The fatty degeneration of lumbar paraspinal muscles on computed tomography scan according to age and disc level.

BACKGROUND CONTEXT: Spinal degeneration can occur not only in the bone and disc but also in muscles. Fatty infiltration (FI) and decreased volume have been described as characteristic changes that occur in muscle degeneration. Many studies about the lumbar paraspinal muscles have been conducted on patients with spine problems. However, the natural changes of age-dependent degeneration in the paraspinal muscles have not been studied properly.

PURPOSE: The purpose of this study is to investigate age- and level-dependent changes of the lumbar paraspinal muscles in the population without lumbar spinal symptoms.

STUDY DESIGN: This study is a retrospective case-control study.

PATIENT SAMPLE: A total of 887 patients who underwent computed tomography scan for abdomen and pelvis (APCT) between January 2013 and December 2013 were enrolled. After excluding 237 patients with medical history of spine surgery, low back pain, myopathy, muscular dystrophy, infectious disease, vertebral fracture, and deformity, 650 patients were finally subjected to this study.

OUTCOME MEASURE: The patients were divided into three age groups: young (20-39 years old), middle (40-59 years old), and old (60-89 years old). The degree of FI was checked twice for multifidus muscle (MF), erector spinae muscle (ES), and psoas muscle (PS) at each disc level from L1 to S1 on APCT by two investigators. The FI was measured as the Hounsfield unit, a mean density (MD) on CT.

METHODS: The age differences were compared with the data of the young group, and the level differences were compared with the data of the L1-L2 level. Student t test and intraclass correlation coefficient were checked for statistical analysis.

RESULTS: The gender ratio was not significantly different among the groups. Comparing with the young group, the MD of MF significantly decreased at L5-S1 in the middle group (p<.05), and at L3-L4-L5-S1 in the old group (p<.05). The MD of ES was significantly decreased at all levels in the old group (p<.05) but not significant in the middle group. Comparing with the L1-L2 level, the MD of MF significantly decreased at L5-S1 in the middle group (p<.05) and at L4-L5-S1 in the old group (p<.05). The MD of ES was significantly decreased at L5-S1 in the young and middle groups (p<.05) and at L4-L5-S1 in the old group (p<.01) compared with those at L1-L2. According to the age- and level-dependent changes of MD in the MF and ES, there was a tendency of progressive increase of FI in the muscles with age, which seemed to start from L5-S1 and spread to the upper levels. The age-dependent fatty degeneration appeared wider in the ES than the MF. The level-dependent FI of the ES showed a similar pattern with the MF, but the change of the ES seemed to start earlier in age than the MF at the L5-S1. There was no significant MD change in the PS according to age and level. Intraobserver and interobserver reliabilities were both high across all of the muscles (0.86-0.94 and 0.83-0.92).

CONCLUSIONS: As a result, the degree of intramuscular fat infiltration seems to be affected by age, disc level, and muscle type. It seems to be more prominent in the extensor muscles, extending from lower to upper levels.