MRI is a noninvasive, radiation-free tool for determining muscle measurements, especially in terms of size and fat infiltration, and it has acceptable reliability.11 In our study, to evaluate paraspinal muscle degeneration, we used the methods reported by Shahidi et al.10 Only 1.5-Tesla MRI was used in consideration of the sample size and image quality. Previous studies have affirmed the feasibility of performing measurements on axial MRI images;11–13 however, any single segment of the paraspinal muscle measurement cannot be representative of the whole spine because of the significant difference in fat distribution.12 Nevertheless, for patients with OVCF in our medical center, all MRI data included the axial image of the mid-height of the injured vertebral body. The measurements in this study were used to investigate the change in the fractured level of the paraspinal muscle using the same MRI scan parameters, thereby avoiding the errors caused by the uneven distribution of fat. With respect to measuring objects for the initial fractured segments ranging from T5 to L5, only the main composition of the paraspinal muscles was considered, except for the psoas, which was mentioned in studies on spinal degeneration and deformities. In view of these findings, we proposed that the outcomes could also represent the credible changes of paraspinal muscles in patients after PKP, and we hope to reflect the relationship between paraspinal muscle degeneration and the incidence of vertebral refractures.
PKP was performed only if the surgical principles were met. All patients were provided with a rigid thoracolumbosacral orthosis brace (TLSO, Hengshui Qianzhong Medical Equipment Co., Ltd., Hengshui, China) and were advised to undergo stabilization for 3 months (equal to the minimum follow-up criteria). All patients were instructed to attend regular follow-ups; 59 of them agreed to undergo another MRI scan, and were proven to not have a refracture. Any case that did not meet the in/exclusion criteria was excluded.
After excluding unusual edema, hemorrhage, and inflammation, the ROI-SI can indirectly reflect the change in fat infiltration of the targeted muscles. Correlation analyses indicated that a stronger paraspinal muscle usually had a higher degree of fat infiltration, whereas the muscle atrophied along with fat infiltration (Fig. 4A, B). These results were similar to those of previous viewpoints12 and supported the reliability of the methods used in measuring SI.
Spinal degeneration includes the vertebrae, discs, and joints, and can be seen in muscles around the spine.6,14 Previous studies have reported changes in the morphology and composition of the muscles in adults. Fat infiltration in the paraspinal muscles has been found to increase progressively with age10,14,15 and is more significant in the lower lumbar region than in the upper lumbar region.14,16,17 In a long-term study, decreased muscle volume was also reported to be age-dependent in the normal population,15 and 15% of skeletal muscle mass was lost every 10 years in individuals older than 50 years of age.18 In the present study, both patient groups showed a decreased in the ROI-CSA accompanied by an increase in the ROI-SI (Table 2). However, patients with refractures had significantly more paraspinal muscle shrinkage along with an increase in the SI compared to the non-refractured group. The CSA/SI ratios showed the same outcomes. In other words, patients after PKP were more inclined to sustain new OVCFs when the quality of the paraspinal muscle deteriorated.
Trunk muscles have been shown to be an important element of spinal stability. Biomechanical studies have reported that paraspinal muscles support the ligaments and vertebrae; therefore, weak paraspinal muscles would result in instability of the spine in patients with osteoporotic fractures. Inversely, stronger muscles can decrease stresses between the vertebrae, intravertebral discs, and facet joints, and play an irreplaceable role at higher loads.6,19
The mass and density of muscle reduced yearly, thus limiting the daily mobility of elderly patients and leading to a higher risk of osteoporotic fractures.8,20 A case-control study of elderly men compared patients with lumbar fractures to healthy volunteers. At an average age of 75 years, the fractured group showed significantly lower CSAs and density in the paraspinal muscles as measured by quantitative computed tomography (CT).20 Research with a long follow-up reported that after performing a progressive back strengthening exercise for 2 years, the strength of the erector spinae muscle was enhanced in postmenopausal women and maintained for 10 years compared with the control group.8 Moreover, people who performed a back muscle exercise had significantly fewer OVCFs in previous studies,8,9 which was similar to the finding of our study.
