Finite element analysis of a novel modular support system for lumbar reconstruction following vertebral column resection

Abstract

BACKGROUND

Posterior vertebral column resection (VCR) is an effective procedure for treating severe rigid spinal deformities. However, this technique carries a high risk of nerve injury associated with the surgical approach. Conventional titanium mesh cages are widely used for anterior column support but have several reported disadvantages, including implant dislocation and subsidence, which may contribute to implant failure.

AIM

To evaluate a VCR modular support system (VCR-MSS) as an anterior-posterior construct for the Indonesian lumbar population.

METHODS

A 2-stage finite element analysis (FEA) framework was used. First, an ASTM F1717-21 polyethylene block model was used to validate the FEA methodology against mechanical testing of a posterior fixation prototype. Subsequently, the validated FEA framework was used to evaluate a conventional mesh cage construct and the proposed VCR-MSS under standardized loading in ASTM-based models and physiological loading in vertebra-based lumbar models.

RESULTS

In the preliminary stage, the ASTM-based model supported validation of the FEA methodology against mechanical testing of the posterior fixation prototype. In the main analysis, the VCR-MSS construct demonstrated greater structural stability than the conventional mesh cage construct. Total deformation was reduced from 6.0443 mm to 0.8435 mm under ASTM-based compressive loading and from 0.1100 mm to 0.0687 mm under vertebra-based flexion loading. In addition, posterior rod stress under ASTM-based compression was reduced from 429.09 MPa to 90.55 MPa, indicating improved load sharing with the VCR-MSS construct.

CONCLUSION

The VCR-MSS construct demonstrated greater stability and reduced posterior rod mechanical burden through integrated load sharing, suggesting its potential as a reconstruction option following lumbar VCR.

Key Words: Posterior vertebral column resection; Finite element analysis; Lumbar reconstruction; Titanium mesh cage; Vertebral column resection modular support system; Morphometric-guided implant design; Mechanics

Core Tip: The vertebral column resection modular support system (VCR-MSS) is a morphometry-guided modular implant specifically designed to match Indonesian lumbar anatomy. Using a 2-stage finite element analysis framework comprising an ASTM F1717-21-based model and a vertebra-based model, we demonstrated that the VCR-MSS exhibited lower total deformation than a conventional mesh cage under all loading conditions. In addition, the VCR-MSS reduced the mechanical demand on the posterior rods, indicating more favorable load sharing within the construct. Collectively, these findings suggest that the VCR-MSS functions as an integrated reconstruction system rather than simply as an anterior spacer.

INTRODUCTION

Lumbar kyphosis, although less common than other spinal deformities, remains a clinically significant condition because of its marked effects on sagittal balance, segmental mobility, and health-related quality of life[1-4]. Patients often present with chronic pain, functional limitations, and progressive sagittal imbalance, which may further restrict daily activities and, in cases of severe deformity, impair cardiopulmonary function[5,6]. The etiology of lumbar kyphosis is multifactorial and includes congenital abnormalities, trauma, infection, and degenerative conditions. However, the clinical severity of the deformity is determined not solely by the magnitude of curvature but also by its rigidity and the degree of compensatory adaptation in adjacent spinal segments[7,8]. Among the available surgical options, vertebral column resection (VCR) is widely regarded as one of the most effective procedures for correcting severe rigid spinal deformity because it permits circumferential release and substantial 3-column correction[9,10]. However, posterior VCR (PVCR) inherently results in profound segmental instability after removal of the vertebral body, necessitating a mechanically reliable reconstruction construct to restore spinal stability[11-14].

In this setting, reconstruction stability depends not only on posterior fixation but also on adequate anterior column support and effective load redistribution across the reconstructed segment. Consequently, implant constructs require rigorous mechanical evaluation, particularly under axial, bending, and distraction loading conditions that reflect physiological and near-failure scenarios[15].

Titanium mesh cages remain a common choice for anterior reconstruction after vertebral body resection because of their wide availability, adaptability, and familiarity in surgical practice[16]. Nevertheless, conventional mesh cages have well-recognized mechanical limitations. Previous studies have demonstrated that trimmed cages often fail to conform adequately to adjacent endplates or sagittal alignment, resulting in reduced effective contact area, increased stress concentration, an elevated risk of subsidence, and higher rates of instrumentation-related failure[17,18]. A finite element study of total en bloc spondylectomy further revealed that poor prosthesis-endplate matching substantially increased endplate stress, particularly during rotational loading, and compromised fixation performance compared with better-matched reconstructions[19].

