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UPWARD PATELLAR FIXATION

This article highlights the locking mechanism of the horse's stifle joint. Interference of soft tissues comprising this mechanism can cause visible, biomechanical lameness.

Hind Stay-Apparatus

Intermittent upward patellar fixation is a condition whereby the horse’s pelvic limb temporarily "locks" in extension. As a result, there is a delay in flexion of the limb. The delay in flexion can range from milli-seconds to over several minutes. A short delay in flexion may manifest only as a subtle pelvic limb asymmetry or lameness; severely affected horses (with a long delay in flexion) may be unable to flex the affected limb without assistance.

Click the icon below to see what upward patellar fixation looks like in the horse:

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To learn more about this and other common gait deficits displayed by horses, we suggest that you read THIS BOOK.

What is the patella?

The horse’s stifle joint is analogous to the human knee. Just like humans, horses have a patella, or "knee cap", which slides along the distal aspect of the femur (thigh bone) during flexion of the joint. The patella slides within a groove (called the trochlear groove) and serves as a fulcrum for the extensor muscles and their tendons as they course over the front of the stifle (or knee) joint. The patella is attached proximally to the quadriceps and biceps femoris muscles and distally to the tibia. In humans, the patella is attached to the tibia by one distal patellar ligament. Horses have 3 distal patellar ligaments: the medial (inside) patellar ligament, the middle patellar ligament, and the lateral (outside) patellar ligament.

How does the horse lock the pelvic limb?

Horses have the ability to lock (or fixate) the pelvic limb in extension. This is possible due to the unique anatomy associated with the horse’s stifle joint. The proximal aspect of the medial femoral trochlea is shaped similar to a hook or ski jump. By placing the space between the medial and middle patellar ligaments over this hook, horses can "lock" their pelvic limbs in extension. Once locked, minimal effort is required to maintain limb extension. A similar locking apparatus in the thoracic limbs allows horses to sleep while standing. Therefore, patellar fixation while standing is a normal process in the horse.

What is upward patellar fixation?

Although patellar fixation is normal in the standing horse, it can produce pelvic limb dysfunction if it occurs during exercise. Inadvertent locking of the patella over the medial femoral trochlea prevents normal flexion of the affected limb(s). Consequently, pelvic limb asymmetry and lameness may become evident.

What causes upward patellar fixation?

There are a variety of things that will precipitate upward patellar fixation in the horse. Some of these include:

Lack of fitness. Lack of quadriceps and/or biceps femoris muscle tone results in an inability to readily pull the patella up and off of the medial femoral trochlea.

Straight or upright pelvic limb conformation. This positions the medial femoral trochlea further distad (lower along the limb) where it is in closer proximity with the patella, facilitating inadvertent upward fixation of the latter.

Downhill top line. Horses with this conformation must extend the hind limbs further under the body to accommodate the forward transfer of weight, thereby increasing the likelihood for inadvertent fixation of the patella.

Negative plantar angulation. Insufficient foot angle can increase the degree of pelvic limb extension, thereby increasing the likelihood for patellar fixation to occur.

Excessive distal patellar ligament length. This places the patella proximad in closer proximity with the medial femoral trochlea, where it can inadvertently catch or lock.

Neurologic disease. A lack of neurogenic input to the hind limbs afforded by neuropathy can reduce tone of the muscles and ligaments associated with the distal patellar apparatus, thereby promoting their biomechanical interference.

Why does PPH get worse when it's cold?

It should be noted that the factors which cause upward patellar fixation are often interrelated. For example, an unfit horse will generally have increased laxity (and therefore increased length) of the distal patellar ligaments. Furthermore, if unfitness is secondary to another disease process (such as neurologic disease), intermittent upward fixation may also occur secondarily. Therefore, it is important to assess the horse as a whole prior to determining the cause for upward patellar fixation.

What are the clinical signs of upward patellar fixation?

