Authors: Yi-An Li, Shih-Liang Shih, Hsin-Chang Chen
Categories: Research, Facetectomy, Finite element study, Interspinous space, Spinal instability
Source: BMC Musculoskeletal Disorders
Resecting the facet joint to relieve nerve pain can lead to spinal instability, deformity, and abnormal pressure on the anterior of the intravertebral disc. To mitigate these issues, surgeons often limit the amount of bone removed during facetectomy or stabilize the spine by fusion to maintain lumbar stability. This study aimed to assess how a M-PEEK rod system influenced the stability of the lumbar spine during graded facetectomy.
Facetectomy was performed on a validated L1-L5 finite element model which was then simulated both with and without the M-PEEK rod system.
In extension, models implanted with M-PEEK in the interspinous space of L3/L4 experienced a 35.2% decrease in range of motion (ROM) at L3/L4, while others saw an 8.4–24.8% increase. For axial rotation, the ROM at L3/L4 increased by 2.2–5.4% in models with the M-Rod, and by 4.9–12.9% in models without the implant. In lateral flexion, the ROM at L3/L4 increased by 8.4–14.3% in models without a PEEK M-Rod (facetectomy only), with adjacent segments experiencing a 6.5% decrease in ROM in the implanted models. Overall, the difference in ROM between the intact and implanted models was minimal.
Facetectomy involving the removal of 50% or more of the facet joint significantly increases range of motion and maximum intradiscal pressure, potentially accelerating disc degeneration, as shown in our finite element study. Stabilizing the segment with an M-PEEK rod may limit excessive motion, providing stability and maintaining intradiscal pressure closer to that of an intact model.
Keywords: Facetectomy, Interspinous space, Spinal instability, Finite element study
Facetectomy is a surgical procedure that requires the removal of one or more facet joints in the spine to reduce nerve compression. The procedure is commonly performed on elderly patients suffering from moderate to severe pain stemming from the spinal nerve root. Although total facetectomy offers maximum decompression to reduce pressure on the nerve root, the complete removal of a facet joint can lead to spinal instability [1–3]. Facet joints are important structures for multi-directional movement of the lumbar spine and for compressive load transmission during extension. In addition to lumbar instability, resecting facet joints can also induce spinal deformation, leading to abnormal loading on the anterior disc. Previous studies [4, 5] indicated that facetectomy may not directly cause injury to the intervertebral disc and capsule, but can cause secondary damage and increase the risk of tears to the annulus fibrosus. Hence, limiting the material removed during facetectomy is important for preserving post-surgical spinal stability.
Although total lumbar facetectomy can be used to completely free a compressed nerve root, and when accompanied by spinal fusion can maintain long-term spinal stability, progressive adjacent segment degeneration and postoperative nerves injuries are common complications reported in clinical follow-up studies [6]. Similarly, while rigid spinal fusion with pedicle screw systems can secure the treated segments, adjacent segment disease can drastically reduce the long-term efficacy of the procedure [7]. Due to the restricted range of motion (ROM) of the fused segments, compensatory movements at adjacent segments can cause overloading on the intervertebral disc and lead to long-term disease [8, 9].
While facetectomy without fusion can preserve a greater ROM at the treated site and reduce compensation motions at adjacent segments, there is still a risk of degeneration of adjacent segments because of the impaired stability [10]. In a clinical follow-up study, Bydon et al. [11] evaluated the outcome of primary decompression per-formed on 500 patients with degenerative lumbar disease. The results showed that 39 of 72 revision cases displayed signs of adjacent segment degeneration. Therefore, con-sideration should be given to how to stabilize the motion segment after discectomy to improve patient outcomes.
