IN A FIX: Causes and Treatment of Upward Fixation of the Patella
If you’ve ever seen a horse with a severe case of upward fixation of the patella (UFP) or “locking stifle”, you know it’s a condition that is impossible to miss. The horse is periodically or permanently unable to flex the affected hind leg(s), which will then stick out and drag behind the horse in an obviously abnormal manner. However, in horses with the far more common but less severe forms of this condition, symptoms can be so subtle that they are often misdiagnosed or overlooked altogether. Partial or intermittent upward fixation of the patella, sometimes called “catching stifle”, can manifest as a mild to moderate hind limb lameness that may mysteriously come and go. Fortunately, both severe and mild cases of UFP usually respond well to treatment, and new developments are making it easier to treat even the most extreme cases.
When in a standing position, all horses have the ability to temporarily “lock” or fixate their stifle joints – which correspond to the human knee. This is a normal occurrence that enables horses to stand and even sleep standing up with minimal muscular exertion. The locking takes place when the patella (knee cap) slides into the upward position, and the medial patellar ligament (MPL) shifts slightly to the inside and hooks over a notch in the medial femoral trochlea, a knob-like structure located at the bottom end of the femur. The patella is then fixed in its upward position, keeping the limb in extension.
Fixation only becomes a problem when the locking mechanism fails to release properly when the limb is called upon to flex. In the most extreme cases of UFP, the horse is completely unable to flex the affected leg without assistance. Most horses react to this locking with surprising calm, but some panic and may injure themselves or others in their frantic attempts to unlock the limb. In the mildest cases of UFP, there is only a brief delay in flexion, without any actual locking of the limb. The delayed flexion is thought to cause the horse some degree of discomfort, but it is often much harder to notice. Horses with UFP may also fall anywhere in between mild and severe, with some locking at times but able to unlock the limb on their own.
Severe or permanent UFP can occur in any breed, but it is most often seen in small equids such as Shetland ponies, miniature horses and miniature donkeys. Partial or intermittent UFP occurs quite commonly in virtually all breeds. Among the standard sized breeds, there appears to be a higher than average incidence in Standardbreds and American Saddlebreds, which may be related to the types of training and activities these horses are typically engaged in. Regardless of breed, UFP is usually present in both stifles, though symptoms are generally seen only on one side. A thorough examination most often shows that the other side is strongly predisposed to develop the problem, and it is not unusual for the “good” side to become symptomatic after treatment of the initially affected side.
The causes of UFP are varied and not entirely understood, but there are a number of recognized factors that can contribute to the problem. The most common of these is a lack of tone or condition in the muscles and ligaments of the stifle joint. One generally accepted theory is that a lack of tone in the quadriceps and/or biceps femoris leaves these muscles unable to release the patella as quickly as necessary. Another theory is that a lack of fitness creates increased laxity in the patellar ligaments, which makes it easier for the MPL to hook over the groove in the medial femoral trochlea. Of course, there are many unfit horses who are never troubled with UFP, and it is worth noting that the condition can occur even in well-conditioned horses. Nonetheless, lack of fitness is viewed as the most common predisposing factor.
Most researchers and veterinarians also believe that “post-legged” horses – those with straighter than average stifles and hocks – are more likely to develop UFP, though this factor is difficult to quantify. Proponents of this theory suggest that straight or upright hind limb conformation brings the femoral trochlea into closer proximity with the patella, so that less extension is needed for upward fixation to occur. Experts generally agree that this is a reasonable proposition, but it is well established that UFP can occur in horses regardless of hind limb conformation. Rarely, trauma to the stifle or pelvis can bring on UFP, while another possible cause may be poor coordination between the flexor and extensor muscles of the stifle.
