fused vertebrae

My 75yr-old father-in-law has recently been told by his GP that he has "some vertebrae fused together in his lower spine". To our knowledge he has not suffered an injury or fall. He was not born with this condition and has led a physically active life until recent lower back pain forced him to cut down on activity. Diagnosis was made following an X-ray. How can vertebrae fuse together like this? None of the books I have read or online forums seem to mention this condition, except where it appears from birth, which is not the case here.

[tippy18]

As you get older the disc between the vertabrae will get thin and cause the vertabrae to fuse together. bone fuses to bone.  I would question my Doctor about this because there may be an underlying reason for this to happen.

[tippy18]

As you get older the disc between the vertabrae will get thin and cause the vertabrae to fuse together. bone fuses to bone.  I would question my Doctor about this because there may be an underlying reason for this to happen.