What Is Patellar Luxation?
The patella — commonly known as the kneecap — sits within a groove at the lower end of the femur and glides smoothly up and down as the stifle flexes and extends. In dogs with patellar luxation, the patella slips out of this groove and moves to the inside (medial) or outside (lateral) of the joint. This is not merely a cosmetic abnormality; luxation causes pain, interferes with normal limb movement, damages cartilage within the groove, and significantly increases the risk of other injuries, including cruciate ligament rupture.
Patellar luxation is one of the most commonly diagnosed orthopaedic conditions in veterinary practice in the UK. It is found across a wide range of breeds, but small and toy breeds are by far the most frequently affected.
Medial Versus Lateral Luxation
The direction in which the patella displaces depends on the underlying skeletal conformation of the dog. Medial patellar luxation (MPL) — where the kneecap slips inward — is far more common and predominantly affects small breeds. It occurs because the groove in the femur (the trochlear groove) is too shallow, the tibial crest is positioned too far inward, or there is abnormal angulation of the femur or tibia that redirects the pull of the quadriceps muscle.
Lateral patellar luxation (LPL) — where the kneecap slips outward — is seen more commonly in large breeds. It tends to carry a less favourable prognosis because the skeletal abnormalities involved are often more complex. Giant breeds including the Great Dane and Irish Wolfhound can be affected, and lateral luxation in these dogs may be associated with significant femoral deformity.
Small Breeds Most at Risk
The breeds that most frequently present with medial patellar luxation include:
- Yorkshire Terriers
- Pomeranians
- Chihuahuas
- Miniature and Toy Poodles
- Maltese
- Bichon Frise
- Shih Tzus
In many of these breeds, the condition is congenital — present from birth — and reflects underlying skeletal conformation rather than injury. A genetic basis is strongly suspected, and the condition is common enough in some breeds that screening by breeders is increasingly encouraged.
Recognising Patellar Luxation
The classic sign of patellar luxation in a small breed dog is an intermittent skipping or hopping gait. The dog will be trotting normally, then suddenly hold the affected leg up for a few strides before it seems to click back into place and the dog carries on as though nothing happened. This occurs when the patella slips out of the groove momentarily and then spontaneously relocates.
Other signs to look out for include:
- Sudden leg-holding — the leg is carried up completely for a short period
- A reluctance to jump up or use stairs
- A crouching posture when walking, particularly on both hind legs in bilateral cases
- Stiffness after rest
- Hind limb weakness over time as muscle mass is lost from disuse
In mild cases, owners may not notice anything at all, and the condition is picked up incidentally during a routine health check. In severe cases, the lameness is constant and distressing.
The Grading System
Patellar luxation is classified on a scale from grade 1 to grade 4 based on how easily the patella moves out of the groove and whether it returns to its normal position spontaneously:
- Grade 1 — the patella can be pushed out of the groove manually but returns immediately when released; does not luxate spontaneously
- Grade 2 — the patella luxates spontaneously during flexion and may or may not reduce on its own; causes intermittent lameness
- Grade 3 — the patella is permanently luxated but can be manually replaced into the groove; the groove is shallow
- Grade 4 — the patella is permanently luxated and cannot be manually replaced; the groove is absent or severely underdeveloped; obvious limb deformity is often present
When Is Surgery Needed?
Grade 1 luxations in dogs that show no clinical signs are often managed conservatively with monitoring, weight management, and exercise modification. However, any dog showing lameness or discomfort, and all dogs with grade 3 or 4 luxation regardless of apparent symptoms, should be considered for surgical correction.
The surgical approach typically combines several techniques to address the underlying structural abnormalities:
- Trochlear groove deepening — either by recession sulcoplasty or trochlear wedge recession, to create a deeper groove for the patella to sit within
- Tibial crest transposition — moving the attachment point of the patellar tendon to realign the quadriceps mechanism
- Lateral retinacular imbrication or medial release — tightening the soft tissues on one side of the joint and loosening them on the other to help keep the patella centred
In severe cases with significant bone deformity, femoral or tibial corrective osteotomy — where the bone is cut and repositioned — may also be necessary. Outcomes are generally very good, particularly for grade 2 and 3 cases. Grade 4 cases with severe skeletal abnormality carry a more guarded prognosis.
The Link Between Patellar Luxation and Cruciate Ligament Injury
Dogs with patellar luxation, particularly those with grade 3 or 4 medial luxation, have a significantly increased risk of cranial cruciate ligament (CCL) rupture. The abnormal forces acting within the joint place chronic stress on the CCL, and studies suggest that anywhere between 15 and 40 per cent of dogs with severe patellar luxation will develop concurrent CCL disease. When both conditions are present, both must be addressed at surgery — failure to do so leads to ongoing instability and poor outcomes.
This is why thorough orthopaedic assessment of the entire stifle is essential before any surgical planning for patellar luxation, and why regular monitoring of grade 1 and 2 dogs is important even when intervention is not immediately required.
Long-Term Management
After surgery, a period of strict rest followed by gradual rehabilitation is essential. Physiotherapy and hydrotherapy help rebuild muscle strength and restore normal movement. Most small breed dogs recover excellently from patellar luxation surgery and return to full activity within two to three months.
For dogs managed conservatively, maintaining a lean body weight is perhaps the single most important intervention an owner can make — excess weight worsens joint loading and accelerates cartilage wear. Regular gentle exercise, avoiding high-impact activities like jumping from height, and attending routine vet checks to monitor for progression are the pillars of long-term conservative care.