Endplate Infraction

b normal-appearing control levels immediately adjacent to painfully deranged discs in comparison to normal-appearing control discs at least one segment removed from the painful and deranged level(s).

Prior investigations1,16 have demonstrated a high correlation between lumbar disc annular tears exhibiting a "high-intensity zone" (HIZ) on T2-weighted, high-field MR images, and painful concordant discography. These discs are often "chemically sensitized"12,26 and are painful with low-pressure and low-volume injection (Figure 6.6A). Internal disc derangement(s) with endplate infraction(s) (Schmorl's nodes) (Figure 6.4)3,15,20-22 have been shown to be frequently painful (see later section, "Thoracic Discography"). It must be noted, however, that with the possible exception of lumbar disc HIZ lesions in back pain sufferers, lumbar discographic response is difficult to predict from MR imaging studies.

The study of control level discs warrants discussion. Based upon our experience,12,13 it is imperative to complete the procedure by studying at least one control level disc that appears to be either normal or less degenerated than the disc(s) under primary investigation. Control levels should be studied whenever a positive response is provoked at suspicious levels. Without the study of a control disc or discs, the validity of response at the painfully deranged level(s) may be questioned. Observing pain-free response to a normal-appearing control level adds validity to the painful response(s) in comparison to studying only the abnormal disc(s) without control(s). It is equally important in surgery planning25 to discographically study control levels to make sure of internal disc integrity at segments that might become marginal discs, adjacent to a contemplated fusion. In cases of lumbar fusion, the best surgical results are obtained when normal and pain-free levels are present immediately above and/or below the level(s) to be fused. If a first control level proves equivocal, a second control level should be studied. If, on occasion, one encounters a patient response that is either equivocal or unexpectedly high at normal-appearing control levels, the entire study must be viewed with skepticism. In our practice, such patients generally are advised by their clinicians to not have surgery. Based upon formal, prospective investigations of clinical subjects and asymptomatic volunteers, clinically suspect discs that appear abnormal on imaging studies have been shown to be more likely to be painful and concordant and given an intensity rating that is high in comparison to less-diseased or normal-appearing control levels.5,40

Lumbar disc injection pressure has been correlated with pain provocation and with outcomes after discectomy and fusion.26 Discs that are intensely painful with both low pressure (<1 atm, or 33 mmHg) and low-volume injection of either saline or contrast have been defined as "chemically sensitive" (Figures 6.2, 6.5, and 6.6), as opposed to mechanically sensitive discs that must be pressurized to provoke a response (Figures 6.1, 6.3, and 6.4). These pain responses appear to be mediated through chemoreceptors within sensory fibers that have grown into the annular tear itself. Often HIZ lesions (meeting strict radiological criteria) prove to be "chemically sensitized" when studied discographically in symptomatic patients.1,16 Although this concept of the "chemically sensitized disc" is valid, it is not a necessity for expe-

Spinal Deformity, Lumbar Fusion, Instrumentation, and Discography 1G7

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