Evidence from other implant studies further supports the observation that limited cage-endplate conformity contributes to stress concentration and postoperative failure[20]. In an investigation of conventional titanium mesh cage reconstruction, the incidence of postoperative cage subsidence was reported to be as high as 90%, with the underlying mechanical mechanism attributed in part to point-type contact between the cage and irregular vertebral endplates. By contrast, studies of anatomic cage designs have demonstrated that broader, more conforming support reduces endplate stress, implant stress, and stress at the screw-bone and fixation interfaces[21]. Similarly, a finite element study found that a novel anatomic titanium mesh cage improved anti-subsidence performance by approximately 51.3% compared with a traditional mesh cage, further emphasizing the importance of the support footprint and contact geometry in spinal reconstruction design[22].

From a mechanical perspective, these findings suggest that lumbar reconstruction after PVCR should be evaluated not solely on whether anterior support is present but also on how effectively that support integrates with the posterior construct and how broadly the load is transferred to the vertebral interface. Narrower or poorly matched anterior support may shift a greater proportion of the load to the posterior rods and screws, whereas a broader, more coordinated support system may improve construct stability and reduce mechanically critical stress concentrations[23-25]. These considerations provide the rationale for evaluating not only conventional mesh cage reconstruction but also a modular system designed to coordinate anterior and posterior support within a single reconstructive framework.

Finite element analysis (FEA) has emerged as a valuable tool for investigating these questions because it enables controlled comparisons of reconstruction strategies under standardized and anatomically relevant conditions[26-28]. Nevertheless, a persistent limitation of spinal reconstruction research is that studies often rely on either simplified construct-level models or vertebra-based anatomic models alone. The ASTM F1717 standard provides a standardized vertebrectomy construct model for comparative static evaluation and has recently been shown to support validation-oriented comparisons between experimental and simulation results. Good agreement between physical testing and FEA suggests its potential utility for early design screening[29]. At the same time, anatomically contextual vertebral models remain essential for assessing how the same reconstruction concepts perform in a more realistic lumbar environment, in which implant geometry, vertebral morphology, and load distribution patterns may influence construct-level mechanical behavior[30,31].

In the Indonesian context, this issue is also relevant to implant design. Population-specific vertebral morphometry may influence implant fit, endplate coverage, and load transfer, yet implant development is often not explicitly guided by local anatomical data. Accordingly, the present study adopts a morphometry-guided approach to evaluate a novel VCR modular support system (MSS), designed to provide broader anterior support and coordinated anterior-posterior reconstruction. The present work employs a 2-stage finite element framework: First, an ASTM F1717-21-adapted polyethylene block construct model serving as a standardized construct-level validation and screening platform; and second, a vertebra-based lumbar model serving as the primary anatomically contextual platform for comparative analysis. Within this framework, posterior fixation without anterior support is used for preliminary validation, whereas the main comparative analysis evaluates posterior fixation with a conventional mesh cage and posterior fixation with the proposed VCR-MSS construct[32-35].

This study represents the first finite element investigation of a morphometry-guided modular reconstruction system specifically developed for lumbar PVCR using anatomical data from the Indonesian population, with evaluation in both ASTM-based and vertebra-specific finite element frameworks.

MATERIALS AND METHODS

Study design

The present study comprised a preliminary ASTM-based validation study followed by the main comparative mechanical analysis. The preliminary study aimed to verify the suitability of the FEA methodology using a posterior fixation construct prototype assembled in a standardized polyethylene block vertebrectomy model adapted from ASTM F1717-21. Mechanical testing and the corresponding ASTM-based finite element model were performed under identical construct conditions to determine whether the computational model reproduced the overall mechanical behavior[36-38]. This validation step was performed solely to evaluate the reliability of the finite element framework. The novel VCR-MSS construct was not subjected to direct mechanical validation but was subsequently investigated using the validated computational framework.

The main study compared two construct models: A conventional mesh cage construct and the proposed VCR-MSS construct. Two distinct modeling approaches were used. First, an ASTM-based construct model was used to evaluate implant strength under standardized compressive and distraction loading in a controlled mechanical environment. Second, a vertebra-based lumbar finite element model was used to assess the mechanical behavior of the same two constructs under physiological loading conditions. Thus, the preliminary study served as a validation-oriented step preceding the main comparative analysis, whereas the main study assessed both the strength-related behavior of the implant constructs under ASTM-based loading and their stress distribution and structural stability under anatomically contextual physiological loading[39-41].

