Worth noting on that study is the low numbers of cats parcipitating. A total of 78 cats and add to that of various breeds. 8 different breeds to be exact. If divided somewhat equal not even 10 cats from each breed. I quote:

...Dr. Smith was unable to determine whether the hip laxity profiles found in this particular pool of cats is representative of the cat population at large...

What is interesting is the relationship between PL and HD the researcher found. PL can be diagnosed without x-rays and according to this study it might be adviceable to x-ray the hips of a cat diagnosed with PL.

I don't x-ray my cats for HD but I do have them checked for PL so should I ever have a cat diagnosed with PL I'd probably have it's hips x-rayed.