What is knownfor certain is that UFP most often appears in young horses, particularly those in early, intense training. “We see it quite often in young horses being trained for racing, including both Thoroughbreds and Standardbreds, or Western work such as cutting and reining,” says equine surgeon and lameness expert, Dr. Dennis Meagher, DVM, PhD, Dipl. ACVS, Professor Emeritus at UC Davis.. “Part of it is the fact that young horses haven’t had time to develop a high level of conditioning, yet in some training programs they’re asked to do a lot of fast work or physically demanding maneuvers. This can put them into a position where they can start to catch the patellas.” Dr. Meagher notes that UFP is not seen as commonly in hunters, jumpers and dressage horses, as these horses generally tend to be started at an older age and brought along more slowly.
The condition is not limited to young horses, however, and even fit, fully grown horses can become susceptible to UFP, particularly if they are abruptly taken out of training and put into confining circumstances such as stall rest, extended quarantine or hospitalization. Under such conditions, horses can quickly lose muscle condition, which makes them more likely to develop UFP.
SYMPTOMS AND DIAGNOSIS
As previously stated, severe cases of UFP are generally easy to recognize, but when a horse is affected by mild UFP, it can manifest very subtly and in a number of ways. Some symptoms are:
- The horse will very commonly take shorter strides with one or both hind legs, and may carry its toes close to the ground as it attempts to avoid flexing its stifles any more than necessary. If the horse actually starts to drag its toes, as some do, there may be noticeable wear on the front surface of the toe. The horse may also swing its hind legs to the outside in an attempt to avoid flexing the stifles.
- The horse may have difficulty pushing off into the canter and may resist cantering, especially on the lead that puts the more affected leg to the inside. Some horses will change leads frequently or cross canter. The canter may also feel very rough or bouncy due to the delay in hind leg flexion.
- The horse may become resistant when asked to circle, as circling demands greater flexion of the stifles and increases the likelihood of them “catching”.
- Riders may feel that the horse is hopping or pulling itself into the trot, rather than pushing from the hind end.
- Soreness may be apparent in the stifles, and rarely, there may be swelling, particularly in long-standing cases.
- There is often a palpable or audible click when the catching patella is released.
- The horse may find it difficult to back up or walk up and down hills.
- Stall resting actually causes symptoms to worsen.
- The affected limb may have a slight jerk when the catching patella releases.
Unfortunately, many of these symptoms can also be associated with a vast array of other hind end lamenesses, making it easy to mistake the condition for some other problem.
Equally unfortunate is when the manifestations of UFP are missed altogether, leading many people to believe that the horse’s resistance to perform certain activities is a training problem. As Dr. Meagher explains, “These horses may begin to refuse to do what’s being asked of them, and sometimes the assumption is that the horse is not cooperating . But the fact is that the horse feels discomfort when the patella catches, so they’re simply getting smart and not wanting to do the things that cause them discomfort.” Therefore, if your normally willing horse starts resisting your training efforts, it is worth taking a close look at how the horse is moving, and what you are asking when you notice resistance. Be aware that a horse can have UFP and you may still be unable to discern anything overtly wrong, as even veterinarians can have difficulty spotting this problem.
For this reason, diagnosis of partial or intermittent UFP is usually a hands-on procedure. “The most common way UFP is identified is by manually pushing the patella up into the fixed position and then having the horse take a step forward,” says Dr. Meagher. “You can lock most horses if they’re tractable to handling, and if the horse is normal, the patella will unlock once the horse takes a step forward. They simply flex their stifle and ignore you. But if they are having problems with it catching intermittently, you can usually make it catch manually, and they will resent it. They’ll very often make a sudden movement or jump away when it pops out because it’s uncomfortable. In some cases, it is also possible to feel sort of a clicking or catching if you keep your hand on the area as the horse is walked forward.”
Other methods of diagnosis include trotting the horse slowly, backing, turning in very tight circles, walking with repeated stops and starts, or walking down a slope, all of which can provoke locking of the patella in an affected animal. The toe of the limb in question should also be examined to see if there is excessive wear caused by dragging of the foot. If UFP is suspected and financial considerations allow, it is best to have the stifle examined by x-ray and ultrasound, because any concurrent stifle problems such as osteochondrosis (OCD) or arthritis may influence treatment choices and prognosis.
