The Effect of Thoracolumbar Injury Classification in the Clinical Outcome of Operative and Non-Operative Treatments

This review assesses the validity of a biomechanical approach using finite element analysis in the Thoracolumbar Injury Classification and Severity Score System (TLICS) by addressing the “gray zone” decision discrepancy of thoracolumbar spinal injuries. A systematic review was performed using the keywords “Thoracolumbar Injury Classification” AND “finite element analysis of the spinal column” to evaluate the validity of the TLICS and finite element analysis of the thoracolumbar spinal column. Results were classified according to the main conclusions and level of evidence. Thirteen articles are included. Four of the articles evaluated the TLICS in comparison to other classification systems of thoracolumbar spinal injuries. A notable finding is that the TLICS had inconsistencies with other classification systems in the treatment of burst fractures without neurological deficits. One article evaluated the TLICS with the inclusion of magnetic resonance imaging (MRI) in the evaluation, which decreased the agreement between the suggested and actual treatment. Among the three finite element analysis studies, limited data have been published on the posterior ligamentous complex (PLC) status when an injury is suspected or indeterminate. The TLICS has been a reliable classification system in the management of single-column fractures and three-column injuries treated with surgical stabilization. Special attention to enhancing the TLICS classification system by eliminating the “gray zone” of a TLICS score of 4 is essential. Biomedical computational modeling evaluating the PLC status of indeterminate or injury suspected is needed to enhance the current TLICS system and to clarify the decision discrepancy in the “gray zone.”


Introduction And Background
Thoracolumbar (TL) fractures are the most common traumatic injuries to the spinal column. The annual incidence of TL injures in the United States is approximately 15,000; the majority of those incidents are due to high-energy trauma resulting mainly from a motor vehicle accident in younger patients [1][2]. Also, nearly 700,000 osteoporotic fractures occur annually in elderly patients [1]. Most commonly, TL injuries occur at the T10 to L2 level [3][4]. The TL junction is more susceptible to injury because there is a transition between the stiff kyphotic thoracic spine and the mobile lordotic lumbar spine [3][4]. Approximately 25% percent of TL fractures result in neurological deficit [5][6].
The four major spinal injuries are compression fractures, burst fractures, flexion-distraction injuries, and translational injuries. Numerous classification systems for thoracolumbar spine injuries have been established. However, there is no current universal acceptance of a classification system for TL spine injuries that facilitates proper communication between treating physicians and helps standardize approaches to treatment. In 1983, the three-column theory was introduced by Francis Denis to classify acute TL spinal injuries [7]. According to this theory, stability is based upon the integrity of two of the three spinal columns [7]. The Denis system may oversimplify complex fractures and may not accurately access the need for operative intervention [8].
In 1994, the AO (Arbeitsgemeinschaft für Osteosynthesefragen System) classification was introduced using a mechanistic approach to separate fractures into 53 different patterns based upon three injury categories and three tiers of subcategorization [5]. The use of 53 different fracture patterns makes routine clinical use of the classification subject to poor inter-and intra-observer agreement [9][10].
In 2005, the Spine Trauma Study Group introduced a classification system for TL injuries named the Thoracolumbar Injury Classification and Severity Score (TLICS). This score assigns numerical values to each injury based upon the morphology, neurologic status, and integrity of the posterior ligamentous complex (PLC) [8]. A patient with a TLICS score higher than five is considered a surgical candidate, and a patient with a score less than three are often non-surgical candidates. The treatment strategy of patients with a total TLICS score of four is unclear. A score of four points remains a gray zone that permits surgeons to use individual clinical judgment to determine surgical options. Another TLICS gray zone shortfall was related to the surgeon's inability to agree on the integrity of the PLC. Definite criteria of PLC injury may be necessary because the differentiation of PLC injury between TLICS score 0, 2, and 3 is very difficult [11].
A classification system is required to facilitate effective communication between spine surgeons, to guide treatment, and to help predict the prognosis. An ideal system should be simple, comprehensive, reliable, and reproducible, with predictive outcomes. Unfortunately, most of the existing classifications have failed to fulfill the above criteria; some are oversimplified while others are too inclusive and complex for routine use. While there is no acceptance of a universal TL spinal injury classification system, it is imperative to understand the evolution of spinal injury classification.

Research significance
Despite multiple methodologies to evaluate patients with TL injuries scored four lines within the gray zone, the standardized classification and treatment of TL spine fractures remains controversial. Thus, comprehensive literature was performed in this study as an attempt to offer the most updated approaches that have been implemented with an efficiency assessment that helps in surgical decisions.

