Biomechanical analysis of lumbar decompression technique and the effect on spinal instability: a narrative review

Introduction

Lumbar laminectomy represents the third most common elective surgery performed in the United States annually with a greater incidence than spinal fusion (1). Isolated decompression surgeries are often performed to treat lumbar degenerative conditions, in particular lumbar spinal stenosis and herniated nucleus pulposus without associated instability or spinal deformity. However, in certain patient populations, decompression only procedures may compromise postoperative outcomes through the development of iatrogenic instability leading to new or recurrent neural foraminal compression (2-5). Ultimately, operative revision with conversion to fusion could be necessary to ameliorate recurring symptoms and restore spinal stability (3,4). However, the degree of iatrogenic destabilization in lumbar decompressive surgery is highly dependent on the patient anatomy and surgical technique, specifically the extent of posterior elements disruption and facet resection (5-7).

Relative to the cervical spine and thoracic spine, the lumbar spine experiences a higher baseline compressive load necessary to support the body weight and is responsible for the majority of physiologic lordosis necessary for maintenance of global sagittal balance (8,9). The increased sagittal orientation of lumbar facets facilitates greater flexion and extension mobility with more limited rotational mobility (10,11). The native lumbar spine generally exhibits a high tolerance to shear disc space forces of up to 1,000 Newtons (N) to resist spondylolisthesis (12). Numerous biomechanical cadaveric and finite element analysis (FEA) models have been developed to study the differences in lumbar segmental motion in flexion, extension, lateral bending, and axial rotation pre- and post-decompressive surgery under physiologic loading parameters. Investigations have compared the relative segmental motion changes after standard laminectomy (bilateral pedicle to pedicle decompression with removal of the lamina and underlying ligamentum flavum), laminotomy (a bilateral keyhole decompression with preservation of midline osseous and ligamentous structures removing the inferior margin of the superolateral lamina and superior marginal of the inferolateral lamina, which is often utilizes in endoscopic approaches) and facetectomy [unilateral of bilateral removal of the superior level inferior articulating process and inferior level superior articulating process (SAP) for complete decompression of the exiting nerve root]. Furthermore, FEA has elucidated the impact of decompressive procedures on intradiscal and facet joint stress. Emerging evidence has attempted to quantify how the extent of pars interarticularis (PI) and facet joint resection or underlying lumbar degenerative changes affect segmental range of motion (ROM) and intradiscal and facet joint stresses. This review study is the first to synthesize the biomechanical effects of decompression technique in relationship to lumbar segmental stability and potential progression to iatrogenic spondylolisthesis. We present this article in accordance with the Narrative Review reporting checklist (available at https://amj.amegroups.com/article/view/10.21037/amj-23-148/rc).

Methods

This narrative review included all biomechanical studies of the lumbar spine that focused on post-decompression segmental ROM and stiffness, intradiscal pressure and strains, and stresses of the disc, facet joint, and pars relative to that of the native preoperative lumbar spine. FEA and cadaveric models of the lumbar spine were both included with preference towards human cadavers, but large animal models were also permissible. The decompression procedures analyzed included unilateral or bilateral lumbar laminotomy, laminectomy, facetectomy, or foraminotomy. Investigations were analyzed according to the type of decompression performed, whether the surgery was unilateral or bilateral, minimally invasive or open approach, the number of operative levels, and the presence or absence of degeneration in the spinal segment. Potential instability associated with novel decompression techniques were included when appropriate but was not the primary objective of this review. The effect of spinal instrumentation or fusion on spinal motion and stresses was not included as this was out of the scope of the current review on post-decompression iatrogenic instability. Additional exclusion criteria included cervical or thoracic spinal models provided the uniqueness of lumbar biomechanics, studies that did not measure spinal instability outcomes pre- and post-decompression, and investigations that were purely clinical and did not include biomechanical modeling. The specific search terms and inclusion and exclusion criteria are further described in Table 1 and Figure 1.

Figure 1 Flow chart illustrating the study selection process for the narrative review on 35 biomechanical studies of the lumbar spine from 1990 until 2022. ROM, range of motion; FEA, finite element analysis.

