Because of the way in which achondroplasia/chondrodystrophy affects cartilage and other connective tissue throughout the dog, in addition to the issues that the dog WILL experience (vastly decreased growth, twisted bone, and spinal disc calcification), there are some problems that are a lot more common in dwarfed dogs.
Growth plate injuries (angular limb deformities): When the growth plate on one of the leg bones is injured (the cells are crushed by a blow or a very bad twist), even if there’s no fracture of the bone the growth plate can just shut down. The bone will end its growth cycle abruptly. However, the other leg bones will continue to grow. The most common place that this happens is in the radius/ulna (the lower bones of the front legs). The ulna stops growing while the radius continues, or the radius stops growing but the ulna continues. The bone that stopped growing keeps the growing one from pushing the leg down evenly. When the other bone continues to grow it buckles and bends outward, because the injured bone binds it to a certain length. This causes a major bow and twist to the entire leg.
Angular limb deformities (which we call “knuckling over” though it’s really not that) are much more common in dwarfed breeds than in other breeds. I’ve not seen a study that describes exactly why, but common sense would indicate that it’s a combination of thin, fragile growth plates that are already dysfunctional and the fact that the dwarfed breeds tend to be solid dogs who land heavily on front legs and are therefore are more likely to crush the growth plate cells. I can’t think of a single dwarfed dog that is not heavy-bodied; the early breeders didn’t shorten the legs of whippets. They shortened the legs of heavy hounds, shepherd-type herding dogs, the big terriers, etc., because the whole point of the short legs was to make dogs who could do the same sorts of jobs as the big dogs, but at a much slower speed or in a smaller area, or to accomodate handlers on foot rather than on horses.
So even as puppies, dwarfed dogs are heavy and sturdy of body. All it takes is for a six-week-old puppy to get her foot caught in the bars of the ex-pen and twist really hard getting it out, or one bad jump of a three-month-old off the porch steps (when the same jump a thousand times has done no damage), and the growth plate will give up the ghost.
Unfortunately, angular limb deformities are not just unsightly; they’re quite painful for most dogs.
Joint issues: Achondroplasia in dogs is poorly understood. So there’s not a lot out there about exactly how each joint is affected by the dwarfism. But we do know certain things. Where bones come together, dwarfed dogs have shorter, wider, shallower joints than longer-legged dogs do. A perfect example of this is in the hips. The dwarfed hip has almost no neck on the femoral head. The femoral head is flattened and tends to form a “cap” rather than a smooth egg shape. And the acetabulum (the socket, which is part of the pelvis), in order to remain functioal with the femoral head, is also wider, shallower, and flattened. If I used the above three sentences to describe a longer-legged dog, I’m practically giving you the textbook definition of hip dysplasia. In a dwarfed dog, the hips are “bad.” The shoulder joint and the elbow joints are “bad.” Of course, that’s if you define “good” as “looks like a sighthound,” which is not what we have, but there’s no question that if a dwarfed hip was submitted as a longer-legged breed the diagnosis would be poor. If a dwarfed elbow was submitted for examination under a longer-legged breed’s name, that elbow would be seen as deformed.
Ligament laxity: There’s definitely SOMETHING going on with achondroplastic ligaments. Humans with achondroplasia are known to have very lax ligaments compared to normal. Dog achondroplasia is NOT the human disorder, but I think the ligament issues may be analogous. You can see this most clearly in the front legs (again) in the wrists. Dog wrists are like marbles packed in a strong rubber tube–there are a bunch of round bones that don’t sit in balls and sockets but are held next to each other by strong ligaments. In most dogs, the marbles stay in a nice compact formation and can flex slightly and return to their former shape. In achondroplastic dogs, the strong rubber tube, it seems, is not so strong. It holds the marbles straight when the leg is just hanging there, but lets the marbles slump over to the side when weight is put on them. The “hush puppy” Basset hound front legs, where the feet end up going completely east-west when the dog stands up, represent the worst of this.
Our boy Bramble (who is half dachshund and half achondroplastic Jack Russell Terrier) has one leg that is a perfect example of ligament failure. If you pick him up by the chest so his front legs dangle, the sections from the wrists to the toes are straight up and down. The toes point to the floor without much deviation. Put him down, and one wrist buckles rather dramatically. His wrist deviates inward toward his chest while his toes deviate outward and end up twisted to the outside of his body. The problem is not with his skeleton–the bones hang together normally. The problem is with his ligaments, which don’t keep the bones together when weight is put on the limb.
Watch Cardigans when they trot toward you. When they pick up the leg (taking weight off the wrist), the “crook” (which is the way we describe the wrist deviation) almost disappears. As the dog puts the leg down, the wrist deviation becomes much more apparent.
Arthritis in the small bones: Lack of proper cartilage = the cartilage deteriorates and bone begins to rub against bone. The bones respond by growing spurs or bumpy bits. Bony changes in a joint are called arthritis; arthritis is painful because, well, bone isn’t supposed to grind against bone. Achondroplastic dogs get spinal arthritis very early, as we saw in the post about vertebral disc disease. They also tend to get arthritis in the bones of their wrists and feet. Several of my friends who are or were groomers absolutely refuse to accept Basset Hounds because in their experience Bassets ALWAYS bite when their nails are done. Picking up and squeezing the foot to get access to the nails is so painful to the dog that he sees no choice but to bite. I think their universally bad experiences are due to the fact that there are so many poorly bred Bassets with really terrible fronts, but the fact that dwarfed dogs get more arthritis is inescapable.
Next up: What does all this mean to us as breeders?
Ruffly Speaking: Railing against idiocy since 2004 