Geometry reconstruction and construct remodeling

A finite element construct model adapted from ASTM F1717-21 was developed using polyethylene blocks as standardized support media. In the preliminary study, this block-based model reproduced the mechanically tested posterior fixation construct prototype and provided a simplified, reproducible mechanical environment for validation-oriented comparison under standardized testing conditions. Within this framework, the posterior construct prototype served as the validation construct for comparing the results of mechanical testing with those of the corresponding ASTM-based finite element model.

For the anatomically contextual model, a vertebra-based finite element model was reconstructed from lumbar computed tomography data in DICOM format obtained from the Department of Radiology, Dr. Saiful Anwar General Hospital, Malang, Indonesia. Before construct assembly, a lumbar morphometric evaluation was performed to characterize vertebral body and pedicle dimensions in the study population. These morphometric data provided the anatomical basis for population-specific implant design, particularly the dimensions of the anterior support component and posterior fixation system. Image segmentation and 3-dimensional reconstruction were then performed to generate the lumbar vertebral geometry used in the vertebra-based finite element framework. The principal morphometric parameters used as design references for finite element construct development are summarized in Table 1.

Table 1 Simplified lumbar morphometric parameters used for implant design and finite element construct modelling.

Parameter	Mean value (mm)	Design relevance	Application in FEA model
ABVH	28.97	Reference for anterior body height	Used to define anterior support height
PBVH	29.24	Reference for posterior body height	Used to adjust posterior construct profile
VW	41.13	Reference for vertebral width	Used to determine implant footprint width
VD	30.57	Reference for vertebral depth	Used to determine implant anteroposterior depth
PL	48.96	Reference for pedicle length	Used to guide pedicle screw length selection
PW	10.56	Reference for pedicle width	Used to guide pedicle screw diameter selection

FEA: Finite element analysis; ABVH: Anterior vertebral body height; PBVH: Posterior vertebral body height; VW: Vertebral width; VD: Vertebral depth; PL: Pedicle length; PW: Pedicle width.

In the main study, two reconstruction assemblies were created within the same modeling framework: A conventional mesh cage with posterior fixation and the VCR-MSS (Figure 1). A schematic illustration of the VCR-MSS implant, depicting the anterior mesh cage component and posterior pedicle screw-rod fixation system, is presented in Figure 2 to illustrate the implant configuration and reconstruction concept. The ASTM-based polyethylene model was used to evaluate these two constructs in a standardized mechanical environment, whereas the vertebra-based model was used to assess the same constructs in an anatomically contextual lumbar setting[42-44].

Figure 1 Finite element analysis models used in the present study. A: ASTM-based polyethylene block construct models; B: Vertebra-based lumbar reconstruction models. In each panel, the conventional mesh cage construct is shown on the left and the vertebral column resection modular support system construct is shown on the right.

Figure 2  Schematic illustration of the vertebral column resection modular support system.

Material properties

Material properties were assigned separately to each component category according to the finite element framework used in this study. In the vertebra-based model, material properties were assigned to the bony vertebral structures and implant components to provide an anatomically contextual simulation environment. Soft tissue structures, including the intervertebral discs, spinal ligaments, and paraspinal muscles, were excluded because the study focused on a comparative biomechanical evaluation of the reconstruction constructs. The posterior fixation system consisted of pedicle screws measuring 6 mm in diameter and 45 mm in length, connected by rods measuring 5.5 mm in diameter, consistent with dimensions commonly used in lumbar finite element reconstruction studies[45-47].

The conventional mesh cage was estimated to provide approximately 50% endplate coverage based on measurements of a commercially available mesh cage relative to the human lumbar vertebral endplate. By contrast, the proposed implant was designed to provide broader vertebral endplate support, with an estimated coverage of approximately 90% based on lumbar morphometric measurements obtained from the Indonesian study population. This design was intended to maximize anatomical endplate contact while maintaining anatomical compatibility within the finite element models. In the ASTM-based construct model, polyethylene blocks were used as the standardized support medium, whereas the fixation and anterior support components retained their corresponding implant material assignments[21].

For all simulations, materials were assumed to be homogeneous, isotropic, and linearly elastic to maintain consistency across construct groups and enable controlled comparative analysis, a common simplifying assumption in comparative implant-related finite element studies[24,25]. The material properties used in the present study are summarized in Table 2.

Table 2 Material properties.