Etiology of and risk factors for spondylolisthesis

In the native spine, the integrity of the posterior ligamentous complex, including the facet capsule, ligamentum flavum, interspinous ligament, and supraspinous ligament, and the posterior bony elements are critical to spinal stability and resistance against flexion, rotational, and shear forces (13). Native spinal instability can occur secondary to traumatic, congenital, degenerative, pathologic, and isthmic etiologies, as a result of discogenic and facetogenic changes and PI fracture among many additional causes. Central and neural foraminal compression may occur as a result of translational forces about the disc space, often requiring operative fusion when severe or progressive (14,15). However, iatrogenic spondylolisthesis, also termed post-laminectomy instability, can also occur in individuals both with and without preoperative spinal instability who undergo isolated decompressive of the posterior spinal elements (4). Biomechanical instability, in this context, refers to aberrant movement of a spinal segment due to decreased physiologic constraint resulting in increased junctional ROM or altered stress distribution under standard multidirectional loading parameters.

Laminectomy procedures resulting in disruption of the spinal extensor mechanism may increase flexion and rotational instability that likely scales with extent of bony resection, ligamentous detachment, and paraspinal disruption (2,4). Over resection of the PI can predispose to postoperative pars fracture (16). Likewise, medial facetectomy causes disruption to facet joint forces resulting in potentially increased facet joint capsular distraction, decreased mechanical constraint in end range motion, and decreased resistance to shear forces (2). Overall, the L4–5 level is most commonly affected by iatrogenic spondylolisthesis which reflects not only the high prevalence of underlying central stenosis but also the relative sagittal facet orientation of this spinal segment. Apart from surgical technique, the degree of iatrogenic instability is impacted by individual spinal anatomy where decreased PI width, narrower and more sagittally oriented facets, increased disc space height, and preoperative segmental translation have been previously identified as possible risk factors (5,7,17,18). Furthermore, the potential impact of paraspinal muscle size and fatty infiltration, facet and disc space degenerative changes, and segmental and global alignment on post-laminectomy instability is yet to be explored in clinical literature. The variable rates of iatrogenic spondylolisthesis in the lumbar spine literature, ranging from 1.6% to 32%, underscores the heterogeneity of patient populations with respect to anatomic risk factors and decompression technique (2). Two early retrospective investigations of wide laminectomy found that 43% (26/61) and 32% (10/31) of patients developed spondylolisthesis in the postoperative setting (19,20). Furthermore, in the largest existing prospective study of decompression for lumbar spinal stenosis, 10% (40/417) required reoperation for the development of recurrent stenosis and/or progressive spondylolisthesis at extended follow-up (21). This uncertainty in clinical literature has inspired biomechanical analyses to further elucidate the relative contributions of patient and operative factors to post decompression spinal stability through controlled parameter cadaveric and FEA investigations.

Description of reviewed biomechanical models

Cadaveric testing involves the use of custom-designed setups and potting techniques to maintain lumbar spine alignment and emulate in vivo conditions. In contrast, FEA is a sophisticated computational technique that breaks down complex structures, like the human spine, into smaller, manageable elements. This allows for detailed analysis of the mechanical behavior and stress distribution within the spine under various conditions. FEA creates two-dimensional and three-dimensional (2D and 3D) models based on high-resolution computed tomography (CT) scans, which can be validated against cadaveric studies for accuracy. Overall, the process of model development involves generating the 3D geometric surface of the lumbar spine using specialized image processing software. Material and tissue properties can be obtained from literature standards and subsequently, solid models and FEA meshes are created to be analyzed (22). Regarding loading parameters, cadaveric testing and FEA studies involved applying diverse loading conditions—axial compression, flexion, extension, lateral bending, and axial rotation—to evaluate lumbar segment stability. Common outcome measurements in both methods include ROM and intradiscal pressure. In cadaveric studies, additional measurements, such as neutral zone (NZ; defined as the zone of minimal resistance as the spine moves away from its neutral position), stiffness, and failure loads provided insights into lumbar stability post-decompression, while in FEA von Mises stress of the annulus fibrosis and facet joints are used to map critical material deformation thresholds (23). Importantly, FEA, unlike cadaveric studies, allows for the customization of anatomical geometry, material properties, and loading conditions for each individual segment within multiple lumbar levels of interest, providing insight into segmental interactions (24). While cadaveric studies can more realistically and directly represent in vivo conditions and validate FEA models, discrepancies stemming from variations in cadaveric specimens’ age, presence or absence of underlying spinal pathology, health status, experimental setups, and testing protocols complicate direct comparisons between studies.

