fat signal seen in adults, the marrow may be bright on a T2 weighted sequence and de- creased in signal on a T1 weighted sequence. Modic first described this MRI finding, and it is called Modic Type 1 change. The finding is due to granulation tissue (not just edema), and therefore much as the granulation tissue around a disc extrusion will enchance after intravenous contrast.
If the signal is dark on T2 and bright on T1 (typical of complete fatty replacement), then this is termed Modic Type 2. There is evidence that Type 1 change is specific for discal pain and perhaps instability [15,18,28,30]. A recent study indicated that moderate end plate changes of both Type 1 and Type 2 may show a high correlation with discogenic pain . Similar inflammatory marrow changes may be seen adjacent to Schmorl´s nodes [26,32,33,34]. It may be that there is a progression from Type 1 to Type 2 as the spine restabilizes, or in the case of spinal fusions, the fusion becomes solid. It is also possible for Type 2 to revert to Type 1 with interval destabilization.
Lumbar spinal central stenosis may be divided into developmental and acquired. Almost all central stenosis is due to developmental stenosis with short pedicles and relatively large facets; or degenerative spondylolisthesis. Developmental central stenosis alone is unlikely symptomatic, but lessens the “reserve“ necessary to protect against central stenosis when the disc and posterior elements degenerate. As the facet joints degenerate there is a decrease in the interfacet angle and often flattening of the posterior thecal sac from the noncompliant posterior epidural fat pad. Absolute measurements of the diameter or cross sectional area of the central canal are of less importance than the observation of the effect on the thecal sac and nerve roots. We
Developmental Central Stenosis
MRI of the Lumbar Spine
Up-Down Foraminal Stenosis with swollen L5 nerve root
categorize stenosis by quarters: <25% cross sectional loss – mild; 25-50% – moderate; 50-75% – moderately severe; and >75% severe.
The anatomic area from the exit of the nerve root from the thecal sac until the entrance zone of the foramen is termed the lateral recess. In reality, lateral recess stenosis is almost always a result of facet hypertrophy, and the area narrowed is therefore the “subarticular gutter“ which is the most cranial portion of the lateral recess.
Foraminal or lateral stenosis is almost al- ways a result of loss of disc height and posterolateral osseous ridging secondary to annular bulging. These findings narrow the foramen in an “up-down“ or “cranio- caudal“ fashion, entrapping the exiting nerve root between the posterolateral ridge and the undersurface of the pedicle. Less commonly isolated overgrowth of enlarged facet joints and facet capsular hypertrophy results in isolated “front-back“ or “ventro- dorsal“ foraminal stenosis. A combination of both types of lateral stenosis would be termed concentric foraminal narrowing. As with disc extrusions, pre or post compressive nerve root swelling may be seen with central and foraminal stenosis.
The knowledge of normal and intradiscal anatomy demonstrated by MRI has grown exponentially over the last few years. While the significance of some findings is clear cut, especially in the evaluation of radiculopathy, in the study of discogenic pain, controversies still exist. As with all imaging studies, the comparison of MRI findings with the clinical evaluation is always essential.
Jay A. Kaiser MD email@example.com
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