Degeneration with aging was the main factor for muscle-bone unit dysfunction. Patients were inclined to have more bed rest or perform fewer daily activities after PKP, although their pain was relieved. The efficacy of prolonged bed rest is uncertain, and complications, such as muscle atrophy, atelectasis, thrombosis, and pressure ulcers, cause concerns.21 Even short-term bed rest can induce acute bone resorption and decrease muscle strength.22 Likewise, atrophied muscles and fat also affect one’s ability to perform activities, increase the risk of falling, and form a cycle of deterioration.23 Patients with vertebral refractures usually were on bed rest for days before hospitalization; therefore, this might have been an unavoidable bias for the measurements in this study. However, the ROI-SI indeed increased significantly in these patients, and it required a long process.
Prolonged bracing treatment after OVCF is controversial. A brace is routinely used for stability after spinal fractures and surgeries. However, there is no strong evidence of the superiority of the brace after PKP; muscular atrophy was also suspected to be related to prolonged bracing. A randomized controlled trial reported that compared with the control condition, wearing a brace did not significantly help OVCF patients in terms of improving the Oswestry Disability Index scores or back pain.24 Inversely, the drawbacks of bracing must be taken seriously. Reinforcing the trunk passively may lower the range of flexion and extension, subsequently leading to a reduction in paraspinal muscle strength.25,26 Ultrasound measurements showed that the lumbar muscles were atrophied after 8-week lumbar belt immobilization.27 All patients in the current study were advised to use the TLSO for 3 months postoperatively. However, the specific processes, including duration and occasion, could not be recorded quantitatively. Based on the results, paraspinal muscle degeneration was seen in both groups, with the refractured group showing more significant deterioration overall than the non-refractured group. Disuse of paraspinal muscles was widespread in osteoporotic elderly patients, and we assumed that prolonged bracing and bed rest may be the components of vertebral refractures; however, no direct evidence of this has been presented yet.
According to the concept of the muscle-bone unit, appropriate loading forces of muscle could stimulate the formation of bone.20 Conversely, lower BMD was also correlated with atrophic skeletal muscle.8,20,23 To our knowledge, long-term low BMD was believed to be a prominent factor for OVCFs, and the preoperative BMD in the present study indicated a poor condition (Table 1). A BMI < 22 kg/m2 was mentioned in a previous study as a strong factor for predicting postoperative refractures.28 A low BMI amplified the stress transferred to the adjacent vertebral bodies, which increased the possibility of new OVCFs.29 Both groups showed similar BMIs and thus provided a comparable baseline; further multivariate analyses of BMD and BMI are required.
Medical injuries of the paraspinal muscles have to be considered. The PKP technique uses trocars, and it is believed to be minimally invasive compared with the open approach. However, damage to the paraspinal muscle is inescapable. In addition, procedures were more difficult to perform in cases of severe degeneration, and repeated adjustment of the needles was related to more damage to the muscles and branch nerves. The unilateral approach causes less muscular injury than bilateral injections, but there is no study on muscular evaluation. It seemed unavoidable that the ROI-SI increases with fat and edema when using T2-weighted images. However, OVCFs in elderly patients were mostly caused by moderate trauma, and patients with obvious muscle edema and inflammation were excluded, which make the results more tenable.
Based on the results of this study, we have discussed the relationship between vertebral fractures and paraspinal muscle degeneration in elderly individuals. In the context of low BMD, people were prone to new OVCFs, but weak paraspinal muscles may also be a risk factor. In clinical practice, prolonged bed rest and bracing treatment are worthy of concern. We hope that this MRI observation could provide a new perspective on refractures after PKP. In addition, a comparison of paraspinal muscles between patients with OVCFs and healthy elderly people is required.
Several limitations of this study need to be noted. First, it was a retrospective study that lacked substantial samples. Only patients with complete 1.5-Tesla MRI data were enrolled to ensure consistency. Second, the final follow-up MRI in the non-refractured group was not entirely random, and potential bias should be mentioned. Furthermore, using a grayscale to estimate SI is an indirect method, although its reliability is acceptable. Further studies using quantitative CT and ultrasound approaches will be more precise.