The innovation of this study was to use quantitative techniques, including QCT and the IDEAL-IQ sequence, to explore the changes of lumbar spine vBMD and paraspinal muscle properties in patients with LDH. Our findings revealed that in LDH patients, the PDFF of paraspinal muscles increases as vBMD decreases, while the CSA of paraspinal muscles decreases as vBMD decreases. Our results also showed that the PS CSA and MF PDFF are independent influential factors of vBMD. This finding suggests a close relationship between muscle mass, muscle size and bone density, indicating that there is an interconnected and interacting system between muscle and bone.
Muscle and bone interact throughout a person's life, and there are currently two mechanisms explaining this interaction. One mechanism involves mechanical loads (19), where the tension generated by the muscle stimulates osteogenic activity in the bone, leading to responses from osteoblasts and osteocytes. The other mechanism involves endocrine factors (20), where the muscle produces biochemical signals during exercise, including hormones and growth factors, which impact the coupling process of bone formation and resorption. Muscle CSA has been widely used to estimate muscle volume as an indirect indicator of muscle strength (21). Muscle fat infiltration is an important manifestation of skeletal muscle aging and reflects the decline in skeletal muscle function and strength (22). Both muscle CSA and PDFF reflect the mechanical tension between muscle and bone. Muscle fat infiltration or muscle atrophy affects the mechanical stimulation of the corresponding area of bone.
However, it is controversial about the relationship between BMD and muscle size. Yang et al. (15) showed that vertebral BMD was associated with paraspinal muscle fat infiltration but not muscle size. This difference may be attributed to variations in the study population, as we focused on an LDH-prevalent population, potentially leading to distinct findings. Our study demonstrated a negative correlation between vertebral vBMD and the paraspinal muscle PDFF, and a positive correlation between vertebral vBMD and the paraspinal muscle CSA, which aligns with the findings of previous similar studies. For example, Li et al. (23) found that both muscle CSA and PDFF in the muscles near the hip joint correlate with proximal femur BMD. This finding suggests that muscle fat infiltration and muscle atrophy may progress simultaneously with age, collectively impacting vBMD. Our study revealed that degenerating paraspinal muscles may contribute to a decrease in vBMD, and this change that is also present in the LDH patients. These findings suggest that we should pay attention to the management of CSA and PDFF of paraspinal muscles in patients with LDH.
We found that among the paraspinal muscles, the PS CSA and MF PDFF were found to be independent influential factors of vBMD. This may be due to the role of the MF as the primary stabilizing muscle of the lumbar spine (24). Anatomical studies have shown that the MF has the largest paraspinal cross-sectional area and is located in the innermost portion of the spine, providing substantial stabilizing support to the spine (25). In addition, the PS is a core muscle group that represents the overall stability of the body, and it originates from the lumbar vertebrae (26). Both the MF and PS exert varying degrees of tensioning load on the lumbar spine. Changes in lumbar paraspinal muscle properties may be an important indicator of lumbar spine BMD (27), and the presence of low BMD also reflects alterations in paraspinal muscle properties. Understanding the relationship between paraspinal muscle properties and BMD in patients with LDH is crucial for comprehending the issue of low back pain and will aid clinicians in selecting appropriate treatment options.
In addition, our study revealed significant differences in age, vBMD and paraspinal muscle properties between males and females. Compared with females, males exhibited greater paraspinal muscle CSA and lower paraspinal muscle PDFF values. Furthermore, males had higher vBMD than females, and gender was also an independent factor of vBMD. Some studies have also reported lower BMD and greater muscle fat infiltration in females than in males (28, 29). This may be because the musculoskeletal relationship can be affected by hormones such as estrogen deficiency (30). The decline in estrogen levels in menopausal women leads to bone mass loss and an increase in muscle fat infiltration (31), leading to a greater degree of muscle fat infiltration in females compared to males.
The results also revealed a strong negative correlation between vBMD and age in LDH patients, and age was also identified as an independent factor of vBMD, which is consistent with the findings of epidemiological studies in the United States (32). Aging has a non-negligible impact on muscle degeneration. On the other hand, the osteoporosis group had a slightly higher BMI than the normal bone density group and the osteopenia group, but there was no statistically significant difference in BMI among the three bone mass groups. This is contrary to findings of Han et al. (33), which reported a statistically significant difference in BMI among the three bone mass groups, with the lowest BMI observed in the osteoporosis group. Low BMI is always recognized as a risk factor for osteoporosis (34). However, the study by Zhu et al. (35) demonstrated that the positive correlation between BMI and BMD was attenuated at high BMI, and that fat mass by DXA were significant predictors of BMD measurement. Therefore, the results of this study suggest that the degree of paraspinal muscle fat infiltration (e.g., intramuscular and intermuscular fat in localized muscles) may not be affected by overall body fat.
Low back pain is a common clinical symptom in patients with LDH, and it is often attributed to fatty degeneration of the paraspinal muscles (36). Kjaer et al. (37) demonstrated a strong correlation between low back pain and paraspinal muscle fatty infiltration in adults. Similarly, Fortin et al. (38) observed a greater fat component in the paraspinal muscles at the herniation level in patients with symptomatic disc herniation, but there was no significant difference in muscle size. On the other hand, there was no correlation between the visual analogue scale (VAS) score and the paraspinal muscle PDFF in patients with low back pain in a study by Li et al. (39) Additionally, Danneels et al. (40) reported no difference in the paraspinal muscle CSA between low back pain patients and healthy control subjects. Previous studies in the LDH population have focused primarily on the association between disc pathology and paraspinal muscles, with limited exploration of the association between muscle and bone. Our study extends the research on LDH by investigating the musculoskeletal relationship, specifically examining the correlation between BMD and paraspinal muscle properties in the LDH population, rather than being limited to the healthy population.
This study has several limitations. Firstly, this was a cross-sectional observational study, making it challenging to establish a causal relationship between vBMD and paraspinal muscle properties, necessitating long-term follow-up studies. Secondly, the study had a small sample size and an uneven distribution across bone mineral density groups. However, the study included patients across a wider age range and maintained a balanced male-to-female ratio, potentially providing insights into bone and muscle development stages throughout life. Thirdly, the study did not account for the dimensions or severity of the patients' LDH, nor did it consider the duration of the disease or the individuals' level of mobility.
In future studies, it may be feasible to validate muscle properties as predictors of osteoporosis. Osteoporosis could be anticipated by simply including an MRI scan sequence, eliminating the need for additional radiation from CT scans. Enhancing screening efficiency and reducing costs could alleviate the societal burden. Additionally, analyzing the correlation between muscle and bone in LDH patients is crucial for comprehending LDH and determining appropriate treatments to mitigate adverse clinical outcomes such as fractures and falls.