Patellar luxation is a condition typically affecting young small and toy-breed dogs but can also be seen in large-breed dogs and cats. While patellar luxation can be caused by trauma, it is usually due to conformational deformities of the rear limbs that will affect muscle pull and alignment leading to luxation of the patella.

Important anatomy:

1.  Quadriceps muscle

a.  Vastus lateralis

b.  Vastus medialis

c.  Vastus intermedius

d.  Rectus femoris

2.  Patella - sesamoid bone of quadriceps muscle

3.  Patella tendon - connecting the patella to the tibial tuberosity

4.  Tibial tuberosity

5.  Medial and lateral joint capsule and fascia (retinaculum)

6.  Trochlear groove and ridges of the distal femur

True traumatic luxation occurs in light of normal bony anatomy and is the result of disruption of soft tissues and forceful luxation of the patella. Luxation can be lateral or medial and animals will be very lame and painful initially. If addressed early, surgery may only need to address the disrupted fascia and joint capsule and further corrective procedures may not be needed if the conformation is normal.

Congenital or developmental patellar luxation can be lateral or medial; however, medial luxation is by far the most common in both large- and small-breed dogs. If lateral luxation occurs, it is generally in larger-breed dogs. Congenital/developmental patellar luxation can be associated with varying degrees of limb deformities, such as increased or decreased femoral neck angle (coxa vara and valga) and torsional and angular deformities of the femur and tibia. Conformational abnormalities of the coxofemoral joint, femur, stifle joint, and tibia are thought to be the underlying cause of patellar luxation as these can change the forces of the quadriceps mechanism.

Patellar luxation can occur in different severity grades (1 through 4).

 Grade 1: The patella is riding in the trochlear groove but it can be manually luxated with the leg in extension. Spontaneous luxation does not or rarely occur and clinical signs are usually absent. This grade is often an incidental finding during physical examination. Bony deformity is absent.
In - can be luxated

 Grade 2: The patella luxates spontaneously although it generally rides in the trochlear groove. Lameness occurs intermittently as the patella luxates and luxation is noted with varying frequency. Mild deformities are present (i.e., internal rotation of the tibia, abduction of the hock). Animals may become painful if cartilage abrades along the patella and trochlear ridge.
In - luxates on own

 Grade 3: The patella is permanently luxated. It can be reduced into the trochlear groove but will luxate again. Bony deformities are present and the use of the leg may be impaired functionally as well as due to pain from eroded cartilage.
Out - can be reduced

 Grade 4: The patella is permanently luxated and cannot be reduced. Severe bony deformities are present. The tibia is rotated to up to 90 degrees and there is femoral valgus or varus. This form is often present in very young animals and, if left uncorrected, will lead to severe, function-impairing permanent deformity.
Out - cannot be reduced

The higher the grade, the more obvious clinical signs usually are.

Most dogs will show intermittent lameness with skipping of steps, holding a rear limb at an abnormal angle or 'locking' up the knee joint before returning to a normal gait. Patellar luxation often occurs bilaterally and signs may be seen in both legs. If cartilage becomes abraded from constant rubbing of bone on bone, lameness and pain will be more severe. With grade 3 and 4 luxations, dogs may also experience a mechanical lameness due to alteration to their muscle-pulley mechanism when the patella is luxated. Concurrent rupture of the cranial cruciate ligament can be seen in 15–20% of dogs - most showing significantly more lameness due to the cruciate tear compared to the luxating patella.

Diagnosis of PL is based on physical exam findings. A thorough orthopedic exam should include gait analysis, a standing exam as well as palpation/manipulation when the animal is laying down. Tracking of the patella is assessed during range of motion. Manual pressure is applied in extension to try to luxate the patella. Internal and external rotation of the tibia can further encourage the patella to luxate. It is important to evaluate the integrity of the cranial cruciate ligament to rule out a cruciate tear. Also, if manual luxation of the patella is associated with a lot of pain, you should suspect that cartilage erosions on the patella and trochlear ridge are present. While radiographs may show luxation, physical examination remains the primary means to diagnose this condition because luxation may be reduced while films are taken. Instead, radiographs are used to assess bony conformation and degenerative joint changes. Patella luxation alone does not usually cause significant DJD; therefore if arthritic changes are present, other causes should be investigated (such as a CCL tear).