Sample selection and preparation are pivotal for addressing variabilities among cadaveric models. Factors such as the extent of degeneration, specimen age, sex, or spinal level can significantly impact results (25). Additionally, nonhuman specimens are sometimes employed to mitigate variability, although this introduces anatomical and tissue property disparities. While human cadaveric spines were seen as the norm for lumbar decompression procedure studies, alternate specimens like porcine and calf have been used for studying the effects of unilateral versus bilateral laminotomy (26-28).

In addition to specimen selection, clinical relevance requires proper specimen preparation and experimental design to minimize discrepancies between in vitro and in vivo conditions. Many studies used fresh frozen cadaveric specimens to retain mechanical properties and minimize the effect of mechanisms like autolysis and freeze-thaw cycles to detrimentally impact tissue properties. The general cadaveric setup included utilizing custom-designed testing fixtures that incorporated hydraulic or pneumatic actuators, direct current motors, and motion analysis systems. Pressure sensors were also used to measure intradiscal pressure for various decompression procedures. These fixtures facilitate precise and controlled loading of axial compression, flexion, extension, lateral bending, and axial rotation to the cadaveric spine specimens. This setup ensures that the spine remained in its anatomically aligned position during compression and moment arm testing. Likewise, in FEA the caudal vertebral level is fixed in 3D space with loading conditions supplied to the top of the cranial most vertebrae. In both systems, 200–500 N axial compressive preloads are commonly applied to simulate baseline disc and joint stresses under body weight. After which, 4–10 Newton meter (Nm) moment arms are applied to mimic physiologic directional loading in everyday activities, such as bending or lifting (25).

Despite many benefits, cadaveric and FEA systems possess limitations, notably the absence of dynamic muscle forces and long-term healing responses, as well as isolated lumbar segment testing’s inability to capture overall spinal compensatory mechanisms (22). Pertinently, it should be recognized that the scope of analysis, including the number of levels encompassed in both cadaveric and FEA models, bears influence on the outcomes and the capacity to evaluate the stresses experienced by adjacent segments both above and below the decompression area. Furthermore, FEA is subject to model assumptions and simplifications that impact accuracy. Notable variations exist between FEA model types, emphasizing virtual construction techniques and included parameters, which, along with variations in age and health status of cadaveric specimens, can impact biomechanical properties and hinder result comparisons.

Effect of lumbar laminectomy on spinal kinematics

Lumbar laminectomy represents the standard of care for central stenosis with neurogenic claudication. In the absence of preoperative spondylolisthesis or neuroforaminal symptoms requiring operative facetectomy, laminectomy is often performed as an isolated decompression procedure without concomitate fusion. However, the extent of destabilization after isolated laminectomy remains difficult to discern in clinical models due to variable patient populations and surgical techniques. In this review, comparing biomechanical study of intact spinal models to simulated single-level bilateral laminectomy demonstrated significant postoperative increase in ROM in all movement directions that were most pronounced in flexion and axial rotation. Provided the significant heterogeneity in model characteristics and loading conditions, percent increases in spinal motion after single-level laminectomy ranged from 7–84% in flexion and 12–133% in axial rotation (Table 2) (27,29-33). Spinal extension (7.3–35%) and lateral bending (4.6–14%) were also significantly affected, although to a lesser extent (26,29,31-33). In cadaveric investigation of single-level laminectomy, the changes in NZ motion were found to be greater than ROM with up to a 175% increased NZ motion in spinal flexion. Likewise, intradiscal pressure in the anterior annulus was found to increase 130% after single-level laminectomy, which underscores the potential postoperative spinal flexion instability (26). Special testing of spinal torsion demonstrated a greater than 30% significant increase in early and late torsional stiffness after single-level laminectomy, likely reflecting the increased rotational mobility (30). However, in other testing of spinal stiffness, there were no significant differences in spinal flexion and extension stiffness after single-level laminectomy (Table 2) (32).

