Ok let me see if I can break this down.....
OFA for an official score must be done after the dog turns 2. The x ray is sent to 3 different orthopedic vets for their oppinion as to how the hip socket, femur ball formation and their fit together look. They are looking for remodeling on the femur head, wear on the edges of the hip socket, and how much Femur head is actually covered in the socket. They are rated excellent, good, fair, and then mildly dysp, and grade 1-3 after that. The 3 scores from the 3 vets are averaged and that is your final score. It is all based on visual inspection of your xrays.
The person doing the xray can GREATLY affect the turnout of your scores, you must must have proper positioning or it can skew your score.
Pennhip measures the laxicity *how loose or tight the tendons holding the hips together*. It must be done by a trained pennhip vet, not any vet can do it. They take a base xray *exactly like ofa* and then they apply a machine that puts pressure on the femurs while holding the pelvis in place, the amount of pressure is determined by the weight and breed of the dog I believe, Then another x ray is taken. Those two xrays are scanned into a computer and run through the pennhip program and it gives you an EXACT measurement of how far the femur head moved when pressure was applied. Thus telling you how tightly your dogs tendons are. They also look for DJD, Degenerative Joint Disease which would cause the bone to degrade over time, causing the dog to become dysplastic even if the tendons were tight. This can be done as young as 4 months, tho I personally believe that a dog should be fully mature before doing any testing.
now with that being said.... let me explain how this pertains to your dog in the real world. lmao.
OFA takes a picture of the dogs joint. It tells you what that dogs joints look like ON THAT DAY. IF the dog is a young dog that is at a healthy weight and has done no work those hips could look pristeen, but allow some age and heavy work on that joint and re xray you could have a completely different picture.
Pennhip, tells you the functionality of the hip joint itself. How well put together is that joint, which in turns tells you WILL IT HOLD UP to work. Period!
IF you have a dog that has BEAUTIFUL hips on the OFA xrays, you grade out as an excellent, AND that dogs Tendons are LOOSE *greater than .60 according to PEnnhip* Then as your dog is working and aging, that Femur head is bouncing around in that socket like a ping pong ball, the tendons aren't holding it in place, and there will be MAJOR grinding and remodling of the hips socket and Femur head. Given enough time *age* or a hard working lifestyle, this dog WILL become dysplastic in it's old age.
Now take the opposit Scenario, you take a dog that grades out as a Fair on the OFA scale. mabey it has poor socket coverage for the Femur head, mabey the dog has small femur heads, or shallow socket lips... any number of things. But this dog pennhips out at .25/.25. Functionality of those hips is near perfect, which means that while the hip isn'y shaped perfectly, they are held together so well that you will have very little wear and tear on them with age and work. Thus your dog has Fair hips, and will always have Fair hips, but never never dysplastic hips.
I personally test for BOTH. I believe that each individual test only looks at half the puzzle. You must know what those hips look like, but you must also know how well they function! One without the other is completely useless in my book. And since Pennhip must take an OFA style xray as thier base xray, I simply ask my vet to double load the film, and send the xray film off to OFA, thus no need for two vet visits, and two prices, I've NEVER in 15 years had a vet have a problem with this, and there is usually only a minimal charge for the extra film.