Enhancing long-term fixation in thoracolumbar injuries: From screw design to bone quality optimization

Abstract

Pedicle screw fixation remains the gold standard for stabilizing unstable thoracolumbar fractures. However, ensuring long-term instrumentation stability continues to challenge both surgeons and implant designers. The study by Bokov et al contributes significantly to this discussion, identifying predictors of pedicle screw loosening such as low bone radiodensity, longer fixation constructs, and extensive decompression. Adjunctive strategies - auxiliary posterior fusion, anterior column reconstruction, and intermediate screw usage - support an individualized, biomechanically sound surgical plan. In this article, we explore the clinical relevance of these findings within spinal trauma care. We emphasize the role of preoperative bone quality assessment, including computed tomography-based Hounsfield unit analysis and magnetic resonance imaging-derived vertebral bone quality score, as modifiable predictors of long-term outcomes. We also discuss innovations in screw design, surface coatings, and patient-specific planning to reduce failure risk. Furthermore, emerging technologies such as finite element modeling and 3D-printed instrumentation may refine patient-specific strategies. By integrating biomechanical principles with personalized surgical planning, future approaches may enhance fixation durability. Ultimately, aligning mechanical stability with biological sustainability is critical to reducing implant failure in complex thoracolumbar trauma cases.

Key Words: Thoracolumbar fractures; Pedicle screw loosening; Bone quality; Finite element modeling; Spinal fixation

Core Tip: This article highlights key factors influencing long-term pedicle screw stability in thoracolumbar fractures, including bone quality, construct length, and decompression extent. Practical strategies such as computed tomography-based Hounsfield unit and magnetic resonance imaging-based vertebral bone quality score assessment, the use of intermediate screws, and anterior column reconstruction are discussed. Technological advances like coated implants and patient-specific planning are emphasized to enhance long-term outcomes. Integration of these approaches may help reduce implant failure in complex spinal trauma cases.

INTRODUCTION

Thoracolumbar injuries are one of the most complex problems in spine surgery. Despite the widespread use of pedicle screw systems, implant failure rates (especially screw loosening) are still a significant clinical problem in long-term follow-up[1,2]. Therefore, ensuring permanent stability requires not only mechanical durability but also biological integration and consideration of patient-specific risks. Recent contributions by Bokov et al[3] refocus on the multifactorial origins of pedicle screw instability, emphasizing factors such as low vertebral bone density, overextension of the construct, and aggressive decompression. Their study supports the principle that stable fixation depends not only on hardware strength but also on careful and biologically sound surgical planning.

BONE QUALITY: A CRITICAL BUT MODIFIABLE RISK FACTOR

Preoperative bone quality assessment stands out as a key determinant of the success of thoracolumbar fixation. While systemic bone mineral density measurements such as dual-energy X-ray absorptiometry are useful, computed tomography-based Hounsfield unit (HU) analysis offers a more sensitive vertebral segment-specific bone density assessment[4,5]. The risk of screw loosening increases significantly in regions with HU < 110.

A newer approach, the vertebral bone quality score (VBQ), is obtained from T1-weighted magnetic resonance imaging (MRI) images and evaluates bone quality with high specificity and sensitivity. The rate of screw loosening is significantly increased in patients with a VBQ ≥ 3.15[6]. In light of these data, methods such as cement augmentation, use of expandable or fenestrated screws, and cortical screw orientation are applicable strategies in patients with low HU or VBQ scores[7].

CONSTRUCT LENGTH AND BIOMECHANICAL EFFICIENCY

The tendency to use long posterior constructs in burst fractures is understandable, but may not always be biomechanically optimal. Extended constructs increase stress at the proximal and distal transition zones, predisposing patients to junctional kyphosis, implant fatigue, and adjacent segment degeneration[8].

Studies support the role of short-segment fixation with intermediate screws at the fracture level as a more biomechanically balanced strategy. This approach shares the axial load more efficiently and limits the moment arm across the implant[9,10].

In cases of vertebral body comminution or anterior column disruption, posterior fixation alone may not be sufficient. Anterior column reconstruction using expandable cages or structural grafts can restore load-bearing capacity and reduce the burden of posterior instrumentation.

Additionally, intervertebral disc integrity has emerged as an under-appreciated variable in corrective care. A 2025 study involving 194 patients found that traumatic disc injury was independently associated with loss of alignment over time, emphasizing the need for this factor to be assessed and addressed before surgery[11].

