The Abnormal Annulus Fibrosus: Can We Infer the Acuteness of an Annular Injury?

Robert M. Quencer

Few issues in neuroimaging are as controversial as the nomenclature used to describe abnormalities of the lumbosacral spine. What immediately comes to mind are the variations in the labeling of various degrees of disk abnormalities (eg, herniation, bulge, protrusion, extrusion). With better MR resolution and a wider understanding of the nature of the disk disease and its manifestations, we are often faced with the task of trying to associate a finding in the disk complex with the underlying pathophysiology. Added to this is the spectrum of abnormalities of the annulus fibrosus and the associated signal intensity changes within the annulus.

Faced with an MR abnormality in the annulus such as a high signal intensity on a T2-weighted image or enhancement on a contrast-enhanced T1-weighted image, one often asks whether these abnormalities represent only acute or subacute changes or whether they are chronic and persistent. In this issue of the AJNR, Munter et al (page 1105) address this issue by analyzing findings in a cohort of 18 patients who had what they defined as annular tears and who underwent follow-up MR studies over a period of 3–64 months. Simply stated, such a study would answer the following important question: If a signal intensity abnormality is present in the posterior annulus, can one say if that represents an acute finding? In addition to affecting the diagnosis and patient care, this observation has clear medical-legal implications. That is, a plaintiff’s attorney may posit that a given annular abnormality represents an acute finding that is traceable to a recent mishap such as a slip and fall or a fender bender.

In Munter et al’s paper, 29 annular tears in the authors’ 18 patients formed the basis of the investigation. Twenty-seven tears were seen at the time of the first study; of those, 25 were radial tears, and two were transverse tears. The persistence of high signal intensity in most tears and the persistence of abnormal annular enhancement in all patients who received gadolinium-based contrast agent over the period of the sequential studies (which lasted years in some cases) makes a strong argument that such findings on a given image are not necessarily acute.

As with many retrospective clinical investigations, certain problems exist, and some questions are left unanswered. The authors acknowledge the limitations of their study; some are worth additional comment. Namely, the authors retrieved cases by finding the words annular tear in old reports; one does not know how many other patients may have such tears that were never mentioned in the original report. Because such patients were not included in the study group, a prospective study would probably have allowed the identification of more patients, increasing the statistical power of the study. The uniform protocol of a prospective study would have allowed the administration of contrast material in all patients, both during the initial study and at follow-up, not just in some patients as in this situation. A prospective study might have allowed for a more uniform follow-up period. The study would have been strengthened and it may have revealed more insights into the clinical importance of annular signal intensity abnormalities if the authors had prospectively evaluated more patients. Their conclusions would have been more cogent had they classified their injuries as radial, concentric, or transverse tears, depending on the extent and location of the signal intensity abnormality. Specifically, these tears could have been correlated with the presence of a normal nucleus pulposus and the presence of degrees of disk bulges or protrusions. As opposed to annular tears, which are commonly associated with disk degeneration, transverse or concentric tears might be expected to appear the same or show little change over time.

Although this article raises a number of questions, it is a valuable starting point for further investigation of the importance of these signal intensity abnormalities on T2-weighted images. It would be of interest to have others examine this issue with a controlled series; however, in the meantime, we have a report to which we can refer when we are confronted with the issue of whether hyperintensity in the annulus is necessarily related to a relatively recent event. The dating of an injury based on abnormal signal intensity or contrast enhancement in an annular tear at follow-up imaging appears to be impossible unless an initial MR image depicts the absence of those findings. Even then, the sensitivity of MR imaging for annular tears remains uncertain. In any event, overinterpretation of the clinical importance of these MR findings is to be avoided.