Component	Material	Young’s modulus (MPa)	Poisson’s ratio	Yield strength (MPa)	Model context
Polyethylene block	Polyethylene	690	0.46	20	ASTM-based model
Cortical bone	Bone	17000	0.30	50	Vertebra-based model
Cancellous bone	Bone	100	0.20	1.67	Vertebra-based model
Conventional mesh cage	Titanium	110000	0.36	793	Vertebra-based and ASTM-based models
Pedicle screws	Titanium	110000	0.36	793	Vertebra-based and ASTM-based models
Rods	Titanium	110000	0.36	793	Vertebra-based and ASTM-based models
VCR-MSS components	Titanium alloy	110000	0.36	793	Vertebra-based and ASTM-based models

VCR-MSS: Vertebral column resection modular support system.

Contact definition

Interactions between adjacent components were defined using explicit contact pairs within the finite element environment. Contact assignments were applied separately to the vertebra-implant and implant-implant interfaces to maintain consistent assembly conditions across the reconstruction constructs. In the vertebra-based model, these interfaces included the pedicle screw-bone, screw-rod, and anterior support-vertebral interfaces. In the ASTM-based construct model, contact definitions were applied between the polyethylene blocks and implant components and between the individual fixation elements. To preserve comparability among the constructs and minimize numerical instability during the solution process, contact behavior was standardized as much as possible across corresponding interfaces. Interfaces intended to function as fixed structural connections were defined using bonded or equivalent tied-contact assumptions, thereby representing ideal continuity between connected components within the construct.

This approach was used to emphasize comparative construct-level stress distribution and deformation behavior under identical computational conditions rather than to model interface micromotion or frictional slip in detail[24]. Bonded contact assumptions therefore standardized the interface conditions across all reconstruction constructs and facilitated direct biomechanical comparison under equivalent computational settings. Because bonded contact represents a fixed interface without relative sliding or separation, friction coefficients were not assigned in the present finite element model. Accordingly, the study did not separately quantify effective contact surface area, interface pressure distribution, or endplate contact stress. Thus, the contact framework provided a stable and standardized basis for comparing the overall mechanical behavior of the constructs rather than isolating the contributions of individual interface-level mechanisms.

Loading and boundary conditions

In the ASTM F1717-adapted construct model, boundary and loading conditions were assigned to replicate a simplified, standardized construct-testing environment. The inferior polyethylene block was fully constrained, whereas loading was applied to the superior block through the posterior fixation construct to generate comparative construct-level mechanical responses. For the comparative FEA of the mesh cage and VCR-MSS constructs, both models were analyzed under identical compressive and tensile loads derived from the lower experimentally observed maximum load of the weaker construct (Figure 3). This approach ensured a fair comparison under the same mechanical demand while remaining within the experimentally supported loading range of both constructs.

Figure 3 Boundary and loading conditions for the ASTM F1717-adapted construct models under compressive and tensile loading. A: Conventional mesh cage construct; B: Vertebral column resection modular support system construct. VCR-MSS: Vertebral column resection modular support system.

The vertebra-based finite element model was evaluated using simplified loading conditions adapted from previous lumbar spine finite element studies. The inferior vertebral region was fully constrained, whereas a400-N compressive preload and an 8-Nm pure bending moment were applied to the superior vertebral region to provide a standardized comparative loading framework. Figure 4 shows the boundary and loading conditions for the vertebra-based lumbar reconstruction model under flexion (compressive) and extension (tensile) loading.

Figure 4 Boundary and loading conditions for the vertebra-based lumbar reconstruction models under flexion and extension loading. A: Conventional mesh cage construct; B: Vertebral column resection modular support system construct. VCR-MSS: Vertebral column resection modular support system.

Mesh generation and convergence

All models were meshed using ANSYS Workbench 2025 R2 with a tetrahedral mesh and a global element size of 1 mm. Mesh refinement was applied to mechanically critical regions, including the pedicle screw interface, rod-screw junction, and anterior support region. Mesh convergence was assessed by progressive refinement, with peak von Mises stress and total deformation monitored at predefined critical locations. Convergence was considered achieved when the variation between successive refinements was less than 5%. The same meshing strategy and convergence criterion were applied consistently to all finite element models to ensure comparability among the evaluated constructs.

Outcome measures

The primary finite element outcome measures were von Mises stress distribution and total deformation under the applied loading conditions. Von Mises stress was used to determine the magnitude and location of stress concentrations within the implant system, particularly in the posterior fixation system and anterior support. Total deformation was used to assess overall structural displacement as an indicator of construct stability.

A yield-referenced mechanical safety assessment was also performed by comparing the peak von Mises stress with the titanium-alloy yield-strength reference. In the ASTM-based model, this assessment reflected construct-level mechanical behavior under high-demand loading, whereas in the vertebra-based model, it was used to determine whether the implant components remained within the elastic range under physiological loading conditions.

