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irritation. It has been shown that chemical mediators such as the enzyme phospholipase A2 and substances such as prostaglandin E, hydrogen and potassium, lactic acid and several polypeptides greatly increase nociceptive sensitivity and played a substantial role in the degeneration of discogenic pain. Continued axial loading of the posterior annulus and those discs sensitive to low pressure appears to be a potential cause of continued axial pain when the anterior column is not fused.
Milette, et al., in 1999 has shown that discography offers greater sensitivity than MRI in detecting annular tears in vivo. The discrepancy between the findings of the two imaging modalities (discography and MRI) is explained possibly by some diffuse alteration of the biochemical environment of the disc that could affect the T2 weighted intensity. A study by Wood in 1999 demonstrated that disc pathology not seen on MRI might be symptomatic on discography. They concluded that they believe that although MRI is useful as a screening examination for patients with mechanical-type pain, it cannot be relied on conclusively to identify certain annular lesions and other potentially painful foci. Brightbill, et al., in 1994 concluded that discography may be useful in patients with persistent symptoms despite a normal or equivocal MRI study. Kornberg suggests that normal disc signal intensity on a MRI does not rule out degeneration. The results of Brightbill's study suggest that structural abnormality of the disc such as seen in internal disc disruption may be missed on an MRI. Saifuddin and others in 1998 discussed several studies that have indicated the direct involvement of the nerve root is not necessary to produce sciatica. McCutcheon and Thompson demonstrated a relationship between posterolateral radial tear of the annulus on CT discography and the reproduction of ipsilateral leg pain. Milette, et al., show that leg pain reproduced at discography could be partially or completely abolished by intradiscal injection of a local anesthetic. They concluded in their study that a tear of the posterior annulus of the degenerative lumbar intervertebral disc can be the source of pain with radiation to the groin, hip, buttock or leg in the absence of nerve root compression.
Indications for Fusion Levels
The accelerated degeneration of the lumbar segments adjacent to a previous fusion has been established. This is documented by Frymoyer, et al., 1979; Harris and Wiley 1963; Lee and Langrana 1984; Lehmann, et al., 1987; Lipson 1983; Unander-Scharin 1951. Lee and Langrana in a biomechanical study of lumbosacral fusion demonstrated increased stress on the adjacent unfused segments, especially the facet joints. Radiographic findings suggest fusion imposes new stresses on the vertebral motion above the fusion. Lehman and Spratt demonstrated excellent degeneration of the (Continued on page 66)
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