Age and bone density significantly affect the incidence of OVCF. As a surrogate of systemic muscle mass, a decreased psoas index also affects OVCF. The CSA of the multifidus muscle did not show a relationship with the incidence of OVCF. However, fatty infiltration of the multifidus muscle affected the OVCF. An increase in fatty infiltration of the multifidus muscle significantly affected OVCF, with and without adjusting for other factors. Furthermore, the severity of fatty infiltration of the multifidus muscle increased the risk of spinal fracture, even though the T-score of BMD indicated osteopenia. Therefore, preserving spinal muscle quality and bone density is essential for preventing OVCF.
Clinically, BMD evaluation provides a guideline for osteopenia and osteoporosis diagnoses.8, 9 A T-score below − 2.5 in BMD has been the gold standard for osteoporosis treatment guidelines. Low BMD increases the risk of bone fracture. However, the T-score of the BMD was not a decisive factor for spinal fractures. Sometimes, there was compression fracture in patients with a T-score higher than − 2.5 in BMD. BMD and other multifactorial factors affect osteoporotic fractures. Multifactorial factors include bone quality, muscle component, social factors, and comorbidity factors.10–12 In patients with chronic kidney disease, BMD showed a relationship with spine osteoporotic fracture risk.13 BMD can assess the macro scale of bone. However, the nanoscale of bone tissue, which has microarchitecture and tissue properties, can influence bone strength and fragility.14 Among these, sarcopenia has received attention as a potential factor influencing osteoporotic fracture, with its increase in the aging population.15 Therefore, researchers have tried to elucidate the components affecting spinal compression fractures associated with frailty and aging.
The multifidus muscle enhances the stabilization of the spine.16 Multifidus muscles have a short extending length, so muscle fibers are packed densely within a small volume. The muscle fibers’ high stiffness increases the resistance of lumbar spine flexion. Therefore, the lumbar back muscle atrophy increases compression and shear force within the disc level.17 The patients who underwent OVCF showed a significant increase in spinal flexion load than those without OVCF.18 Therefore, multifidus muscle atrophy increases the flexion force and decreases the spinal segment stability, resulting in an increased risk of OVCF. However, in this study, the CSA of the multifidus muscle did not show significant differences among the groups, not even between the fracture and control groups. In addition, fatty infiltration of the multifidus muscle significantly affected the OVCF. As atrophy of the back muscles is generally present in elderly patients, preserving muscle fibers is directly correlated with muscle function. Along with muscle atrophy, fatty infiltration of the skeletal muscle (myosteatosis) is also an essential frailty process.19, 20 Therefore, we presume that PMF but not CSA is a more accurate factor for OVCF.
Sarcopenia is a generalized muscle disorder, including low muscle strength, low muscle quantity, and low physical performance.21 Systemic muscle atrophy can influence the atrophy of localized spinal muscle mass and function, increasing the incidence of a spinal fracture.22, 23 This is because core muscles stabilize the spinal segment. However, this hypothesis remains controversial. Furthermore, some studies reported that the psoas index and/or back muscle atrophy were not independent risk factors for spinal compression fracture.24, 25 This discrepancy might be due to a simple error in measuring CSA without considering a fatty change. We reported that PMF but not CSA of the multifidus muscle was an independent risk factor for OVCF.
Identifying the factors associated with OVCF is essential for preventing further fractures. The T-score of BMD is a robust and quantitative predictive factor for osteoporotic fracture risk.26 However, it should be considered that the T-score sometimes shows discordance with osteoporotic fracture risk. Degenerated lumbar spine showed uncertainty for BMD measurement for aging.27 Measurement of bone density can be affected by calcifying structures around the spine, which are anterior and posterior longitudinal ligament, ligament flavum, aorta, and interspinous ligament.28, 29 Therefore, prediction of fracture risk is a challenge concerning the degenerative spine. The PMF in the P-BMD group significantly decreased than that in the control group. Back muscle quality had a significant impact on OVCF independent of BMD. This study showed that preserving muscle quality is essential for preventing compression fractures in the elderly.
This study has some limitations. First, we did not analyze the entire area of the back muscles. We only used the L4-5 and L5-S1 levels as surrogates of the back muscles to compare the control and fracture groups with various fracture sites. However, there was no significant difference between the PMF at the fracture and lower lumbar levels. In a previous study, back muscle degeneration was correlated with aging. Therefore, the analysis of the multifidus muscle in the lower lumbar region can represent the degree of systemic back muscle degeneration.