Spine surgeons shall pay more attention to bone nonunion after PLIF-PSF. Lumbar paraspinal muscles may play an important role due to the fact that they can maintain overall spinal alignment, mobility, and stability [19–21]. It has been confirmed in some studies that the degeneration of paralumbar muscles, such as atrophy and fat infiltration, would impair the paralumbar muscle function, deteriorate the spinal activity and stability, and increase the bone nonunion rate in lumbar fusion surgery [8, 9]. However, these studies are performed based on precise calculation methods, such as the functional cross-sectional area (FCSA) and fat infiltration (FI). Besides, these methods cannot provide rapid and accurate muscle health evaluation for busy spinal surgeons, and would exert an impact on the prediction of postoperative bone union and the formulation of surgical plans. In this study, four simple indicators, including the LIV, PL–CSA/BMI, Goutallier classification, and new muscle health grade (good muscle and poor muscle) are proposed for the reason that they do not require cumbersome threshold measurement techniques and staining techniques of Image J and/or Osiri X [22]. At the same time, the difference of MRI imaging will affect the threshold determination and staining result, which will aggravate the difference of rFCSA and FI, and affect the judgment of the occurrence of postoperative bone nonunion, while LIV, PL-CSA/BMI, Goutellier classification will not be affected by these techniques [22–24].
Takayama et al. maintained that LIV was closely related to CSA of lumbar multifidus and erector spinae muscles, and it can present the degree of paralumbar muscle atrophy [10]. In this study, the PL–CSA/BMI is also included because it corrects the patient's habits in order to correctly measure the relative size of the cross-sectional area [14]. In our study, we found that although there were statistical differences in LIV and PL-CSA/BMI at the L4-L5 segments, LIV and PL-CSA/BMI at the L3-L4, L4-L5 and L5-S1 segments were not independent predictors of postoperative bone nonunion in binary logic regression analysis, which is consistent with the study of Choi et al. that the atrophy of the multifidus and erector spinae muscles is not an important predictor of bone nonunion [8]. At the same time, it can be found that the proportion of patients with the Goutallier classification grade ≥ 3 (high fat infiltration) in the nonunion group is significantly higher than that in the union group. As per previous studies, the fusion rate would decrease with the increase of the fat content in paralumbar muscles, due to the possible fact that the increase of fat infiltration is related to the decline of muscle strength [9, 25]. Because LIV and PL–CSA/BMI represented the degree of paraspinal muscle atrophy, while the Goutallier classification reflected the degree of muscle fat infiltration. Previous studies have shown that type I muscle fibers are more prone to fat infiltration, while type II are preferred to atrophic changes [26–28]. It is concluded that a higher state of fat infiltration with less type I muscle fibers results in dysfunctional balance and posture, thus increasing the probability of bone nonunion. This may be that fat infiltration is more different than muscle atrophy in the occurrence of postoperative bone nonunion. However, the Goutallier classification is not an independent predictor in the regression model. This can be explained that the Goutallier classification is a qualitative visual grading method, which is related to fat infiltration (FI), but its accuracy is unfavorable.
Virk et al. [15] proposed a new muscle health grade standard, which closely correlated with health-related quality of life scores (HRQOLs). The VAS-leg, SF-12PHS, ODI, and PROMIS scores of the good muscle group were significantly different from those of the poor muscle group (P < 0.05). The new muscle health grade includes the PL–CSA/BMI and Goutallier classification, which includes the degree of muscle atrophy and fat infiltration at the same time, thus providing a more objective and complete evaluation of muscle health. In this study, it can be found that the proportion of patients with poor muscles in the nonunion group is higher than that in the union group. Meanwhile, the muscle health grade (good muscle and poor muscle) in the regression model is an important independent predictor of bone nonunion. It may be explained that the muscle health grade takes muscle atrophy and fat infiltration into account at the same time. Compared with the PL–CSA/BMI or Goutallier classification, the muscle health grade can more effectively reflect the severity of muscle degeneration. Severe muscle degeneration may aggravate bone nonunion in two ways. As is revealed in previous studies, the increase of the bending moment transmitted by internal fixation devices will increase the risk of bone nonunion, while serious muscle degeneration will have a poor effect on the reduction of the bending moment [29]. In addition, paralumbar muscle tissues can provide important blood vessels for accelerating lumbar interbody fusion [30], and severe muscle degeneration may hinder this process. Furthermore, it can be found in this study that diabetes in patients receiving PLIF-PSF at the L4-L5 or L5-S1 segments is an independent predictor of bone nonunion in regression analysis. Neil et al. [31] showed that type 2 diabetes had a unique effect on bone healing. Diabetes can produce growth factor resistance, which leads to an increased level of platelet-derived growth factors (PDGFs), thus reducing the quality of the formed fusion mass, and ultimately affecting bone nonunion. Based on the above findings, it can be recommended that spinal surgeons shall adopt the new muscle health grade before surgery to predict postoperative bone nonunion, and actively control blood glucose in patients with diabetes, so as to reduce the occurrence of postoperative bone nonunion.
There are some limitations in this study. Firstly, only the single segment lumbar fusion is included, while the double-segment and multi-segment lumbar fusion are not involved. Secondly, no postoperative MRI is performed to evaluate the muscle injury, which may reduce the predictive value of paralumbar muscles for bone nonunion. Meanwhile, the CT evaluation of bone nonunion is not performed after surgery, which may reduce the reliability of bone nonunion [6, 32]. Thirdly, due to the retrospective study, some data may be unavailable or incomplete, resulting in the exclusion of some patients and the reduction of the sample size. Despite these limitations, it can be believed that this study is the first effort to use a rapid and accurate muscle evaluation method to predict postoperative lumbar bone nonunion, which is expected to facilitate the work of clinical surgeons.