Horses with intermittent upward patellar fixation will exhibit clinical signs during their attempt to flex the pelvic limb from an extended position. In acute severe cases, the pelvic limb may stay locked in extension. The horse may not be able to flex the stifle and tarsus without assistance. In some instances, the condition may temporarily resolve only to recur after taking a few steps. These signs are quite obvious and diagnosis is relatively simple if the condition is severe. Most of the time, however, there is only a "catching" of the patella as it slides up and over the hook and the limb does not truly lock in extension. In this scenario, there may only be a mild pelvic limb asymmetry or lameness. This type of lameness can be easily confused with other problems and therefore may present a dilemma in regard to accurate diagnosis.

Following are common clinical signs associated with mild to moderate forms of intermittent upward patellar fixation:

Non-weightbearing pelvic limb lameness.

Resistance in the canter.

Consistent lead changes or cantering on the wrong lead.

The canter is very rough or bouncy.

Swelling, heat, and/or pain may be associated with one or both stifle joints.

The horse drags his/her hind toes during exercise.

Resistance and/or difficulty when walking up and down hills, or when backing up.

Lameness is most severe when the horse is first taken out of the stall.

Lameness becomes more obvious following an extended period of stall rest.

The horse does not respond to anti-inflammatory (e.g. Phenylbutazone) therapy.

As with many cases of pelvic limb lameness, secondary abnormalities such as thoracolumbar ebaxial (back) and proximal thoracic suspensory ligament soreness may also be present. These are generally detected during the passive lameness evaluation and are suggestive of chronic pelvic limb asymmetry/ lameness.

How is upward patellar fixation diagnosed?

Clinical signs are characteristic and, if the limb is locked in extension (i.e. the case is severe), diagnosis is simple. As previously mentioned, however, most cases are mild and diagnosis may be more difficult. A detailed history and careful clinical evaluation are essential parts of a proper workup. One helpful diagnostic aid involves placing the horse in one or more situations where prolonged pelvic limb extension is normally required. Such situations include walking up and down hills, the trot-to-canter transition, and backing-up. When confronted with these situations, the affected horse will either 1) demonstrate upward patellar fixation by temporarily locking the pelvic limb, or 2) cheat by switching leads, swinging the limbs to the outside, avoiding pelvic limb extension, etc.

Many times, a slight hitch or "catch" is visible as the pelvic limb begins to flex from an extended position. This "catch" is most easily detected by visualizing the point of the hock as the horse picks the limb up to advance it cranially. Infrequently, an audible "snap" or popping sound is also evident during exercise (particularly when walking).

In many instances, upward patellar fixation can be produced in affected horses by manually forcing the patella upward and outward. The examiner may actually be able to keep the pelvic limb locked in extension using minimal effort.

Since the problem is usually secondary to conformation and/or level of fitness, it is almost always bilateral (i.e. it usualy affects both side of the horse). However, affected horses historically exhibit clinical signs in one pelvic limb. It is not until the more affected limb is successfully treated that a problem in the contralateral limb manifests.

Occasionally, the BUTE TEST is utilized to differentiate between upward patellar fixation (which is biomechanical in nature) and issues that are pain- and/or inflammatory-mediated (such as synovitis/ arthritis).

How is upward patellar fixation treated?

There are several forms of treatment for upward patellar fixation in the horse:

Exercise

Lack of fitness results in decreased thigh muscle and patellar ligament tone. With decreased supporting muscle and ligament tone, it becomes easier for the patella to lock on the femur and harder for it to replace within the trochlear groove. In subtle cases of upward patellar fixation where conformation is relatively good, increased exercise alone may resolve the problem.

We frequently ask the client to grade the level of their horse’s current level of fitness on a scale of 1 to 10 (1=very unfit; 10=extremely fit). We suggest achieving a fitness level of at least 7-8 (if possible) prior to pursuing other forms of treatment. This will rule out unfitness as a major contributor to the problem as well as increase the effect of other therapy.

Corrective Shoeing

Since fixation of the patella occurs when the pelvic limb is extended, prolonging the extension phase of the stride can make "unlocking" more difficult. Alternatively, shortening the amount of time the pelvic limb spends in extension allows the horse to unlock his/her patella before the distal patellar ligaments become excessively tight. Since the conformation of the distal pelvic limb and/or the toe length is intimately related to pelvic limb breakover, the farrier can frequently alleviate the problem via corrective trimming/shoeing. Rolling and/or rockering the toe of the shoe, applying a full (egg-) bar shoe, and/or the use of wedged pads (when needed) are commonly used techniques. In many cases, we are able to help the pelvic limbs break over before intermittent upward patellar fixation occurs.

