A Medial Patella Luxation (MPL) or ‘dislocating kneecap’ is a common problem in toy breed dogs. Owners will typically notice a small skip in the dog’s step as they walk. The dog may even run on three legs before spontaneously returning to normal as if nothing has happened. What actually happens is the kneecap (patella) slips out of the smooth groove in which it normally sits during flexion and extension of the leg and instead sits medially (towards the opposite leg). With the patella in this position, the knee cannot extend properly and stays slightly bent. Sometimes the patella will spontaneously pop back into the groove where it belongs, but for some dogs, getting a kneecap back where it belongs and normal extension of the rear leg is only achievable with surgical correction.
Approximately 50% of cases involve both knees (stifles). Medial patellar luxations are graded on a scale of 1 to 4, depending on the severity. While displaced (or luxated), the entire weight-bearing stress through the rear leg is altered, which, over time, leads to changes in the hips and long bones and ultimately development of arthritis. The severity of these changes depends on the degree of the luxation, and on how long that degree of luxation has been occurring. Long-term stresses and altered conformation of the joint can lead to a ‘bow-legged’ appearance, requiring more complicated surgical correction.
Grade 1 luxations generally do not require surgical repair. Dogs with Grade 2 luxations will likely benefit from surgery, and this is discussed with the owner to determine how much the lameness affects the dog’s day-to-day life. Surgical repair is necessary for Grade 3 or 4 cases.
Unfortunately, a luxating patella can also predispose little dogs to rupturing their cruciate ligaments. These are the internal crossing ligaments within the knee which help prevent internal rotation and instability. A dog rupturing their cranial cruciate ligament is the canine equivalent of an athlete tearing their ACL. Keep an eye out in future newsletters for an article on cranial cruciate ligament repair.
Poppy is a ten-year old Maltese cross who presented to us with a history of intermittent ‘skipping’ lameness in her left hind limb and an ongoing issue with a dislocated kneecap which would pop in and out of its groove. This was worsened after stepping on something during a walk, leading to a non-weight-bearing lameness, at which point she was brought into the clinic. On physical exam, she was not using her left hind leg, her left stifle was painful and unstable and she had luxating patellas in both stifles. To reduce her discomfort and to assess her knees and hips, she was sedated for x-rays and an orthopaedic exam, which revealed a Grade 3 MPL in her right stifle and a Grade 4 MPL with a fully ruptured cranial cruciate ligament in her left stifle. This resulted in severe instability in her left knee and was preventing her kneecap from popping back into place. Surgery was the only option to return proper function to her leg.
There are a variety of surgical techniques available to help correct a luxating patella. The techniques that are used depend on the severity of the joint disease and the dog's stifle conformation. Most commonly, a combination of techniques is used to achieve the best result.
‘Lateral Imbrication’ involves tightening the joint capsule to reduce the likelihood of the patella slipping and confining it to the patellar groove (also known as the trochlear groove). This technique alone can be appropriate for mild cases, but is most often used in conjunction with other techniques.
‘Trochlear Modification’ involves altering the depth of the trochlear groove in which the patella usually sits. In toy breed dogs, this groove is often congenitally shallow, which allows the patella to slip. This groove is lined by very important, slippery cartilage known as hyaline cartilage, which we want to preserve in order to reduce pain and arthritis and allow ease of movement of the joint. The hyaline cartilage is peeled away, the bone underneath sliced out to deepen the groove and the cartilage replaced.
In severe and long-term cases of patellar luxation where the ‘bow-legged’ appearance and conformational changes have already set in, the tibias (long bones in the lower leg) will have rotated. This results in the tibial crest where the thigh muscle attaches rotating inward. In this case, the crest has to be removed and repositioned in order to straighten out the leg. Severe rotation of the tibias can even involve having to cut through the entire bone to de-rotate it back into place and securing it with orthopaedic pins. This is referred to as a ‘Tibial Tuberosity Transposition’ or ‘TTT’.
Intra-articular surface of Poppy's left knee. Her torn cruciate ligament is at the central red area. Permanent changes to her cartilage (white areas) are already apparent, such as osteophytes.
Post-operative stifle x-rays (lateral).
Post-operative stifle x-rays (anterior-posterior).
In cases of bilateral MPL (when both knees are affected), some surgeons will prefer to repair each limb separately, allowing at least 8 weeks of recovery between surgeries so that the dog has one ‘good leg’ while the surgical leg is healing. In young patients, it can be a good idea to perform surgery on both legs at the same time to prevent conformation issues in the second leg. In Poppy’s case, due to the severe luxation in her second ‘good leg’ and the prolonged recovery time of dealing with multiple surgeries, we believed it beneficial for her to correct both stifles during the same anaesthetic.
Poppy at her 10-day post-operative check, a bit reluctant to use her left hind leg.
Proper at-home after-care and physiotherapy are essential in these orthopaedic cases in order to improve the animal's chances of a successful recovery. Poppy was put on 8 weeks of strict cage rest, with controlled leash walks for the bathroom, daily gentle physio and passive range of motion exercises with regular check-ups at the vet to assess her healing before gradually returning to normal activity. Luckily for her, after a successful surgery and a lot of diligent and loving, at-home care from her owner, Poppy has made a wonderful recovery and is back to walking around, running alongside the bicycle and enjoying life.