Abstract
Background Excessive stress on the pedicle screws or inadequate load-sharing with surrounding spinal components increases the mechanical demand and the risk of loosening or breakage. The lumbar interfacet cage (FFX device) is designed to prevent spinal instability and facet motion, enhancing facet joint fusion.
Objective The present study aimed to compare the biomechanical performance of a lumbar interfacet cage when associated with a pedicle screw construct, compared with pedicle screws alone and pedicle screws associated with lumbar interbody cages, using the FE method.The authors hypothesized that implanting additional lumbar interfacet cages would reduce mechanical stress on pedicle screw constructs.
Study Design Comparative biomechanical study by finite element (FE) method.
Methods A validated FE model for the lumbar spine was used to assess stress variations on pedicle screw constructs and discs in the prefusion stage following surgery. Modeled scenarios included (1) a short pedicle screw construct (L4/L5), with and without bilateral lumbar interfacet cage device placement and with or without interbody fusion cages, and (2) a long pedicle screw construct (L2–S1), with and without lumbar interfacet cage placement at all levels.
Results Both facet and interbody cage placement in conjunction with short L4/L5 pedicle screw constructs significantly reduced mechanical loading on pedicle screws and rods compared with the pedicle screw construct alone. The placement of lumbar interfacet cages used in combination with pedicle screw constructs in long L2 to S1 constructs also significantly reduced stress loading on pedicle screws and rods, especially at the lower extremity of the construct.
Conclusions The placement of facet cages in conjunction with pedicle screws can improve the load distribution of the construct, enhancing its stability and durability. This approach may help reduce the rate of pedicle screw loosening and breakage, which are complications commonly associated with long pedicle screw constructs.
Clinical Relevance Pedicle screw loosening and breakage remain frequent complications in lumbar fusion, especially in long constructs. This finite element analysis demonstrates that adding lumbar interfacet cages to pedicle screw constructs significantly reduces mechanical stress on screws and rods. By improving load distribution in both short and long constructs, the technique may decrease the risk of screw loosening and implant failure before fusion, potentially improving construct durability and reducing reoperation rates.
Level of Evidence 5.
- lumbar facet joint
- FFX
- PLIF
- TLIF
- finite element analysis
- lumbar spine
- biomechanics
- pedicle screw loosening
- pedicle screw breakage
Introduction
Pedicle screw fixation following decompression is currently considered the standard technique for achieving fusion and spinal stability in patients with lumbar spinal stenosis.1 It is also widely used in long surgical constructs, such as those for scoliosis patients.2 While improvements in posterior pedicle screw systems have significantly enhanced their strength, rigidity, and overall performance for spinal surgeries, these have also contributed to increased loading at the bone-screw interface that may contribute to pedicle screw loosening and breakage.3–5
Pedicle screw loosening and breakage are influenced by a combination of biomechanical, surgical, and material factors. Uneven or excessive stress on the screw or inadequate load-sharing between the pedicle screw and surrounding spinal components increases the mechanical demand on the screw. Pedicle screw loosening has been reported to occur in up to 25% of nonosteoporotic and 40% of osteoporotic patients undergoing pedicle screw fixation for lumbar degenerative diseases.6–8 Pedicle screw loosening can result in localized back or radicular pain, paresthesia or numbness, spinal instability, pseudarthrosis, reduced range of motion, and the potential for adjacent segment disease due to increased stress on adjacent spinal levels.9,10 While advancements in surgical approaches and pedicle screw system technologies have resulted in a reduction in the incidence of hardware fractures, there is a reported breakage rate of 10% to 12% for pedicle screws and/or rods.11 Factors that increase the risk of pedicle screw breakage include obesity, female sex, and smoking, while long-segment fusion with the placement of posterior interbody fusion devices tends to lower the risk of screw fracture.12
