When a horse is standing normally, its stifle is locked into a weight-bearing capacity. When this locking occurs, the medial patella ligament fastens over the end of the femur (medial trochlear). When the horse goes to move, this patella ligament must unfasten, allowing the patella to ‘unlock’ and the stifle and hock to flex. “Stifle lock”, or “upwards fixation of the patella”, occurs when the ligament becomes stuck.
Cause of Stifle Lock
Whilst often the direct cause for stifle lock is not fully understood, many feel it is a hereditary condition. Factors which influence its likelihood include muscular condition (particularly quadriceps), conformation, lack of fitness and immaturity. Injury may also lead to a breakdown of the unlocking mechanism of this joint.
Symptoms of Stifle Lock
Stifle lock in its more severe form may present as the horse’s hind leg extending out and backwards rigidly, with its fetlock resting on the ground. Horses may also present with forms of the condition and not have the obvious extension. In cases the horse’s stifle locks intermittently when the horse is moving. The hind leg will appear to collapse briefly, the stifle often unlocking itself with a loud click. In its most mild form stifle lock can be quite difficult to diagnose.
Stifle lock can result in secondary inflammation, damage to the ligaments and osteoarthritis in the stifle joint.
Diagnosis of Stifle Lock
In order to diagnose stifle lock the horse’s gait must be examined by a veterinarian. After ascertaining that a locking ligament problem is presenting, x-rays may be recommended to rule out other causes, and also to check for secondary wear and damage to the joint.
Treatment of Stifle Lock
Lack of fitness results in decreased thigh muscle and patellar ligament tone. Muscle strength training to improve the quadriceps muscle can result in improvement of milder cases. Suitable exercise regimes will vary depending on the horse and the severity of the condition. In subtle cases of stifle lock where conformation is relatively good, increased exercise alone may result in resolution of the problem.
Properly balancing the breakover of the foot can help to optimise the biomechanics of the hindlimb and has been shown to be beneficial, particularly in milder cases.
Oestrone sulfate injections
Intramuscular injections of a substance known as oestrone sulfate is sometimes used in horses suffering from stifle lock. Oestrone sulfate is believed to have the beneficial effect of altering the tension of the patella ligaments and therefore improving the biomechanics of the apparatus. Little scientific study has been done on the use of oestrone sulfate in cases of stifle lock, however, anecdotally, it has been shown to be beneficial for some horses exhibiting intermittent stifle lock. As oestrone sulfate is a hormone, when used in mares it can cause them to show signs of oestrus (heat), which can be undesirable. Horses are usually prescribed one to two weekly intramuscular injections for up to 4 weeks. Horses which respond well may then require injections on an as need basis.
Directly injecting the patella ligament/s with counterirritants
Injecting the medial patella ligament ± the middle patella ligament with mildly irritant substances (e.g. iodine 2% in an oil base) may be done with the aim to achieve a localised inflammatory reaction. Causing inflammation of the ligament ultimately has the effect of fibrosis or scarring, which in turn causes the ligament to become shorter, thicker and less elastic. This can reduce the likelihood of stifle lock occurring and has been shown to have good results in horses suffering from mild or intermittent stifle lock.
Surgery is usually reserved for very severe cases or those that have not responded well to more conservative and non-invasive treatment options.
Medial patella ligament desmotomy
The traditional surgical treatment option is a medial patellar ligament desmotomy. The purpose of this surgery is to completely cut the medial patellar ligament so it cannot catch or lock over the medial trochlear ridge of the femur. Complications reported from this technique include fragmentation of the distal patella and osteoarthritis of the stifle joint. Consequently, this technique is usually considered a last resort in riding horses. Nevertheless, this approach has excellent success rates, even in severely affected horses.
Medial patella ligament splitting
This technique involves “splitting” the medial patella ligament. This has a similar effect as counterirritant injections in that it results in localised inflammation and ultimately scarring and thickening of the ligament. It has been shown to have excellent success rates without the potential complications of a medial patella ligament desmotomy.
Most cases of stifle lock can have a favourable outcome, through either a holistic approach with conservative therapies, or via surgery.