1. Luxatio patella

2. LUXATIO PATELLA CASE REPORT
A 2 year old FS miniature poodle, weighing 3kg (body type 5) presented to the hospital for a progressive history of hopping on one leg. The owners describe their dog running & periodically skipping/hopping on the right hindleg and then going back to normal. They describe that it happens about times a day. The dog is otherwise fine, non-painful and willing to exercise normally.

3. Pathophysiology
Patella luxations may be congenital or developmental, and there may be a genetic component.  This may explain why we see them in the same breeds over and over again.  The cause of patella luxations is related to the complex function of the hip joint, femur, patella, stifle joint and tibia.  Some conformations that have been incriminated are coxa vara or coxa valga, and femoral and tibial deformities.  These conformations can cause a shift of the patella out of the trochlear groove of the femur due to a shift of the weight-bearing forces over the patella.  There is wearing of the trochlear ridge and flattening of the groove, which leads to abnormal articular cartilage in the groove. 

4. Patella luxations have four classifications
Patella luxations have four classifications.  Clinically, the amount of lameness associated with this condition is variable.  Some dogs will have patella luxations their entire lives with no clinical symptoms.  Others will suddenly become more symptomatic after a period of soundness.  Animals with grade 4 luxations are often severely affected and have severe skeletal deformities as well.
Grade 1 The patella is in the groove normally and can be manually luxated.  It returns to the groove spontaneously when released.
Grade 2 Mild anatomic changes are present.  The patella is in the groove more than it is out, but can be manually luxated and will luxate spontaneously.  The patella will remain luxated until relocated manually or by extension and derotation of the joint.
Grade 3 Mild to moderate anatomic changes are present.  The patella is out of the groove more than it is in.  It will luxate spontaneously, can be manually reduced, but will luxate upon release. 
Grade 4 Moderate to severe anatomic changes are present. The patella is out of the groove all the time and cannot be manually replaced. Surgical intervention is required for reduction. Click here to open a picture of the stance of a dog with bilateral grade 4 luxation.

5. Radiographs These radiographs illustrate a grade 4 patellar luxation
Radiographs These radiographs illustrate a grade 4 patellar luxation.  Note the position of the patella, the rotation of the femur, and the deformity of the tibia on the cranial-caudal view.  On the lateral view, note the position of the patella. 

6. Trochleoplasties can be done one of four ways.
Abrasion trochleoplasty involves rongeuring or rasping the articular surface of the groove down to subchondral bone. This will fill in with fibrocartilage and remain deep as long as the patella rides in the groove.
The patellar groove is deepened with a rasp.
The end result is a more stable patellar groove.

7. A wedge recession trochleoplasty involves cutting and removing a V-shaped wedge out of the groove. The resulting groove is enlarged, and the removed wedge is replaced into the groove. This retains the original hyaline cartilage in the groove and is particularly good in young animals.
A hobby saw is used to remove a wedge-shaped portion of the patellar groove.
The groove is widened by removing an additional piece.
The original wedge is replaced in the groove, recessed from its original position.0

8. Trochleoplasties,
continued:
Other options include trochlear chondroplasty, which involves elevating the hyaline cartilage flap with no bone, rongeuring the underlying bone and then replacing the cartilage flap in this deepened groove. This only works in dogs younger than 10 months, because their cartilage is thicker and more pliable than older dogs. Block trochleoplasty (seen below) is virtually identical to a wedge recession, except the removed piece is rectangular instead of wedge-shaped, and some surgeons feel it provides a deeper recess and greater stability.

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