What Is a Luxating Patella?
A luxating patella is a condition in which the patella — commonly called the kneecap — slips out of its normal position within the trochlear groove on the femur. The trochlear groove acts as a channel that guides the patella during leg movement; when this groove is too shallow, poorly shaped, or when surrounding structures are misaligned, the patella can pop out of position, causing intermittent or persistent lameness.
Luxation can occur in two directions. Medial luxation, where the kneecap shifts inward towards the body's midline, is by far the more common form and is seen predominantly in small breeds. Lateral luxation, where the patella shifts outward, is less frequent overall but more commonly associated with large and giant breeds. The condition may be hereditary — and therefore present from a young age — or acquired following trauma.
The Grading System
Veterinary surgeons classify luxating patella using the Putnam grading system, which runs from Grade I to Grade IV. Understanding these grades helps owners appreciate why treatment recommendations differ between cases.
Grade I
At Grade I, the patella sits correctly within the groove during normal movement and can only be displaced if manually pushed out during examination. It returns spontaneously to the groove the moment pressure is released. Most Grade I cases are entirely asymptomatic and are found incidentally during routine health checks. Surgical intervention is rarely needed at this stage, though monitoring and weight management are advised.
Grade II
At Grade II, the patella luxates spontaneously when the leg is flexed and remains out of the groove until manually repositioned or until the dog extends and rotates the leg. The classic clinical sign is an intermittent skipping gait — the dog hops for a few strides on three legs, then places the foot down and continues walking normally. This can look alarming but is not always painful. Surgery is often recommended at Grade II to prevent progression and to protect the joint from abnormal wear.
Grade III
At Grade III, the patella is permanently luxated but can be manually replaced into the groove during examination. The dog will typically show a more consistently abnormal gait and may adopt an unusual hindlimb posture, often appearing crouched or cow-hocked. Activity levels may be reduced. Surgical correction is strongly recommended at this stage.
Grade IV
Grade IV represents the most severe form. The patella is permanently luxated and cannot be replaced manually. Significant conformational bony abnormalities are present. The dog may walk with great difficulty or appear unable to extend the stifle fully. Surgery is required, though cases at this grade are more complex and carry a more guarded prognosis than lower grades.
Breeds Commonly Affected
Medial luxating patella is extremely common in small breed dogs. Yorkshire Terriers, Chihuahuas, Toy Poodles, Pomeranians, Maltese and Bichon Frises are among the most frequently affected. The condition is so prevalent in these breeds that screening is routinely recommended before breeding. Lateral luxating patella is seen more often in large breeds, including Labrador Retrievers and Great Danes. Cats can also develop luxating patella, though it is less frequently diagnosed than in dogs.
Signs to Watch For
The most recognisable sign is the intermittent skipping gait described under Grade II — the dog suddenly lifts one hindleg for a few steps, as if shaking something off its foot, then resumes normal movement. Some owners describe their dog suddenly yelping or crying out, followed by a brief period of lameness. In more severe cases, the dog may consistently carry one hindleg or develop an obviously abnormal hindquarter posture.
Over time, untreated cases can result in muscle wasting in the affected hindlimb, progressive joint changes, and increasing discomfort. It is worth noting that the degree of visible lameness does not always correlate directly with the grade — some Grade III dogs adapt surprisingly well, whilst some Grade II dogs are clearly uncomfortable.
Diagnosis
Diagnosis is based primarily on orthopaedic examination. A veterinary surgeon will assess the patella in a conscious patient, manually testing whether the kneecap can be displaced and to what degree, grading the condition accordingly. Radiographs are taken to evaluate the bone structure of the stifle, measure relevant angles, and rule out concurrent hip dysplasia — which co-occurs with luxating patella in some small breeds at a higher rate than expected by chance.
Association With Cruciate Ligament Disease
Medial luxating patella is a recognised predisposing factor for canine cranial cruciate ligament rupture (CCLR). The abnormal biomechanics created by a displaced patella alter the forces acting on the stifle joint over time, placing additional strain on the cruciate ligament. Dogs diagnosed with luxating patella — particularly at higher grades — should be monitored closely for early signs of cruciate disease, and preventative weight management is especially important in these individuals.
Treatment Options
Grade I cases are generally managed conservatively with regular monitoring, weight management and, where appropriate, physiotherapy to maintain muscle strength around the joint. Grades II through IV typically warrant surgical correction.
Surgery for luxating patella commonly involves a combination of three techniques. Trochleoplasty deepens the trochlear groove so the patella sits more securely; this may be performed as a block recession (removing and replacing a block of bone and cartilage to deepen the groove whilst preserving the cartilage surface), a wedge recession, or an abrasive recession. Tibial tuberosity transposition repositions the bony attachment point of the patellar ligament to correct the alignment of the extensor mechanism. Soft tissue reconstruction tightens the joint capsule on the side opposite to the direction of luxation to provide additional support.
The specific combination of techniques used depends on the grade, the size of the dog, and the degree of bony deformity present.
Recovery and Prognosis
Following surgery, most dogs require six to eight weeks of strictly restricted exercise — lead walks only, no jumping or running. Physiotherapy is recommended to rebuild muscle mass and promote a normal gait pattern. Return to normal activity is gradual and guided by the operating surgeon.
The overall prognosis following surgical correction is generally excellent for Grade II and Grade III cases in small breeds. There is a risk of re-luxation in approximately five to ten per cent of cases, and this is more likely in dogs with severe bony abnormalities. Osteoarthritis continues to progress — particularly in Grade III and IV cases where joint damage was already established before surgery — but appropriate weight management and joint support can slow this considerably. Regular veterinary check-ups help monitor joint health in the long term.