Locking Stifle (Upward Fixation of the Patella) in Horses: Causes, Treatment & Prognosis

The stifle is a complex joint in the horse’s leg, comparable to the human knee. It plays a key role in the overall function and support of the hind leg.

The equine stifle joint is comprised of bones, ligaments, cartilage, and muscles that work together to enable movement and provide stability to the horse.

A locking stifle, or upward fixation of the patella (UFP), occurs when the kneecap becomes temporarily stuck on the femur (thigh) bone. This causes the hindlimb to become locked in an extended position, making it difficult for the horse to flex the leg. ^[1][2][3]

This condition can significantly affect a horse’s mobility and comfort, often appearing as sudden stiffness in the hind leg, difficulty moving, or toe dragging.

A locking stifle can vary in severity, ranging from mild, awkward movements to a persistent lock that may require surgery to correct. By learning about the causes, symptoms, and available treatment options, horse owners and caretakers can make informed decisions when managing this condition.

Stifle Joint Anatomy in Horses

The horse’s stifle joint is an important structure in the hind leg, with a similar function to the human knee but more complex. It acts as a large hinge, enabling smooth and efficient movement while supporting the horse’s weight and maintaining stability during various activities.

The stifle joint has two main compartments: ^[3][4]

• Femoropatellar Joint (FP): The area where the patella (kneecap) sits and slides smoothly over the femur (thigh bone). This joint helps the horse’s leg bend and straighten efficiently, playing a critical role in the horse’s gait and stride.
• Femorotibial Joint: The area where the femur connects to the tibia (shin bone).

The femorotibial joint is further divided into two separate sections: ^[3][4]

• Medial Femorotibial Compartment (MFT): The inner part of the joint where the femur and tibia meet on the inner side of the leg.
• Lateral Femorotibial Compartment (LFT): The outer part of the joint where the femur and tibia connect on the outer side of the leg.

These two compartments work together to support the horse’s body weight and allow for smooth movement of the hind leg. They ensure the forces exerted during movement are distributed evenly across the joint.

The Stay Apparatus

The passive stay apparatus is a unique anatomical mechanism in horses that allows them to stand for extended periods with minimal muscular effort, enabling them to conserve energy. This system acts like a built-in “lock” for the hind legs, helping to keep the horse upright while resting. ^[5][6][7]

The stay apparatus works by: ^[5][6][7]

• Stifle Joint Locking: The patella (kneecap) can hook over a ridge on the femur (thigh bone), locking the stifle joint in place. This is achieved through the action of the quadriceps muscles and the patellar ligaments.
• Hock (Tarsus) Joint Stabilization: The reciprocal apparatus links the stifle and hock joints, ensuring the hock is also stabilized when the stifle is locked. This system involves the superficial digital flexor tendon and the peroneus tertius muscle.
• Fetlock and Pastern Joint Support: The suspensory ligament and digital flexor tendons support the fetlock and pastern joints in the hindlimbs, preventing overextension and maintaining joint stability.

To release the stay apparatus, the horse contracts its quadriceps muscles, which flexes the stifle joint and moves the patella off its locking ridge on the femur. This action allows the stifle to unlock.

Locking Stifle

A locked stifle occurs when the patella (kneecap) gets stuck in an extended position on the femur (thigh bone), preventing normal joint movement. There are two main types of locked stifle, or upward fixation of the patella, in horses: persistent and intermittent.

• Persistent upward patellar fixation: Usually occurs when the horse is at rest. In this condition, the kneecap remains continuously locked over the medial trochlear ridge of the femur, causing the stifle joint to stay extended. As a result, the affected hind limb is unable to bend. The leg may release suddenly, either spontaneously or after being manipulated. ^[3]
• Intermittent upward patellar fixation: Occurs sporadically, often in response to the horse changing gait or turning. During these episodes, the patella seems to catch on the distal femur as the leg moves forward, causing an unusual movement of the patella. The affected leg may jerk forward before releasing. ^[2]

A mild form of a locking stifle is delayed release of the patella while in motion, causing the horse’s leg to move in a jerky manner. This is typically seen when the horse is slowing down. ^[1][2]

Symptoms

The symptoms of locking stifle are all related to movement and the horse’s gait. Specific symptoms include: ^[1][3][8]

• Extended Hindlimb: The affected hind leg is often held extended away from the body, with the fetlock joint flexed. This position can persist for minutes, hours, or much longer, depending on the severity of the condition.
• Stiffness and Lameness: The horse may display noticeable stiffness and lameness in the affected leg, especially in chronic cases.
• Reluctance to Move: There is difficulty or hesitation in moving, especially when initiating movement or transitioning between gaits.
• Jerky or Abnormal Gait: The horse may exhibit a jerky or irregular gait, particularly apparent at the walk and during turns.
• Sudden Leg Release: The leg may suddenly release with a characteristic snap or jolt, returning to a normal position and function.
• Toe-dragging: The horse might drag its toe or show signs of dragging the affected leg during movement as a result of the awkwardly extended leg.

Note that a locking stifle can be unilateral, affecting one hindlimb, or bilateral, impacting both hindlimbs simultaneously.