RESULTS

In the preliminary validation study, the ASTM-based finite element model of the posterior fixation construct under compressive loading showed a maximum total deformation of 13.296 mm and a peak von Mises stress of 1736.2 MPa. Stress was concentrated primarily in the rod segment near the screw-rod junction, identifying this region as the principal mechanical vulnerability of the construct. This finding was consistent with the failure-prone screw region identified experimentally in the preliminary mechanical test, supporting the suitability of the ASTM-based finite element model as a validation-oriented screening tool for identifying critical stress regions under standardized testing conditions.

ASTM-based comparative study

Compressive loading: Under ASTM-based compressive loading, the VCR-MSS construct demonstrated markedly lower total deformation than the conventional mesh cage construct. Total deformation was 6.0443 mm in the conventional mesh cage construct compared with 0.84345 mm in the VCR-MSS construct. At the component level, anterior component deformation was 1.8514 mm in the conventional mesh cage construct and 0.13481 mm in the VCR-MSS construct. Rod component deformation was 5.9869 mm and 0.55594 mm, respectively.

For von Mises stress, the conventional mesh cage construct demonstrated a total peak stress of 2035.2 MPa, whereas the VCR-MSS construct demonstrated a total peak stress of 1306.5 MPa. The anterior component showed the highest component stress in the conventional mesh cage construct, reaching 1236.8 MPa, while rod component stress was 429.09 MPa. In comparison, the VCR-MSS construct demonstrated an anterior component stress of 1167.9 MPa and a substantially lower rod component stress of 90.545 MPa.

In the yield-referenced mechanical safety assessment, the anterior-component peak von Mises stress exceeded the titanium-alloy yield-strength reference in both constructs under ASTM-based compressive loading, although anterior stress was slightly lower in the VCR-MSS construct than in the conventional mesh cage construct. By contrast, posterior rod stress remained below the same reference threshold in both constructs and was substantially lower in the VCR-MSS construct. These findings indicate a more favorable posterior mechanical safety profile for the VCR-MSS construct under compressive construct-level loading (Figure 5).

Figure 5 ASTM-adapted polyethylene compressive model comparing the conventional mesh cage construct and the vertebral column resection modular support system construct, showing von Mises stress and total deformation under compressive loading. VCR-MSS: Vertebral column resection modular support system.

Tensile loading: Under ASTM-based distraction loading, the VCR-MSS construct also demonstrated lower total deformation than the conventional mesh cage construct. Total deformation was 0.69253 mm in the conventional mesh cage construct compared with 0.17447 mm in the VCR-MSS construct. At the component level, anterior component deformation was 0.21341 mm and 0.024626 mm, while rod component deformation was 0.69253 mm and 0.11428 mm, respectively.

For von Mises stress, the conventional mesh cage construct demonstrated a total peak stress of 306.09 MPa, with an anterior component stress of 305.07 MPa and rod component stress of 48.211 MPa. In the VCR-MSS construct, total peak stress was 780.77 MPa, with the same peak value identified in the anterior component (780.77 MPa), whereas rod component stress was substantially lower at 18.541 MPa.

In the yield-referenced mechanical safety assessment, the anterior-component peak von Mises stress remained below the yield-strength reference in the conventional mesh cage construct and approached the same threshold in the VCR-MSS construct under tensile loading. However, posterior rod stress remained below the reference threshold in both constructs and was markedly lower in the VCR-MSS construct. Thus, the VCR-MSS construct maintained a more favorable posterior mechanical safety profile under tensile construct-level loading (Figure 6).

Figure 6 ASTM-adapted polyethylene distraction model comparing the conventional mesh cage construct and the vertebral column resection modular support system construct, showing von Mises stress and total deformation under distraction loading. VCR-MSS: Vertebral column resection modular support system.

Vertebra-based FEA

Flexion (compressive) loading: Under flexion (compressive) loading, the VCR-MSS construct demonstrated lower total deformation than the conventional mesh cage construct. Total deformation was 0.11004 mm in the conventional mesh cage construct compared with 0.068676 mm in the VCR-MSS construct. At the component level, anterior component deformation was also lower in the VCR-MSS construct, at 0.022952 mm compared with 0.033512 mm in the conventional mesh cage construct. Rod component deformation followed the same pattern, with values of 0.11004 mm in the conventional mesh cage construct and 0.068676 mm in the VCR-MSS construct.