Once you have a confirmed diagnosis of UFP, there are various treatment options ranging from simple exercise to surgery. In most cases, especially those that are not severe, the initial prescription is usually to try to improve the horse’s overall condition and coordination, and specifically, to tighten the muscles and ligaments in the stifle joints through exercise. Certain exercises are beneficial for this purpose, while others should be avoided. Beneficial exercises include:
-working in deep, soft footing
-trot poles & cavaletti
-turn out in a hilly pasture
The main goal of these exercises is to encourage the horse to lift and reach with its hind feet, flex through the hocks and stifles, and push off properly with its hind end. Exercises to avoid are anything involving turns and circles, particularly tight ones. Longing and round pen work are therefore best avoided. The ideal exercise for a horse with UFP would be long trots up and down rolling hills on a path without sharp turns. If you aren’t lucky enough to have access to those perfect hills or are limited to an arena, try setting up trot poles or cavaletti on whatever straightaways you do have, and slow to a walk when you have to go around a tight corner.
If exercise alone doesn’t solve the problem, there are other conservative forms of treatment worth trying before considering surgery. The most common involves the injection of a counterirritant, usually an iodine in oil solution, into or around the patellar ligaments. The resulting desmitis (inflammation of the ligament) causes the ligaments to thicken and tighten slightly, which is often enough to prevent the patella from locking inappropriately. According to Dr. Meagher, “This treatment is very effective for horses that catch occasionally, or horses that catch for a split instant, but don’t lock completely.” He advocates that this treatment should be done in a series of very small injections of the counterirritant up and down the full length of both the medial and the middle patellar ligaments.
However, some veterinarians do not feel comfortable injecting iodine directly into the ligaments – especially the proximal (upper) third, which has the greatest effect on the condition. The danger is that an incorrectly placed injection could cause severe damage by entering the joint, as the proximal part of the MPL is close to the synovial membrane. To avoid this possibility, some veterinarians only inject the counterirritant around the ligaments and/or into the distal (lower) third of the MPL, where there is less risk of damage. Good results have been reported using this method of counterirritant therapy, but it may be less reliable than the method described by Dr. Meagher. In most cases, counterirritant injection therapy is performed in conjunction with putting the horse on an exercise program, as conditioning is such a key factor in managing UFP.
A less conventional but promising treatment for UFP is acupuncture, which can be performed with traditional acupuncture needles, or by the injection of vitamin B-12 into the appropriate acupuncture points. Canadian veterinarian Michael Perron, B.Sc. (Agric.), DVM, CVA of Surrey, British Columbia, has been performing acupuncture on horses with UFP for over ten years, and reports excellent results. “Intermittent or partial fixation of the patella is fairly common in my practice, a large part of which is young Standardbreds and Warmbloods. I should emphasize that my primary treatment for UFP is a proper conditioning program, but if that alone doesn’t resolve the problem, I find that acupuncture generally does. I’ve used acupuncture to treat UFP on probably 100 horses or so, and since I’ve been using this method, I have never had to resort to a surgical procedure for UFP.” According to Dr. Perron, most of the cases he sees are in the mild to moderate range of UFP, but he has had a few that were quite severe, and he says that even these were resolved with a combination of acupuncture and exercise.
When performing acupuncture, Dr. Perron most often uses injections of B-12 into acupuncture points around the stifle, rather than traditional needles, although he does use both, depending on the individual presentation and the temperament of the horse. “Some horses won’t hold still long enough to get the full benefit from traditional needles, and the B-12 injections continue to stimulate the acupuncture points for a longer period. The B-12 also creates a mild inflammatory response which is desirable in the treatment of UFP.”