Lower stress levels on screws correspond to a reduced risk of loosening and fracture. The placement of posterior interbody cages with lumbar pedicle screws is often used to enhance the stability and durability of spinal constructs to lower the rate of pedicle screw loosening and breakage.12–14 The use of interbody devices reduces the rate of pedicle screw loosening and breakage by better distributing loads across the spine, resulting in less stress on the posterior pedicle screw construct. The placement of interbody devices also increases the surface area for fusion, which enhances bone growth between the vertebrae. This bone growth and resultant spinal fusion reduce the micromotion that has been shown to contribute to pedicle screw loosening and breakage.15,16
The “three-column spine” model described by Louis17 divides the spine into 3 separate columns; an anterior column comprising the vertebral bodies and intervertebral discs, and 2 posterior columns formed by the facet joints and their supporting structures. This anatomical framework emphasizes the mechanical role of these columns in maintaining spinal stability. Based on the mechanical role associated with the 2 posterior columns, the lumbar interfacet cage was designed as a posterior lumbar stabilization facet cage to prevent spinal instability and facet motion, enhancing facet joint fusion (Figure 1).18,19 Two devices are surgically implanted at each level, 1 between the facet joint surfaces on each side, and bone graft material is placed inside and posterior to the implant. Using a validated finite element (FE) model for the lumbar spine, Simon et al. previously reported that modeled facet joint stress at adjacent lumbar spine levels for interfacet cages prior to fusion was significantly less than for pedicle screws.20 Since reducing the biomechanical load on pedicle screws could reduce the risk of screw loosening and implant failure, we hypothesize that the combination of pedicle screw and lumbar interfacet cage placement can reduce the stress on pedicle screw constructs prior to bony fusion.
Image of the interfacet cage (FFX; Courtesy of SC MEDICA).
The goals of the present study were to compare biomechanical loading on short pedicle screw constructs at L4/L5 alone to the use of these pedicle screw constructs with either interfacet cages or an interbody lumbar cage and to compare the stresses on long pedicle screw constructs with and without interfacet cages at all levels between L2 and S1.
Methods
FE Model
A previously validated 3-dimensional L1 to S1 intact lumbar spine FE model for measuring the biomechanical impact and the level and patterns of stress produced by the different constructs was used for the present study (Figure 2a).20 The interfacet cage was modeled based on the FFX device (SC Medica, Strasbourg, France) when defining the model parameters.
Finite element model of lumbar spine (L1–S1) used for the present study. (a) Lateral view. (b) Disc alone. (c) Posterior view.
The shape and dimensions for each vertebra were adjusted to the mean size (50th percentile) of a healthy spine as reported in the literature.21–23 The disc was built of 3 separate elements (Figure 2b) in order to produce a lateral rigidity 3 times lower than the anteroposterior rigidity24 with the 2 lateral elements having a lower rigidity than the medial element. Capsular ligament stiffness was simulated by a torsional spring added between the 2 vertebrae, simplifying the model since torsional rigidity is very high and difficult to reproduce in a model.24
A mesh was generated on the above geometry (Figure 2a and c) using Creo Simulate 3.0 (PTC Inc., Boston, MA, USA). This software uses the p-version of the FE method, which utilizes higher degree polynomials as opposed to increasing the number of elements.25 This method enables the ability to check whether the solution converges with only a single mesh step as opposed to iteratively refining the mesh using the h-method. As with the previous study, we used the most linear solver for the model to reduce calculation time and computer processing capacity. Additionally, since previous studies have demonstrated that the most important parameters influencing spine mobility are disc height and facet orientation,21,24 we did not model other ligaments except for the analysis of disc rigidity.
The final lumbosacral model included 32,000 tetrahedron elements with the sacrum being considered fixed, with a pure moment load applied to the superior L1 endplate to produce flexion, extension, lateral bending, and axial rotation (Figure 3).26 After assembling all parts of the model, adjustments were made to respect the anatomical lordosis of the spine and to ensure load-displacement curves that matched what is reported in the literature.27,28
Finite element model of lumbar spine. (a) Pedicle screw construct alone (L4/L5). (b) Pedicle screw construct (L4/L5) with interfacet cages. (c) Pedicle screw construct (L4/L5) with interbody cage (L4/L5). (d) Pedicle screw construct alone (L2/S1). (e) Pedicle screw construct with interfacet cages (L2/S1).
