Does lumbar paraspinal muscles improve after corrective fusion surgery in degenerative flat black?

Background: Degenerative flat back (DFB) is characterized by sagittal imbalance resulting from the loss of lumbar lordosis (LL). Extensive degeneration and weakness of lumbar paraspinal extensor muscle (PSE) are thought to be the main cause of DFB. This study is to evaluate correlation between preoperative PSE conditions and angular severity of DFB and to evaluate correlation between preoperative PSE conditions and degree of improvement of DFB obtained by corrective surgery. Materials and Methods: Forty five patients with DFB who took magnetic resonance image (MRI) preoperatively and conducted simple radiography and three-dimensional gait analysis before and 6 months after corrective surgery were included. To determine the severity of PSE atrophy, the ratio between cross-sectional area of PSE and disc was calculated from L1-L2 to L4-L5 on MRI. To assess the degree of fat infiltration, the signal intensity of PSE was measured. Static parameters of spinopelvic segment were measured by simple radiography. Dynamic parameters of spinopelvic and lower limb joints were obtained by three-dimensional gait analysis. Results: In static parameters, thoracic angle was correlated with atrophy and fat infiltration of upper PSE. Thoracic angle was less improved after surgery, as atrophy of upper PSE was more severe. In dynamic parameters, thoracic angle showed correlation with upper PSE conditions, whereas lumbar angle had correlation with middle to lower PSE conditions. While thoracic kyphosis was less improved after surgery, as atrophy of upper PSE was more severe, LL was less improved, as atrophy and fat infiltration of PSE from L1-L2 to L4-L5 were more severe. Conclusions: Severity of atrophy or fat infiltration of PSE showed correlation with degree of angular deformity in patients with DFB and with less improvement after corrective surgery. Dynamic parameters showed more prominent correlation with PSE conditions than static parameters and also showed segmental specificity between PSE and angular deformity.