Dr. Perron explains that from the perspective of traditional Chinese medicine, disease is viewed as an imbalance in the body’s energy or “chi”, and acupuncture is designed to rebalance the chi. “Of course I have a more Western perspective,” he adds, “and in my view, what we are doing with acupuncture is signaling the body to re-establish normal physiology and function.” He also believes that acupuncture may work by mitigating sensations of pain, thereby helping a horse to feel and move better through the prescribed conditioning period. “There has been quite of lot of scientific analysis done on acupuncture in general, and it has been proven to have both neurological and physiological effects. However,” he says, “I do not pretend to understand entirely how it works, and I’m not sure anybody really knows.”
Those in the scientific community are quick to point out that there has been very little research done on acupuncture in horses, and that there is no scientific data to support the efficacy of acupuncture for the treatment of UFP in horses. Dr. Perron understands why such skepticism remains regarding the effectiveness of acupuncture, as he used to share that view. However, he now says, “I’ve been using acupuncture for so long with such good results that it has become an everyday part of my practice. I guess I tend to forget that it’s considered unusual by some people.” He would like to see formal studies conducted to verify the benefits of acupuncture for the treatment of UFP, but for now, he has satisfied himself that the procedure works.
Perron speculates that other veterinarians may be inadvertently performing acupuncture when they treat UFP by injecting counterirritants around (but not into) the patellar ligaments. “We see similar results with both treatments,” he says, “but I couldn’t tell you whether their treatment works because they’re unintentionally stimulating acupuncture points, or whether the acupuncture works solely for its counterirritant effects. This would be an interesting area for study.”
Whatever non-surgical methods are tried, horse owners should also be aware of the surgical options that can almost always resolve UFP if conservative efforts fail. For many years, the standard surgical procedure for the treatment of UFP has been medial patellar ligament desmotomy (MPLD), which involves severing the medial patellar ligament so that upward patellar fixation is no longer possible. In current practice, the recovery period for a horse that has had MPLD is generally three to five months, during which time many veterinarians feel that the horse should not be worked.
Short-term complications of MPLD can include swelling, pain and premature opening of the sutured wound, and it is therefore recommended that non-steroidal anti-inflammatory drugs be administered. As for any long-term complications, that remains a subject of much debate. While MPLD remains the procedure of choice for many veterinarians, it fell out of favor with many others when world renowned equine surgeon and researcher, Dr. C. Wayne McIlwraith, BVSc (DVM), PhD, FRCVS, DSc, Dipl. ACVS, of Colorado State University, published a paper which concluded that MPLD caused instability in the stifle that led to disease and fragmentation. Frequently quoted and widely interpreted as a condemnation of the procedure, McIlwraith’s 1989 paper continues to influence many veterinarians who have shied away from performing MPLD.
McIlwraith himself, however, points out that the study he conducted in 1989 used normal horses – horses without UFP – as test subjects, and that oddly enough, the conclusions of the study do not seem to transfer to horses that actually have the condition. “Initially, what happened was that I had fifteen cases of patellar fragmentation, and twelve of them had had patellar desmotomies. It therefore seemed like we were getting the fragmentation in association with MPL desmotomies, which was why some people had said they never liked the procedure. In the old days, a lot of horses got their medial patellar ligaments cut when they didn’t need it, for example just about every American Saddlebred got it done routinely, and there were quite a few cases in my initial group that were Saddlebreds.
“So, we then did a study where we cut the medial patellar ligament on 12 normal horses and we looked at them once a month for three months, then we radiographed them and arthroscoped them and they all had some degree of disease on the distal aspect of the patella. Eight out of twelve horses had a fragment that showed on an x-ray, and all twelve showed fragments with arthroscopy. Long term, we went in and took the fragments out, and with that subsequent surgery, the horses were ultimately fine.”
McIlwraith then explains that while his study proved that MPLD leads to fractures in normal horses, other studies have concluded that this complication is much less common in horses that have confirmed cases of UFP. “There’s a sort of growing consensus to that effect,” says McIlwraith. “It’s an observation that was initially made by Dr. Joe Foerner in Chicago a long time ago, and it has since been repeated by Andy Bathe, formerly of Cambridge University.”