Comparative Studies
The model was utilized to compare the stresses on the pedicle screws and rods in short and long constructs. For the short construct, scenarios compared at L4/L5 were pedicle screws and rods alone, pedicle screws and rods implanted in conjunction with interfacet cages, and pedicle screws and rods implanted in conjunction with an interbody cage. For the long construct, pedicle screws and rods alone implanted from L2 to S1 were compared with pedicle screws and rods implanted in conjunction with interfacet cage placement at all levels from L2 to S1.
The FE model presumed bilateral placement of the pedicle screws (with rods) and interfacet cages. For each comparison, the reference model was arbitrarily loaded at a value within the validated range of loads of the model: at 3 Nm for L4/L5 assemblies and 1 Nm for the L2/S1 assemblies to produce a reference displacement. The loads for the subsequent models were then varied until a displacement that differed by less than 3% was obtained. These loads were then considered to be at constant displacement, enabling the comparison of constraints at different points of the model for the analyses performed.
All constructs were modeled in the prefusion state, as in clinical reality: (1) pedicle screws typically loosen or constructs fail before fusion occurs, and (2) once fusion has taken place, screw loosening or construct failure would not be clinically relevant. Pedicle screws, interbody cages, and interfacet cages were numerically fixed at the bone-implant interface, and no bone bridge was added to the model to simulate fusion. As facet cages only produce an effect in extension, all scenarios were loaded in extension.
Results
Short Pedicle Screw Construct (L4/L5) With and Without Interfacet Cages
The maximum displacement for the model for the pedicle screw construct alone at L4/L5 was 22.59 mm compared with 22.82 mm for the pedicle screw construct plus the interfacet cages (Figure 4a and b). This 1% (0.23 mm) difference is less than 3% and therefore well within the desired comparison conditions.
Displacement (a) pedicle screw construct alone (L4/L5). (b) Pedicle screw construct (L4/L5) with interfacet cages. (c) Pedicle screw construct (L4/L5) with interbody cage (L4/L5). (d) Pedicle screw construct alone (L2/S1). (e) Pedicle screw construct with interfacet cages (L2/S1).
The maximum stress on the pedicle screw–only construct (L4/L5) was 224 ± 0.65 MPa compared with 96 ± 0.73 MPa for the pedicle screws plus the interfacet cages (Figure 5a and b). This −128 MPa (−57%) difference is greater than the 0.73 MPa maximum error and is therefore significant. This demonstrates that the stress on the pedicle screw construct is significantly lower when interfacet cages are implanted in conjunction with the L4/L5 pedicle screw construct.
Pedicle screw stresses during prefusion extension. (a) Pedicle screw construct alone (L4/L5). (b) Pedicle screw construct (L4/L5) with interfacet cages. (c) Pedicle screw construct (L4/L5) with posterior lumbar interbody fusion cage (L4/L5). (d) Pedicle screw construct alone (L2/S1). (e) Pedicle screw construct with interfacet cages (L2/S1).
The maximum stress on the discs for the pedicle screw–only model (L4/L5) was 2.9 ± 0.65 MPa compared with 3.0 ± 0.73 MPa when the interfacet cages were added to the pedicle screw construct (Figure 6a and b). Since this difference is less than the 0.73 MPa maximum error, there is no difference in loading on the discs when interfacet cages are implanted in conjunction with the L4/L5 pedicle screw construct.
Disc stresses during pre-fusion extension. (a) Pedicle screw construct alone (L4/L5). (b) Pedicle screw construct (L4/L5) with interfacet cages. (c) Pedicle screw construct (L4/L5) with interbody cage (L4/L5). The missing disc is a result of numerically fixing the interbody cage at this level. (d) Pedicle screw construct alone (L2/S1). (e) Pedicle screw construct with interfacet cages (L2/S1).
