A new quantitative measure of disc degeneration

Background Context The ability to adequately measure a phenomenon is critical to studying and understanding it. Since 1957, a variety of subjective visual grading methods have been used to assess disc degeneration, but these have been limited by gross ordinal scales and imprecision, as well as suboptimal reliability. Conceptually sound, objective, precise measurements are needed to advance knowledge of disc degeneration and its causes, progression, and consequences. Purpose This study aimed to investigate the reliability and validity of a new system (“SpIn” for spine insight) to quantitatively measure lumbar disc degeneration or pathology. Study Design This is a measurement study using cross-sectional and longitudinal data. Patient Sample The subjects were 108 men from 35 to 63 years of age at baseline. Outcome Measures SpIn measures were validated using age, Pfirrmann grade, and other magnetic resonance imaging (MRI)-based disc and vertebral measurements associated with degeneration. Methods The lumbar spine was imaged using a 1.5 T Magnetom MRI scanner at baseline and a 1.5 T Avanto scanner at 15-year follow-up, forming two scanner-age groups. After the disc was manually traced on mid-disc axial MR images, image analysis software automatically measured distances, areas, and mean signal of regions of interest to calculate the new ratio-based disc degeneration measurements (SpIn). Repeated measurements were conducted on 30 subjects to estimate intra- and inter-rater reliability. Univariate methods and multiple regression modeling were used to compare associations of SpIn values and Pfirrmann grade, as a reference standard, with age and other degenerative and morphologic changes over follow-up. The MRI data used in the study were collected with support from the National Institutes of Health (NIH) National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) and the Finnish Work Environment Fund. One author (TV) has a patent interest in SpIn. Results Intra-rater and inter-rater measurements for SpIn yielded correlation coefficients of at least 0.98. Associations with age were clearly weaker for Pfirrmann grade than for SpIn. The variance in age explained by axial SpIn values ranged from 15.0% to 23.4% (adjusted R²), depending on spinal level and scanner-age group, as compared with 5.9%–12.9% for Pfirrmann grade. Although both SpIn values and Pfirrmann grades were associated with familial aggregation, associations were generally higher with Pfirrmann grade. Baseline SpIn values and Pfirrmann grade were both associated with subsequent, structural degenerative changes in lumbar discs and vertebrae over the 15-year follow-up, but all associations were stronger with SpIn. Conclusions SpIn provides a highly reliable, objective, continuous digital measurement of disc degeneration, which uses routinely acquired MRI and could benefit related research.