Previously, our team developed a pedicle screw-based supporting structure with an “M” geometry (M-PEEK rod) that acted as a non-fusion interspinous spacer. The design achieved a secure fixation with pedicle screws and did not alter the ROM or disc stress of adjacent segments compared to other interspinous devices [12]. Our design offers a flexible bridge support on incomplete posterior element of vertebra condition to limit the exceed extension motion and preserve range of motion in treated segment. The fixed type of M-PEEK rod is differed from other interspinous posterior devices, the secured and stability was depending on pedicle structure without considering quality of posterior element of vertebra. Most interspinous posterior devices must be secured in a high-quality posterior element of vertebra condition. If the spinal fusion is required for a patient treated by M-PEEK rod before, it was just to replace M-PEEK by the titanium rod and extend the fixation segments. The purpose of this study was to evaluate the stability of the spinal motion segment after graded facetectomy when implanted with the M-PEEK rod system using a validated L1-L5 finite element model.
A finite element model of the L1–L5 lumbar spine segment was created using the software ANSYS 2021 (ANSYS Inc., Canonsburg, PA, USA). The model (Fig. 1a) was developed using geometry from a physiologically accurate spinal model that included intervertebral discs and vertebrae (Zygote Media Group, Inc.). The material properties of the components in the model shown in Table 1 were sourced from literature [13–17]. The model included six ligaments which were represented by 2-node tension-only spring elements (Fig. 1b) and were modeled as nonlinear with insertion points approximated to typical anatomy [14]. The cortical bone, cancellous bone, and disc were represented using SOLID187 element. Within the disc, twelve double cross-linked fibrous layers were integrated into the ground substance, with the stiffness of the fibers progressively increasing from the outermost to the innermost layer. Meanwhile, the nucleus pulposus was simulated as an incompressible fluid using 8-node fluid elements. Frictionless contact elements were used to simulate the facet joints. Additionally, frictionless contacts were assumed between the M-PEEK rod and the interspinous regions of the vertebrae. Bonded contacts were established at the following between the tulips of the pedicle screws and the M-PEEK rod, and between the pedicle screws and the pilot holes in the vertebrae. We defined the global element sizes for each part of the model and applied local sizing controls to refine the mesh in areas of particular interest. For intricate geometries of the lumbar spine segment modes in this study, we selected the tetrahedral meshing method to capture detailed geometrical features and better replicate the clinical situation where hex-meshing might be impractical [18]. After generating the mesh, we evaluated its quality using the built-in metrics and made necessary refinements, mesh sensitivity and convergence test to ensure optimal results. The convergence test revealed three mesh the coarse model with 20,531 elements and 31,357 nodes; the normal model with 120,865 elements and 184,561 nodes; and the finest model with 155,638 elements and 239,914 nodes. These densities were selected to assess the range of motion (ROM) changes in the intact model. Given that the change was within 1.03% (less than 0.2°), the finest mesh density was chosen.
Fig. 1 L1-L5 spine segment a) finite element mesh; b) model containing supraspinous interspinous ligament (ISL), ligament (SSL), capsular ligaments (CL), posterior longitudinal ligament (PLL), transverse process ligaments (TPL), ligamentum flavum (LF), and anterior longitudinal ligament (ALL)
The model was validated by comparing the range of motion against experimental data from a published in vitro study and finite element analysis results from literature [12, 15–17]. When subjected to a 400 N follower load combined with a moment of 10 Nm under physiological motions, our intact model demonstrated that the moment increased in increments of 0.36 Nm until the range of motion (ROM) of the model (L1-L5) reached 16.21° in flexion, 10.25° in extension, 15.33° in left lateral flexion, and 8.55° in left torsion (Table 2). The ROM observed in this study is similar to that observed in our preliminary study [12]. The resulting model consisted of approximately 155,638 elements and 239,914 nodes.
A compressive follower load was emulated using a two-node link element, which was affixed near the geometric center of each vertebra and maintained tangency with the spinal curve. By contracting the link elements, a 400 N compressive force was applied to replicate physiological compressive loading [12]. Ultimately, the selected follower load paths effectively limited the range of motion (ROM) of each segment to within 0.4° across all FE models.
The intact model was subjected to a 400 N follower load in conjunction with a 10 Nm moment to simulate physiological motions. For the implant models, they were first subjected to a 400 N follower load and then the moment was progressively increased until the overall range of motion (ROM) closely matched that of the intact model. The differences in ROM among the implanted FE models were kept within a controlled range of 0.65° (as shown in Table 2). Additionally, all FE models were constrained at the lower surface of the fifth vertebra. The percentage differences in ROM were compared against the intact model.