The farrier will also want to make sure that your horse’s plantar angles are appropriate. Deficient plantar angulation can also precipitate biomechanical interference of the stifle as explained HERE.

Hormonal Therapy

The administration of estrogen has shown to prove benefical for some horses exhibiting intermittent upward patellar fixation. The presence of estrogen within the body of the horse may affect the tension of various supporting ligaments. These include the collateral, suspensory, cruciate, and distal patellar ligaments. Altering distal patellar ligament tension helps to relocate the patella, thereby potentially making upward patellar fixation more difficult. This in turn may alleviate clinical signs.

It should be noted that estrogen is also a powerful behavior modificator in the horse. It is often used for stallions and geldings that are excessively difficult to handle, aggressive towards people or other horses, or overly anxious at shows and other events. Estrogen is very effective at reducing anxiety and resistance as well as improving overall behavior in these horses. Treatment usually consists of 2 injections of estrogen (25mg) in the muscle twice weekly for 5 consecutive weeks, then as needed therafter.

Administration of estrogen to mares usually causes them to exhibit clinical signs of estrus (heat). Since this change in behavior is generally undesirable, we typically do not recommend its use in mares.

Learn more about the use of hormonal therapy in horses HERE.

Intraligamentous Infusion of Counterirritant

This form of therapy is usually referred to as "blistering". Blistering involves the inject of an irritative substance into soft tissue(s) in an attempt to create an inflammatory reaction. The irritative substance usually consists of iodine 2% in an almond oil base. This substance can elicit an inflammatory response for up to 30 days depending on the amount used and the location of injection. It is important to remember that fibrosis and scar tissue formation within normal soft tissues will occur as a result of severe inflammation. As you know, scar tissue does not function like normal soft tissue. Therefore, blistering in certain areas may inhibit proper function of associated soft tissue. It is for this reason that The Atlanta Equine Clinic typically does not institute blistering as typical form of treatment for soft tissue problems.

However, in the case of intermittent upward patellar fixation, we gain a biomechanical advantage by replacing normal tissue with scar tissue. The infusion of counterirritant within and around the medial and middle patellar ligaments results in the elicitation of an intense inflammatory reaction by the horse’s body. With inflammation, fibrosis and scarring of the patellar ligaments occur. During the scarring process, soft tissues will contract (shorten). As the patellar ligaments shorten, the patella is pulled up and over the hook of the medial femoral trochlea and into its normal position within the trochlear groove. At this point, it becomes more difficult for the horse to lock the patella and easier to flex the pelvic limb from an extended position. In our hands, this from of treatment has been extremely effective in a vast majority of cases involving intermittent upward patellar fixation.

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Medial Patellar Desmotomy

The medial (or inside) patella ligament is one of the key structures (along with the patella and middle patellar ligament) that is required to lock the patella on the femur. Since the problem represents the horse’s inability to quickly disengage the patella from the medial femoral trochlea, surgical resection of the medial patellar ligament results in complete resolution of the problem. Once the medial patellar ligament is resected, upward patellar fixation becomes impossible and the clinical signs associated with this condition disappear. Consequently, this has become a very popular form of treatment for horses with intermittent upward patellar fixation.

It is extremely important to note, however, that the medial patellar ligament also performs another function: stabilization of the patella within the trochlear groove of the femur. Without tension from the medial patellar ligament, the patella becomes unstable within the femoropatellar joint. Femoropatellar synovitis and frequently osteoarthritis result. Since the stifle is high-motion in nature, chronic inflammation within this joint poses a significant concern in regard to future performance soundness. Persistent femoropatellar joint inflammation typically needs to be addressed on a continual basis and often requires considerable maintenance therapy. It is for this reason that The Atlanta Equine Clinic views this form of treatment inappropriate except for the most severe of cases that have proven refractory to the other forms of therapy.

Learn more about medial patellar desmotomy in horses HERE.