Review Methods
A systematic literature review of available literature was performed to identify all studies dealing with accessing the validity of the Thoracolumbar Injury Classification and the finite element biomechanical models of the TL spinal column. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed to identify the articles [12]. Using the Medline and PubMed databases, a descriptive and up-to-date review of the spine trauma literature was gathered in Table 1. The terms "Thoracolumbar Injury Classification" and "finite element analysis of the spinal column" were used in the main entry search on June 19, 2020. Two authors (C.J.S. and M.M.A.) reviewed the search results. We included only retrospective or prospective clinical publications, evaluating the reliability of the commonly used TL injury classification systems (i.e., TLICS and AO) along with biomechanical analysis studies using finite element (FE) of the thoracolumbar spine. The exclusion criteria consisted of literature reviews, case reports, personal communications, or paper presentations. The final articles were selected according to the evidence-based medicine criteria proposed by Wright et al. [13]. An electronic search yielded 417 studies. After 106 duplications were removed, 311 studies remained; 267 were excluded after review of the abstract and full-text articles, leaving 44 eligible studies. An additional 31 articles were then excluded based on additional inclusion and exclusion criteria. A total of 13 articles published between 2015 and 2020 met the inclusion criteria ( Figure 1).

Results
The results reported were subdivided into findings related to the Thoracolumbar Injury Classification and finite element analysis of the thoracolumbar spinal column.

Thoracolumbar Injury Classification
Compared to the AO recommendations, TLICS may be more reliable in guiding the surgical management of unstable TL burst fractures without neurological deficits, as the AO system had recommended conservative treatment. However, the patient had an unstable burst fracture [14].
Yuksel et al. report that for patients with neurological deficits, both the TLICS and AO scoring systems recommended surgical treatment [14]. Of the 37 patients without neurological deficit, all patients received TLICS > 4, whereas 19 out of the 37 patients received an AO score between 1 to 3 points in which AO recommends conservative treatment (  In the Pneumaticos et al. (2016) study, the mean follow-up was 28 months in which conventional treatment for a TLICS score of 4; TL fractures were reported as equally valid to those scoring <3 [16].

Finite Element Analysis of the Thoracolumbar Spinal Column
Guo et al. analyzed the TL burst fractures under vertical impact loads using the FE method [17]. At high energy conditions (56 J), the burst fracture occurred on the L1 segment, and the fracture pattern was observed in clinical practice.
Sterba et al. analyzed ligament mechanical properties on the lumbar spine in posterior-anterior impact loading conditions [18]. At high velocity (10 m/s), a major or complete rupture was determined, scoring an additional three points according to TLICS leading to a total TLICS score > 5. Additionally, Wu et al. reported the significance of the supraspinous ligament (SSL), as its failure led to the greatest change in range of motion (ROM) and instantaneous axes of rotation (IAR) under flexion [19]. Thus, the SSL plays a crucial role in maintaining the stability of the thoracolumbar spine during injury.
A finite element study was performed by Wu et al., where the biomechanical role of the TL ligaments of the PLC was analyzed [19]. The study evaluated the effect of consecutive ligament failure and the role of ligaments in maintaining the stability of the injured TL spine. A consistent increase in the ROM and location of IAR as the ligaments were consecutively removed was determined by the model. A notable finding was the SSL had the greatest influence on the change in the ROM and IAR under flexion, allowing the PLC to maintain the stability of the TL spine during injury.
Finite element modeling can provide a practical understanding of compression injuries of different patient types. Takano et al. used modeling to analyze the vertebral stress concentration of a healthy subject compared to an individual with an osteoporotic L1 vertebral compression fracture [20]. Under five basic vertebral physiologic motions, higher stress, and the strain exhibited by the osteoporotic subject, finite element analysis provides a useful method to evaluate injury patterns of the spine and a comprehensive understanding of each patient's condition, which is crucial in determining the best surgical treatment.
Hsieh et al. reported the use of a hybrid fixation method combining vertebroplasty and cement-augmented screws for securing TL burst fractures that resulted in a stiffer construct and lower stress on the pedicle screws [21]. The hybrid fixation method presented in this study showed that the use of cement-augmented screws does not increase the risk of adjacent level vertebral fracture.