In a FEA model directly comparing unilateral and bilateral single-level laminectomy, the percentage ROM increase after unilateral laminectomy was less than half that of bilateral laminectomy in all physiologic loading directions and less than 1/10th the ROM specifically in spinal flexion (34). Post-unilateral laminectomy, the overall magnitude of ROM and segmental stress change was generally minor (<5%) with the exception of facet joint forces which increased more than 20% (35). While there are no existing studies that have directly compared spinal mobility changes after single-level versus multiple level laminectomy, indirect evidence from investigations with similar experimental design suggests that flexion/ extension mobility is up to two times greater after multi-segmental laminectomy (36-39). Specifically, in two L3–5 laminectomy studies, the upper L3–4 level was associated with greater ROM (maximum increase: 300% L3–4 vs. 99% L4–5), however, annular stress was maximized at the lower L4–5 segment (stress increase: L4–5 260% vs. L3–4 50%) (38,39).

Mechanical instability of lumbar laminotomy compared to laminectomy

Unilateral or bilateral laminotomy, known as a keyhole decompression, may be performed as a minimally invasive alternative to traditional laminectomy in scenarios such as discectomy where wide posterior decompression is not necessary (40). While laminotomy relative to laminectomy does preserve the midline structures and posterior tension band, the potential post-operative stability benefit is a subject of ongoing research. Cadaveric investigations have demonstrated that single- and multi-level lumbar laminotomies are significantly less destabilizing when compared to full, facet-sparing, laminectomies in the same region (Table 3) (27,31,36,38,39,41,42). In particular, in a three-level simulated cadaveric decompression, there was a 100% increase between post-laminotomy and post-laminectomy ROM (36). There is, however, conflicting evidence on whether single-level post-laminotomy ROM changes are significant relative to the intact model. In the reviewed studies with significant changes, axial rotation was generally the direction of greatest post-laminotomy instability (ROM increase: 12–47%) (31,41,42). Lateral bending was least affected by single-level laminotomy with less than a 10% post-procedural ROM change across investigations (36,41-43). For unilateral laminotomy the increase in rotational motion was most pronounced for rotation in the direction of decompression (31,39,41,44). In limited direct comparisons, there were no significant ROM differences between unilateral and bilateral laminotomy regardless of whether bilateral decompression was performed (31,39,44). Similar to the isolated laminectomy studies, the post-laminotomy maximum increase in NZ motion was greater than the maximum increase in ROM for all physiologic directions (41). Furthermore, there were no significant differences in post-laminotomy spinal stiffness (36,43).

The role of facetectomy on lumbar spinal instability

While partial facetectomy is often required to achieve adequate decompression of lateral recess and neuroforaminal stenosis, the extent of lumbar facet resection that is tolerated without necessitating concomitant spinal fusion is a subject of ongoing debate. Clinical gestalt commonly describes a 50% facet resection threshold as the upper boundary for performing isolated decompression surgeries. However, there is a lack of high-quality evidence to support this threshold and potentially important patient factors of facet size, orientation, and tropism and facet and disc degenerative underlying changes remain largely unaccounted for. As such, this review investigates the biomechanical effects of varying grades of both unilateral and bilateral medial facet resection in wide laminectomy and lateral facet resection in foraminotomy. Currently, the role of facet orientation and facet tropism on postoperative instability is yet to be characterized in the biomechanical literature.