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Causes

The exact cause of a locking stifle isn’t completely understood, but several factors are known to contribute to this condition. Horses with poor muscle strength or excessively straight hindlimb conformation may be predisposed to this condition. ^[1][9]

Injuries to the stifle (most notably the femoropatellar joint) can disrupt the normal mechanics of the joint. Damage to the ligaments, tendons, or cartilage around the joint can interfere with the smooth movement of the patella. ^[1][3]

Certain breeds, like Shetland ponies, are more commonly affected, suggesting hereditary predisposition. Multiple horses within the same family can develop locking stifles, indicating a possible genetic link. More research is needed to fully understand the specific genetic factors involved. ^[1][2][3]

Young horses in early training, still growing and lacking sufficient muscle strength, are particularly prone to locking stifles.^[1][2][9]

Diagnosis

Diagnosis of stifle issues begins with physical examination of the horse, assessment of symptoms, and a medical history. To provoke and observe locking, the veterinarian may trot the horse slowly, back them up, or have them turn in tight circles. ^[2]

They may also manually test the patella by forcing it upward and outward to check for locking. Excessive wear on the toe from dragging and diagnostic imaging of the stifle joints can help confirm the diagnosis. ^[1][3]

Differential Diagnosis

When diagnosing a locking stifle in horses, veterinarians must differentiate it from other conditions that might present with similar symptoms.

Key differential diagnoses include: ^{[1][2][3][10][11][12][13]}

• Coxofemoral Luxation: Dislocation of the hip joint can make the hindlimb appear extended, similar to a locking stifle. However, unlike a locking stifle, coxofemoral luxation usually results in more severe lameness and difficulty moving. Additionally, coxofemoral luxation may occur secondary to a locking stifle. It is suggested that when the patella is locked, the quadriceps muscle might contract strongly in an attempt to flex the stifle, potentially leading to dislocation of the hip joint.
• Low-Grade Hindlimb Ataxia: Lack of coordination can cause abnormal gaits and difficulty moving. Unlike a locking stifle, ataxia usually affects the horse’s balance and coordination rather than causing the limb to remain fixed.
• Stringhalt: This condition is characterized by an exaggerated, jerky movement of the hindlimb, which can resemble the jerky movement seen in milder locking stifle cases. Horses with stringhalt may also exhibit toe-dragging.
• Arthritis: Degenerative joint diseases in the hip or stifle can cause stiffness and discomfort, which might be confused with the signs of a locking stifle. Arthritis typically results in a more consistent level of lameness and may show joint changes on X-ray.
• Tendon or Ligament Injuries: Damage to the tendons or ligaments around the stifle or hindlimb can cause lameness and altered limb movement.

Diagnostic Imaging

In cases where the locking is partial or the diagnosis is uncertain, further diagnostic imaging may be required. Diagnostic imaging such as X-rays or ultrasound of the stifle joint can help rule out other conditions and assess the joint’s structure. ^[3]

Treatment

When treating a horse with a locking stifle, or upward fixation of the patella (UFP), veterinarians consider both conservative treatments and surgical options.

Each approach has its own benefits and considerations, depending on the severity of the condition and the individual horse.

Conservative Management

Conservative treatments for locking stifle in horses focus on managing the condition without surgery. For mild or intermittent cases, increasing controlled exercise can effectively build muscle strength and improve the horse’s overall condition. For young or growing horses, management typically involves careful monitoring and patience, as the condition often resolves with maturity. ^[3]

In addition, counterirritants like iodine-based solutions may be injected into the patellar ligaments to induce controlled inflammation. This process helps form fibrous tissue, which can stiffen the ligaments, improve their function, and reduce the likelihood of the patella getting stuck. ^[8]

Corrective shoeing and trimming also play a role by adjusting how the hoof interacts with the ground. Adding a heel wedge to the shoe can improve stifle joint movement and reduce tension in the patellar ligament, helping the patella move smoothly. ^[1][2][14]

Surgery

 
When conservative treatments fail and a definitive diagnosis is made, surgery is warranted. Medial patellar ligament desmotomy (MPD) is a surgical procedure used to correct severe cases of locking stifle in horses. The surgery aims to release the patella and restore normal movement to the stifle joint. ^[1][3]

Under standing sedation and local anesthesia, a small incision is made over the medial patellar ligament. The surgeon carefully cuts the ligament, allowing the patella to move freely and preventing it from locking on the femur’s ridge. The incision is then sutured and bandaged. ^[14]

Unfortunately, MPD surgery can lead to several complications. Long-term issues may include femoropatellar joint instability, which can result in osteoarthritis, inflammation, and ligament problems, potentially causing persistent lameness. Short-term complications often involve swelling and pain. ^[1][3][9]

Clinicians now recommend at least two months of stall rest after MPD surgery to ensure proper healing, prevent complications, and support effective rehabilitation. This rest period allows the surgical site to recover fully and reduces the risk of complications such as joint instability. ^[2][3]

Prognosis

The prognosis for horses with a locking stifle is generally positive when the condition responds well to conservative treatment. Mild cases often resolve without causing long-term problems, since many young horses outgrow the condition as they mature. ^[3]

However, horses with severe cases that require surgery generally have a more guarded prognosis. While recurrence of locking stifle after surgery is rare, MPD surgery can lead to both short-term and long-term complications, and recovery outcomes can vary among horses.

Effective post-surgery care is essential for improving recovery and overall outcomes. ^[2][3]

Summary

Locking stifles, or upward fixation of the patellas, occur when the horse’s patella gets stuck on the femur (thigh) bone, causing the leg to lock in an extended position.

• The cause of this condition is not fully understood, but a genetic component is suggested
• Many horses outgrow this condition as their bodies mature and they develop strength and conditioning in the legs and joints
• Conservative management includes controlled exercise, rest, and targeted injections to stimulate fibrous tissue growth
• Surgical correction is required in severe cases, but carries a risk of secondary complications