Discussion
As shown in Table 1, the evidence for evaluating the validity of the TLICS is favorable in the last decade. The TLICS has been a reliable classification system in the management of single-column fractures treated conservatively, and three-column injuries (flexion/extension distraction injuries and fracture-dislocations) treated with surgical stabilization [22][23]. However, limited data have been published addressing the TLIC score of 4 or gray zone in which there is a lack of standardization of surgical or non-surgical management among treating physicians.
A notable finding is that the retrospective evaluation of the TLICS had inconsistencies with other classification systems in the treatment of burst fractures without neurological deficits. Additionally, a significant finding reported by Yuksel et al. is the treatment decision discrepancy between the TLICS and AO classification systems in unstable burst fractures ( Table 2) [14]. Standardization of TL injury scores is crucial to guiding proper surgical management among treating physicians.
Despite the increased reliability of the TLICS in the management of unstable burst fractures, the Thoracolumbar AOSpine Injury Score was recommended as more reliable than the TLICS system in the treatment of burst fractures, fracture classification, and morphology [24][25][26]. A potential explanation for the inconsistencies is that the TLICS system is that it does not consider particular factors such as segmental kyphosis, loss of vertebral height, and degree of canal compromise for guiding surgical treatment. A limited evidence-based relationship with the patient's outcome reported the inclusion of these factors [9]. The TLICS system does not account for these factors for guiding surgical treatment. The TLICS classification system has left an ambiguous zone for burst fractures (2 points) when evaluating the PLC as an injury suspected, or indeterminate of the PLC has scored 2 points, giving a total TLICS score of 4 points. Another probable reason for inconsistencies in the retrospective evaluation of the TLICS is according to the type of radiologic assessment, and the TLICS score can change.
While initial MRI of the spine is not standard of routine care at many trauma centers, studies have proposed that when the presence or disruption of the PLC was not clear on computed tomography (CT), an MRI would be useful. A potential explanation for the finding reported by Dawkins et al. is differences in the final PLC score as some raters may have increased the score, and some kept it the same [15]. Also, an MRI was ordered in cases in which the surgical decision-making process was not clear from CT alone.
An additional problem noted in this study is that the evaluation of TLICS validity is the lack of a gold standard for measuring the treatment of thoracic and lumbar spinal trauma (TLST). In the current literature, many studies have evaluated the outcome of specific injury patterns. Few studies have assessed the longterm patient-based outcomes in the conservative or surgical management of their injuries. However, the lack of a universally accepted standard classification system for TL injuries has limited our ability to further access the utility of the TLICS in terms of evaluating patient's TLICS score at the time of injury and treatment and their long-term reported outcomes. Special attention to enhancing the TLICS classification system by eliminating the gray zone of a TLICS score of 4 is necessary. Biomedical computational modeling may be used on the TL spine to enhance the current TLICS classification by standardizing treatment among treating physicians.
As many researchers have proposed classification systems and extensive description patterns in clinical observation, the finite element method has been reported as a useful tool to verify the fracture patterns and provide the spinal injury score [17]. A limitation of the Guo et al. study was the failure model of the PLC was simplified and not analyzed [17]. To eliminate the gray zone in the current TLICS, the PLC must be analyzed using the finite element method to evaluate the PLC status further when an injury is suspected or indeterminate (2 points, burst fracture-2 points, TLICS-4 points).
While both the Sterba et al. [18] and Wu et al. [19] studies analyze the PLC biomechanical properties, there is limited data on the finite element analysis evaluating the PLC status of indeterminate or injury suspected. A key component to eliminating the gray zone in the TLICS classification system is evaluating the PLC status to standardize and guide surgical treatment in patients with TL spinal injuries.
Based on the findings reported by Hsieh et al., finite element biomechanical analysis has demonstrated a useful technique for evaluating surgical treatment approaches of burst fractures [21]. The use of finite element modeling has proven to be an efficient tool to access postoperative outcomes by evaluating the biomechanics of hardware-related failures.
Biomechanics computer models of the spine have been developed using a wide range of approaches such as finite element models of various complexity. These models enhanced our understanding of the spine, especially with the increasing power of computers. The complexity of the human spine and variations in material properties and boundary conditions make it a suitable candidate for finite element modeling [21]. Moreover, the finite element method often provides significant advantages by providing a post-treatment assessment for spine injuries, such as TL burst fracture (TLBF), and where there are such individual variations, allowing cause-effect relationships to be isolated and thoroughly explored.
This literature review suggested that the use of TLICS is safe, especially when treating single-column or three-column spinal injuries. Given that there is no universal acceptance of a classification system for TL injuries that helps to standardize approaches to treatment among interpreting physicians, the use of finite element analysis provides a useful tool to enhance the TLICS system. Special attention to TLICS application is necessary for the treatment of TLBF by eliminating the current gray zone. However, unstable TLBF inconsistencies in the total score among the TLICS and AO systems have led to a lack of standardization in surgical management. In patients with a TLICS score of 4, often from a burst fracture (2 points) and PLC injury suspected or indeterminate (2 points), their treatment plan is decided by the surgeon's clinical judgment, leading to inconsistencies in the treatment approach among surgeons. Eliminating the gray zone will likely provide universal acceptance of a single classification system used by treating physicians, preventing discrepancies in scoring TLF. Finite element analysis offers a precise method to evaluate the PLC. However, limited studies have addressed uncertain PLC status. In such circumstances, an accurate assessment of neurological deficit plays an essential role.
Further studies using finite element analysis of TL spinal fractures would improve the TLICS classification system. Moreover, the analysis can provide a good understanding of post-treatment of several TL fracture patient types. For stable TL fractures, the biomechanical computational framework for accessing them quantified the effect of treatment aid in the evaluation of vertebral fractures and the understanding of factors contributing to fracture risk. Additionally, the biomechanical computational framework of unstable and stable TL fractures post-treatment using finite element analysis provided the most efficient tool to analyze the surgical hardware used and its long-term effect on the spinal system.

Conflicts of interest:
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