The study conducted by Dr. Andrew Bathe, MA, Vet MB, Dipl. ECVS, DEO, MRCVS, was a retrospective analysis of 49 horses with confirmed UFP that had undergone desmotomy. The study concluded that MPLD has an extremely high success rate, as 98% of the test subjects showed no evidence of UFP after surgery, and 94% achieved their intended athletic use. However, the study does state that 18% of the horses suffered some kind of post-surgical complication, including fragmentation of the patella, undesirable bone formation and synovitis, and three of the horses never returned to full athletic function.
As for why the horses with UFP show a markedly lower rate of patellar fragmentation than normal horses when desmotomy is performed, Dr. Bathe suggests that this could be due to the straighter stifle conformation frequently observed in horses with UFP, but Dr. McIlwraith says, “We really don’t know why this happens.” He also believes that the current general practice of resting a horse for three to five months after MPLD may prevent much of the damage they used to see after performing the procedure, as it was formerly thought that a horse had to be put back into work immediately to prevent the ligament from growing back together.
Dr. Meagher, however, feels that the lower rate of complications seen today can be greatly attributed to the fact that MPLD is now rarely performed on horses who are not true candidates for the procedure, and he does not believe that such a long rest period is necessary. Says Meagher, “My post operative approach was 3-4 days of hand walking followed by light jogging under saddle for 10-14 days, and then return to normal conditioning program. I saw no increase in complications with that regime. The bottom line is that although MPLD has fallen in and out of favor at various times, the facts show that it has been very successful in treating the most severe cases of this condition for decades, and it continues to be a perfectly valid procedure.”
THE FRENCH REVOLUTION
MPLD has undoubtedly helped many horses over the years, yet virtually all veterinarians agree that it should only be considered if conservative efforts have failed, and some remain of the opinion that MPLD has a higher rate of complications and a longer recovery period than is desirable. One such veterinarian is equine surgeon and researcher Dr. Aziz M. Tnibar, DMV, PhD, Dipl. ECVS, of Paris, France. Having long been dissatisfied with MPLD, Dr. Tnibar developed an alternative surgical technique that seems to be quietly revolutionizing the treatment of UFP. Called medial patellar ligament splitting, Tnibar’s procedure not only has an excellent success rate in resolving UFP, but it is also minimally invasive, does not destabilize the patella, requires no drugs for aftercare and allows the horse to be returned to work immediately.
“The rationale for MPL splitting is to induce a localized desmitis on the proximal third of the ligament without transecting it,” states Tnibar. “This subsequently leads to a localized thickening of the ligament, which both prevents the ligament from hooking too easily over the notch in the medial ridge of the femoral trochlea, and permits the MPL to be more easily disengaged from the ridge, thus resolving UFP.” The rationale is very similar to the one behind counterirritant therapy, but Dr. Tnibar believes splitting to be a superior procedure for several reasons. As he explains, “The injection of counterirritants is associated with irregular results, and even, though very rarely, with disastrous damage to the joint if iodine is inadvertently injected into the joint capsule. Another reason is that injections are generally performed in the distal third of the MPL, whereas the ligament hooks at its proximal third, where the splitting procedure is performed.”
A further advantage of splitting is that, unlike desmotomy, it can be used regardless of whether or not there are other problems with the stifle. According to Dr. Tnibar, “Desmotomy is strictly counterindicated in cases where there is femoropatellar or femorotibial joint damage, as the induced joint instability from desmotomy can worsen these conditions. Splitting is not counterindicated in such cases.”
Though it takes time for a new procedure to trickle down into the everyday practice of veterinary medicine, Dr. Tnibar’s splitting technique is gaining in popularity as veterinarians discover its various advantages. “We’ve tried Dr. Tnibar’s procedure,” says Dr. McIlwraith, “and I know a number of other people who are using it. I think it’s being practiced quite a bit, and overall, it’s a very successful procedure.” Asked whether he prefers the new procedure to desmotomy, McIlwraith says, “I believe Dr. Tnibar’s technique is better. You’re not cutting the ligament so you’re not creating any instability. I think it is preferable to try splitting first.”
