Femoropatellar instability leading to patellar luxation is a common cause of lameness in dogs.
The condition can vary from complete, irreducible luxation of the patella and severe lameness, to mild instability without associated clinical signs. The luxation can be lateral, medial or proximal and the cause can be congenital or traumatic.
The most common cause of patellar luxation is congenital medial patellar luxation in small dogs such as miniature Poodles, Chihuahuas and Yorkshire Terriers.
Medial patellar luxation is the most common luxation in all sizes of dogs. Lateral luxation is much rarer, and in larger dogs carries a poorer prognosis. Lateral luxation may also occur in horses.
Signs usually appear before 2 years of age.
Cats can also suffer from patellar luxation, but much less commonly than dogs.
The cause is though to be due to the underdevelopment of the medial femoral condyle, which allows displacement of the patella.
Acute traumatic patellar luxation can be medial or lateral and is associated with acute, non-weightbearing lameness and pain on manipulation of the joint.
Congenital medial luxations are graded according to clinical signs and palpation findings:
- Grade 1: Patella luxated medially easily, returns when released, no crepitation or bony deformity and no/infrequent clinical signs
- Grade 2: Spontaneous luxation occurs, with a non-painful, skipping lameness. There may be mild bony deformities such as internal rotation of the tibia and abduction of the hock.
- Grade 3: The patella is permanently luxated, but can be manually returned. There is marked tibial rotation and a shallow trochlear groove. It is often bilateral and results in crouched gait.
- Grade 4: Permanent, non-reducible luxation of the patella. Tibia is rotated 60-90° relative to the sagittal plane. Severe bony deformities develop if not corrected. Crablike posture and abnormal gait will be present.
There may be muscle atrophy in the affected limb.
History, clinical signs and palpation usually allow diagnosis of the condition and permit grading of congenital medial patellar luxation.
Radiographycan document the luxation and is useful to assess bony deformity and degenerative joint changes.
Skyline views of the distal femur are useful pre and post-operatively to assess the depth and contour of the femoral trochlea.
Treatment should be individual for each patient, as there are such varying presentations and causes for the condition.
Traumatic luxations should be repaired by suturing the medial or lateral fascial defect.
Grade 1 medial luxation is treated conservatively and the dog is reevaluated if lameness develops.
Grade 4 cases are treated surgically early in life to prevent severe bony deformity and disability.
Dogs with grade 2 or 3 with only mild lameness and degenerative changes usually do not need corrective surgery unless lameness is severe.
Cranial cruciate ligament rupture may occur in any dog with medial patellar luxation due to the strain placed on the ligament, and should be treated surgically.
The decision for surgery is made if lameness is of concern to the owner and a disability to the dog.
Surgical correction requires realignement of the extensor mechanism and stabilisation of the patella in the femoral trochlea.
This can be achieved by:
- Tibial tuberosity transposition
- Femoral trochleoplasty to deepen the groove: abrasion trochleoplasty, recession trochleoplasty
- Wedge resection
- Releasing incisions and imbrication of the capsular tissues
- Lateral rotation of the tibia
Post-operatively: exercise restriction for 4-6 weeks and passive range-of-motion exercises, weight loss and NSAIDs are recommended.
The luxation can recur if there was inadequate correction of the deformity.
Prognosis is favourable for small dogs with grade 1-3 medial patellar luxation.
Large dogs with medial patellar luxation also do well if the luxation is corrected before cartilage erosion has occurred.
Grade 4 luxation can be corrected in dogs less than 3-4 months of age. After that, deformity is severe and the prognosis is guarded.
Lateral patellar luxation has a good prognosis if it occurs alone, but the prognosis is guarded if it occurs along with other limb deformities.
|Patellar Luxation Learning Resources|
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|Small Animal Orthopaedics Q&A 16|
Pasquini, C. (1999) Tschauner's guide to small animal clinics Sudz Publishing
Slatter, D. (2002) Textbook of small animal surgery Elsevier Health Sciences
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