Cysts: diagnosing these defects in horses’ bones *H&H VIP*

Dramatic but intermittent lameness was the first clue that something was amiss with Sarah Clarke’s young warmblood horse, Hiawatha La Perle (Hamlet).

When the four-year-old gelding was presented to us at the University of Liverpool equine hospital for investigation, there were very few clinical findings. However, a bone scan revealed a significant uptake of radioactive material — a “hot spot” — in his left elbow, and a subsequent joint block rendered him sound.

Further X-rays confirmed the presence of a defect in the bone, known as a cyst, in the elbow joint.

Most bone cysts are thought to arise due to osteochondrosis, which is part of a group of developmental conditions usually seen in younger, growing animals. Osteochondrosis develops from an interruption of the normal growth pattern involving the cartilage at the end of the bones.

Cartilage in these regions acts as a template upon which bone is formed. Retention of this cartilage, or a  lack of the mineralisation necessary to turn it into bone, can create loosely attached “flaps” — a condition known as osteochondrosis dessicans (OCD). Alternatively, a flap can fold inwards and is retained within the bone as a cyst.

The failure of cartilage and bone to develop properly can occur for a number of reasons, including genetics, periods of rapid growth, dietary imbalances and trauma. The higher incidence of cysts in certain types of horse, such as large, fast-growing animals, and on some stud farms, may be due to genetics or nutrition.

Bone cysts diagnosed in older horses have possibly remained dormant or are related to a traumatic incident of the joint. It is thought that damage to the cartilage surface leads to a cystic cavity in the bone.

The prognosis is generally poor, because secondary arthritis frequently ensues.

Hole in one

Bone cysts typically develop in the fetlock, coffin joint, pastern joint, shoulder and elbow, although they are most commonly seen in the stifle. Diagnosis begins with examination of the affected joint or limb. The bony structures are usually evaluated with X-rays, which may show an area of bone loss — typically circular or oval-shaped — close to or involving the joint surface.

Not all cysts cause lameness and they can occasionally be incidental findings. Proving their significance is important for correct diagnosis and management, so your vet may advise a joint block or an alternative imaging technique, such as a bone scan, to see if a cyst is active and contributing to the lameness.

A cyst may also cause episodes of intermittent and often severe lameness, with the horse appearing relatively normal in-between. This may be due to changes in pressure caused by joint fluid entering and/or exiting the cyst, leading to pain. Additionally, the cyst lining is a source of inflammatory substances which, once released, can lead to acute flare-ups.

Management of a horse with a bone cyst may be conservative, involving rest and anti-inflammatories, or surgical. The most important considerations are usually the location of the cyst and the access to it, especially if surgery is planned. Other factors include the horse’s age and the presence of concurrent joint disease, such as arthritis. Horses older than three years with a stifle cyst have been shown to have a poorer prognosis.

For treatment of stifle cysts, where we have most information relating to outcome, direct injection with steroids results in around 75% of horses returning to soundness. Injection can be via ultrasound or arthroscopy (keyhole surgery). The advantage of surgical injection is that the rest of the joint can be evaluated at the same time, which may provide valuable prognostic information such as the overall quality of the cartilage or the presence of other injuries.

Gaining access

Another surgical technique is curettage (removal) of the cyst lining, via arthroscopy or from outside the joint. The method depends upon access and visualisation: certain cysts, such as those in the elbow, are not easily accessible.

Since curetting alone can lead to the cyst becoming larger, the cavity is usually packed with bone graft or similar material. The graft contains a “scaffold” which stimulates bone cells to grow on its surface, which may lead to resolution of the cyst.

This does not always happen: around 60% of horses with a stifle cyst return to use following curettage, falling to around 35% if the horse is older than three. Less information is available about other joints and the outcome is considered to be less favourable.

A newer technique involves placing a screw across or close to the cyst. This may be restorative, either by altering the forces across the defect or allowing an influx of cells to promote healing. Although promising results have been published — around 75% of horses with a stifle cyst returned to use — this technique is relatively new and requires a degree of expertise to perform.

Ref Horse & Hound; 21 February 2019

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