Five finite element models of the L1-L5 segment were developed in this a) model A is the healthy intact condition (Fig. 1a); b) model B (Fig. 2a) was modified from model A to include an implanted polyetheretherketone M-rod (M-PEEK) in the interspinous space of L3/L4; (c) model C was developed from model B with an additional 30% excision of the superior articular process on both sides (Fig. 2b); (d) model D is from the model B with a 50% excision of the superior articular process on both sides (Fig. 2c); (e) model E is from the model B with a 100% excision of the superior articular process on both sides. Another three finite element models were also developed in this study to compare the segmental ROM (Fig. 3) without a M-PEEK rod after excision of the superior articular process on both sides (model C’, model D’, and model E’).
Fig. 2 a) Model B was modified by inserting an implant M-PEEK rod into the interspinous space of L3/L4 without excision of the superior articular process. b) Model C was derived from Model B by performing a 30% excision of the superior articular process. c) Model D was derived from Model B by performing a 50% excision of the superior articular process
Fig. 3 Method to determining the ROM (θ)
In comparison to the intact model in extension, the ROM at the L3/L4 level decreased by 35.2% in models B, C, D, and E (Table 2), while it increased by 8.4%, 15.2%, and 24.8% in models C’, D’, and E’ (Table 3), respectively. In contrast, the ROM at L1/L2, L2/L3, and L4/L5 increased by 14.3%, 12.3%, and 17.6% in models B (Fig. 4), C, D, and E, respectively, but decreased by less than 10% in models C’, D’, and E’ (Fig. 5; Table 3). In flexion, the ROM of model E at L3/L4 increased by 1.6% relative to the intact spine model, while the ROM at adjacent segments changed by less than 1% in all models.
Fig. 4 The contour plot shows the node displacement (in mm) of model B in extension
Fig. 5 The percentage difference in range of motion (ROM) compared to the intact model at the implanted and adjacent levels when placed in flexion, extension, axial rotation, and lateral flexion
Compared to the intact model, in axial rotation, the ROM at L3/L4 increased by 2.2–5.4% in models with the M-Rod, and by 4.9–12.9% in models without the implant. The ROM of the adjacent segments in all implanted models under axial rotation was similar, ranging from 1.2–6.3% decreased of the intact model (Fig. 4, Table 2). Similar results were also observed in the models without implants, with the ROM ranging from 3.2–8.5% of the intact model. In lateral flexion, the ROM at L3/L4 increased by 8.4–14.3% in models C’, D’, and E’, while the ROM of adjacent segments decreased by approximately 6.5% in the models with an M-Rod (models B, C, D, and E).
Lumbar facet joints are complex structures that primarily function to stabilize the spine while allowing flexibility. Guha et al. [19] reported that lumbar decompression without fusion performed to relieve nerve pain often results in spinal instability, which can lead to degenerative lumbar stenosis. In practice, surgeons usually try to limit the amount of the facet joint removed to reduce port-operative instability. Pedicle screw fixation is considered a safe option for maintaining stability at the treated site, but adjacent segment disease is a common complication when using stiff instrumentation [7]. This study investigated whether spinal stability could be maintained after facet joint excision if the operative site was bridged with a M-PEEK rod system. Finite element models were developed with different grades of facetectomy to determine how the M-PEEK rod system affected spinal motion and intradiscal pressure.
The results of this study showed that the ROM of the treated segment when placed in flexion, extension, lateral flexion and rotation increased in all facetectomy models without a M-PEEK rod system in comparison to the intact model. When over 50% of the facet joint was removed, the ROM in all movements increased rapidly, especially in extension and rotation. When 100% of the facet was removed, the ROM increased by nearly 130%, 80%, 100%, 130% in extension, flexion, lateral flexion, and rotation in comparison to the intact model, which may increase the risk of instability in the spinal motion segment. In a finite element study by Ahuja et al. [2], spinal mobility was found to increase when over 30% of the facet joint was removed. However, Ahuja did not assess the change in ROM in different directions. Other studies [20, 21] have shown that removing sections of the facet joint leads to a greater ROM at the treated site, in particular when the segment in placed in extension, lateral flexion, and rotation. This is consistent with the findings of our study.
