Patella luxations (Proceedings)


The patella or kneecap is the largest sesamoid bone in the body and one of 4 in the stifle, the other three being the 2 of the gastrocnemius and the popliteal muscle. The patella lies within the tendon of insertion of the quadriceps muscle. The part of the tendon between the apex or distal aspect of the patella and its insertion on the tibial tuberosity is called the patellar ligament.

The patella or kneecap is the largest sesamoid bone in the body and one of 4 in the stifle, the other three being the 2 of the gastrocnemius and the popliteal muscle. The patella lies within the tendon of insertion of the quadriceps muscle. The part of the tendon between the apex or distal aspect of the patella and its insertion on the tibial tuberosity is called the patellar ligament. The caudal surface of the ovate shaped sesamoid should articulate with the trochlea or trochlear sulcus of the distal femur. In animals with patella luxations, the patella displaces from the normal positioning within the trochlea.

Luxations are typically congenital or developmental but can also be traumatic. 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 patella luxations in small animals occur in both the dog and cat. In dogs, luxations occur in both large and small breeds although luxations in small breeds are more common. The most common luxation is medial regardless of the size of the animal. Lateral luxations, although less common, are seen more often in large breed dogs. Since the patella plays an important role in the entire extensor mechanism, abnormalities along the system can influence the function and position of the patella and vice versa. Patellar luxations may also be associated with cranial cruciate ligament deficiency, abnormal coxofemoral joint confirmation such as increased or decreased femoral neck angle (coxo valga/vara) and hip dysplasia, angular deformity of the femur or tibia (congenital or traumatic), abnormalities of the quadriceps muscle or a patellar ligament that is too long or short. At times it can be difficult to determine cause and effect. The entire system should be evaluated prior to delving into surgical correction. Greater than 50% of patients have bilateral luxations.

Clinical signs

Most owners will report that the dog uses the leg normally but will periodically skip, hop, kick out the leg or stretch the leg when the patella luxates. As the disease progresses, the degree or frequency of clinical signs may increase. Each time the patella luxates there is potential cartilage damage on both the underside of the patella and the trochlea. Dogs with continuous medially luxated patellas, especially young growing dogs, may have a 'bow-legged' appearance or genu varum. Dogs with continuous laterally luxated patellas, especially young large breed dogs, may have a 'knock-kneed' appearance or genu valgum. Some pets and owners have developed an almost symbiotic relationship in regards to patella luxations. When the pet luxates a patella they may approach the owner or 'signal' to the owner. The owner then massages, stretches or rubs the leg ultimately reducing the patella. Concurrent rupture of the cranial cruciate ligament can be seen in 15-20% of dogs. These dogs may show significantly more lameness due to the cruciate tear compared to the luxating patella along with progressive development of osteoarthritis.

Grading or Severity

Patella luxations are grade one through four (1-4) based on increasing severity of the luxation. Surgical treatment is considered for luxations greater than grade1.

     • Grade 1: The patella can luxate when manual pressure is applied but reduces when the pressure is released. Spontaneous luxation does not occur or rarely occurs. Flexion and extension of the joint is normal.

     • 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.

     • Grade 3: The patella rides out of the trochlear groove but can still be manually reduced. Once reduced, after flexion and extension of the stifle, the patella generally reluxates. Abnormalities of the soft tissues surrounding the stifle along with deformities of the femur and tibia may be present.

     • Grade 4: The patella rides out of the trochlear groove and cannot be manually reduced. There may be a shallow or absent trochlear groove, 80-90° rotation of the tibial plateau, displacement of the quadriceps mechanism, abnormalities of the soft tissue support structures of the stifle along with deformities of the femur and tibia.

Palpation of Patellar Luxations:

The initial evaluation should start with observation of the gait at a walk and trot. Dogs with grade 2 luxations may periodically hop, skip, kick out the leg or stretch the leg when the patella luxates. Dogs with grade 3 and 4 medial luxations may have a 'bow-legged' appearance or genu varum. Dogs with grade 3 and 4 lateral luxations may have a 'knock-kneed' appearance or genu valgum. While the patient is standing I palpate both rear limbs assessing for thigh muscle circumference and current position of the patella. The cranial head of the Sartorius muscle can be palpated on the cranial aspect of the thigh. This muscle is followed distally to a patella. Normally this should align on the cranial aspect of the limb. With medial luxations the distal aspect of the Sartorius will palpate slightly medial in reference to the proximal aspect. The reverse is true of lateral luxations. I also palpate the stifle itself. The cranial aspect of the stifle should have a convex boney feel to it if the patella is positioned properly. If the patella is luxated, the most cranial aspect of the stifle may have a convex or flat boney feel to it since the convex patella is no longer positioned within the groove. With one hand supporting the patient under the pelvis, I grab the distal femur/stifle and circumduct the hip. I have found that some patellas luxate easily this way where as others require direct manual pressure. After I have completed a thorough gait and standing exam, I evaluate the patient in lateral recumbency. Tracking of the patella is assessed during range of motion. Witht eh hip neutral or flexed and the stifle in extension, manual pressure is applied to luxate the patella. Internal and external rotation of the tibia crest 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. In some patients sedation may be required to perform a thorough exam.


