Upright plain radiographs in two planes are the initial imaging study of choice. They aid in ruling out pathologies such as deformity, fractures, or metastatic cancer as underlying causes of back pain and, often supplemented by other imaging modalities, are evaluated for signs of degeneration. Findings in degenerative discs include disc space narrowing, endplate sclerosis, “vacuum” phenomenon within the disc, and osteophytes. Flexion and extension views may be helpful if instability is suspected.
Magnetic Resonance Imaging (MRI) is a more sensitive imaging study for the evaluation of degenerative disc disease. Findings on MRI scan include disc space narrowing, loss of T2 signal within the nucleus pulposus, endplate changes, and signs of internal disc derangement or tears (). High Intensity Zones (HIZ) have been found in close to one third of patients undergoing MRIs for low back pain and have been used as a marker for internal disc derangement. However, the accuracy and reliability of these HIZs has been questioned [19
Disc space narrowing and degenerative changes at the L3-L4 level (arrow) on sagittal T2-weighted MRI.
Modic et al. were among the first to radiologically characterize vertebral endplate changes that are associated with degenerative disc disease [21
]. The Modic classification system includes three types of changes, and grading has been shown to be reliable and reproducible [23
]. In Type I, there is increased signal on the T2-weighted sequence and decreased signal intensity on the T1 sequences indicative of marrow edema. Type II is characterized by fatty infiltration of the marrow as demonstrated by hyperintense T1 and T2 images. Finally, Type III demonstrates hypointense signals on T1 and T2 sequences, which corresponds to endplate sclerosis. The Modic types are summarized by .
Table 1 Modic changes as illustrated by Jones et al. .
Pfirrmann et al. further examined and characterized intervertebral disc pathology using MRI [24
]. The degree of disc degeneration were graded I through V. Grade I discs are white, and homogenous on T2 sequences. Grade II discs are white, but somewhat inhomogenous with banding. Grade III discs are grey with unclear distinction between the nucleus and annulus. Grade IV discs are inhomogenous and dark without distinction between the nucleus and annulus. Finally, Grade V discs demonstrate a collapsed disc space. The Pfirrmann grading system is depicted by .
Table 2 Pfirrmann grades as illustrated by Pfirrmann et al. .
While plain radiographs and MRI provide information regarding the health of the intervertebral segment, they do not provide any information regarding the segments impact on clinical symptoms. The use of discography has attempted to identify specific degenerated discs as pain generators [25
]. Provocative discography involves the injection of contrast dye into the nucleus. Computed tomography is used to evaluate for extravasation of dye indicating annular tears. The patient's symptoms and intradiscal pressure during the injection are also recorded. If the pain on injection is similar to their back pain, then the discogram is considered concordant. Also, if pain is produced at low pressures, it is felt that there is symptomatic annular disruption or internal derangement. However, if the pain is different or produced at high pressures of injection, the test is often considered discordant. Still, low-pressure discography has been found to have false positive rates of up to 25% in asymptomatic individuals and may accelerate disc degeneration [26