I don't know why some Dr's ask for MRI's and some work only from xrays. Maybe it depends on what they see in the preliminary xrays and individual possible problems surrounding the bone.

I had the same great surgeon for both of my TKR's and my reverse TSR. For the knees, I only had a new set of x-rays taken, no MRI's. Both knees recovered well without problems.

For the shoulder, I had x-rays, CT scan and MRI's done to determine whether I would have a regular or a reverse procedure, then another special MRI to provide my measurements for a custom made implant. I think the first CT and MRI's were to determine whether my rotator cuff was in any shape to save (it was not.)

So in my case, I'm assuming my surgeon thought my knees would be fairly routine, so didn't require the MRI's, while he did need it for more information on my shoulder.