Stifle lock or upward fixation of the patella is a commonly occurring condition that has been diagnosed and dealt with in the equine population for hundreds of years. Here, we take a more detailed look into the locking stifle.
Stifle lock is seen as an occasional occurrence in some horses and as a constant problem in others. Some breeds of ponies are particularly susceptible due to their conformation and other horses acquire it through injury. Over time there have been various treatments used, from changes in exercise, to medications and surgeries.
The stifle is the equivalent to the knee in humans and is composed of three individual joints. The medial and lateral (inside and outside) femorotibial joints are compartments formed by the articulation of the femur and the tibia. The femoropatellar joint is the connection between the femur and the patella (human kneecap). There are several ligaments associated with the stifle, the patellar ligaments — being the more commonly dealt with in the horse — the cruciate ligaments and the collateral ligaments. The stifle also contains a medial and lateral meniscus that are pieces of cartilage that sit between the femur and tibia and act to reduce the forces produced in any one spot during flexion and extension.
The patella glides up and down a groove in the femur as the stifle extends and flexes. When the horse flexes the limb, the patella moves down the groove and, as the horses straightens or extends the stifle, the patella moves up the groove. Attached to the patella are three ligaments called the medial, middle and lateral patellar ligaments. Medial refers to the inside ligament, lateral to the outside ligament, and the middle patella ligament is the large ligament at the front. The medial patellar ligament connects via a piece of fibrocartilage to the patella, whereas the middle and lateral patellar ligaments connect directly onto the patella.
When the horse extends the stifle joint to stand and fully weight-bear on the limb, the patella moves to its highest position on the groove. This allows the fibrocartilage attached to the medial patellar ligament to sit up over a ridge on the groove (trochlear tubercle). This effectively locks the stifle in this position, which helps the horse, together with the stay apparatus of the hindlimb, to rest using minimal energy and to sleep whilst remaining upright. Shifting the weight on the limb unlocks the stifle and allows the limb to flex freely.
Unfortunately, in some horses the locking mechanism fails to unlock, leaving the horse with a “locked stifle”.
There are many reasons the locking mechanism fails: some reasons are short term and require minimal input to rectify, others have an underlying conformation issue which is not able to be fixed easily (if at all) but will need to be managed. Horses with straight hind limb conformation (typically 140+ degrees) are particularly susceptible to the condition, as are young, immature horses with poor or weak muscling behind, and those with less than ideal foot conformation (long toe with uneven balance between the inside and the outside of the foot).
WHAT TO LOOK FOR
What do we see when a horse locks its stifle? The classic sight is readily identified with the leg locked in an extended position, typically out behind the horse with the toe dragging on the front surface of the foot. If the horse tries to walk forward and can’t unlock the stifle first, there will often be a drag mark on the ground surface or dirt on the front of the hoof wall. I have been called to euthanise horses that owners have seen trying to walk like this and they have assumed the leg was broken.
In many other cases, the leg is only locked momentarily and all that is observed is a brief jerking in the hind limb action, colloquially termed “catching stifle”. Instead of a fluent movement of the hind limb as the horse goes to flex the limb and move forward, it starts the cycle with a jerky movement as the locked stifle suddenly unlocks. It must be remembered that the stifle and hock always extend or flex in unison. This may happen for one or multiple steps as the horse starts off and then the catching resolves and the horse appears fine.
An x-ray of a right lateral stifle.
Other clinical signs can include stumbling during a workout, or in some athletic animals, skitching around the corners at high speeds (instead of a nice fluent action as they travel around the bend, the horse puts in short jerky steps until they straighten up). In horses that only show catching during their workout, it is more commonly seen as the horse decelerates (goes from a trot to a walk). These cases are harder to diagnose as most of these horses show no overt signs of lameness and so when are assessed at rest, they can appear normal. Palpation of the ligament can sometimes elicit a locking, but this is not a reliable test for diagnosing upward fixation. I find it more reliable to back a horse or circle them in tight circles to look for evidence of the patella fixating. This is one of the reasons when purchasing a new horse, it is important to circle and back them in order to look for subtle changes in the gait, which may indicate a problem. Most horses will not show evidence of lameness unless the condition has been going for an extended period and secondary stifle inflammation becomes a concern.
