Figure 178

Disc space infection following discec-tomy. Lateral X-ray of the lumbar spine of a patient following L1-L2 discectomy, showing intervertebral disc space narrowing with endplate irregularities and anterior new bone formation suggesting postoperative disc space infection (see also MRI, Figure 17-17A).

uptake may indicate that a pseudoarthrosis has developed, that a surgical infection has evolved, or that a degenerative process such as adjacent level disc degeneration or the development of Modic type III changes has taken place.

In cases of pseudoarthrosis, there is continuous motion with impact loading on the bone surfaces at the non-union site, resulting in osteoblast cell activation and increased technetium-99 diphospho-nate uptake at the non-union site. Pronounced uptake at the site of attempted fusion, when seen more than a year after surgery, is highly suspicious of a pseudoarthrosis. Single photon emission computed tomography (SPECT) might further delineate the site of non-union.

Combined technetium-99 and gallium-67 citrate scintigraphy is an accurate test for evaluating patients suspected as having postoperative disc space infection. Although dual-phase (i.e., blood pool and bone imaging) scintigraphy has been reported as being insensitive in detecting vertebral osteomyelitis, the sensitivity of delayed bone imaging has been reported to be excellent, ranging from 86 to 100%. The concern is that bone scan may be nonspecific in the early postoperative period due to noninfectious inflammatory reaction. Gallium imaging with SPECT analysis may be helpful in such situations. A disproportionate focal increased uptake on both the technetium and gallium scans is indicative of vertebral osteomyelitis.

COMPUTED TOMOGRAPHY (CT)

CT yields excellent images and anatomical details of the bony structures and, as such, provides important data in patients with residual central, lateral, or foraminal stenosis. Although CT is an outstanding tool in the diagnosis of lumbar disc herniations in a "virgin" spine, it is less effective in distinguishing between postoperative scarring and recurrent disc herniation. The previous laminectomy site is easily recognized on the axial CT images, when a portion of the lamina is missing. In cases of microdiscectomy where bone resection was not performed, the operative site may be recognized by the missing ligamentum flavum.

On CT, the extradural scar appears as a soft tissue lesion with a higher density than the thecal sac and a lower density than the disc. The scar appears anteriorly at the discectomy site and/or laterally and posteriorly. The dural sac is usually retracted toward the scar; and the scar does not compress the sac (as opposed to disc herniation). The scar is usually enhanced with IV contrast (see later discussion), but the accuracy of differentiating between scar and residual or recurrent disc herniation with CT is low.

Residual stenosis, especially foraminal narrowing, is one of the most frequent causes of failed back surgery syndrome and is easily detected on postoperative CT imaging (Figure 17-9).

The CT examination can reveal changes outside the spinal canal, such as atrophy of the multifidus and fat replacement of the muscle fibers.

CT with IV iodinated contrast medium may be occasionally used to demonstrate active infection of scarring. The ability of the IV con-

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