3D analysis of fatty infiltration of the paravertebral lumbar muscles using T2 images - a new approach.

PURPOSE Factors influencing paraspinal muscle degeneration are still not well understood. Fatty infiltration is known to be one main feature of the degeneration cascade. The aim of this cross-sectional study was to illustrate the 3D cluster of paraspinal lumbar muscle degeneration on T2-weighted MRI images using our newly developed software application 'iSix'. METHODS Mono- (Mm. rotatores), multi- (Mm. multifidus) and pluri-segmental (M. erector spinae) lumbar muscles groups were segmented on T2-weighted MR sequences using a novel computer-assisted technique for quantitative muscle/fat discrimination. The degree of fatty infiltration of the three predefined muscle groups was compared on a 3-dimensional basis, with regard to segment involvement and age. General linear models were utilized for statistical comparison. RESULTS N = 120 segments (age: 52.7; range 16-87 years) could be included. The overall relative fatty infiltration of the mono-segmental muscles was higher (21.1 14.5%) compared to the multi-segmental (16.0 8.8% p = 0.049) and pluri-segmental muscles (8.5 8.0%; p = 0.03). Mono-segmental muscles on the levels L4/5 (22.9 ± 10.2 [CI 17.6-28.2] %) and L5/S1 (27.01 ± 15.1 [CI 21.4-32.7] %) showed a significant higher amount of fat compared to the levels L2/3 (8.2 ± 6.8 [CI 2.2-14.2] %; L4/5 vs. L2/3, p = 0.03; L5/S1 vs. L2/3, p = 0.02) and L3/4 (13.2 ± 5.4 [CI 8.6-17.7]%; L4/5 vs. L3/4, p = 0.02; L5/S1 vs. L3/4, p < 0.01). Multivariate linear regression analyses revealed age and Pfirrmann grade as independent factors for fatty muscle degeneration. CONCLUSIONS 3D analysis of fatty infiltration is an innovative tool to study lumbar muscle degeneration. Mono-segmental muscles are more severely affected by degeneration compared to multi-/pluri-segmental muscles, especially at the L4/5 and L5/S1 level. Age and disc degeneration independently correlate with muscle degeneration