IMAGING STUDIES FOR FAILED BACK SURGERY S YNDROME

In patients with failed back surgery syndrome, magnetic resonance imag ing (MRT) can be the best first-line imaging study because it simplifi es the diagnosis. This update is based on over 600 cases. MRI shows th e scar tissue at the surgical site, persistent evidence of disk hernia tion for several weeks after surgery, and evidence of local and region al edema in one-fourth of cases. The edema is most marked between two months and two years after the operation and can misleadingly suggest discitis. MRI is the best investigation for detecting recurrent hernia tion at the same vertebral level or another level. Herniated disk mate rial is seen as a mass that does not enhance after gadolinium, in cont rast to the vascularized scar tissue. Free fragments are often clearly visible within the scar tissue. Fragments that migrate to the epidura l space can give rise to granulomatous reactions. Scar tissue can be s een in the epidural space and within the disk; it can show enhancement after gadolinium for several years. The scar can be atrophic or hyper trophic and can encase or impinge on the dural sac and nerve roots. Pa thological fibrosis cannot be differentiated from ordinary scar tissue . Arachnoiditis causing adherence of the nerve roots to the dura mater or to each other occurs in 5 % to 10 % of cases. Nerve root enhanceme nt after gadolinium is seen in three-fourths of cases. Bone lesions ar e common, especially some time after surgery; they are usually accompa nied with other lesions. Hematomas are seen in less than 10 % of cases . Infections are similarly rare (0.25 % each for discitis and epidurit is). The diagnosis of discitis is difficult and requires percutaneous biopsy of the disk, especially when MRI shows fluid within the disk, w ith decreased signal intensity on T 1 images, increased signal intensi ty on T2 images, and no enhancement after intravenous gadolinium.