral cartilages. When this happens, a small line often remains between the upper and lower areas of mineralisation that can closely resemble a fracture line. This can cause confusion, leading to people wrongly assuming that this ‘fracture’ is the cause of lameness.

OTHER INVESTIGATIONS Bone scans and nuclear scintigraphy may provide more information about the relevance of sidebone to the lameness shown by the horse. Positive uptake in these tests may be seen if the sidebone is active or inflamed as a result of trauma or fracture. In this setting, a bone scan can detect differences between the medial and lateral

cartilages or between the cartilages of one foot and the cartilages of the other foot in terms of inflammation levels. This technique is probably of most use in confirming a true clinical problem associated with sidebone, as active inflammation suggests that the cartilage is the cause of the lameness or is at least contributing to it.

TREATMENT

If sidebone is proved to be the cause of lameness, treatment starts with the restoration of good foot balance and the compensation for the loss of shock-absorbing potential of the foot through remedial farriery. A common farriery technique used in these cases is to shoe the affected horses with flat, wide, webbed shoes with a rolled toe, which are wide at the quarters and heels and extend beyond the ground surface at the heels to support the heel regions and allow for expansion. Nails should not be used behind the mid-quarter of the foot.

These interventions often abolish the lameness in affected horses quickly, but the radiographic appearance of the collateral cartilages is unlikely to return to normal.

SIMILAR CONDITIONS

QUITTOR

Another condition associated with the collateral cartilages is quittor. This occurs when the collateral cartilage is damaged, normally due to external trauma. The cartilage or, more frequently, the upper part of it becomes infected and loses blood supply, resulting in a discharging wound in the area. The damaged and infected cartilage acts as a foreign body and will require removal for the area to heal.

PALMAR HYPEROSTOSIS Occasionally, only the back part of the collateral cartilage mineralises, leading to a condition called palmar hyperostosis. This is an uncommon clinical finding associated with lameness. In these cases, horses have mineralisation of the collateral cartilages of the front feet with a loss of mineral from a portion of the coffin bone. An association with poor foot balance is suspected in these cases but not proven.

The surfaces on which you work your horse can have an effect on the development of sidebone.