The patella or knee cap is a small bone buried in the tendon of the quadriceps muscles of the thigh. The patella normally rides in a femoral groove within the stifle. The patellar tendon attaches on the tibial crest, just below the knee. The quadriceps muscle, the patella and its tendon form the “extensor mechanism” and are normally well-aligned with each other. Patellar luxation is a condition where the knee cap rides outside the femoral groove when the stifle is flexed. It is further characterized as medial or lateral, depending on whether the knee caps rides on the inner or on the outer aspect of the stifle.
Incidence of Patellar Luxation
Patellar luxation has been described as the most common congenital anomaly in dogs. Patellar luxation affects both knees in 50% of all cases, resulting in discomfort and bilateral lameness. In most studies, small-breed dogs are 10 times more commonly affected than large dogs, especially the Boston terrier, Chihuahua, Pomeranian, miniature poodle and Yorkshire terrier . A recent study of breed susceptibility for developmental orthopedic diseases found a predisposition for patellar luxation in 32 breeds, including larger dogs such as Chinese Shar Pei, flat-coated Retriever, Akita and Great Pyrenees.
- Dogs with patellar luxation are born with the disease (congenital) in 82 % of cases
- Both knees are affected in 50% of all cases
- The luxation is medial in 98% of small dogs, around 80% in medium and large-breed dogs and 67% of giant-breed dogs
- Females are 1.5 times more likely to be affected than males.
Although a specific cause of patellar luxation remains unknown, early diagnosis of bilateral diseases in the absence of trauma and breed predisposition are suggestive of a congenital or developmental misalignment of the extensor mechanism. Possible causes include:
- Abnormal conformation of the hip joint, such as hip dysplasia
- Malformation of the femur, with angulation and torsion
- Malformation of the tibia
- Deviation of the tibial crest, the bony prominence onto which the patella tendon attaches below the knee
- Tightness/atrophy of the quadriceps muscles, acting as a bowstring
- A patellar tendon that may be too long or too short
Although the femoral groove into which the knee cap normally rides may be shallow or absent in a majority of cases, patellar luxation is no longer considered an isolated disease of the knee.
Signs and Symptoms
Clinical signs associated with patella luxation vary greatly with the severity of the disease: this condition may be an incidental finding detected by your veterinarian on a routine physical examination or may cause your pet to carry the affected limb up all the time. Most dogs affected by this disease will suddenly carry the limb up for a few steps. . As the disease progresses in duration and severity, this lameness becomes more frequent and eventually becomes continuous. In young puppies with severe patella luxation, the rear legs often present a “bow-legged” appearance.
Grades of Patella Luxations The severity of patella luxation has been graded on a scale of 0 to 4, based on orthopedic examination of the knee. Surgical treatment is typically considered in grades 2 and over:
- Grade I: Can be luxated, but remains in place without direct force
- Grade II: Can be luxated without direct force and can be reduced by manipulation
- Grade III: Spontaneous luxation found at least once in standing - Reducible
- Grade IV: Irreducible luxation.
Exam, Screening Tests, and Imaging
The diagnosis of patella luxation is essentially based on palpation of an unstable knee cap on orthopedic examination.
Additional tests are often required to diagnose conditions often associated with patella luxation:
- Palpation to assess damage to ligaments
- Radiographs of the pelvis, knee and occasionally tibias to evaluate the shape of the bones in the rear leg and rule out hip dysplasia
- 3-dimensional computed tomography provides an image of the skeleton of the entire rear legs; this advanced imaging technique helps the surgeon plan surgery in cases where the shape of the femur or tibia needs to be corrected
- Blood work and urine analysis are usually recommended prior to anesthesia.
What Will Happen If Patella Luxation is Left Untreated?
Every time the knee cap rides out of its groove, cartilage (the normal lining of bones within joints) is damaged, leading to osteoarthritis and associated pain. The knee cap may ride more and more often out of its normal groove, eventually exposing areas of bone. In puppies, the abnormal alignment of the patella may also aggravate the shallowness of the femoral groove and lead to serous deformation of the leg. In all dogs, the abnormal position of the knee cap destabilizes the knee and predisposes affected dogs to rupture their cranial cruciate ligament, at which point they typically stop using the limb.
What Options are Available for Treating Patellar Luxation?
Patellar luxations that do not cause any clinical sign should be monitored but do not typically warrant surgical correction, especially in small dogs. Surgery is considered in grades 2 and over (See above). One or several of the following strategies may be required to correct patella luxation:
- Reconstruction of soft tissues surrounding the knee cap to loosen the side toward which the patella is riding and tighten the opposite side
- Deepening of the femoral groove so that the knee cap can seat deeply in its normal position
- Transposing the tibial crest, bony prominence onto which the tendon of the patella attaches below the knee. This will help realign the quadriceps, the patella and its tendon
- Correction of abnormally shaped femurs where the knee cap rides outside of its groove most or all the time. This procedure involves cutting the bone, correcting its deformation and immobilizing it with a bone plate .
The procedures that will best address the problem are selected on an individual basis.
Postoperative Care : Pain medications may be prescribed for a week after surgery. Physical therapy, with compresses and gentle, passive flexion and extension of the knee may be recommended shortly after surgery. Exercise is typically limited to leash walks for 6 weeks depending on the procedures performed and factors affecting the healing capacities of your pet. Radiographs may be repeated at regular intervals to monitor healing.
Over 90% of owners are satisfied by the progress of their dog after surgery.
Osteoarthritis is expected to progress on radiographs. However, this does not necessarily mean that your dog will suffer or be lame as a result. Keeping your pet trim and encouraging swimming/walking rather than jumping/running will help prevent or minimize clinical signs of osteoarthritis.
Some degree of knee cap instability will persist in up to 50% of cases. This does not cause further lameness in the majority of cases. Migration or breakage of surgical implants used to maintain bones in position occurs occasionally. Infection is a rare complication.
Because some breeds are predisposed to this condition, dogs diagnosed with patella luxation should not be bred.