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Luxating Patellas

 

Also referred to as medial patellar luxation, MPL, lateral patellar luxation, LPL, dislocating knee cap


 

In a normal stifle, the patella (knee cap) is buried in the tendon of the quadriceps muscles, the extensor muscles which run down the front of the thigh. This tendon attaches to the tibial crest just below the knee. The patella should ride smoothly up and down in a groove within the femur and the extensor mechanism (quadriceps muscle, patella and its tendon) should be well aligned with each other.

In a stifle with a luxating patella the knee cap rides outside of the femoral groove when the knee is bent or flexed. This luxation/dislocation can happen either medially (patella rides on the inner aspect of the femur; most common, especially in little dogs) or laterally (patella rides on the outer aspect of the femur). 

Diagnosed in over 7% of puppies, luxating patellas are one of the most common orthopaedic condition in dogs.

Breed predispositions: The condition affects primarily small dogs, especially breeds such as miniature poodles, Chihuahuas, Pomeranians, Boston and Yorkshire terriers. The incidence in large breed dogs has been on the rise over the past ten years, with breeds such as Chinese Shar Pei, Flat-Coated Retrievers, Akitas, and Great Pyrenees now considered predisposed to this disease.

Bilateral incidence: Over 50% of animals affected will have luxating patella in both knees.

Developmental or traumatic?:


Luxating patellas occasionally result from a traumatic injury to the knee, causing sudden severe lameness of the limb. However, most cases are devlopmental and the cause is likely multifactorial.

In dogs with developmental patellar luxation the femoral groove into which the knee cap normally rides is commonly shallow or absent. Early diagnosis of bilateral disease in the absence of trauma and breed predisposition supports the concept that patellar luxation results from a congenital or developmental misalignment of the entire extensor mechanism. Developmental patellar luxation is therefore no longer considered an isolated disease of the knee, but rather a consequence of a complex skeletal abnormalities affecting the overall alignment of the limb, including:

abnormal conformation of the hip joint, such as hip dysplasia

malformation of the femur, with abnormal angulation and rotation

malformation of the tibia

deviation of the tibial crest, the bony prominence onto which the patella tendon attaches below the knee

tightness/atrophy of the quadriceps muscles

excessive length of the patellar ligament

In short - most dogs who suffer from luxating patellas - have bowed legs....

PLEASE NOTE that, because there is evidence that this condition is at least in part genetic, dogs diagnosed with patellar luxation should not be bred.

 

Surgical correction of luxating patellas should be considered as soon as the animal is symptomatic. The typical early presentation is a dog who intermittently “bunny hops” on its hind limb(s). Early cases may not show lameness but will run normally and soundly, with just intermittent hopping on both back legs at once or intermittently running on three legs and holding one up. As the condition progresses, legs may bow, cartilage wear and bone-on-bone contact produce pain so we can see lameness and joint swelling.

 

Grading luxating patellas

Luxating patellas are graded according to their ability to return to normal position.

Grade 1 – can be luxated manually but return to normal position automatically when pressure is released. Flexion and extension of the joint are normal

Grade 2 – mild changes of limb conformation may be present and the patella may be easily luxated manually. The patella remains luxated when pressure is released.

Grade 3 – medial displacement of the quadriceps muscle is noted +/- deformities of the tibia and femur. The patella is luxated most of the time but can be manually reduced to a normal position.

Grade 4 – the patella is permanently luxated and cannot be repositioned. The femoral groove will be shallow / absent, the quadriceps muscle will be medially displaced, soft tissues of the stifle may show abnormalities and the femur and tibia may have marked abnormalities. There may be an 80-90 degree medial rotation of the proximal tibia.


 

Does your dog need surgery?

Grade 1s typically do not require surgery but are just monitored for progression. Grade 2 or over usually warrant surgical correction as they cause pain and structural abnormalities. Often if a grade 2 patella is causing problems it will gradually become a grade 3 or 4 as the structures around the joint alter to allow the patella to luxate more easily. The more severe the grade - the harder, more invasive and potentially more expensive they can be to fix. In some cases - a stitch in time, genuinely does save nine.

What does surgery for medially luxating patellas involve?

Your animal will have an initial consultation with Dr Chris Franklin to assess the severity of patellar luxation and changes to surrounding structures.

The next step in the decision making process for surgery is to get xrays of the affected knee(s). Our usual approach is to book your animal in for a 24 hour stay during which we will

1. take pre anaesthetic bloods to check for any underlying medical conditions that we should be aware of prior to proceeding with an anaesthetic

2. put your dog on an intravenous drip including a constant rate infusion of pain relief

3. administer a general anaesthetic and epidural anaesthetic

4. take xrays of the affected knee

5. perform surgical corrected of the stifle. This depends on the chronicity and severity of the condition and usually involves all, or a combination of, the following

- block recession trochleoplasty

-tibial crest transpositon

-lateral retinacular imbrication

Block recession trochleoplasty

This is a method of deepening the femoral groove. We cut a block from the base of the femur, including articular cartilage and bone, remove a plate of bone from beneath this block then the block of bone is replaced. This process maintains the healthy articular cartilage surface but deepens the groove in which the patella runs.


 

Tibial crest transpostion

This is a process whereby the crest of the tibia, which is what the medially displaced extensor mechanism attaches to, is cut from the front of the tibia and reattached in a more lateral position.This involves fixation with pins and a tension band wire.


Lateral retinacular imbrication

This involves shortening the soft tissues that have stretched on the lateral side of the knee. It is done by trimming excess tissue away and placing sutures to tighten the fibrous joint capsule and lateral edge of the patellar tendon.

Post-operative xray:


Are there any other choices for extreme cases?

Some grade 4 dogs (and rarely grade 3) will have such a severe angular deformity of the bones in the back leg, that trying to straighten the mechanism of the quadriceps and patella alone will not work. In these cases it becomes necessary to straighten the leg, rather than the muscle mechanism. 

Dr Franklin performs 2-3 of these cases a year, as they are not that common. The surgery involves cutting the femur and removing a wedge to allow the bone to straighten once it is reconstructed. A post operative x-ray is shown below.


    Post-operative expectations

Your dog will be kept in the hospital on a pain relief drip for the rest of the day and the night after the surgery. They will be able to come home the next day. Strict room rest must be enforced when your dog is to be left home alone. Antibiotics and pain relief will be dispensed to prevent wound infections and inflammation post operatively. As for exercise, initially we suggest going out on a lead to go to the toilet only, with leash walking starting once your dog is using the leg well. Initially just 5 minutes a day and building up gradually, increasing walk length by 5 minutes each week. No off lead playing / jumping on and off furniture / playing with other dogs for 8 weeks.

By 8 weeks we expect your animal to have built up to 2 x 30 minute lead walks a day and be comfortable with a good range of movement and able to start off leash exercise.

Please note that it is normal for dogs to need intermittent use of anti-inflammatories throughout the 6 weeks post-operative period to maintain good range of movement and good muscle build up.

Rechecks will be scheduled for 3 days, 10-14 days, 4 weeks and 8 weeks post operatively. These will be free of charge unless further medication or xrays are required.

Physiotherapy (first consult included in the charge of your surgery and can be done on site here at the at East Mornington Referral Hospital) from the 3 day check