Patellar luxation

What it is

Patellar luxation is the slipping of the patella out of the trochlear groove of the femur. Although it can be caused by trauma, in the great majority of cases it is congenital or developmental: the puppy is born with a faulty alignment of the extensor mechanism (quadriceps, patella, patellar ligament and tibial tuberosity) which, deviating from the axis of the femur, pulls the patella out of the trochlea with every contraction of the quadriceps. That abnormal pull in turn disturbs the growth of femur and tibia, and the deformities worsen with time.

Who it affects

Medial luxation is by far the most common and mainly affects small and toy breeds: Yorkshire terrier, Chihuahua, Pomeranian, toy poodle, Boston terrier, Maltese. It is also seen in large breeds, where lateral luxation, associated with genu valgum, is proportionally more frequent than in small dogs. It is bilateral in a substantial share of cases and is usually diagnosed in young animals, although low grades may go unnoticed until adulthood.

Anatomy involved

• Femoral trochlea: shallow or even flat, with hypoplastic trochlear ridges.
• Femur: varus (bowed) in medial luxation; valgus in lateral luxation; femoral torsion may be present.
• Tibial tuberosity: displaced medially (or laterally), which deviates the line of pull of the patellar ligament; internal rotation of the tibia.
• Soft tissues: retinaculum and capsule contracted on the side of the luxation and stretched on the opposite side.

Grades

• Grade I: the patella can be luxated manually but returns to the trochlea on its own when released. Lameness is rare.
• Grade II: it luxates spontaneously with stifle flexion or manual pressure and stays out until reduced by extension or manipulation. Typical intermittent lameness: the dog "skips a step" with the limb flexed and then bears weight normally.
• Grade III: permanently luxated, but can be reduced manually; it luxates again when released. Obvious bony deformities and a crouched stance.
• Grade IV: permanently luxated and cannot be reduced. Severe deformity with tibial rotation; the animal walks with flexed stifles and cannot extend them.

Diagnosis

Palpation of the patella with the stifle in extension and flexion to assign the grade. Lateral and craniocaudal radiographs show the position of the patella, femoral varus or valgus and osteoarthritis; the "skyline" view assesses the depth of the trochlea. Computed tomography is used to plan corrective osteotomies. The cranial cruciate ligament must always be examined, since its rupture frequently accompanies chronic luxations.

Treatment

Grade I without lameness is managed without surgery. Surgery is indicated for grades II to IV with clinical signs and combines techniques according to the deformities present: trochleoplasty (wedge or block recession) to deepen the groove, tibial tuberosity transposition to realign the patellar ligament, release of the contracted retinaculum and imbrication of the stretched side. In marked femoral varus a corrective osteotomy of the distal femur is added. The prognosis is good in low grades and worsens with deformity and established osteoarthritis.

In the cat

Patellar luxation is less common in the cat than in the dog, but not rare. It is almost always medial and often bilateral. The Devon Rex and Abyssinian are predisposed, and an association with hip dysplasia has been described. Bony deformities of the femur and tibia are usually less marked than in small dogs.

Many cats with low grades show no signs or only difficulty jumping, and the condition is found on examination. Cats with signs show intermittent lameness, a crouched gait or reluctance to jump, and in high grades difficulty extending the stifle. Diagnosis is by palpation and radiographs as in the dog. Surgery (trochleoplasty, imbrication of the stretched side, release of the contracted side and, if needed, tibial tuberosity transposition) is indicated in cats with clinical signs, and most improve afterwards.

Key studies in the cat

• Smith et al. (1999), a cross-sectional study of 78 cats. 58% had medial patellar luxation, mostly grade 1 and bilateral in 71%, and 32% had hip dysplasia; 24% had both. The association between the two was weak (cats were 3 times more likely to have both than either alone). Only 11 of the 78 had pelvic limb signs.
• Loughin et al. (2006), 42 cats. 34 had bilateral luxation, 95% of affected joints were medial and only 17% of cats had a history of trauma. Low-grade luxations could cause lameness as severe as high-grade ones. The outcome was excellent in 23 of 35 joints treated surgically and in 8 of 17 treated without surgery.
• Rutherford et al. (2015), 85 surgeries in 71 cats from four referral centres. Complications occurred in 26% (major in 20%, reluxation in 5%). The risk was higher with a previous femoral fracture on the same side, grade 4 luxation, tibial tuberosity transposition and previous stifle surgery.
• Mills et al. (2024), 44 stifles in 39 cats treated with polyethylene prostheses that raise the trochlear ridges. Minor complications occurred in 20% and major ones in 7%. At 2 years or more, 74% had osteoarthritis with mineralisation, yet function was good or very good in 93%. One of the authors was involved in the commercial development of the implant.