Assessment of association between low back pain and paraspinal muscle atrophy using opposed-phase magnetic resonance imaging: a population-based study among young adults.

STUDY DESIGN: A cross-sectional imaging study of young adults. OBJECTIVE: To evaluate whether severity of low back symptoms predicts atrophy in the paraspinal muscles of young adults. SUMMARY OF BACKGROUND DATA: Although an increased fat content of the lumbar muscles has been observed among adults with chronic LBP, there is limited knowledge of this association in younger populations. METHODS: The population-based study sample consisted of 554 subjects (321 females and 233 males) from the 1986 Northern Finland Birth Cohort. Latent Class Analysis (LCA) was used to cluster the subjects according to the low back symptoms and functional limitations at 18, 19, and 21 years. The mean age of the subjects at the time of the MRI (magnetic resonance imaging) was 21 years (range, 20-23). Muscle atrophy was evaluated by assessing the fat content of the paraspinal muscles using Opposed-Phase MRI. The cross-sectional areas (CSAs) of the erector spinae and multifidus muscles were also measured. RESULTS: LCA analysis produced five clusters differing in symptoms, ranging from a cluster (n = 65) in which subjects had high likelihood of symptoms and functional limitations at all time points, to a cluster (n = 165) with no pain ever. The fat content of the multifidus muscles was significantly higher among women than men (14.0% vs. 5.3%, P < 0.001), but it was not significantly associated with symptom severity. The CSA of both erector spinae and multifidus muscles were significantly larger among men than women (P < 0.001 in all of the muscles), but were not associated with pain severity. CONCLUSION: Low back symptoms and functional limitations over a 3-year period were not associated with increased fat content or a reduction in the cross-sectional area of lumbar paraspinal muscles among young adults.