Paraspinal Lean Muscle Mass Measurement Using Spine MRI as a Predictor of Adjacent Segment Disease After Lumbar...

OBJECTIVE. The purpose of this study was to compare paraspinal muscle mass between patients with and without adjacent segment disease (ASD) after lumbar fusion. MATERIALS AND METHODS. Fifty patients with ASD (mean age, 61.4 years; ratio of male to female patients: 13:37; mean body mass index [BMI; weight in kilograms divided by the square of height in meters], 25.1) were matched to 50 control patients on the basis of age, sex, BMI, and fusion segment. The total cross-sectional area (CSA) and functional CSA (FCSA; i.e., the area containing lean muscle tissue only) of the paraspinal muscle group (the multifidus and erector spinae muscles) and the psoas muscles were measured on preoperative MRI. The ratio of the FCSA to the total CSA and the skeletal muscle index (SMI; calculated as muscle area [expressed as centimeters squared] divided by the square of the patient's height in meters]) were calculated and compared between the two groups with use of the independent-sample t test. RESULTS. The mean FCSA (2178.6 mm² vs 2594.0 mm²; p = 0.004), the ratio of the FCSA to the total CSA (45.4% vs 52.2%; p = 0.001), and the SMI of the FCSA (8.8 vs 10.6; p = 0.001) of the paraspinal muscle group were significantly smaller in patients with ASD compared to the control group. When the paraspinal and psoas muscle groups were combined, the mean FCSA (3680.8 mm² vs 4268.2 mm²; p = 0.013), the ratio of FCSA to total CSA (53.3% vs 58.6%; p = 0.004), the SMI of the total CSA (27.7 vs 29.3; p = 0.049), and the SMI of the FCSA (14.9 vs 17.3; p = 0.002) were significantly lower in patients with ASD than in control patients. CONCLUSION. Patients with ASD had smaller lean muscle mass (FCSA), a lower ratio of FCSA to total CSA, and a lower SMI of the FCSA of the paraspinal muscle group on pre-operative MRI, compared with control patients.