Introduction to Patellar Luxation
Patella (kneecap) luxation is a common orthopedic problem affecting both our small and large canine friends. This condition typically results from abnormalities in bony development of the hind limbs. The patella is a small bone that is part of the extensor (quadriceps) mechanism. It sits just inside the trochlear groove at the end of the femur (thigh bone). It helps to direct the pull of the quadriceps muscles in a way that a pulley redirects a cable. The patella is found within the patellar ligament, which attaches to the point of the tibia (shin bone). This mechanism allows for pain free and efficient extension of the stifle (knee). If the patella is luxated either to the inside or outside of the stifle then this efficiency is severely affected causing pain and stress around the stifle. Furthermore, if the patella is able to pop in and out then wear and tear on the cartilage of both the patella and trochlear ridge occurs resulting in osteoarthritis.
How does this occur?
Patella luxations occur from a developmental disorder that occurs after birth. The exact causes are still unknown; however, it is thought that changes beginning at the hip are partially responsible. The changes at the hip result in a sequence of skeletal changes of the thigh and shin bones. The changes noted are outward bowing of the thigh bone causing an inward angle as well as external torsion of the end of the thigh bone. Furthermore, the top shin bone will develop with an inward angle with internal rotation or external torsion so that the point of the shin will be facing towards the inside of the leg. Because of these changes the extensor mechanism has a tendency to pull the patella to the inside of the knee. Theses changes are noted with the most common type of patella luxation, known as a medial patella luxation (MPL). If a lateral patella luxation occurs (LPL), the changes noted are opposite of an MPL. Lastly, the trochlear groove where the patella should be located is often times under-developed leaving it shallow or absent.
Clinical Signs and Diagnosis
Often times an intermittent lameness will be noted beginning at a young age. It is common to note that dogs will have a bunny-hopping gait or will tend to skip. If the patella is popping in an out dogs will sometimes kick their leg out almost as if stretching it while walking or running. This occurs when the patella luxates and the dog is trying to get it back into a normal place. Sometimes a cry or yelp may be heard when the patella luxates, and the dog may hold the leg up and not use it. With long standing patella luxations muscle loss may be noted in one or both hind limbs. An orthopedic examination is needed to diagnose the problem. On exam the patella’s are located, if they are found within the correct location they are attempted to be luxated; on the other hand if the patella’s are found either inside or outside the knee then they are attempted to be put back into place. After the examination the grade of luxation is determined on a scale of 1-4. A grade 1 patella luxation is classified as the patella being in the correct location; it can be luxated but pops back into place. This is differed from a grade 2 patella luxation where the patella is in the correct location, but once luxated will not easily pop back in. Grade 3 and 4 luxations, the patella is always in a luxated position, while with a grade 3 the patella can be reduced versus a grade 4 the patella can not be reduced. Radiographs (X-rays) are taken to determine the location of the patella, if there is any underlying arthritis, as well as any bony abnormalities.
Choosing the Right Treatment
For mild luxations (grade 1 and non-clinical grade 2), surgery is typically not indicated; however, formal rehabilitation to strengthen the quadriceps muscles and improve patellar tracking is beneficial. Furthermore, formal rehabilitation may help in preventing worsening of the clinical signs and avoidance of surgery in select cases. Surgery is indicated in cases of grade 3 and 4 luxations as well as with grade 2 luxations that are causing clinical signs. Your dog would be placed under general anesthesia; small incision is made on either the inside or outside of stifle. Once the stifle joint is exposed the trochlear groove will be evaluated. If it appears shallow then a deepening of the groove will take place (trochleoplasty). This is completed by removing a small block of cartilage, deepening the underlying bone and replacing the cartilage back. Additionally, the soft tissues on the side towards the luxation will be released while the soft tissues opposite the side of luxation will be tightened (imbricated). In some cases, a small cut is made in the point of the shin bone and the bone is moved towards (tibial tuberosity transposition) the side opposite the luxation to improve the overall alignment. This small cut in the bone is then held in place by small wires. If excessive rotation is noted an anti-rotational suture may be placed. In more severe cases the end of the thigh bone may be cut and straightened and held stable by a bone plate.
What happens after surgery?
After surgery you dog will need a period of rest and relaxation of about 8-12 weeks. This means no running, jumping, or playing. They will need to be taken outside on leash to urinate and defecate; excessive climbing up and down stairs or on and off furniture should be avoided. We recommend when not directly supervised that patients be placed in a crate, small laundry room or bathroom, or a small portion of the house sectioned off so that your dog can’t over do it. Excessive activity will lead to implant breakdown, soft tissue injuries, or delayed healing.
The staples/sutures will be removed, or incision evaluated at approximately 2 weeks after surgery and radiographs will be needed at either 6 or 8 weeks and possibly 12 weeks after surgery to evaluate healing. At these rechecks an orthopedic exam will also be performed to ensure the surgical site is healing as expected.
Just as with people we recommend physical rehabilitation beginning 2 weeks after surgery. Rehabilitation will involve once weekly formal rehab sessions along with at home exercises. We have noted quicker healing, maintenance of muscle mass and range of motion, and superior outcome in the patients that undergo formal rehabilitation. Furthermore, rehabilitation offers an outlet of energy in controlled manner so that your dog is still able to maintain some activity while healing. Patients that have been treated conservatively usually require once to twice weekly rehabilitation for a period of about 3-6 months.
Following surgery, we recommend that patients begin oral joint supplements, maintain a healthy body weight, and remain active once healed from surgery. These things will be the beginning blocks along with surgery to minimize and slow down the progression of OA.
David Dycus, DVM, MS, CCRP, DACVS-SA
Board-Certified Veterinary Surgeon
Beale's Best Contributor