Association between final local kyphosis angle and residual back pain in traumatic vertebral fractures at the thoracolumbar junction with posterior stabilization using percutaneous pedicle screws

Introduction

There are reports of an association between final local kyphosis and residual back pain (RBP) traumatic thoracolumbar fracture (1-4) and reports of no association (5-8), with no consensus. Percutaneous pedicle screw (PPS) fixation of thoracolumbar fractures has become more widespread in recent years, but no previous reports have examined the association between final local kyphosis and RBP after surgery with PPS. The aim of this study was to investigate whether there is an association between the final local kyphosis angle and RBP in patients with traumatic vertebral fractures at the thoracolumbar junction who underwent single posterior surgery with PPS and implant removal after fracture healing. A second goal was to determine the optimal cut-off value for the final local kyphosis angle with and without RBP. This article is presented in accordance with the STARD reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-24-69/rc).

Methods

The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the ethics committee of the Toyama Prefectural Central Hospital (No. 6455) and informed consent to use patient data was obtained from all patients. The study included data from patients who underwent posterior spinal stabilization using PPS between February 2013 and March 2022 for traumatic thoracolumbar fracture without neurological deficits (Figure 1). Patients were included if they sustained a single level high-energy trauma fracture at T11–L2 (fragility fractures such as osteoporosis were excluded), were younger than 70 years old at the time of injury, had undergone surgery with a single posterior approach (additional posterior decompression or anterior approach were excluded), had the implant removed after fracture healing, and had outcomes documented by imaging and back pain assessments 3–12 months after implant removal. Surgery was indicated for A3-type fractures with more than 20° of vertebral kyphosis, A4-type, or B-type fractures. For cases meeting these criteria, age, gender, Injury severity score, level of fracture, AO classification, the McCormack Load sharing classification (9), and range of stabilization were collected. In addition, imaging assessment was performed with the patient in the supine position to evaluate vertebral kyphosis angle, local kyphosis angle (Cobb angle), and the percentage of anterior, middle and posterior vertebral body compression at the time of injury and at the final follow-up after implant removal (Figure 2) (10). The Cobb angle was chosen because it is the most commonly used method for measuring kyphosis angle and has small intra- and inter-observer variability (10,11). The Numerical Rating Scale (NRS) and the Oswestry Disability Index (ODI) were used to assess the level of RBP. Regarding the ODI, the questions on item 8 on sexual life are sometimes omitted in studies with young people (12) and were therefore omitted in the present study. The patients were divided into two groups, with and without RBP [Group RBP (+) and Group RBP (−), respectively].

Figure 1 Flowchart of patient inclusion. PPS, percutaneous pedicle screw.

Figure 2 Illustration of the measurement method. (A) Vertebral kyphosis angle; (B) local kyphosis angle (Cobb angle); (C) anterior vertebral body compression percentage: 2b/(a + c) ×100; (D) middle vertebral body compression percentage: 2b/(a + c) ×100; (E) posterior vertebral body compression percentage: 2b/(a + c) ×100. a, b, c: length of white line on each vertebra.

Statistical analysis

All statistical analyses were conducted using IBM SPSS Statistics for Windows (IBM Corp, Armonk, NY, USA). All data are expressed as mean ± standard deviation. A Mann-Whitney U test, Chi-squared test, or Fisher’s exact test was used to compare each variable between groups. Differences with P<0.05 were considered significant. Cut-off values were calculated using the Youden index with receiver operating characteristic (ROC) curves.

Results

Seventeen males and 8 females were included in the study. The mean age of the patients was 41.5±16.2 years, the time of implant removal was 16.6±6.2 months after initial surgery and the final imaging evaluation was 6.0±2.9 months after implant removal. There were 12 patients in the RBP (+) group and 13 in the RBP (−) group. The level of back pain in the RBP (+) group was averaged 2.0±1.2 on the NRS and 14.4%±9.8% on the ODI. The two groups were significantly different only for the final local kyphosis angle, with no significant differences for the other variables (Table 1). The ROC curve of the final local kyphosis angle for RBP had an area under the curve (AUC) of 0.88 (P<0.01; Figure 3). The optimal cut-off value for the final local kyphosis angle for RBP calculated from the Youden index was 15.85°, with a sensitivity of 0.92 and a specificity of 0.85.

Figure 3 The ROC curve of the final local kyphosis angle for residual back pain (P<0.01, AUC =0.88). ROC, receiver operating characteristic; AUC, area under the curve.

