Patella luxation (Proceedings)

Congenital patella luxation (MPL; LPL)

Synonyms

Medial patella luxation (MPL), Lateral patella luxation (LPL)

Etiology

Coxa vara (bowleg), and excessive retroversion of the femoral head causes of MPL. Coxa valga (knock-kneed) cause LPL. Traumatic patella luxation is not discussed here.

Pathogenesis

The patella is a type A (primary function is articulation) sesamoid bone located in the tendon of insertion of the quadraceps muscles. The origins of the quadriceps muscles are the proximal femur and immediately cranial to the acetabulurn (rectus femoris m.). The quadriceps m. follows a straight line, by necessity, to its insertion at the tibial crest. If the quadriceps muscles are displaced medial, then the patella must also be displaced medially. Coxa vara, and to some extent excessive retroversion, displace the origin of the quadriceps m. medially. In the 6 week old dog, altered quadriceps muscle pull causes permanent bone changes in 2-4 weeks. The resulting deformities are bowing and torsion of the distal femur, bowing and torsion of the proximal tibia due to compression from the medially located quadriceps. The trochlear groove is shallow because pressure from the patella on the articular-epiphyseal growth plate is necessary to retard bone growth locally and form a trochlear groove. Absence of the patella due to luxation results in failure of the trochlear groove to achieve a normal depth. Rotational instability of the stifle occurs because of stretching of the lateral joint capsule and other lateral supporting structures. The severity of MPL is progressive until about 6 months of age. Grade II - IV MPL in a juvenile dog can be expected to progress and surgical correction should not be delayed. The grade of MPL is stable after 6 months of age. However, DJD will be progressive and there is a report of 15% increased risk of cranial cruciate ligament rupture in dogs with MPL.

Signalment

MPL is a very common juvenile joint disease which necessarily develops in dogs less than 6 months of age. MPL and LPL are developmental diseases, not "congenital" and the conventional name indicates. However, dogs may not be presented until months or years later when DJD or cruciate rupture become clinical. Predominantly small breeds of dogs are affected, although it does occur in large dogs. Lateral patella luxation is a less common occurrence and typically occurs with coxa valga associated with hip dysplasia in large breeds of dogs. Feline patella luxation is rare and occurs in association with bilateral congenital hip luxation. There is no sex predisposition. A review of 124 cases of patella luxation reported that in small breeds 98% are medial and 2% are lateral; in medium breeds 90% are medial and 10% lateral; large breeds have 80% medial and 20% lateral; giant breeds 66% medial and 33% lateral.

History

Grade 1 and 2 MPL are often an incidental finding on orthopedic examination. History often reveals the dog carries his leg for a few steps then returns to weight-bearing, without evidence of pain. Onset is insidious. Dogs with grades 3 and 4 patella luxation have more severe and consistent lameness and deformity of the leg(s).

Clinical exam

Observation of a dog with MPL from the rear will demonstrate a bowlegged stance with the point of the hocks (calcaneal tuber) pointed laterally. Walking the dog may or may not carry the leg for a few steps then return to weight-bearing. MPL occurs bilaterally in most cases. With grade 4 MPL the dog is unable to extend the affected leg to the floor. Palpation and manipulation of the stifle is not painful. Medial patella luxation is graded during physical examination by extending the stifle and determining the location and ability to reduce the patella.

Grade 1: The patella is located in the trochlear groove. It can be manually luxated, but reduces itself as soon as manual pressure is released.

Grade 2: The patella may be found either in or out of the trochlear groove, and is easily manipulated to the other location, where the patella remains when manual pressure is stopped.

Grade 3: The patella is located out of the trochlear groove. It can be manually reduced, but reluxates as soon as manual pressure is released.

Grade 4: The patella is out of the trochlear groove and cannot be reduced. As the grade of medial patella luxation gets worse, so does the degree of bowing of the femur and tibia, rotation of the tibia and shallowness of the trochlear groove.

"Sloppy" patella is when the patella can be luxated both medially and laterally. It is important to evaluate for this condition during examination so appropriate surgical correction for the lateral luxation (imbrication of the medial joint capsule) can be performed.

When evaluating patella luxation, the amount of internal rotation of the tibia is important. A straight tibia helps reduce the patella, while internal rotation (which can be extreme in some dogs) pulls the patella medially.

Ancillary exam(s)

Radiography is only useful in documenting the severity of DJD, and assessing the amount of osseous deformity if osteotomies are required for treatment of grade 3 & 4 MPL. Synovial fluid analysis also documents DJD.

Treatment(s)

Goals of surgery are to straighten the quadriceps mechanism (quadriceps-patella/trochlear groove-tibial crest), achieve adequate depth of the trochlear groove and to effect minimal postoperative pain (both acute and chronic) so the patella will stay in the trochlear groove and the dog will use the leg. Some authors state a set list of procedures for a given grade of MPL. Others advocate use of prosthetic sutures to pull the patella and/or tibial crest laterally rather than straighten the quadriceps mechanism. It is our belief that each case is unique and any combination of the procedures described below should be used as needed to achieve the surgical goals.

