**1. Introduction**

Burst fractures in the thoracolumbar (TL) spine are biomechanically characterized by the compression and failure of the anterior and middle spinal columns [1]. The management of TL burst fractures remains challenging, and different treatment strategies are available. These include posterior instrumentation (PI), anterior reconstruction, and three-column spinal reconstruction (TCSR) with combined PI and anterior reconstruction having been reported and deliberated on in the literature [2–4]. Among the different approaches, TCSR combined PI and anterior reconstruction with PMMA augmentation or titanium strut graft has been shown to provide immediate stabilization and restore spinal integrity in highly comminuted burst fractures [2]. Clinical studies have reported the advantages of TCSR over stand-alone PI or anterior-only surgery, including better neurological improvement, stability, restoration of sagittal balance, and less implant failure [2–4].

However, the rigid nature of the constructs increases the risk of adjacent segment complications. Adjacent compression fractures or adjacent disc degeneration at the proximal junctional level are devastating and result in proximal junctional failure (PJF) [5]. PJF can

**Citation:** Wong, C.-E.; Hu, H.-T.; Huang, Y.-H.; Huang, K.-Y. Optimization of Spinal Reconstructions for Thoracolumbar Burst Fractures to Prevent Proximal Junctional Complications: A Finite Element Study. *Bioengineering* **2022**, *9*, 491. https://doi.org/10.3390/ bioengineering9100491

Academic Editors: Franz Konstantin Fuss, Christina Zong-Hao Ma, Zhengrong Li and Chen He

Received: 10 August 2022 Accepted: 19 September 2022 Published: 21 September 2022

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lead to spinal cord compression, spinal instability, and kyphotic deformity, which often require a second surgery. Reported risk factors for PJF include osteoporosis, older age, greater preoperative sagittal imbalance, and longer-segment fixation [6,7].

Compared to conventional long-segment PI (LSPI), which involves instrumentation at two levels above and below the index level, short-segment PI (SSPI) had less stiffness and less increase in stress on the adjacent levels but was associated with an increased risk of implant failure [8,9]. In contrast, other studies advocated that SSPI could provide sufficient stabilization [10]. Given the incongruent results, controversy remains in the choice of posterior fixation techniques [11,12]. Moreover, the complexity of TL reconstruction was increased by the different anterior vertebral column reconstruction materials, including Polymethyl methacrylate (PMMA) cement and titanium cages, which have both been widely used in vertebral body reconstruction [13]. Although previous studies have demonstrated similar clinical and radiographical outcomes between PMMA and titanium cages in TL reconstruction [14], their effects on the proximal junctional level and the biomechanics of PMMA and titanium cage-based reconstruction constructs have not been evaluated. Since PI resulted in the redistribution of the spinal loading between the anterior vertebral graft and the pedicle screw-rod construct [15], TCSR constructs involving combined anterior and posterior instrumentation should be evaluated as a whole. Given the biomechanical complexity of the TL region and the paucity of clinical and biomechanical evidence, the decision-making of selecting an optimal spinal reconstruction strategy remains controversial but appears to be important.

To address the knowledge gap, we designed a finite element (FE) study to investigate the biomechanical performance of different TCSR constructs. Furthermore, we thought to find the optimal strategy to reduce the burden on the proximal junctional level. We established FE models of T10-L3 TL segments and the simulated failure of the L1 vertebral body to represent a burst fracture. Reconstructions with PI and TCSR constructs were simulated and compared. The range of motion (ROM) in flexion, extension, lateral bending, and axial rotation of the whole model and the reconstructed vertebra was analyzed. The mechanical burden of each construct on the reconstructed level, proximal junctional vertebra, disc, facets, and the construct itself was also compared. The objective of this study was aimed to compare and optimize the design of thoracolumbar reconstruction constructs by systematically investigating their biomechanical properties and how they affect the proximal junctional level. The knowledge gained from this study can provide help spine surgeons select an optimal TL reconstruction construct to minimize proximal junctional complications.