Discussion

Burst fractures can be considered mechanically stable if there remains preservation of the posterior osteo-ligamentous complex and facet capsules (13). In stable burst fractures, nonoperative treatment with a brace or cast has been shown in many studies to be effective (8,14). But an increasing fracture kyphosis will be seen over time in most individuals (13). The significant increases in the fracture angle or significant increases in pain have been suggested by some as an indication for consideration of operative treatment (15,16). Operative treatment offers some advantages over nonoperative treatment such as earlier rehabilitation and may restore sagittal alignment (6,17). Therefore, the decision for surgery depends on the vertebral destruction such as the degree of kyphosis or axial compression, and any neurologic involvement (13). However, no specific rule for surgical intervention for burst fractures without neurologic involvement has been established because there is no consensus on the association between final local kyphosis and RBP after traumatic vertebral fracture. Some report that surgery is ultimately less painful than conservative treatment (7), while others report that it is more painful (8). The invasiveness of the surgery itself may also contribute to the eventual pain, which may make it difficult to establish the indications for surgery. On the other hand, unstable burst fractures with significant disruption of the posterior osteo-ligamentous complex (AO B-type) are typically treated with an operative approach, as the healing capacity of torn ligamentous structures is much less than that of bone (18). Based on this background, in this study surgery was indicated for fractures associated with A3-type fracture with more than 20° of vertebral kyphosis, A4-type, or B-type fractures. Spinal stabilization using PPS for traumatic thoracolumbar vertebral fracture has been used widely in recent years because it has advantages over conventional open surgery, such as shorter surgical time and less blood loss with no increase in blood loss in early surgery (19,20). Previous reports have not been limited to only those cases with or without implant removal, or at least no reference was made to the presence or absence of implant removal (1,3,6-8). This study was limited to patients who had undergone implant removal. Implant removal after stabilization of thoracolumbar fractures using PPS is considered highly satisfactory in terms of patient satisfaction due to reduced RBP and discomfort (21). The concept of posterior stabilization with PPS without bone grafting is considered only as temporary stabilization, thus implant removal is performed after a period of recovery at our institution. In addition, this study was limited to patients who underwent imaging and back pain assessment 3–12 months after implant removal. The reason for this is that within 3 months of implant removal there might still be wound pain due to the surgery, and after 12 months there would be factors other than the trauma, such as degeneration that could affect the pain.

This is the first report of the association between final local kyphosis angle and RBP in patients with traumatic vertebral fractures at the thoracolumbar junction who have undergone PPS fixation and implant removal after fracture healing. The results showed an association between the final local kyphosis angle and RBP. There are two hypotheses as to why the final local kyphosis angle is associated with RBP. The first is that local kyphosis causes muscle soreness due to the strain on the muscles to correct the posture. The other one is that discogenic pain may be caused by disc degeneration, as the present results showed a significant difference in the final local kyphosis angle rather than the final vertebral kyphosis angle.

As there is a clear association between final local kyphosis angle and RBP, the cut-off value for the final local kyphosis angle for RBP is of some interest. Few reports to date have reported the cut-off value for the final local kyphosis angle for RBP. Schulz et al. reported 12° as a cut-off value for the final local kyphosis angle for RBP in combined anterior-posterior surgery (2). We examined cut-off values for posterior surgery, which is more common than anterior-posterior combined surgery. The results showed that the optimal cut-off value of the final local kyphosis angle for RBP was approximately 16°. This value has a sensitivity of 0.92 and a specificity of 0.85, which is considered to be a reliable guide to treatment. Causes of final local kyphosis include poor surgical reduction and post-operative loss of correction. Surgery using PPS has been reported to result in poor reduction compared to conventional open surgery (19). However, by using a reduction procedure such as the joy-stick technique, a similar appropriate reduction to conventional open surgery can be obtained (22). In this study, screws were not inserted at the fracture level except in one case. It has been reported that percutaneous screw fixation combined with intermediate screws at the fractured vertebra could more effectively restore and maintain fractured vertebral height (23). Based on the results of the present study, insertion of screws into the injured vertebrae may be a useful treatment option. On the other hand, the main cause of post-operative correction loss is reported to be degeneration of the injured disc (24-26). The recurrence of kyphosis due to disc degeneration can occur because the disc can be injured during the original trauma (24). Anterior fusion is therefore reasonable to prevent kyphosis caused by disc degeneration. However, the degree of RBP in the RBP (+) group was mild, with an average of 2.0 for the NRS and 14.4 for the ODI, so we do not consider this to be sufficient evidence to recommend additional anterior surgery. Therefore, further studies are needed for cases with severe back pain.

There are some limitations of the present study. The sample size is small. It is possible that some variables would be significantly different between the groups if the sample size were larger. In addition, factors other than imaging findings, such as physical function, have been reported to be associated with RBP (27), and larger sample sizes are needed to examine these factors. It has been reported that superior clinical outcomes depend on restoration of sagittal alignment (28). Just as pelvic incidence values vary widely from person to person, so does the appropriate lumbar lordosis angle. Patients with large pelvic incidence may be more prone to low back pain as a result of having to perform greater compensatory functions to ultimately achieve the required lumbar lordosis (29). This report does not examine this point. Further studies should include sagittal alignment and pelvic parameters. Furthermore, in this study, imaging assessment was performed at an average of 22.6±6.0 months after posterior stabilization and 6.0±2.9 months after implant removal. There is a possibility of further progression of kyphosis in the future, and long-term follow-up is necessary.

Conclusions

There is an association between the final local kyphosis angle and RBP. The optimal cut-off value of the final local kyphosis angle for RBP was approximately 16°.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The author has completed the STARD reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-24-69/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-69/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-69/prf

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-24-69/coif). The author has no conflicts of interest to declare.

Ethical Statement: The author is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the ethics committee of the Toyama Prefectural Central Hospital (No. 6455) and informed consent to use patient data was obtained from all patients.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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