Patellar Luxation

Figure 1: Three-dimensional computed tomography showing the anatomy of a canine hind limb with medial patellar luxation, viewed from the front (left) and from the outside of the leg (right). The extensor mechanism begins near the hip, includes the quadriceps muscle group, the patella (knee cap), and the patellar tendon, and attaches to the tibial crest. In this condition, the knee cap tracks outside the femoral groove.

The patella (knee cap) is a small bone embedded within the tendon of the thigh’s extensor muscles (the quadriceps). Under normal conditions, the patella glides within a groove in the femur (thigh bone) at the knee (Figure 1). The patellar tendon attaches to the tibial crest, a bony prominence on the tibia (shin bone) just below the knee. Together, the quadriceps, the patella, and the patellar tendon form the extensor mechanism, and they are normally aligned. Patellar luxation (dislocation) occurs when the patella moves outside the femoral groove when the knee bends (Figure 1). It is classified as medial or lateral, depending on whether the patella displaces toward the inner or outer side of the knee.

Patellar luxation is among the most common orthopedic disorders in dogs and is diagnosed in 7% of puppies. It is most common in small dogs, particularly breeds such as Boston Terriers, Yorkshire Terriers, Chihuahuas, Pomeranians, and Miniature Poodles. Over the last ten years, the frequency in large-breed dogs has increased, and breeds including Chinese Shar Pei, Flat-Coated Retrievers, Akitas, and Great Pyrenees are now considered predisposed. In about half of cases, both knees are affected, which can lead to discomfort and reduced function.

Figure 2a: Preoperative “skyline” knee radiograph showing a shallow femoral groove with the patella positioned outside the groove.
Figure 2b: Intraoperative view of the same femoral groove before correction.
Figure 3: Preoperative CT evaluation of a dog with severe luxation in both knees and femoral malformation (bowing) in each limb.

Patellar luxation can occasionally be caused by traumatic injury to the knee, producing sudden severe lameness. In most dogs, however, the exact cause is not clear and is likely multifactorial. In non-traumatic cases, the femoral groove is often shallow (Figures 2a and 2b) or even absent. Early identification of bilateral disease without trauma, combined with breed predisposition, supports the idea that patellar luxation reflects a congenital or developmental malalignment of the entire extensor mechanism. For this reason, developmental patellar luxation is no longer viewed as a knee-only disorder, but rather as the outcome of complex skeletal abnormalities that alter overall limb alignment, including:

• abnormal hip joint conformation, such as hip dysplasia
• femoral malformation with abnormal angulation and torsion (rotation) (Figure 3)
• tibial malformation
• deviation of the tibial crest (the attachment point of the patellar tendon below the knee)
• tightness or atrophy of the quadriceps muscles, acting like a bowstring
• an overly long patellar ligament

Because evidence suggests a genetic contribution, dogs diagnosed with patellar luxation should not be bred. Patellar luxation can cause pain and osteoarthritis in the stifle (knee) joint and can lead to lameness that may progress to avoidance or non-use of the limb.

Clinical signs vary widely depending on disease severity. Patellar luxation may be found incidentally during a routine exam, or it may cause the dog to hold the affected leg up continuously. Many dogs will suddenly lift the limb for a few steps (a “skip”), and owners may notice the dog shaking or stretching the leg before normal use returns. As the condition becomes more severe and longstanding, the episodes occur more often and can eventually become constant lameness. In young puppies with severe medial luxation, the hind legs may appear “bow-legged,” and this appearance often worsens as the puppy grows. In large-breed dogs with lateral luxation, a “knock-kneed” stance may be observed.

Diagnosis is primarily based on orthopedic examination, with palpation demonstrating an unstable patella (Figure 4). Additional tests may be needed to identify associated problems and to help an ACVS board-certified veterinary surgeon recommend the most appropriate treatment. These may include:

• palpation of the knee under sedation to assess ligament injury
• radiographs of the pelvis, knee, and tibias to evaluate bone shape in the hind limb and assess for hip dysplasia (Figure 5)
• three-dimensional CT (CAT scan) to image skeletal features of the entire hind limbs; this is especially useful when surgical planning requires correction of femoral or tibial shape (Figure 3)
• blood tests and urinalysis as a pre-anesthetic precaution

Over time, the patella may dislocate more frequently, wearing away cartilage and eventually exposing bone, which leads to arthritis and pain. Other knee structures may become increasingly strained, potentially increasing risk for cranial cruciate ligament rupture. In puppies, the malalignment can also contribute to significant limb deformity.

Figure 4: Palpation of a dog with lateral patellar luxation (patella positioned on the outer side of the femoral groove).
Figure 5: Radiographs of a dog with right-sided luxation: the patella is visible on the inner side of the knee (solid arrow). The dog had undergone surgery on the left knee six weeks earlier; the patella (dashed line) is in a normal position. Wires (block arrow) were used to transpose the tibial crest toward the outer side of the knee.

Patellar luxations that do not produce symptoms are typically observed rather than surgically corrected, particularly in small dogs. Surgery is most commonly considered for grade 2 or higher luxations (Figure 6). Surgical management is generally more challenging in large-breed dogs, especially when luxation occurs together with cranial cruciate ligament disease, hip dysplasia, or excessive angulation of the long bones.

One or more of the following strategies may be used:

• Soft tissue reconstruction around the patella to release the side toward which the patella is displaced and tighten the opposite side.
• Deepening of the femoral groove to allow the patella to sit securely in the proper track (Figure 7), using one of several possible techniques.
• Tibial crest transposition (Figure 5), repositioning the bony attachment point of the patellar tendon to realign the quadriceps, patella, and tendon.
• Femoral corrective surgery in selected cases where femoral deformity causes the patella to luxate most or all of the time; this involves cutting the bone, correcting alignment, and stabilizing it with a bone plate (Figure 7).

The surgeon selects the most appropriate combination of procedures for each individual dog based on examination findings.

Figure 6: Standard grading system for patellar luxation.
Figure 7: Postoperative view showing a deepened femoral groove with the patella now well-seated, and the tibial crest moved outward to restore alignment of the patellar tendon and thigh muscles. Pins (arrow) have been placed in the tibia to hold the tibial crest in its new position.

The surgeon who performed the procedure is best positioned to provide individualized postoperative instructions and establish a tailored recovery plan.

More than 90% of owners report satisfaction with their dog’s improvement after surgery. Prognosis may be less favorable in large dogs, particularly when patellar luxation is accompanied by additional abnormalities such as excessive long-bone angulation or hip dysplasia.

Recurrence of patellar instability is uncommon. Implant migration or breakage and postoperative infections are rare.