For von Mises stress, the VCR-MSS construct demonstrated a higher total peak stress than the conventional mesh cage construct under flexion. Total peak stress was 179.19 MPa in the conventional mesh cage construct and 226.26 MPa in the VCR-MSS construct. The anterior component represented the principal stress-bearing region in both constructs, with peak values identical to the total peak stress: 179.19 MPa in the conventional mesh cage construct and 226.26 MPa in the VCR-MSS construct. Rod component stress was substantially lower in both constructs, measuring 19.551 MPa in the conventional mesh cage construct and 22.354 MPa in the VCR-MSS construct. Compared with the titanium-alloy yield-strength reference, peak von Mises stress in both the anterior component and posterior rod remained below the reference threshold for both constructs, indicating that these components remained within the elastic range with an adequate margin relative to yielding (Figure 7).

Figure 7 Vertebra-based finite element analysis comparison of the conventional mesh cage and vertebral column resection modular support system constructs under physiological flexion loading, showing von Mises stress and total deformation. VCR-MSS: Vertebral column resection modular support system.

Extension (tensile) loading: Under extension (tensile) loading, the VCR-MSS construct again demonstrated lower total deformation than the conventional mesh cage construct. Total deformation was 0.13876 mm in the conventional mesh cage construct compared with 0.053294 mm in the VCR-MSS construct. Anterior component deformation was likewise lower in the VCR-MSS construct, at 0.016659 mm compared with 0.033397 mm in the conventional mesh cage construct. Rod component deformation followed the same pattern as total deformation, with values of 0.13876 mm in the conventional mesh cage construct and 0.053294 mm in the VCR-MSS construct.

For von Mises stress, the VCR-MSS construct demonstrated a lower total peak stress than the conventional mesh cage construct during extension. Total peak stress was 181.19 MPa in the conventional mesh cage construct and 90.754 MPa in the VCR-MSS construct. The anterior component again showed the highest component stress in both constructs, with values of 181.19 MPa in the conventional mesh cage construct and 90.265 MPa in the VCR-MSS construct. Rod component stress remained lower than anterior component stress in both constructs, at 25.019 MPa in the conventional mesh cage construct and 16.935 MPa in the VCR-MSS construct. Compared with the titanium-alloy yield-strength reference, peak von Mises stress in both the anterior component and posterior rod remained below the reference threshold for both constructs, indicating that these components remained within the elastic range with an adequate margin relative to yielding (Figure 8).

Figure 8 Vertebra-based finite element analysis comparison of the conventional mesh cage and vertebral column resection modular support system constructs under physiological extension loading, showing von Mises stress and total deformation. VCR-MSS: Vertebral column resection modular support system.

Consistency between ASTM-based and vertebra-based findings

Across both the ASTM-based and vertebra-based analyses, the VCR-MSS construct consistently demonstrated lower total deformation than the conventional mesh cage construct. Under several loading conditions, the total peak stress was identical to the anterior component stress, indicating that the global stress maximum occurred in the anterior construct region. Similarly, under several loading conditions, total deformation was identical to rod component deformation, indicating that the greatest global displacement occurred in the posterior rod system.

Overall, these findings indicate that the VCR-MSS construct provided greater structural stability than the conventional mesh cage construct, as reflected by consistently lower total deformation across both standardized and anatomically contextual models. However, the pattern of stress redistribution varied according to loading mode and construct region.

DISCUSSION

This study compared the mechanical performance of a conventional mesh cage construct and the novel VCR-MSS using two complementary analytical environments: A standardized ASTM-based construct model and a vertebra-based lumbar finite element model. The principal finding was that the VCR-MSS construct consistently demonstrated lower total deformation than the conventional mesh cage construct across both models. This pattern was observed under compressive and distraction loading in the ASTM-based model and under flexion and extension in the vertebra-based model. Collectively, these findings suggest that the VCR-MSS construct provides greater structural stability than the conventional mesh cage construct in lumbar PVCR reconstruction[9,24,25,33]. To the best of the authors’ knowledge, this represents the first finite element investigation of a morphometry-guided modular reconstruction system specifically developed for lumbar PVCR using anatomical data from the Indonesian population and evaluated within both ASTM-based and vertebra-specific finite element frameworks.

A central mechanical difference between the two constructs appears to be how the load was shared between the anterior component and posterior fixation system. In the conventional mesh cage construct, high anterior component stress was accompanied by substantial rod component deformation, particularly in the ASTM-based compressive model. This pattern suggests that although the mesh cage participated in load bearing, the posterior rod system still carried a substantial proportion of the global displacement demand. By contrast, the VCR-MSS construct markedly reduced both total and rod component deformation, while rod component stress was also substantially lower. These findings suggest that the VCR-MSS construct may improve construct stiffness by reducing posterior mechanical demand and redistributing load more effectively through the integrated reconstruction system[24,25].