In all models implanted with an M-PEEK rod, the ROM in extension was similar to the intact model. This may be due to the polymer rod providing support to the spine. Given that the initial height of the PEEK rod was the same in all models, and the interface between the spinous process and PEEK rod had a defined contact condition, this may explain the similar ROM in all M-PEEK rod models in extension. In a previous interspinous distraction decompression study [12], we found that the M-rod system provided a similar support function at implanted segments. In flexion, bending, and rotation, the ROM in all M-PEEK rod models was similar to the intact model. This may be because the contact region between the PEEK rod and interspinous process acted as a hinge for sagittal rotation and anteroposterior/mediolateral movement of the segment when there was no facet joint to resist such movements. Ahuja et al. [2] found that progressively increasing the amount of the facet joint removed resulted in greater spinal mobility in anteroposterior and mediolateral directions during flexion. Because the M type rod (M-PEEK rod) is a concave shape that can control motion of the segment by interspinous lower anatomic curve resistances and constrains of interspinous ligament.
When 30% of the facet joint was removed in the model without M-PEEK rod fixation, the maximum intradiscal pressure in the treated segments slightly increased during all motions. When over 50% of the facet joint was removed without M-PEEK rod fixation, the maximum intradiscal pressure on the posterior annulus increased significantly in extension. Removing 50% and 100% of the facet joint resulted in a 15% and 32% increase in intradiscal pressure, respectively. Additionally, when 50% of the facet joint was removed, the maximum intradiscal pressure increased by over 20% when the model was placed in rotation. Zeng et al. [1] found that bilateral facetectomy caused a 23.6% and 59.3% increase in intradiscal pressure when the segment was placed in extension and axial rotation, while there was only a slight increase in pressure during flexion and lateral flexion. A finite element study by Li et al. [5] identified stress concentrations at the posterior annulus with large facetectomies. The greater pressure would increase the risk of intervertebral disc degeneration at the operative segments. This study also assessed the decrease in stress on the posterior annulus in the models implanted with an M-PEEK rod when placed in extension and lateral flexion. The results showed that the maximum intradiscal pressure was 40–50% lower in the M-PEEK models then in the intact model, and the initial contact between the L4 spinous process and M-PEEK rod was an important factor for distributing the loading through the posterior part of the transition segment in all loading conditions. In flexion, axial rotation, and lateral flexion, there was little difference in the maximum intra-discal pressure between the M-PEEK models and the intact mode (less than 3% difference in pressure). The M-PEEK rod acted to restrict the ROM of the operative segment, which resulted in the facetectomy models having a maximum intradiscal pressure similar to the intact model.
This study has some limitations. Only a specific segment in the lumbar spine was treated and simulated. In addition, disc degeneration or other physiological conditions were not considered in the facetectomy models, or how these conditions may impact the ROM and stress on the annulus. The screw threads were simplified in our model, so the actual screw stress may be different than calculated. This is acceptable because the stress on the screws was not a focal point of this study and is not expected to impact the findings for the lumbar spine.
Facetectomy with 50% or more of the facet joint removed resulted in a significant increase in ROM and maximum intradiscal pressure at the operated segment. This may accelerate disc degeneration or cause a tear in the annulus fibrosus. Stabilizing the segment with an M-PEEK rod restrained excessive motion at the facetectomy segment, which may provide a certain degree of stability, resulting in intradiscal pressure as close to the intact model as possible. If over 50% facetectomy of the facet joint is performed on a patient, the use of an M-PEEK rod may provide some benefits for segment stability and help prevent excessive loading on the intradiscal pressure.
Not applicable.
YA and HC carried out the study and drafted the manuscript. YA, HC and SL participated in the study design and discussion of the results. YA and SL constructed the testing models, performed the biomechanical analysis. All of the authors read and approved the final manuscript.
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No datasets were generated or analysed during the current study.
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The authors declare no competing interests.
No datasets were generated or analysed during the current study.