Last year we worked with a litter that showed the power of genetics. A couple moved into our area and their initial visit to our clinic was for a pregnancy check of their three-year-old terrier. The exam showed nothing remarkable about the dog except the puppies were due in about three weeks.
We didn't see the owners or their dogs for about four months. They had not been very aggressive about placing the puppies and still had all five of them. They made an appointment to have the litter checked for a limping problem. It seems that each of the puppies would occasionally limp on one of their hind legs. While playing or running the puppy would suddenly start carrying one of his hind legs up off the ground and sometimes would yelp in pain as the leg was brought up. If the affected pup laid down or just stood still and rested the leg briefly, he could then use it normally. Three of the puppies would hold up one rear leg one day and then the other the next day; the other two puppies always seemed to favor their left rear leg. And, just to confuse matters more, on some days none of the puppies ever limped but ran and played normally!
After having the situation explained to us but before examining the puppies, our minds ran wild with diagnostic possibilities. We thought about the flooring they might be using, Lyme Disease, puppy abuse, and possible alien capture. We work with many breeders and have dealt with numerous litters over the years but never had we heard of anything like this. On exam however, everything fell into place. All the puppies and the mother suffered from the same condition: each had an abnormality of the knee joint.
The bones that make up this joint in the hind leg are the femur above, the tibia below, and the patella (or kneecap). There is a fairly deep groove in the femur that maintains the left-to-right position of the kneecap as it slides up and down as the joint bends. In this group of dogs, the groove in the femur was shallow and the patella would slide to the inside of the leg and prevent the joint from working correctly. This is a fairly common problem in all terrier breeds and is referred to as a luxating patella, meaning that the patella has slipped out of its correct place. The patella in this abnormal position ties up the leg and can only return to its normal position after the quadriceps muscles relax.
While dogs with luxating patella can only be cured or corrected with surgery, some may exist comfortably with the condition for a long time. It all depends on how often the patella goes out of position, how much pain it causes, how it affects the animal's movement, or the amount of arthritis that occurs.
What was abnormal about this litter was how early the animals were affected. We usually don't see signs until they are adults. The fact that the entire litter was affected is also unusual. Most veterinarians believe that this condition is passed from generation to generation genetically and cases like this just provide additional evidence.