This is a fairly common problem of ponies and young horses. Stifle lock is referred to upward or proximal patellar fixation. In a moderate to severe form the limb becomes ‘locked’ in extension causing the limb to drag behind in extension, hopping along on 3 legs until it is suddenly ‘released’ in a rapid, jerky motion. This is because the failure to flex the stifle will also mean that hock cannot flex due to the reciprocal apparatus. This exaggerated releasing movement can sometimes be confused with stringhalt.
Anatomy review
In the normal horses the stifle joint is passively fixed in extension to allow the horse to stand without fatiguing. This fixation is controlled by action of the quadriceps femoris muscle group. Muscle contraction causes the stifle joint to extend which involves the patella (knee cap) moving proximally over the medial ridge of the trochlea, where it is anchored in place by the middle and medial ligaments of the patella and the medial parapatellar fibrocartilage. The medial ridge of the trochlea is much larger than the lateral ridge. There are three patellar ligaments, the medial, intermediate and lateral ligaments, but it is the medial ligament and the medial parapatellar fibrocartilage that play the most important role in locking the stifle. The patella is then dislodged by contraction once again of the quadriceps femoris and ‘turning’ by the action of the biceps femoris muscle.
Failure to dislodge the patellar will result in fixed extension of the stifle and hock.
Predisposing factors
There are several factors that can predispose a horse to this condition. Based on anatomy there is a lot of focus on the quadriceps muscle group. Horses that have poor quadricep muscle tone, through a lack of training, conditioning or being confined to a stall, are predisposed to upward fixation of the patella.
Another focus is the medial patellar ligament itself. There may be circumstances where the ligament becomes stretched and looses tone, long enough to reach over the medial trochlear ridge. This could occur with a sudden traumatic hyperextension of the hindlimb. It is also plausible that persistent upward fixation could further stretch the medial ligament.
There are conformational issues that may predispose to stifle lock. Horses with straight pelvic limbs are thought to be at risk. High medial hoof walls and elongated toes leads to hyperextension and outward limb rotation, predisposing to upward fixation of the patella.
There is a genetic component with some small breeds predisposed to upward fixation, particularly Shetlands and Shetland cross ponies.
Signs
The condition can be present in both hind limbs, and the signs can be quite variable in both severity and frequency. The condition can be mild and intermittent with a small delay in stifle flexion, resulting in a jerky gait. In more severe cases the horses can be severely debilitated.
The condition may become apparent when the horse is circled or walked down a slope.
Treatment
To unlock a persistently locked patella, push medially and slightly upwards with one hand on the lateral aspect of the patella while pulling on the horse’s tail. Alternatively, have the horse walk backwards while you attempt to dislodge the patella.
In the mild form of the disease the best first line approach is to improve quadriceps tone by training and conditioning. Some people have recommended intramuscular oestradiol, although this seems to have lost favour. Internal blistering using a sclerosing agent has also been advocated by some, injected over the medial and middle patellar ligaments, taking great care not to enter the joint (especially the medial femorotibial joint). Examples include 2% iodine in peanut oil.
If conservative treatment is unsuccessful, splitting the medial patellar ligament (desmoplasty) may be used. This can be done standing and involves making a series of stab incisions into the ligament over approximately half its length. The theory is the ligament thickens through fibrous healing and is and less likely to entrap.
If the condition remains than cutting the medial patellar ligament (desmotomy) is viewed as a last resort. The condition has lost favour, because it is associated with an increased risk of patellar fracture (fragmentation of the apex of the patella), although in some cases does not result in lameness.
Tags: Musculoskeletal system