The differences in the stresses on the facets at each level (excluding L4/L5) for the pedicle screw construct alone model compared with the pedicle screw construct plus interfacet cage model were not significant, ranging from 0.35 MPa for L5/S1 to 1.40 MPa for L3/L4 for a maximum error of ±0.73 MPa (Table). At L4/L5, the stresses on the facets for the pedicle screw–only construct were negligible (0.05 MPa). The stresses on the facets for the pedicle screw plus interfacet cage construct were 11.5 MPa, demonstrating a significant transmission of 11.45 MPa from the pedicle screws and rods to the facets resulting from the insertion of the interfacet cages.
Maximum von Mises stress (MPa) in the facets by procedure type.
Short Pedicle Screw Construct (L4/L5) With Interbody Vs Facet Cages
The maximum displacement for the model for the pedicle screw construct with interbody cage at L4/L5 was 22.75 mm compared with 22.82 mm for the pedicle screw construct plus the interfacet cages (Figure 4b and c). This 0.07 mm (0.3%) difference was less than 3% and therefore well within the desired comparison conditions.
The maximum stress on the pedicle screws and rods for the pedicle screw construct with interbody cage at L4/L5 was 92 ± 0.44 MPa compared with 96 ± 0.73 MPa for the pedicle screws plus the interfacet cages (Figure 5b and c). Since the maximum error of 0.73 MPa is lower than the difference between the 2 groups (4 MPa), the load on the pedicle screws and rods with the facet cages was higher than with interbody cages.
The maximum stress on the discs for the pedicle screw construct with interbody cages at L4/L5 was 3.0 ± 0.44 MPa compared with 3.0 ± 0.73 MPa for the pedicle screws plus the interfacet cages (Figure 6b and c). Therefore, there is no difference in loading on the discs when either interbody or facet cages are implanted in conjunction with a pedicle screw construct.
The differences in the stresses on the facets at each level (excluding L4/L5) for the pedicle screw construct with interbody cages compared with the pedicle screws plus the interfacet cage model were not significant, ranging from 0.2 MPa for L2/L3 to 1.6 MPa for L1/L2 for a maximum error of ±0.73 MPa (Table). At L4/L5, the stresses on the facets in the pedicle screw with interbody cage construct were negligible (0.004 MPa), while in the pedicle screw plus the interfacet cage construct, the stresses (11.5 MPa, ie, a difference of 11.496 MPa) were significantly transmitted from the pedicle screws/rods to these facets by the interfacet cages.
Long Pedicle Screw Constructs With and Without Interfacet Cages for All Levels (L2–S1)
The maximum displacement for the model for the pedicle screw construct alone from L2 to S1 was 2.04 mm compared with 2.00 mm for the pedicle screw construct plus the interfacet cages (Figure 4d and e). This 2% (0.04 mm) difference is less than 3% and therefore well within the desired comparison conditions.
The maximum stress on the pedicle screw–only construct (L2–S1) was 125 ± 0.43 MPa compared with 48 ± 1.25 MPa for the pedicle screws plus the interfacet cages. The maximum stresses were borne at the extremity of the long construct in S1 (Figure 5d and e). This –77 MPa (–62%) difference is greater than the maximum error of 1.25 MPa and is therefore significant. This demonstrates that the stress on the pedicle screws and rods is significantly lower when interfacet cages are implanted in conjunction with this pedicle screw construct.
The maximum stress on the discs for the pedicle screw–only model (L2/S1) was 1.95 ± 0.43 MPa compared with 2.23 ± 1.25 MPa when the interfacet cages were added to the pedicle screw construct (Figure 6d and e). Since this difference is less than the 1.25 MPa maximum error, there is no difference in loading on the discs when interfacet cages are implanted in conjunction with this pedicle screw construct.
The differences in the stresses on the facets at each level for the pedicle screw–only construct (L2/S1) compared with the pedicle screw construct plus interfacet cage model were not significant, ranging from 0.04 MPa for L1/L2 (nonfused level) to 2.35 MPa for L2/L3, for a maximum error of ±1.25 MPa (Table).