IMPLANT SURFACE TECHNOLOGY AND OSSEOINTEGRATION

In patients with thoracolumbar spine fractures treated with pedicle screws, loosening or inadequate osseointegration of the pedicle screw may result in early complications such as loss of correction, screw pull-out, screw loosening, and pseudoarthrosis in the postoperative period. Achieving robust osteointegration at the bone-pedicle screw interface is critical in osteoporotic patients[12]. Therefore, polymethyl methacrylate-based cement augmentation with fenestrated screws is widely preferred to strengthen the bone-screw interface and increase pullout resistance. However, due to the non-biological bonding, late screw loosening and polymethyl methacrylate-related complications can occur with this method. Therefore, in recent years, in vivo and in vitro studies have increased on designs such as hydroxyapatite-coated screws, 3D-printed porous titanium and tantalum screws. In particular, several studies have shown that hydroxyapatite-coated implants increase pull-out resistance and possess osteoconductive properties[13,14].

FINITE ELEMENT MODELING AND PATIENT-SPECIFIC SOLUTIONS

Finite element modeling (FEM) has emerged as a revolutionary tool for predicting the biomechanical behavior of spinal implants and developing patient-specific surgical strategies[15,16]. In particular, FEM can simulate the pullout resistance of screws, the stress distribution at the bone-screw interface, and the effects of techniques such as cement augmentation in patients with osteoporotic bone[17]. For instance, FEM has been used to compare short-segment vs. long-segment constructs in thoracolumbar burst fractures, demonstrating superior stress distribution with intermediate screws[9]. Case-specific modeling has also guided screw trajectory optimization in osteoporotic patients, reducing failure risk[18].

In adult spinal deformity surgery, ideal sagittal alignment can be calculated based on the patient’s pelvic incidence[19]. In parallel with this approach in thoracolumbar injuries, FEM allows the calculation of the “ideal” alignment most appropriate to the patient’s spinopelvic parameters, and personalized prebend rods can be produced accordingly. This innovative approach has the potential to significantly reduce both mechanical complications and the risk of pseudoarthrosis by minimizing intraoperative rod bending errors and overcorrection stresses imposed on the implant.

COST-EFFECTIVENESS AND ACCESSIBILITY CONSIDERATIONS

While coated screws, cement augmentation, and 3D-printed implants show biomechanical promise, their integration into routine care depends on cost-effectiveness and availability. Hydroxyapatite-coated screws and porous implants remain costly and may be limited to tertiary care centers. Similarly, patient-specific 3D-printed rods are promising but resource-intensive. Future studies must assess the economic feasibility of these technologies, balancing biomechanical benefits with healthcare costs.

CLINICAL IMPLICATIONS

Identifying and addressing predictors of pedicle screw loosening - such as low bone density, long constructs, and inadequate anterior support - can significantly reduce mechanical complications following thoracolumbar instrumentation. Preoperative HU and MRI-based bone quality evaluation should be integrated into routine planning. Adopting enhanced screw designs, surface coatings, and patient-specific implant strategies may optimize long-term stability and improve surgical outcomes in spinal trauma. An overview of the clinical implications and recommendations is provided in Table 1.

Table 1 Risk factors and mitigation strategies for screw loosening.

Risk factor	Mitigation strategy
Low bone density (HU < 110)	HU/VBQ-based screening, cement augmentation, coated screws
Long posterior construct	Use intermediate screws, consider short-segment fixation
Multilevel decompression	Augment anterior column, limit construct length
Osteoporotic bone	Expandable/fenestrated screws, nanocoated implants, PMMA augmentation
Traumatic disc injury	Assess via MRI, consider anterior support to maintain correction
Fracture at thoracolumbar junction	Biomechanical modeling, optimized trajectory selection

HU: Hounsfield unit; VBQ: Vertebral bone quality; PMMA: Polymethyl methacrylate; MRI: Magnetic resonance imaging.

Postoperatively, anti-osteoporosis therapy is essential for patients with reduced bone mass. Bisphosphonates and denosumab have been shown to improve bone quality and reduce fracture risk, while newer agents such as romosozumab may further enhance bone strength, potentially supporting implant fixation[20]. Standardized treatment protocols, however, remain under discussion, and further clinical research is required.

CONCLUSION

Pedicle screw fixation continues to play a central role in the treatment of thoracolumbar trauma, but true long-term success depends on aligning mechanical stability with biological sustainability. The findings of Bokov et al[3] remind us that effective fixation is both an art and a science, requiring balancing of hardware design, construct mechanics, and patient-specific risk factors. Integrating HU and MRI-based bone quality scores, leveraging innovative surface technologies, and utilizing biomechanical modeling tools will allow surgeons to personalize care with greater precision. As the future of spine surgery moves toward personalization and digital planning, we must ensure our implant strategies evolve accordingly.