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 osteoarthritis; therefore if arthritic changes are present, other causes should be investigated (such as a cranial cruciate ligament tear). Large breed dogs or dogs with more severe and chronic luxations tend to have more osteoarthritic changes than small breed dogs. Radiographs are not limited to orthogonal views of the stifle. Radiographs of the femur and tibia may also be indicated to assess for deformities/abnormalities of the hip, femur and tibia.

Surgical management:

Surgical stabilization of the patella typically falls in to two categories – bone reconstruction or realignment and soft tissue reconstruction. Soft tissue reconstruction alone cannot make up for a failure to perform bone reconstruction/realignment when it was indicated. The most common procedures involving the bone are deepening of the trochlear groove or sulcus, movement of the tibial crest and, occasionally, corrective osteotomy of the femur or even tibia.

Different techniques to deepen the trochlear groove:

     1. Abrasion trochleoplasty: Cartilage and bone are removed from the trochlear groove with a rongeur, rasp/file or high-speed burr, leaving the defect to fill in with fibrocartilage.

     2. Recession trochleoplasty: Hyaline cartilage is maintained by cutting a wedge or a block within the trochlear groove with a Hobby saw or sagital saw. Bone is removed underneath and the wedge or block is replaced again in its recessed position. Block recession has the advantage over wedge recession that a larger area can be recessed, especially proximally in the groove where the patella luxates more commonly.

     3. 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 is too thin and cannot be separated from underlying bone anymore.

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

Alignment of the extensor mechanism:

     1. Tibial tuberosity transposition (TTT): An osteotomy is performed to cut the tibial tuberosity and patella tendon insertion taking care to leave the distal periosteal attachment intact. This is typically performed with a mallet and osteotome, sagital saw or bone cutters. The tuberosity is then moved an appropriate distance laterally (in MPL) or medially (in LPL) to aid in alignment of the quadriceps-mechanism. In addition the crest may be moved distally in cases of patella alta or high-riding patellas. The bone is reattached to the proximal tibia using two k-wires in small breed dogs and cats or a pin/tension band in large breed dogs.

     2. Antirotational suture: In medial luxations, a lateral suture is placed in the same fashion as an extracapsular cruciate ligament repair (around the lateral fabella, under the patellar ligament, through a hole in the proximal tibial tuberosity) and tightened to rotate the tibial and tibial tuberosity into a more normal position. The suture should be a heavy gauge, nonabsorbable material such as nylon or flurocarbon. In lateral luxations, the suture is placed around the medial fabella instead. Since the suture will ultimately fail, this technique is typically not sufficient to permanently correct medial luxations, especially in larger breed dogs, 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. Normal anatomic femoral varus angles of 5-7.50 have been reported. It has been suggested to correct varus angles great than 10-120 although no definitive studies have been performed to date. Femoral torsion may also need corrected. 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.

Soft tissue correction:

     1. Fascial release: In the case of a medial luxation, 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 a 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: In the case of a medial luxation, 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 femoro-patellar articulation while the quadriceps-mechanism can adjust to its new position.

Radiographs may be performed after surgical repair to document patellar position for the owner and assess implant placement.


Overall the prognosis for return to function after patellar surgery is good. Complications include reluxation, infection, implant migration or failure and continued lameness. The larger the dog or the more severe the grade of luxation prior to surgery, the higher the complication rate. Tibial tuberosity transpositon and trochlear deepening are reported to reduce the risk of reluxation and major complications.

Postoperative care:

A soft padded bandage (modified Robert Jones) may be placed over night to aid in compression and support of the soft tissues. Alternately the stifle may be iced regularly (10-15 minutes 3-4 times a day) to reduce swelling and pain. The patient is typically restricted to short controlled leash walks with no running, jumping or playing for 6-12 weeks depending on the exact surgical procedures performed and surgeon's preference. Reevaluation and radiographs may be performed 6-8 weeks post surgery to assess limb use, patellar position, patient comfort and healing of osteotomies.

Medical management:

Since osteoarthritis has been shown to develop with patella luxation or after stifle surgery, medical management of osteoarthritis may be indicated. This includes, but is not limited to, weight management, joint supplements (glucosamine and chondroiten), omega 3 fatty acids, as-needed use of nonsteroidal anti-inflammatory drugs (NSAID's), low-impact exercise and physical rehabilitation. Physical rehabilitation alone has not been shown to correct or prevent patella luxations however it can help with strengthening and pain management both pre and postoperatively.

Additional reading:

Textbook of Small Animal Surgery, Ed: Douglas Slatter, 3rd edition. Pub. Saunders of Elsevier Science, Philadelphia, 2003

Handbook of Small Animal Orthopedics and Fracture Repair, Ed: D Piermattei, G. Flo, C. DeCamp, 4th edition. Pub. Saunders of Elsevier Science, Philadelphia, 2006

Small Animal Surgery, Ed. T. Fossum, 3rd edition. Pub. Mosby of Elsevier Science, St. Louis, 2007

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