Discussion
The present study simulated the biomechanics of several spinal fusion alternatives in order to compare their effects on spinal stability and load distribution. A previously validated 3-dimensional L1 to S1 intact lumbar spine FE model20 was used to compare the loads in short and long pedicle screw constructs across 5 scenarios. The objective of the study was to provide biomechanical evidence that the implantation of interfacet cages in conjunction with pedicle screw constructs can reduce stress on pedicle screws and rods, as pedicle screw breakage and loosening are well-documented complications of these constructs. Two-thirds of reoperations for lumbar spine surgery are due to implant failure, specifically pseudarthrosis across the lumbosacral junction.29 Thus, achieving solid fusion at the lumbosacral junction remains a challenge, especially in long constructs from the thoracic spine to the sacrum/pelvis, with secure sacropelvic fixation a necessary component.
The design and placement location of the interfacet cages are intended to prevent facet motion while promoting a more anatomical and less invasive bony fusion option compared with conventional spinal constructs when used as a stand-alone device.30 The present study confirmed our hypothesis that the device can also improve load distribution when used in hybrid constructs with short or long pedicle screw constructs. This study provides in vitro evidence of a clinical potential for the interfacet cages to reduce the risk of pedicle screw loosening and breakage. The presence of the interfacet cages was favorable when used with pedicle screw constructs to transfer the mechanical load from screws and rods already at the prefusion stage. This was consistent when modeled across single-level (L4/L5) and multilevel (L2/S1) scenarios. For the latter, the mechanical load was particularly reduced at the S1 level. While the interfacet cages appeared to be less effective than an interbody cages in reducing stress to the screws and rods, several points need to be considered when comparing interbody vs facet cages. First, as shown in Figure 7, most of the load reduction on pedicle screws/rods could be achieved with facet cages. This raises questions about the benefit/risk ratio of using more invasive interbody cage placement when correction of lordosis is not needed.31 Second, the use of facet cages at all levels is also more technically achievable in long constructs compared with interbody cages.
Maximum load (MPa) on the pedicle screws and rods.
FE modeling provides a valuable option to obtain both qualitative data and quantitative data related to stress and strain distribution, contact forces, and displacement field that may help the understanding of mechanisms and compare different cases.32 FE analysis eliminates the problems associated with reproducibility of the results when using cadaver studies and also enables the ability to study the same anatomical location of the spine, avoiding the need for statistical analyses to compare results collected on spines with variations in morphology, bone, and ligament quality.33,34
Limitations
There are several potential limitations to assessing spinal loading using a biomechanical in vitro study. The present model used did not assess other factors that could affect stresses on spinal constructs like body weight, musculature, and varying body movement. Our model was designed to gain an understanding of the general effects of various lumbar fusion alternatives on spine biomechanics in order to compare differences between these differing techniques and/or devices. Our model was also limited to linear laws of behavior, with the points of contact being the only nonlinear features. This resulted in limits on the loads it could be validated for. Last, our model was limited to the capabilities of the software utilized. Additional limitations of the model were reviewed as a part of our previous publication, which reviewed the validation of our model.20
Conclusion
The findings of the present study suggest that the placement of interfacet cages in conjunction with lumbar pedicle screw constructs can reduce stress on pedicle screws and rods and improve load distribution. This provides in vitro evidence that this combination may be able to reduce pedicle screw loosening and breakage. Longitudinal clinical data associated with the clinical use of the lumbar interfacet cages with pedicle screw systems is needed to validate this hypothesis.
Footnotes
Funding Funding to support the development and analysis of the finite element models utilized for the present study and payment of the article processing charge was provided to LSCF by SC Medica (Strasbourg, France).
Declaration of Conflicting Interests Lionel Simon and Romain Buttin are paid consultants of SC Medica. Robin Srour reports being listed as a designer in the patents of the FFX device and having a relative that is employed by SC Medica. Rachid Saddiki and Thierry Dufour have previously received consulting fees from SC Medica. The remaining authors report no conflicts of interest.
Disclosures Thierry Dufour reports royalties and consulting fees from Highridge Medical. Rachid Saddiki reports consulting fees from RIWO, Centinel, and Cousin Spine. Robin Srour reports royalties from Spineart.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2025 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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