Patella luxation (loose kneecap)

Patella luxation is a problem commonly seen in dogs and occasionally seen in cats. The patella (a.k.a. knee cap) is a “floating” bone associated with the knee or stifle joint. Patellar luxation most commonly occurs in small breed dogs such as poodles, pomeranians and chihuahuas, but can also occur in larger breeds. Most patellar luxations involve displacement of the patella to the inside of the knee, which is referred to as a medial patellar luxation (MPL). Patellar luxations can also be to the outside or lateral (LPL) – although this is much less common. Medial patellar luxations are associated with a “bow-legged” confirmation while lateral patellar luxations are associated with a “knock-kneed” posture.

Anatomy

The patella rests just above and in front of the knee joint in the distal femoral trochlear groove of the femur. Normally this groove provides a stable region for the patella to rest. In dogs with patellar luxation, this groove often does not develop adequate depth which increases the instability of the patella.

The patella has a flattened oval shape and is surrounded by the tendons of the quadriceps muscle groups. The tissue that surrounds the patella continues down to attach to the front aspect of the top of the tibia at the tibial tuberosity. The tissue between the patella and the tibial tuberosity is called the patellar ligament. In dogs with medial (inside) patellar luxation, the tibial tuberosity is located too far to the inside of the knee. In dogs with lateral patellar luxation, the tibial tuberosity is located too far to the outside of the leg. The patella is surrounded by additional stabilizing tissues including the joint capsule of the knee joint and strong fascial tissues of the limb. In dogs with medial patellar luxation, these additional tissues are excessively tight on the inside of the knee and excessively loose on the outside of the knee. Some dogs also have a curved femur or tibia (shin bone) that can contribute to the patellar luxation. All of the above variations from normal anatomy contribute to the instability of the knee-cap and are the focus of correction with surgery.

What animals are commonly affected?

Small breed dogs are affected by patella luxations much more often than large breed dogs. Female dogs are affected approximately 1.5 times as often as males. Both knees are affected in 50% of all animals.

Are some dogs more severely affected than others?

Yes. Patellar luxations are divided into four different grades depending on the tendency of the patella to stay in or out of its groove. The four grades are as follows.

Grade 1: The patella can be luxated with hand manipulation, but returns to its normal location without assistance and is almost always in an appropriate position. Grade 1 luxations are infrequently associated with any lameness.

Grade 2: The patella will luxate spontaneously, but is most often in its normal position. Significant lameness can be associated with grade 2 luxations.

Grade 3: The patella will luxate spontaneously and is most often located out of its normal position. It can however be returned to its normal location with hand manipulation. Significant lameness is often associated with grade 3 luxations.

Grade 4: The patella is always located out of its normal position and cannot be returned to normal with hand manipulations. This grade is often associated with a severe “bow-legged” or “knock-kneed” limb appearance, and often causes severe lameness. Grade 4 luxations can cause debilitating lameness especially in large dogs.

What are the common clinical signs?

Typical dogs with patellar luxation have an intermittent non-weight bearing lameness. They will be completely normal most of the time, but become completely non-weight bearing intermittently when the knee cap is luxated.  When it returns to its normal position in the trochlear groove, the animal will place normal weight on the affected limb. Owners will occasionally describe a popping noise in their pet’s knee or will be able to feel the instability in the knee cap when touching their pet’s back legs. Most animals do not seem especially painful when the patella luxates, they just simply do not use the limb with the patella outside of its normal position.

How is a patellar luxation diagnosed?

A patellar luxation is diagnosed based on a physical examination by your veterinarian. Radiographs (x-rays) of the rear limbs are most often taken to document the severity of the bone deformity.

What else could be causing the problems with my pet?

A definitive diagnosis is generally very straightforward for dogs with patellar luxation. This is especially true if the lameness has been present since your dog was a puppy. If your dog is older, and the severity of lameness has recently worsened, other causes of lameness should be evaluated. Concurrent, potential problems include other problems of the knee such as rupture of the cranial cruciate ligament, knee infection, joint cancer or immune diseases such as polyarthrtitis. Other potential diagnoses include hip dysplasia, herniated intervertebral disc or problems with the femur or tibia (i.e. bone cancer or fungal infection).

What are my treatment options?

Surgery is recommended for animals with grades 2, 3 and 4 luxations that have significant lameness. Dogs with grade 3 or 4 patellar luxation generally will have significant lameness and therefore surgical correction is recommended. The goal of surgery is to keep the patella in its appropriate location at all times. This is most often accomplished with three surgical maneuvers.

(1) Trochleoplasty – As previously mentioned, the trochlear groove, located at the end of the femur, is abnormally shallow in dogs with patellar luxations. The groove is therefore evaluated in all dogs at the time of surgery. If it is abnormally shallow then the groove is deepened. One Surgery to deepen the groove is called trochleoplasty.

