We studied 98 people (50 men and 48 women) without symptoms of back pain, from 20 to 80 years old (mean age, 42.3 years). Volunteers were recruited by distributing flyers in the hospital, mailing an announcement to all staff physicians, and advertising in the hospital newspaper. Participants did not need to be affiliated with the hospital. Applicants completed a consent form approved by the Investigational Review Board and were interviewed by one of us. Those with a history of back pain lasting more than 48 hours or any lumbosacral radiculopathy were excluded (about 20 patients). To reduce bias in the interpretation of the MRI scans, abnormal scans from 27 people with back pain were selected and mixed randomly with the scans from the 98 people without symptoms.

The level of physical activity was scored as follows: 0, no exercise; 1, occasional exercise (less than weekly); 2, weekend exercise; 3, workouts three or four times a week; and 4, workouts five or more times a week or regular workouts that included strenuous activity such as weightlifting or horseback riding.

All MRI scans were obtained at Hoag Memorial Hospital with 1.5-T imagers (Signa, General Electric, Milwaukee; and Magnetom SP4000, Siemens Medical Systems, Iselin, N.J.). The studies consisted of four spin-echo sequences: a coronal localizer with a repetition time and echo time (TR/TE) of 400/15 msec, a sagittal view with a TR/TE of 300-600/11-23 msec, an axial view with a TR/TE of 700-900/11-15 msec, and a sagittal view with a TR/TE (dual-echo sequence) of 2500-2600/16-21 and 90-105 msec. Technical specifications included a slice thickness of 3 and 4 mm for sagittal and axial sequences, respectively; a field of view of 26 and 20 cm for the sagittal and axial images, respectively; and a matrix of 192 by 256. The T 1 -weighted axial sequences were stacked slices extending from the inferior aspect of L3 through the inferior aspect of S1. There were two excitations for the T 1 -weighted axial and sagittal images, with one excitation for the T 2 -weighted sagittal images.

All studies were read at the Cleveland Clinic by two experienced neuroradiologists familiar with the MRI imagers used. The readers did not know the clinical status of the subjects. All identifying information and dates were obscured. Readings were carried out in groups of 9 to 11 studies per session, which included 1 to 4 studies from people with symptoms. The readers independently evaluated the status of the 5 intervertebral disks in the lumbosacral spine in all 125 subjects (a total of 625 disks).

The terms used to classify disks were defined as follows: normal, no disk extension beyond the interspace; bulge, circumferential symmetric extension of the disk beyond the interspace (around the end plates); protrusion, focal or asymmetric extension of the disk beyond the interspace, with the base against the disk of origin broader than any other dimension of the protrusion; and extrusion, more extreme extension of the disk beyond the interspace, with the base against the disk of origin narrower than the diameter of the extruding material itself or with no connection between the material and the disk of origin. This terminology was selected on the basis of the findings of a companion study that evaluated interobserver and intraobserver variability when different nomenclatures were used to describe disk abnormalities in the same 125 MRI studies. In that study, all scans were read independently at least twice by the two neuroradiologists (evaluator 1 and evaluator 2), with a minimum of two weeks between the readings. The data in the current study are based on the second reading in the companion study, in which the terms we selected (normal, bulge, protrusion, and extrusion) were used for the first time. With these definitions, an interobserver agreement of 80 percent (for all 125 subjects) was found (kappa = 0.59)9.

Nonintervertebral disk abnormalities were assessed on the basis of a consensus by two other readers at Hoag Memorial Hospital. The following abnormalities were recorded: Schmorl's nodes, facet arthropathy, spondylolysis, spondylolisthesis, annular defects, and stenosis of the central canal or neural foramen. The criteria for stenosis of the central canal and neural foramen were obliteration of the epidural fat with flattening of the thecal sac and obliteration of the perineural fat, respectively10.

For statistical analyses, the prevalence of disk abnormalities observed by the two readers was determined according to the subjects' sex, age, and physical-activity score, with the use of a generalized linear model for correlated binary data11. All tests of significance were two-tailed.