It has been proposed that fusing the spine and stopping motion at that level is enough to prevent further trauma to the spinal cord. For many patients fusion stabilizes or even improves the neurologic condition.
Stopping further trauma seems to be an important part of recovery. With less compression, there is better blood supply to the area. And autopsy studies show that even when MRI signals show there's a problem, in actual fact, once the mechanical factors (pressure and movement) are eliminated, repair can take place.
The mismatch between findings on MRI and patient function remains a mystery. Some people with no findings have extreme symptoms. Others with no symptoms at all have significant changes in their MRIs or other imaging studies.
Right now, it's not clear how to predict who will have good long-term results. Age and duration of symptoms have been shown to be important variables in some, but not all, studies. Further study of this problem is needed before doctors can assure patients one way or the other about the final results.