Dynamic changes of elasticity, cross-sectional area, and fat infiltration of multifidus at different postures in men with chronic low back pain

Background context: Multifidus cross-sectional area was often measured in chronic low back pain (LBP) patients to estimate the muscle activity for spinal stability. However, such estimation may be inadequate as the contribution of muscle elasticity in muscle activity is ignored. In vivo quantitative data on multifidus elasticity is therefore important for the study of muscle contractile function in response to motor control for spinal stability in chronic LBP patients.

Purpose: The purpose of this study was to quantify the elasticity, cross-sectional area, and fat area of the multifidus for the contractile function and the distribution of deformable muscle tissue and nondeformable fat tissue at different postures in patients with and without chronic LBP.

Study design/setting: This is a prospective study. Force-deformation data of the multifidus were acquired using ultrasound elastography. The anatomical changes of the multifidus were measured on the cross-sectional images of the multifidus acquired using B-mode ultrasound imaging.

Patient sample: The sample comprised 12 adult male patients with chronic LBP and 12 asymptomatic male controls.

Outcome measures: The outcome measure was the elasticity of the multifidus at the L4 level for the assessment of muscle contractile function when patients were in the prone, upright, and 25° and 45° forward stooping positions. The cross-sectional area and fat area were also measured on the B-mode ultrasound images of the multifidus acquired at the same vertebral level and the postures.

Methods: With the patients in each of the prone, upright, and 25° and 45° forward stooping positions, ultrasound elastography and B-mode ultrasound imaging were performed on the left and right multifidus at the L4 level. The elasticity of multifidus indicated by the effective Young's modulus was derived from the force-deformation data acquired using ultrasound elastography. The cross-sectional area and fat area were assessed on the B-mode ultrasound images. The effective Young's modulus, cross-sectional area, and fat area were analyzed with multivariate general linear model analysis to investigate the possible effects of LBP and posture.

Results: There was an increasing stiffness of multifidus demonstrated by increasing effective Young's modulus from the prone to upright position and 25° and 45° forward stooping positions. Differences in multifidus stiffness between chronic LBP patients and asymptomatic controls were shown in the upright and 25° and 45° forward stooping positions but not in the prone position. The cross-sectional area of the multifidus increased from the prone position to the greatest value in the upright position and decreased in 25° and 45° forward stooping positions. Smaller multifidus cross-sectional area was demonstrated in chronic LBP patients than that in controls at all postures. No effect of posture on fat area within the multifidus was shown although the fat area within the multifidus was larger in chronic LBP patients.

Conclusions: Different, changing patterns of elasticity and cross-sectional area were identified in the multifidus in relation to posture. Increased stiffness of multifidus in response to the physiologic range of static loads and smaller cross-sectional area was characterized in the chronic LBP condition for spinal stability. Ultrasound elastography offers in vivo assessment of muscle contractile function of deep trunk muscles, which benefits the future investigation of the neuromuscular regulating mechanism in LBP. It can also be applied to refine the palpatory skill for the physical assessment in sports training and physical therapy.