Bone chips in joints are a relatively common occurrence, and often cause lameness and joint swelling. The horse may resent flexion of an affected joint and may be lamer at trot following a flexion test. There are two main causes of bone chips — osteochondrosis (OCD) and trauma.

OCD is a developmental orthopaedic disease characterised by abnormal development of the joint surfaces. Both the cartilage and underlying bone layer (the subchondral bone) are affected, resulting in the breaking away of fragments or flakes.

Different joints are affected more commonly in certain breeds — warmbloods often have OCD chips in the fetlocks, hocks and stifles, for example. While OCD occurs with reasonable frequency in thoroughbreds and sport horses, it is rarely seen in pony breeds.

As OCD fragments are developmental, there is no association with exercise. But clinical signs are often seen when a horse first begins work, when the extra activity can cause chips to move.

Chips that occur as a result of exercise-induced trauma are most commonly seen in racehorses, typically in the fetlock and knee. These occur exclusively when horses are in exercise, although the amount of time in training before chips can be seen can vary substantially.

Traumatic bone chips can be caused by direct impact or following a “twist” of the joint, when a fragment of bone is pulled off within the attachment of a ligament (avulsion fracture). Patella (stifle) fractures and chips are commonly sustained hitting a fence, while fractures in the hock (the lateral malleolus of the tibia) most commonly occur following a fall.

A bone chip in the stifle joint presents as joint distension

Exercise-induced bone fragmentation is most commonly seen in racehorses. This X-ray shows a bone chip from the radius in the knee

The majority of problematic chips cause lameness and swelling of the affected joint, which can be identified upon examination before a definitive diagnosis is made with X-rays.

In cases in which joint swelling is less obvious, nerve blocks are required to determine from where the lameness is arising. Injection of local anaesthetic into the affected joint will make the horse sound, prompting X-rays to be taken.

In some instances, chips are first identified on survey X-rays taken prior to purchase.

A bone chip in the stifle joint presents as joint distension

A bone chip in the stifle joint presents as joint distension

Factors affecting removal

The decision regarding if and when a chip should be removed is not always straightforward.

If a horse presents lame with a swollen joint and is unable to work, then treatment is obviously necessary. Surgical removal almost always gives the best long-term outcome for the horse and is usually recommended.

There are exceptions, however. If the chip is small and still attached, an injection of anti-inflammatories into the joint can be an effective treatment, and one which allows a horse to return to work after only a couple of days. This may be an attractive option when a chip is discovered mid-competition season.

The drawback is that in some cases this can worsen the situation. If the joint is made comfortable and the horse is able to exercise normally, the chip may cause more permanent damage in the joint.

Arthroscopy reveals an OCD fragment in the hock. OCD chips  are developmental and occur most frequently in the hock, stifle or fetlock

Arthroscopy reveals an OCD fragment in the hock. OCD chips are developmental and occur most frequently in the hock, stifle or fetlock

Decisions regarding treatment become more difficult when chips are discovered on survey X-rays, in a horse without any lameness or joint swelling. In these instances, the chip is almost certainly not causing a problem — but this does not mean that it won’t do so in the future.

Generally, chips are best removed in young horses that haven’t yet started work, as the disruption to their work programme will be less than if left until a problem occurs.

Conversely, a chip discovered in an older horse may be better left alone. Provided that he has experienced no trouble in the past and has a good recent competition record (giving some assurances that the chip hasn’t been limiting performance), there is a good chance that the chip isn’t — and won’t — be problematic.

Not all chips are equal. The specific location, size and circumstances of the fragment will determine whether your vet recommends its removal.

When surgery is needed

: fragments can be the result of trauma, such as a direct impact from hitting a fence. The arthroscopic image, top, and X-ray below, show a stifle with fragmentation from the patella. There are further fragments visible on the X-ray free-floating in the joint from the femoral ridges

Fragments can be the result of trauma, such as a direct impact from hitting a fence. The arthroscopic image shows a stifle with fragmentation from the patella

Anti-inflammatory injections into the joint will treat the inflammation caused by the chip, but not the chip itself. While this may be all that is required in certain cases, removal by arthroscopic (keyhole) surgery is usually needed for a permanent cure.

The size and location of the chip determines how complex the surgery will be. Repair of large chips may be more effective by placement of a screw, a procedure also performed arthroscopically.

Because the incisions are small, arthroscopic surgery causes the horse minimal discomfort and recovery is much quicker than with open joint surgery. A typical case is in the hospital for only a couple of days.

Recovery time is usually dictated by the severity of the injury rather than the surgery. Horses can start exercising soon after being operated upon, with most beginning to walk within the first week or two and resuming ridden exercise after two months. Surgery typically costs £2,000-£3,000; the exact amount is dependent on the specific injury.

Although a routine procedure for experienced surgeons, chip removal is never without risk. Surgery is usually carried out under general anaesthesia, which involves a small risk of fatality. Other complications, such as infection, can also occur and there are costs and time off work to consider. For these reasons, surgery is not always the right option.

While a chip left in place may never cause a problem, those that have already caused lameness or joint swelling will almost invariably continue to do so.

: fragments can be the result of trauma, such as a direct impact from hitting a fence. The arthroscopic image, top, and X-ray below, show a stifle with fragmentation from the patella. There are further fragments visible on the X-ray free-floating in the  joint from the femoral ridges

Fragments can be the result of trauma, such as a direct impact from hitting a fence. This X-ray shows a stifle with fragmentation from the patella. There are further fragments visible on the X-ray free-floating in the joint from the femoral ridges

Freely floating chips can move around the joint, getting caught between the joint surfaces as the horse exercises and causing permanent damage to the cartilage. Inflammation and irritation caused by the chip creates an environment within the joint that leads to degeneration of the cartilage surfaces. The resulting arthritis can end a horse’s career.

Removal of large chips that comprise a significant part of the normal joint surface is often associated with a worse outlook. Chips in certain locations, such as at the base of the sesamoid in the fetlock joint, can also be problematic, because of the loss of attachment site for important ligaments.

The vast majority of horses, however, return to work after surgery without any untoward effect.

Ref: Horse & Hound; 14 January 2016