Component-level analysis is important for understanding why the lower deformation observed in the VCR-MSS construct did not consistently correspond to lower total von Mises stress. Under several loading conditions, the VCR-MSS construct exhibited higher total or anterior component stress than the conventional mesh cage construct. This finding should not necessarily be interpreted as unfavorable mechanical behavior. Rather, it suggests that the VCR-MSS construct may transfer load more directly through the anterior or modular region, producing a more concentrated internal stress pattern while simultaneously limiting global construct motion. The yield-referenced mechanical safety assessment further supports this interpretation. In the vertebra-based model, peak von Mises stress in both the anterior component and posterior rod remained below the titanium-alloy yield-strength reference, indicating that the titanium components remained within the elastic range under physiological loading. In the ASTM-based model, anterior component stress approached or exceeded the same reference under selected loading conditions; however, this finding should be interpreted in the context of a standardized, high-demand, near-failure construct-level environment rather than physiological in vivo loading. Within this context, the lower posterior rod stress observed in the VCR-MSS construct indicates that the new construct reduced the stress burden on the posterior fixation system, thereby producing a posterior unloading effect through greater load transfer to the anterior reconstructive region. In other words, the conventional mesh cage functioned primarily as an anterior support adjunct, whereas the VCR-MSS functioned as an integrated load-sharing reconstruction system. Previous studies of prosthesis-endplate matching and anatomical cage design have similarly demonstrated that improved anterior conformity and broader support interfaces can reduce stress concentration and improve load distribution. Furthermore, mechanical investigations of vertebral body replacement and fixation strategies have shown that construct design strongly influences stress localization and the pattern of load sharing between the anterior and posterior columns.

In the ASTM-based model, this load-sharing pattern was particularly evident under compressive loading. Compared with the conventional mesh cage construct, the VCR-MSS construct reduced total deformation from 6.0443 mm to 0.84345 mm and decreased rod component stress from 429.09 MPa to 90.545 MPa. Despite the observed total peak stress, the substantial reduction in rod stress indicates that the posterior fixation system was mechanically protected to a greater degree. From a reconstructive perspective, these findings may indicate favorable mechanical conditions by reducing the mechanical burden on the posterior fixation system and improving immediate construct stability. Although FEA cannot directly predict clinical outcomes such as rod fracture, implant subsidence, or long-term implant survival, lower rod stress and reduced construct deformation may indicate lower mechanical demand on the reconstruction construct, supporting a potential biomechanical advantage of the VCR-MSS over a conventional mesh cage. Standardized ASTM-based finite element studies have shown that such models are useful for construct-level validation and early screening before more anatomically contextual simulations. In the validated ASTM F1717 study, strong agreement between experimental and FEA results was observed in most comparisons, supporting the role of standardized testing frameworks in early implant assessment.

Under distraction loading, the VCR-MSS construct again demonstrated lower total and rod component deformation than the conventional mesh cage construct. The very low rod component stress observed in the VCR-MSS model under distraction further supports the interpretation that the integrated construct reduced the mechanical burden on the posterior fixation system. At the same time, the equivalence of total stress and anterior component stress under this condition indicates that the global stress maximum occurred in the anterior region. This finding further suggests that the anterior component served as the principal stress-bearing region in the VCR-MSS construct during distraction. The vertebra-based model provided additional anatomical context for these findings. Under both flexion and extension, the VCR-MSS construct demonstrated lower total deformation than the conventional mesh cage construct, indicating improved segmental stability under physiological loading. However, the stress pattern varied with loading direction. During flexion, the VCR-MSS construct showed higher total and anterior component stress, whereas during extension, it showed lower stress than the conventional mesh cage construct. These findings suggest that the mechanical behavior of the VCR-MSS construct is influenced by loading direction and that its advantage is more consistently reflected in reduced deformation than in universally lower stress magnitude[24,25]. Overall, the VCR-MSS consistently reduced total deformation, whereas its effect on peak stress depended on loading direction. Direction-dependent stress redistribution has also been reported in other spinal finite element studies comparing cage-based reconstructions, vertebral body replacements, and posterior fixation strategies. These studies have shown that construct performance may vary under flexion, extension, bending, and rotational loading rather than demonstrating consistent superiority of a single construct across all motion directions.

