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Patellar Luxation

What is patellar luxation?

The patella (referred to in humans as the kneecap) is a small bone at the front of the knee, gliding up and down within a groove at the end of the thigh bone (femur) as the knee flexes and extends. In some animals, the patella can luxate (dislocate) out of its normal groove, resulting in lameness, pain and osteoarthritis.

Patella luxation is an extremely common developmental condition of dogs. It is relatively rare in cats. Medial patella luxation is common especially in small breed dogs. Lateral patella luxation is less common but can cause more problems, especially in large breed dogs.

The patella (knee cap) doesn’t sit in the centre of the groove on the lower femur. This is because three structures are not in a straight line:

  1. Bone at the front of the tibia – the tibial tuberosity
  2. The groove in the lower femur – the femoral sulcus.
  3. The muscles and tendons pulling on the patella

What are the symptoms?

A luxating patella can affect both cats and dogs of any breed, but is most common in small or miniature dogs and is rarer in cats. Luxating patella in cats is most commonly found in domestic short-haired cats. Most animals start showing signs as puppies or young adults, although mature dogs can also suffer from patellar luxation.

Although the signs of patellar luxation can vary, you might notice your pet ‘skipping’ as they walk, with their hind leg carried for a few steps before quickly returning to normal. This happens when the patella slips out of the groove and stops when it goes back in again. Some animals will limp continuously, or – if both knees are affected – they may have difficulty walking, or move with a stiff gait.

Patella luxation grading system

Grade 1: the patella can be pushed out of the groove but spends most of time in the groove
Grade 2: the patella approximately half the time in the groove and half the time out of the groove. It can easily be manipulated in or out but has no tendency to stay in either position.
Grade 3: the patella spends almost all the time outside the groove and with pressure can be pushed back into the groove.
Grade 4: the patella spends all the time outside the groove and even with pressure the patella cannot be pushed back into the groove.

When the patella is out of the groove, the groove becomes shallower, or the ridge may be worn away, making the problem worse. Abnormal loading of the joint can, in time, lead to cranial cruciate ligament injury.

Causes of medial patella luxation are not fully understood but include genetic factors, trauma when growing or ligament problems. Many dogs may have a combination of several causes. The luxating patella rubs on the edge of the ridge or the rough bone to the side of the ridge. This wears away the cartilage, eventually causing pain and arthritis.

The diagnosis

Your vet will normally refer you to a specialist orthopaedic surgeon if they suspect your pet has a luxating patella, or if you’ve noticed a ‘skipping’ motion as they walk. Once referred, Swift’s orthopaedic specialists will assess your pet, often using advanced imaging to get a clear overview of your their condition and assess the severity of the luxation.

How do we treat patellar luxation?

The treatment your pet requires depends on the severity of the luxation. Once they’ve examined your pet, the specialist will assign the luxation a diagnostic grade based on how mobile the kneecap is relative to the groove. There are four recognised diagnostic grades, with 1 being the lowest and 4 being the most severe.

For grade 3 and 4 patellar luxation, surgery is strongly advised, and may also be recommended for grade 2 cases if your pet is presenting persistent lameness. Surgery for patellar luxation is incredibly effective, freeing 90% of dogs from lameness and dysfunction.

Complications in treating patella luxation

  1. Infected implants- this usually occurs due to the patient licking at the wound. This can also occur during surgery, recovery or through haematogenous spread (through the blood stream).
  2.  Implant failure- very rare and normally due to extreme activity.
  3.  Bone fracture- very rare and normally due to extreme activity.
  4. Seroma- relatively common but insignificant fluid accumulation after surgery, normally improves with no treatment.

Signs to watch out for:

  1. Sudden worsening of lameness that lasts for more than 12 hours.
  2. Smelly, yellow or abnormal discharge from the wound
  3. Swelling more than two days after surgery. This is often seroma, but it’s worth having a check.

Post-operative care

To ensure the health of your dog after surgery, it is recommended that the following post-operative procedures are followed:

  1. Elizabethan (Buster) collar at all times for two weeks.
  2. Medications as prescribed – this is normally pain relief for 4-6 weeks and sometimes antibiotics.
  3. Restricted exercise. Until the bone has healed, your dog should be rested in a cage at all times except short lead walks to go to the toilet. This normally takes 6-8 weeks.
  4. If there are any problems, please contact your vet: it is better to have a recheck appointment when everything is normal than miss a problem. If everything is going as expected, please schedule a recheck appointment with the regular vet 4-5 days and sometimes 14 days after surgery. If you are worried about the wound then please contact us. Signs to watch out for include redness, heat, pain or significant worsening of any lameness
  5. A recheck appointment for radiographs under sedation at six weeks after surgery is highly recommended. Your dog will need to be starved for this from midnight as sedation will likely be needed.
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