If a dog owner evaluate the status of the dogs hips, that's not considered strange in any way. But for a feline breeder to do the same? Well, that tend to cause a lot of questions. But yes, even cats can be affected by hip dysplasia (HD).
Hip dysplasia is a defect in the socket joint of the pelvis. It's a hereditary defect and it cause the socket to be more shallow than it normaly should be. This result in a missfit of the femural head and cause the two surfaces to rub against each other, degenerating the cartilage between them in the process. The body can not produce new cartilage and, hence, eventually bone is rubbed against bone. This is very painfull and the body try to repair the damage by adding new bone, which will make things even worse.
In generall cats are good at hiding pain, it would not benefit them in the wild to show another cat that they are... So a cat could suffer from HD without the owner acctually realising it. Instead they are good at adapting their stride and walk more carefull or not walk that much at all. They may not be so keen on jumping up and down on tables, for example. Cats with mild HD may not suffer from it at all, however some cats do have such problems with their HD that they have to be put out very early in life, some even before one year of age. Since cats rarely show their pain this problem has rarely attracted any attention. According to researchers this is probably why some breeds tend to have a higher frequency of this problem than others, it simply has not been recognized and breed away from.
It's not established but it is assumed that HD is a polygenic recessive disease. This implies that there are several genes involved that add to the risk for the cat to develop HD or not (as for dogs). The cat has to inherit the defect from one or several genes from both parents in order for the symptom to show. This is probably why HD is so tricky to get rid of, since you could mate two cats that are perfectly normal, have breed normal kittens before in other combinations, but suddenly get a litter affected by HD. You just have mixed the wrong set of genes in that specific combination. This is also why it's important to test every generation for HD, even if the parents or their parents have been classified as normal.
Which genes that are involved, is not known. It sure would have been easier that way... Instead it is recommended that each Maine coon used in breeding is evaluated for its hip status first. This is done by radiographic evaluation by an expert. It will not guarantee that the offspring is not affected, but it will limit the risk compared to not knowing the parents status at all. This can be seen in the statistics from the Swedish health programme registered in PawPeds.
Recommendations
The Swedish health programme recommends that all Maine coons older than 10 months are tested for HD before they are used in breeding. The reason is to decrease the level of HD within the breed. In the Swedish system hips are given a grade for each joint, and its breeding status is made based on the "worst" hip result. The grades are "normal", "1", "2", and "3", where normal-grade 1 is accepted in breeding. A cat with grade 1 should only be mated with a cat that is normal. Cats with grade 2 or 3 are classified as moderate to severe HD and is not recommended to be used in breeding. OFA also accept plates from cats for evaluation and will provide an average result for both hip joints. Their recommendation is to test the cat at 2 years of age, evaluations prior to that are only preliminary. OFAs results are published on OFAs webpage, PawPeds results are published on PawPeds website.
Offsprings from parents mated according to PawPeds recommendations do get better HD-status, compared to the status of offsprings from parents where the status is not known. Breeding according to the recommendations have also increased the number of cats with good HD-status over a period of 10 years.