Another notable observation from the present analysis is that under multiple loading conditions, total peak stress was identical to anterior component stress, whereas total deformation was identical to rod component deformation. Mechanically, this finding implies that the anterior component frequently represented the region of global peak stress, whereas the rod component was often the dominant site of global displacement. This pattern helps clarify the functional distinction between these regions: The anterior component primarily governed local stress transfer, whereas the rod component primarily reflected whole-construct motion. The VCR-MSS construct appeared to maintain anterior participation in load bearing while reducing deformation of the posterior fixation system.

The present findings are also relevant to the broader issue of subsidence and fixation-related complications following anterior reconstruction. Previous studies have demonstrated that insufficient cage-endplate conformity, smaller support areas, and higher local cage stress are associated with increased stress concentration and a greater propensity for subsidence[21,22,34]. In a study of cage size related to total en bloc spondylectomy, smaller-diameter cages exhibited markedly higher axial and torsional stress than larger cages, supporting the concept that broader anterior support enhances stress resistance and construct rigidity. Similarly, studies of anatomical titanium mesh cages have shown that improved endplate matching reduces endplate stress, cage stress, and fixation-related stress compared with conventional designs[21,22,45].

From a methodological perspective, the present study also benefits from the combination of standardized and anatomically contextual models. ASTM-based models offer reproducibility and construct-level comparability, whereas vertebra-based models provide more physiologically relevant insight into motion-dependent stress transfer[33]. Nevertheless, the findings should be interpreted within the context of a computational mechanical investigation. Although the finite element framework was supported by a preliminary ASTM-based validation study using a posterior fixation construct, direct experimental validation of the VCR-MSS implant itself was beyond the scope of the present work. Therefore, the mechanical advantages demonstrated for the VCR-MSS should be regarded as computational predictions that warrant further confirmation through dedicated mechanical testing, cadaveric studies, and future clinical investigations. The preliminary model examined the posterior fixation construct without anterior support as a validation platform, whereas the primary comparative analysis evaluated two anterior reconstruction constructs: A conventional mesh cage and the VCR-MSS. This dual-model strategy is supported by previous studies showing that standardized test configurations remain useful for validation and screening but may not fully capture anatomical load redistribution under more realistic spinal conditions. In addition, the selection of axial preload and bending moment in vertebra-based spinal FEA has commonly been guided by previous mechanical studies using follower-load or physiological-moment concepts, helping to maintain consistency with established loading frameworks[43]. Because bonded contact suppresses interfacial micromotion, the present model may underestimate local slip and endplate contact pressures. Consequently, the findings should be interpreted as comparative construct-level trends rather than direct predictions of implant-endplate interactions or subsidence. An additional limitation is that the finite element model did not include soft tissue structures; therefore, the results represent comparative construct-level mechanics rather than a complete physiological representation of the lumbar spine.

Limitations

The present study should be interpreted within the scope of a comparative FEA evaluating the initial mechanical performance of the proposed VCR-MSS construct. The computational models incorporated commonly accepted simplifying assumptions regarding material behavior, interface conditions, and anatomical representation. In addition, bonded contact was used to represent rigid fixation at the implant interfaces; thus, interface micromotion and frictional effects were not considered. Surrounding soft tissues, including spinal ligaments and paraspinal musculature, were not modeled, and their stabilizing effects were therefore not represented. These assumptions enabled a standardized construct-level comparison but do not fully replicate the in vivo spinal environment. The mechanical evaluation was limited to axial compression. Bending and lateral loading conditions were not investigated because of limitations in the available testing facilities.

Furthermore, validation was performed at the component level using established material properties and previously validated finite element modeling approaches; however, experimental validation of the assembled VCR-MSS construct has not yet been performed. Accordingly, the findings should be regarded as computational evidence supporting the mechanical feasibility of the VCR-MSS concept rather than definitive evidence of its overall mechanical performance. Future studies should include whole-construct mechanical testing, fatigue assessment, and clinical evaluation to further establish the performance and translational potential of the proposed reconstruction system.

CONCLUSION

The VCR-MSS construct demonstrated a more favorable mechanical profile than the conventional mesh cage construct, primarily through lower total deformation and lower rod component stress under standardized and physiological loading conditions. These findings suggest that the VCR-MSS construct may provide improved load-sharing behavior by preserving anterior structural participation while reducing posterior mechanical demand. Thus, the VCR-MSS appears to function less as a simple anterior spacer and more as an integrated reconstruction system in lumbar PVCR. Future studies should focus on experimental mechanical validation of the proposed VCR-MSS using standardized mechanical testing and cadaveric models, followed by evaluation under cyclic loading conditions to assess long-term mechanical durability. Clinical investigations will subsequently be required to determine the safety, effectiveness, and translational applicability of the proposed implant in patients undergoing PVCR.