Abrasion trochleoplasty is deepening the trochlear groove with a file, threaded Steinmann pin, or burr. The groove should be deepened until 50% - 100% of the patella's cranial to caudal depth will sit in the groove (as with the other techniques of deepening the trochlear groove). Excessive deepening will hold the patella in place but will cause chronic pain and prevent the dog from using the leg. This technique destroys the hyaline cartilage, which is replaced with fibrocartilage. The technique is simple, but tends to cause more postoperative morbidity in large dogs.

Wedge resection of the trochlear groove deepens the trochlear groove by removing the trochlear groove, deepening the bone bed from which it came, then replacing the trochlear groove. A fine toothed "hobby saw" is used to make a cut from the lateral trochlear ridge that is directed caudally to the midline of the bone (approximately the origin of the caudal cruciate ligament) with the saw blade placed lengthwise from proximal to distal. A similar cut is started at the medial trochlear ridge and meets the first cut caudo-centrally. The trochlear groove with its subchondral bone is lifted out and the cut in the femur is deepened with a second cut or a flat file. The wedge is then replaced, resulting in a deeper trochlear groove. Pins or other fixation devices are not needed to hold the wedge in place. The advantage of this technique is that it preserves normal hyaline cartilage. Although it is technically simple in larger dogs (> 15 lbs), it becomes technically more difficult as the dog becomes smaller.

Block resection of the trochlear groove is similar to the wedge resection. Rather than a pie shaped wedge, a brick shaped block is made by parallel craniocaudal cuts at each trochlear ridge with a "hobby" saw which are joined by a distal to proximal osteotome cut in the coronal plane. Wedge vs block resections are selected based on width to depth ratios and surgeon preference.

Chondroplasty is a groove deepening technique that is possible only in young dogs (i.e. < 5 months). The hyaline cartilage is cut with a scalpel and the cartilage elevated from the subchondral bone with a periosteal elevator, a procedure that is not possible in older dogs without tearing the cartilage. Subchondral bone is removed to the desired depth with rongeurs or other instruments, and the cartilage laid back in place. This procedure also preserves hyaline cartilage and is possible in any size stifle. If the cartilage is damaged during elevation then trochleoplasty is performed.

Lateral imbrication of the joint capsule and retinaculum pulls the patella laterally. The amount of tightening is based on judgment to make the patella track in the center or slightly lateral in the trochlear groove. The most important sutures are from the center of the patella distally toward the tibial crest, and should be placed first to evaluate effectiveness. It is important to imbricate the joint capsule since tightening the retinaculum alone will not prevent luxation.

Medial desmotomy releases the medial pull of the joint capsule and retinaculum. This should not be done unless the patella cannot be reduced otherwise. Equal tension from the medial and lateral joint capsules pulls the patella caudally into the trochlear groove. Medial desmotomy allows the patella to displace cranially and makes it easier to luxate, perhaps laterally.

Tibial crest transposition realigns the quadriceps mechanism by moving the tibial crest laterally (for medial patella luxation). An osteotome is used in the medial to lateral plane. The tibial crest is left attached distally by a small amount of bone and periosteum to minimize the risk of avulsion of the transposed crest by pull of the quadriceps. A new bed is created for the tibial crest laterally and the crest is "broken" (breaking the distal attachment without breaking the periosteum) over to its new position. A K-wire or 2 are used to fix the tibial crest in place.

Derotational suture is placed laterally from the lateral femorofabellar ligament through the tibial crest, as a lateral suture for cranial cruciate ligament rupture. The tibial crest is held in external rotation while the derotational suture is tied. Unlike, the tibial crest transposition, a derotational suture constantly holds the patella tendon insertion laterally, and does not have the risk of avulsion of the patella tendon insertion.

Osteotomy of the distal femur and/or proximal tibia may be required in grade 3 and will be required in grade 4 MPL to straighten the leg. Fixation is typically with bone plates.

Prognosis/client education

Prognosis varies from excellent with grade 1 to terrible with grade 4. The second most important predictor is whether surgery is performed unilaterally or bilaterally; if patella luxation exist bilaterally then surgery should be performed bilaterally to promote self induced physical therapy on both rear legs. Grade 2 patella luxation will usually respond well to surgical correction if DJD and/or cranial cruciate rupture are not present. If CCL rupture is present, surgery to stabilize the CCL takes precedence, and surgery for patella luxation (other than joint capsule imbrication) should be delayed as the postoperative treatments for these two surgeries are diametrically opposed. If the leg carrying lameness is chronic then difficulty may be encountered in getting the dog to bear weight on the leg despite an excellent surgery. These dogs are light weight and can get along very well on three legs, plus the habit of carrying the leg. Any pain in the stifle after surgery will also cause the dog not to use the leg. If the dog is < 6 months old the grade of MPL may worsen and surgery should not be delayed. Dogs > 6 months old are not urgent.