Sidebone is the name given to the ossification (bony formations) of the flexible collateral cartilages of the distal phalanx (coffin bone) in the foot. These are found either side of the coffin bone in some horses protruding very little and in others, protruding up towards the level of the pastern joint. These lateral cartilages support the hoof wall and provide support and cushioning to the heel. Sidebone is a frequent finding in horses that undergo radiography (x-ray) of the feet and is most commonly seen in the front feet.

Sidebone has a number of causes. It is thought to be a normal ageing process and is therefore often seen in older horses; it is also related to concussion of the foot which is caused by regular work on hard ground; poor foot conformation (inherited and due to incorrect trimming and/or shoeing).
Horses with the following conformational abnormalities have an increased susceptibility to sidebone than others:

  • Narrow, upright feet
  • Unbalanced feet
  • Toe-in or toe-out conformation

Heavy horses, such as the Draught, are also more susceptible than other lighter-built horses and ponies.

You may be able to feel the bony formations just above the coronary band the cartilages in a normal foot are pliable; if your horse has sidebone, these cartilages will have undergone ossification and will feel rigid.
Sidebone rarely causes lameness, however if some of the bony formations fracture, rub together or push against other sensitive structures in the foot, this can potentially cause pain and manifest as lameness.
If your horse is lame, care should be taken not to mistake sidebone as the cause of lameness where there may be other underlying causes; your vet will need to perform diagnostic analgesia (nerve/joint blocks), take radiographs and potentially use further imaging modalities such as gamma scintigraphy (bone scan) and magnetic resonance imaging (MRI) to accurately determine the source of pain before giving a definitive diagnosis.

As a general rule, sidebone doesn’t cause many cases of lameness unless the lateral cartilage fractures and becomes unstable. If your horse is showing signs of being lame he/she will require a full diagnostic¬†lameness work-up¬†in order to arrive at an appropriate diagnosis, determine the prognosis and provide a suitable treatment plan.
Where fractured sidebones are causing a problem, your horse will require a prolonged period of rest as with any fracture and possibly some non-steroidal anti-inflammatory drugs, e.g. phenylbutazone (bute). Repeated radiography can be used to assess the healing progress. Once a callous has formed bridging the gap between the bone fragments, the degree of lameness should improve. Where poor conformation/foot imbalance is causing an underlying problem, remedial farriery or trimming will gradually encourage better hoof growth and hoof balance and may help to improve your horses clinical appearance.
In extreme cases of foot pain a neurectomy (surgical removal) of the palmar digital nerves which supply sensory innervation to the foot may be performed to permanently alleviate any pain your horse may be experiencing. This procedure is only used as a final end-point in the treatment of foot pain, and will only be performed if all other methods of treatment have failed and euthanasia is the only remaining alternative option.
Many horses lead full and active lives with sidebone, however if it happens to be associated with lameness, then the prognosis is guarded due to the limited treatment options available.

Many horses develop sidebone regardless of the way they are managed/exercised or shod/trimmed. Avoiding excessive intense work on roads or hard ground is advisable as this can contribute to the premature formation of sidebone. You should also ensure that your horse receives regular hoof care to prevent imbalances and uneven weight-bearing which is responsible for many conditions of the foot.