Radiographic changes for MPL could include:

 Coxa vara - decreased femoral neck angle

 Distal femoral varus and genu varum - medial bowing of the distal femur with bow-legged stance

 Medial displacement of the tibial tuberosity - this varies with degree of MPL but can be up to 90 degrees

 Shallow trochlear groove

Radiographic changes for LPL could include:

 Coxa valga - increased femoral neck angle

 Distal femoral valgus with genu valgum - lateral bowing of the distal femur with knock-kneed stance

 External femoral torsion

 Lateral displacement of the tibial tuberosity

 Shallow trochlear groove

Treatment can be conservative for low-grade patella luxation, especially if dogs are not showing persistent lameness and have overall good function. If pain is present or lameness becomes more prominent however, surgery should be considered. For higher grades, surgical correction is usually indicated, especially in grade 4 luxation since function is severely impaired. The goal of surgery is to realign the extensor mechanism (quadriceps muscle, patella, patella tendon, and tibial tuberosity) and allow the patella to track in the trochlear groove. Depending on the degree of changes, surgery usually involves the following four procedures: deepening the trochlear groove to capture the patella, moving the quadriceps into better alignment, and adjusting joint capsule and fascial tension through a releasing incision on one side of the joint and imbrication on the other. Soft-tissue procedures such as fascial/retinacular release and imbrication are generally not sufficient as the only corrective measure.

Different techniques to deepen the trochlear groove:

1.  Abrasion trochleoplasty: the trochlear groove is deepened by removing cartilage and bone with a rongeur or high-speed burr, leaving the defect to fill in with fibrocartilage.

2.  Chondroplasty: only used in very young animals - hyaline cartilage is elevated, subchondral bone is removed, and the cartilage flap is repositioned over the deepened groove; in older animals, the hyaline cartilage cannot be separate from underlying bone anymore.

3.  Recession trochleoplasty: hyaline cartilage is maintained by cutting a wedge or a block within the trochlear groove, removing bone underneath, and replacing the wedge or block again in its recessed position. Block recession has the advantage over wedge recession that a larger area can be recessed, therefore ensuring good femoropatellar contact.

The goal of these procedures is to recess the patella at least 50% into the trochlear groove.

Different techniques to realign the quadriceps mechanism:

1.  Tibial tuberosity transposition: an osteotomy is performed to cut the tibial tuberosity and patella tendon insertion taking care to leave the distal periosteal attachment intact. The tuberosity is then moved an appropriate distance laterally (in MPL) or medially (in LPL) to aid in alignment of the quadriceps mechanism. The bone is reattached to the proximal tibia using two K-wires.

2.  Antirotational suture: for MPL a lateral suture is placed in the same fashion as for extracapsular cruciate ligament repair (suture loop from a hole through the proximal tibial tuberosity around the lateral fabella) and tightened to rotate the tibial and tibial tuberosity into a more normal position. Since the suture will ultimately fail, this technique is typically not sufficient to permanently correct MPL and is not commonly performed. It can, however, be used in dogs with cranial cruciate tears and concurrent mild MPL.

3.  Corrective osteotomy: if severe deformity of the bone is present, correction of angulation is performed to align the quadriceps mechanism. Several different techniques can be employed such as closing and opening wedge osteotomy. Rigid fixation, usually with internal plate fixation, is required after osteotomy. Most large-breed dogs with higher-grade patellar luxation require corrective osteotomies - 'local' correction at the level of the stifle is not enough to permanently eliminate luxation because the abnormal forces acting on the patella usually prove too strong.

Corrective soft-tissue procedures:

1.  Fascial release on side of luxation: in the case of MPL, the medial aspect of the stifle joint capsule as well as fascia is cut to allow the patella to be moved laterally into the groove. In high-grade luxation, a large gap usually remains after release of these tissues, which can be closed with subcutaneous tissue or a fascial strip from the other side of the patella.

2.  Fascial imbrication on opposite side of luxation: in the case of MPL, the now more redundant lateral aspect of the stifle joint capsule and fascia are tightened (either by cutting out a strip of tissue or using an everting or overlapping suture pattern). Ultimately the goal is to have both sides (medial and lateral) of the patella be tightly sutured to force the patella into its new groove. This will provide good femoropatellar articulation while the quadriceps mechanism can adjust to its new position.

A soft padded bandage is usually applied after surgery for several days. Dogs must be exercise restricted to short leash walks to the bathroom only with confinement otherwise until the tibial tuberosity osteotomy has healed (usually this takes anywhere from 4 to 8 weeks depending on the age of the dog). Postoperative complications may be K-wire migration, seroma formation around the tips of K-wire, delayed union of the tibial tuberosity osteotomy, fracture of the tibial tuberosity, fracture of the distal femur due to aggressive trochlear deepening, and development of degenerative joint disease. With proper surgery, most dogs with grade 1 and 2 luxation should have a good to excellent prognosis for return to normal function. Owners of dogs with grade 3 and 4 luxation must also be informed about the potential for reluxation (especially with grade 4). If bilateral patellar luxation is present, surgery is often staged to provide a 'good' leg to walk on for a few weeks while the other one is healing. In very young dogs, bilateral surgery may have to be done to avoid further bony deformation.