What can go wrong?
Upward fixation of the patella — ‘locking stifle’
Although the patella locking mechanism is a helpful adaptation in the horse, sometimes it can go wrong, and this results in the patella being stuck in the locked position. This occurs when the hindlimb is extended back further than normal and the limb becomes stuck in extension. It’s more likely to occur in horses with particularly straight hindlimbs and especially in young horses and ponies with poor muscle condition. In these cases, the weak thigh muscle fails to pull and release the patella.
If there has been trauma to the stifle region, in particular to the patella or its ligaments, it might become more difficult for the patella to be moved around and it may therefore remain in the locked position. Often a locking stifle becomes more obvious when the horse is stabled, and he will be seen holding and extending the limb back while he flexes his fetlock.
If this problem is identified, the horse can be given an exercise programme to follow, with the aim of building up his thigh muscles. Exercises may include daily lungeing, or walking up hills, and as much turnout as possible. A conditioning and exercise programme is usually most effective in young horses. In other instances surgery may be required. The prognosis for this condition is good, however, as most cases respond well to management.
Luxation of the patella
Usually the trochlear groove is deep enough for the patella to stays within it, but if it’s too shallow the patella will slide sideways out of the groove. This condition tends to be seen in miniature breeds or in foals and is thought to be an inherited condition. An affected horse can be seen standing in a crouched position, unable to extend his stifle. He may also have a stiff gait. Surgery is required to treat the problem to ensure that the patella stays within the groove. Because the condition is hereditary, breeding from affected animals is not recommended.
Fractured patella
A fractured patella can be caused by a kick to the stif le area, the leg hitting a jump, or getting stuck while trying to climb over a gate. Generally the horse will show signs of lameness, with heat, pain and swelling around the area. There may also be a skin wound indicating where the injury occurred. X-rays of the area will be taken to see the extent of the injury. In some cases, box rest and pain relief will prompt healing, but some horses will require surgery to remove fragments of bone, or to repair large fractures.
Osteoarthritis
Osteoarthritis can occur following one of the conditions mentioned above, but it can also be seen after trauma or injury to the joint.
The horse is likely to be lame and there may be swelling. X-rays will be taken to assess the area, checking for fractures or bone fragments. Steroid injections into the joint around the patella can be given and, as part of the recovery, the horse will need to follow a controlled exercise programme. If he doesn’t have severe osteoarthritis, it can often be managed with anti-inflammatory medication or joint injections, but if the disease is advanced it is probably unlikely that he will return to full work. Other causes
There are many other injuries that can affect the stif le joint, but the key point is to identify that the horse isn’t moving correctly and ask your vet to examine the affected leg. They will perform diagnostic tests, such as X-rays, to assess the patella and the stif le joint.
Once the problem has been diagnosed, a plan can be put in place to manage or treat the condition.