April 7, 2015
“Call me Pretzel”, the note said. I peered over the top of it to look at my patient. She was aptly named, wriggling and dancing on the exam table, tongue flashing as she air kissed in happy anticipation of meeting me. “Well, alright then! Pleased to meet you, Pretzel!” I greeted her. With a surge of energy she tried to jump towards me and instead collapsed in a heap as her back leg buckled under her. I righted her with one hand and glanced at her chart again. “Sometimes lame” it read.

I looked again at Pretzel, a little toy breed dog weighing less than 10 pounds. She was just over 1 year of age. Her owner reported that although her little dog usually ran around happily, recently she had noticed that sometimes she picked up one of her back legs when running, or might kick it out behind her then resume her pace. Pretzel didn’t like to jump as much as she used to, and occasionally gave a little cry when landing on her rear legs. As I examined her, I gently pressed on her tiny kneecaps, or patella’s, and felt the right kneecap slip out of place, bumping over the groove at the bottom of the upper thigh bone, the femur, that was supposed to hold it in place. The kneecap slid to the inside of her leg – but as soon as I straightened the leg, it popped back into place. Diagnosis : Luxating patella, also called a dislocating kneecap.

Dislocating kneecaps are very common in many toy breeds. In some breeds it is estimated that over 50% of all puppies are so afflicted, a hereditary predisposition that is evident by a year of age or sooner. Some people believe that as these toy breeds are bred for ever smaller “extreme” sizes more anatomical problems crop up. Certainly these dogs were bred for looks, as lap companions and not as “working dogs” –  attention to strong teeth, big muscles and performance were not a first priority.

The knee of a dog is an interesting joint, being very similar to that of a human in general construction. Kind of bony, it is held together by tendons and ligaments, without much muscle to cover and protect it. Hard working, it must flex and bend with every step taken, as the entire backside of the body is supported on it. So can you picture all this? The big heavy femur with this small sliding patellar bone-and-ligament-show that connects and allows bending when attached to the lower tibia/calf bone. What keeps the patella in place? It is supposed to be nestled deep in the valley of the femoral groove, and helped to stay there by being firmly attached in a straight line to the bony ridge of the tibial crest below it.

In toy breeds, the femoral groove is often formed FLAT. Imagine! With nothing to cup it, the patella is at the mercy of the inner pull of the thigh muscles and off it goes, dislocating sideways towards the inside of the knee. Because the dog was born that way, it doesn’t know that that’s not the way it should be; truthfully, although they walk a little oddly, most young dogs don’t show serious discomfort. But as the patella bobs back and forth over the rim of the distal femur, it rubs ever so slightly, just a touch of friction….and eventually arthritis. At first just a little breakdown of the cartilage covering the bone. Over time, as year pass, from an erosion of the cartilage little pits and bumps appear and the knee becomes more painful. Eventually some dogs become so crippled they can no longer use the leg at all. Surgery is too late by then.

Pretzel, happily, was young and could benefit greatly from corrective surgery. It was easy to recommend it – all the telltale signs were there, from the hard pop when her knee dislocated (painful!) to the changes in activity her owner recognized at home. After x-rays to be certain she had no other orthopedic problems that might affect the outcome and to document the flattened femoral groove, she was taken to surgery – there is no other remedy for such a knee – and her knee rebuilt. A new groove was fashioned deep into the femur and the surrounding joint capsule and ligaments were tightened to hold it in place while healing. Although Pretzel’s leg was straight, in some crooked-legged breeds the tibial crest will also be transplanted.

In case you’re wondering, large breed dogs also can develop dislocating knees, though for most it is caused by some kind of accident or trauma. Surgery is always a necessity in these cases.

During surgery we looked at her original bone, where a thin line of angry red could be seen on the inside rim where Pretzel’s patella had been rubbing as it popped back and forth. We couldn’t undo that damage, but we could stop it right there, and with no further rubbing there was a good chance she would have minimal arthritis in that joint as she aged. Once the groove was deepened, the kneecap sat firmly in place! Pretzel awakened to a fully functional knee for the first time in her life. Helped by anti-inflammatory and pain medications she was walking gingerly in the first few days; had her stitches out by the second week, and began swimming in her bathtub at home as a form of non-weight bearing physical therapy. Within 6 weeks Pretzel was flying across rooms again, taking couches in a single bound, and dispensing crazy doggie kisses to all comers.

Christine B. McFadden, DVM
Valley Animal Hospital, Merced