Increased intramuscular fatty infiltration without differences in lumbar muscle cross-sectional area during remission of unilateral recurrent low back pain.

Lumbar muscle degeneration is a common feature in non-specific low back pain (LBP). It is hypothesized that degenerated muscles might compromise spinal stability and lead to further injury/pain. However, little is known about lumbar muscle morphometry after resolution of LBP. Therefore, this study investigated the extent of lumbar muscle atrophy and fatty infiltration in individuals who are at risk for a recurrence of LBP. Thirteen participants in remission of unilateral recurrent LBP were compared to 13 healthy controls, comparable for age, weight, length and level of physical activity. Total, lean muscle and fat cross-sectional area (CSA) of lumbar multifidus (MF), erector spinae (ES) and psoas (PS) were investigated on T1-weighted Magnetic Resonance Imaging (MRI), bilaterally and at 3 lumbar levels (L3 upper, L4 upper and L4 lower endplate). In addition, a muscle-fat-index (MFI) was calculated reflecting the amount of fatty infiltration in lean muscle tissue. No significant differences for total, lean muscle and fat CSA were found between people in remission of recurrent LBP and the control group. Conversely, MFI was increased bilaterally at the 2 lowest lumbar levels. There were no differences between the previously painful and non-painful side of the LBP group for any of the parameters. These results show a generalized increase in intramuscular fatty infiltration in lean muscle tissue in the absence of macroscopical signs of muscle degeneration after resolution of LBP. These findings reflect a decreased muscle quality, but not quantity, and might indicate a pathophysiological mechanism contributing to recurrence of LBP.