Another supporter of the procedure is equine surgeon Dr. Alberto Serena, DVM, MRCVS, of the College of Veterinary Medicine at Auburn University. He has been using Tnibar’s technique since early 2003, and he is convinced that it is superior to MPLD. Says Serena, “Besides the fact that it is relatively less invasive – stab incisions versus 2-3cm long skin incision and desmotomy – I think it is preferable because it doesn’t seem to change the mechanics of the stifle joint in the early post-operative period, and the horse doesn’t need to be rested at all or receive any anti-inflammatory drugs such as phenylbutazone (bute) because the inflammation caused by the procedure is desirable in order to thicken the medial patellar ligament.”
The only objections to the procedure seem to be that it was designed to be performed under general anesthesia, with the guidance of ultrasound technology. These aspects add to the cost of the procedure, making MPL splitting more expensive than desmotomy. In addition, general anesthesia always carries some degree of risk, particularly for aged or debilitated animals.
However, as veterinary surgeons performing MPL splitting are becoming more familiar with the technique, some are modifying the procedure in ways that eliminate these drawbacks. “The procedure itself is technically not difficult at all,” says Dr. Serena. “After the first few cases, I now perform the procedure without ultrasound guidance, since I think that the medial patellar ligament’s margins can be felt quite well by palpation alone, and penetration of the femoropatellar joint is easily avoidable. As well, during a discussion at a national meeting, one veterinarian reported that he performed the splitting procedure with the horse standing, sedated, and with the aid of local anesthesia.”
Asked how he feels about his procedure being modified in these ways, Dr. Tnibar says, “What really matters is to perform an accurate splitting to create good, localized desmitis without damaging the underlying structures. I have also performed some surgeries without ultrasound guidance, and I think that once a surgeon is familiar with this easy technique, he or she can choose to do that, as well. In regards to dispensing with general anesthesia, I have not personally performed the surgery with the horse standing, but I am planning to try it in the future. Probably not on a Shetland pony, however!” Indeed, the positional awkwardness of trying to perform surgery on the stifle of a standing equid (especially a small one) may mean that some vets will always prefer the use of general anesthesia for MPL splitting, but even so, many owners will likely be willing to spend a bit more if it means fewer complications and no long, post-surgical lay up period for their horses.
As for Dr. Tnibar, he is pleased to know that his technique is beginning to catch on in North America, but he would be equally pleased to see something even better come along. He is particularly interested to find out more about the use of acupuncture in the treatment of UFP. “I am not familiar with this use of acupuncture being performed by Dr. Perron,” he says, “but if it works very well, maybe we will have no need of my surgical procedure!” He is hoping to discuss the acupuncture technique with Dr. Perron, who is equally interested to learn more about Dr. Tnibar’s procedure.
The new techniques for the treatment of UFP are certainly exciting, but it is worth keeping in mind that they do not have the extensively researched and documented track records of the more established forms of treatment. As Dr. Meagher says, “The use of ligament splitting or acupuncture for the treatment of UFP appears to show promise. With time and a larger number of cases, the full value and limitations of these procedures will become evident.” Meanwhile, horse owners should not feel pressed to follow the latest medical trends, should their own veterinarians recommend a more conventional avenue of treatment.
Horses and their owners will undoubtedly benefit from the ongoing explorations into the treatment of upward fixation of the patella. As a growing body of research brings us new treatments and an ever-deepening understanding of equine biomechanics, we will no doubt come to a more thorough understanding of the causes of this sometimes baffling condition. Though there is still much to learn about UFP, rest assured that if your horse has this condition, chances are excellent that it can be resolved and you’re your horse will return to soundness.
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