How big is your dog?  It is hard to tell from the photo but some breeds are more prone to disc disease than others.  Your dog  could have had a disc that was partially out of place before surgery and not causing any problems, then it slipped out of place and is now putting pressure on the spinal cord.  This is more common in small to medium breeds and the dog is usually painful somewhere along the spine.  In larger dogs we more commonly see FCE (fibrocartilaginous emboli) which can occur anytime with or without trauma or surgery.  These dogs tend to be nonpainful but have decreased use of 1 or both legs due to a piece of cartilage from the spine that becomes lodged in a blood vessel in the spinal system.

Your vet is right in that only on MRI, CT scan, or myelogram can "see" the spinal cord.  MRI is best and usually costs at least $1500.  Treatment ranges from steroids to "tincture of time". A consult with a neurologist may be worthwhile and usually runs $125 to $200 depending on where you live.

Pam and Tazzie

I would give acupuncture a try.  My Pitbull Kona was diagnosed with 4 bulging discs in all areas of her spine back in 11/2006 (we did an MRI).  She couldn't use her back legs and steroids did nothing so we started acupuncture.  After 4 weeks she was walking again!  She is still with us but needs acupuncture every 5 weeks due to the severity of her condition.  She is not a tripawd but I don't think that would matter as far as your dog's prognosis, unless your dog has osteosarcoma.  Sometimes that cancer can come back in the spine and the symptoms would mimic a bulging disc but the dog would be very painful.

Pam