(2) Tibial Tuberosity Transposition – The attachment of the patellar ligament to the tibia, the tibial tuberosity, is detached and moved towards the outside of the joint in dogs with medial patellar luxation. In dogs with lateral luxation, the tibial tuberosity is moved towards the inside of the knee. The tuberosity is then stabilized in its new location with small pins, and typically heals during the next 8 weeks.

(3) Lateral Imbrication and Medial Release – The tissues that surround the patella are loosened or released on the inside of the knee and tightened or imbricated on the outside of the knee in dogs with medial patellar luxation. The exact opposite is performed in dogs with lateral patellar luxation.

In dogs with grade 4 patellar luxation, more aggressive surgery is often required. This involves straightening of the femur and/or tibia with complete bone cuts (a.k.a. osteotomies), and stabilizing the bones with bone plates.

What are the chances that my animal will improve?

The chances of improvement following surgery to correct patellar luxations is very good in dogs with grade 1, 2 or 3 luxations. With surgery, the patella should become completely stable and the lameness should resolve. In dogs with grade 4 luxations, the likelihood of improvement is less certain and dependent on the severity of the bone deformities.

Are there any potential complications associated with surgery?

Any surgical procedure has the potential for complications. Fortunately, surgery for a patellar luxation is commonly performed and complications are rare. Potential complications include problems with general anesthesia, surgical infection, migration of the surgical implants (i.e. pins that stabilize the tibial tuberosity transposition), and failure of the surgery to completely stabilize the patella and resolve lameness. Greater than 90% of dogs with surgical repair of grade 1, 2 and 3 patellar luxations will not have any significant complications and will return to normal, or near normal, function with surgery.

How will I take care of my pet following surgery?

Care of the incision: You should monitor your pet’s incision at least twice daily for evidence of infection. Infection generally appears as a painful red swelling associated with the incision. Other incisional problems include separation of the incision line. This is most commonly associated with infection or a pet licking or chewing at the incision.  We send all of our patients home with an Elizabethan collar (a.k.a. lampshade) to protect the incision from licking and chewing until the sutures or staples are taken out at approximately 2 weeks following surgery.

Activity restriction: Following surgery, your pet will need to be confined to a crate or small room for 8 weeks. Slippery floors such as hard wood and ceramic tile should be avoided. Your pet should not be allowed to play with other animals or children, and should be kept away from stairs and furniture. He/she should be taken outside 4 to 5 times per day to urinate and defecate. Your pet should be on a short leash at all times, when outside, for at least 8 weeks.

Physical therapy: Physical therapy is beneficial following surgery for patellar luxation repair. Starting 3-4 days after surgery, you should perform passive range of motion exercises with him/her at least 3 times per day on the operated knee. We can also instruct you regarding physical therapy with Canine Rehabilitation of Nashville if indicated.

Recheck examinations: Rechecks are generally scheduled for 2 weeks following surgery, for suture removal, and 8-10 weeks for final recheck examination. If your pet is doing well at that time no additional rechecks will be needed.

Medications: We will provide you with instructions about medications to be taken following knee surgery. Generally some type of pain medication will be administered for 10-14 days following surgery.

General care: Following any surgery, you should make sure your pet is eating and drinking normally without vomiting, diarrhea or prolonged anorexia. You should also be aware of your pet’s general attitude and make sure he/she interacts normally with your family members, is able to sleep normally and is not unusually lethargic. If you notice anything significantly out of the normal in reference to the above habits you should contact your veterinarian

Summary

Patellar luxation involves displacement of the knee-cap from its normal position towards the inside of the knee, with medial luxation, and toward the outside with lateral luxation.

Small dogs are affected more often than large breed dogs, and females are affected 1.5 times more frequently than males.

Patellar luxations are divided into medial (inside) and lateral (outside) categories with four degrees of severity. Grade one luxations are the least severe and grade four luxations are the most severe.

Dogs with patellar luxation most often have an intermittent non-weight bearing lameness that does not generally appear painful.

A definitive diagnosis is made upon physical examination, most often with the aide of radiographs of the entire limb.

Surgery is recommended for animals with grade 2, 3 or 4 luxation and a history of lameness.

Surgery generally involves deepening the distal femoral groove, movement of the tibial tuberosity and manipulation of the tissues that surround the kneecap.

The prognosis for a complete resolution of lameness following surgery with grade 1,2 or 3 luxation is excellent. The prognosis for return to normal or near normal function with grade 4 is less predictable.

Activity restriction following surgery is required for 8 weeks to promote the best chance of a return to normal function and avoid potential surgical complications.