The horse will naturally unlock its stifle by shifting weight to the opposite hind limb and contracting the quadriceps muscle and rotating the leg outwards slightly, unhooking it from the tubercle and allowing the patella to slide back down the groove. In cases where the stifle is locked, and the horse is unable to free itself, backing the horse or using a rope to pull the leg forward can help.
VARIOUS TREATMENTS
Treatments are varied according to the reason the horse is locking its stifle. For those young, immature horses or horses that are poorly muscled behind, exercising is often all that is required to “cure” the condition. These horses often show signs of locking after they have been standing still for a while, such as when they first walk out of their stall or after being tied up. The aim of the exercise is to build/strengthen up the hindquarters of the horse, so exercising uphill, in deep sand or over small poles can work. Any underlying foot imbalance should be corrected, and if the horse has long toes, then shortening the toe to increase the breakover of the foot can also help. Sometimes the inside of the foot is rounded to encourage the foot to breakover slightly to the inside in order to reduce the incidence of locking.
Chiropractors, spinal manipulators and physiotherapists have also played a role in alleviating this condition in some cases.
“In some horses
the locking mechanism
fails to unlock.”
A recurrence of the locking can happen when these horses are out of work for an extended time, presumably due to the loss of muscle strength.
Chiropractors, spinal manipulators and physiotherapists have also played a role in alleviating this condition, potentially as a result of them improving the neurological or muscular function of the structures involved in locking and unlocking the stifle.
The use of an estrogen related compound (oestradiol benzoate) has been used successfully to help horses with catching stifles, although some debate its usefulness. I have found it quite successful in many cases where the catching is most likely related to a weak hind end and the use of oestradiol is combined with a good exercise program. It is not totally clear why the oestradiol works, but the two most likely thoughts are that it causes a relaxation in the patellar ligaments so they slip off the tubercle more easily, and the second that it improves the strength of the muscles involved in lifting the patellar up over the tubercle to release the lock.
If exercise and corrective shoeing/hoof care do not rectify the issue, the next step is to inject the patellar ligaments with an irritant to cause some inflammation and thickening of the ligaments to prevent them from becoming fixed. The two more commonly used products here are an iodine in oil solution or a chemical, sodium tetradecyl sulphate (commercially known as Fibrovein). This treatment can certainly help relieve this condition, however, care must be used when doing this procedure as inadvertent administration into the stifle joint, which is near the ligaments, can have disastrous consequences.
Strengthening the hindquarters of the horse through exercising uphill can help. Image by Elli Birch/Boots & Hooves Photography.
SURGICAL PROCEDURE
If medical solutions don’t resolve the locking patellar episodes, then surgery becomes the next option available. The most traditional form of surgery is what is referred to as a medial patella desmotomy or “cutting the inside patellar ligament”. This surgery has been around for many years and can often be done in the standing horse relatively simply and quickly. It was very popular several decades back but lost a lot of favour due to the increased incidence of horses fracturing their patella or succumbing to stifle arthritis following the procedure. Many of the horses that fractured their patella were horses that had their ligament cut and then continued in work. Since recognition of these complications, the practice has been to rest the horse for three months after the surgery to allow the patella to accommodate to its new position in the groove and this has reduced the number of complications recorded.
Increasing in popularity over recent years has been the practice of “splitting the ligament” (medial patellar ligament desmoplasty). Instead of cutting the medial patellar ligament, the ligament is split 8-15 times using either a small scalpel blade or a 14g needle, leaving the ligament in one piece but causing a lot of inflammation to occur. It is very popular with some veterinarians; however, some studies have shown that the long-term effect and overall satisfaction for owners is less than flattering. It does, however, have fewer complications than the traditional cutting surgery.
The caudal to dorsal view (back to front angle) of the right stifle.
“Surgery is reserved for those
cases that have not responded
to medical therapies.”
I have traditionally cut the medial patellar ligaments and found this to have given the best results for both the horse and the owner. I should add that surgery is reserved for those cases that have not responded to medical therapies. Radiographs of the stifles to eliminate any underlying bone pathology should be performed prior to surgical intervention. In most cases I cut both the left and right medial patellar ligaments as even though the problem may only be visible in one limb, it is often bilateral. This is a result of me having cut only one ligament in some horses, only to have the owners return months later to have the other side cut due to it catching. The exception to this would be for cases where a trauma has occurred and only one hind limb is showing signs of a marked locking or a failure to unlock, in which case I would just cut the one side.
This article was first published in Equestrian Life, July/August 2019. EQ
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