In three studies modeling foraminotomy or foraminoplasty with endoscopic techniques, total unilateral resection of the lateral aspect of the SAP had the greatest effect on intradiscal pressure (6–11% increase with pre-load) and spinal rotation in the ipsilateral (maximum 83% increase) and contralateral (maximum 39% increase) directions (45-47). Lateral facet resection of less than 30% and basement foraminoplasty procedures resulted in minimal changes in ROM in each direction and intradiscal pressures (both <5% change) that were not significant (45-47). Regardless of the extent of simulated endoscopic facet resection, superior adjacent segment mobility and intradiscal pressures and strain were largely unaltered (Table 4) (45,46).

Ten cadaveric and FEA studies modeling single-level unilateral and bilateral medial facet resection were reviewed with variable results summarized in Table 4 (24,35,42,44,48-52). In subgroup analysis of biomechanical models, single-level unilateral and bilateral medial facetectomy of 25% or less had minimal effect on post-operative ROM (<10% change in all directions), intradiscal pressure and stress, and anterior-posterior translation with shear stress (Table 4) (48,50). Medial 50% facetectomy also generally resulted in minimal post-procedure ROM or annular stress changes, however unilateral 50% facetectomy did increase contralateral rotation and contralateral facet joint forces 20% and 25%, respectively (48-50). Furthermore, 75% unilateral or bilateral medial facetectomy was found to be the threshold for substantial increase in segmental torsional motion and translation of the above SAP (up to 100%) (48). Single-level total unilateral facetectomy resulted in less than 20% increase in ROM but notably impacted contralateral facet joint forces (108% increase in extension) and discal properties including 11% increase in intradiscal pressure, 24% increase in annular stress in contralateral rotation, and approximately 20% increase in anterior shear (24,35,48,52,53). Whereas single-level total bilateral facetectomy altered segmental motion with a 24–40% increase in flexion/extension and 100–131% increase in bilateral axial rotation ROM (35,44,51). Single-level total bilateral facetectomy had a relatively larger impact on discal behavior in extension and axial rotation where simulated post-procedure intradiscal pressure increased 24% and annular stress increased 59% (42,49).

Overall, the results of multi-level partial and total medial facetectomy biomechanical studies exhibited similar trends to those of the aforementioned single-level procedures (Table 4) (37,54,55). Analogous to the single-level facetectomy findings, 30% unilateral and bilateral multi-level medial facetectomy altered multidirectional segmental motion less than 10% (54). However, unlike the single-level graded facetectomy subgroups, 45% medial facet joint excision threshold increased flexion extension mobility and intradiscal pressure up to 30% (54). In total facetectomy, multi-level relative to single-level procedures demonstrated similar effect on ROM (maximum 140% increase in rotation) but a more profound effect on intradiscal pressure with up to a 70% increase post-procedure (37,54,55). In a FEA investigation directly comparing the directionality of multi-level unilateral versus bilateral total facetectomy instability, when testing in flexion and extension, unilateral facetectomy had greater medial-lateral mobility compared to bilateral facetectomy with greater anterior-posterior mobility (54).

The effect PI resection on lumbar spinal instability

Similar to the etiology of spondylotic spondylolisthesis, there is concern that partial intraoperative resection of the PI during spinal decompression may predispose to iatrogenic PI fracture and resultant segmental listhesis. However, there is ongoing uncertainty regarding the amount of medial or lateral PI resection that is permissible based on individual PI anatomy and the cortical yield point in physiologic loading conditions. In a FEA simulating graded unilateral medial PI resection with laminectomy, 50% or more PI resection significantly increased PI stresses in all loading directions and 25% PI resection significantly increased PI stresses in flexion and rotation (16). Specifically, in the 75% PI resection group, PI stresses increased up to 585% in flexion and 946% in rotation which approached the yield strength of cortical bone, signaling the likelihood of PI fracture with repeat loading (16). Two additional FEA studies modeled unilateral lateral PI resection with and without concomitant posterior decompression and found that bending in the ipsilateral direction and bilateral rotation had the greatest impact on PI stress (both up to 100% increase) (53,56). Furthermore, the magnitude of PI stress after removal of the lateral pars increased notably with lower lumbar position (L4 vs. L3) and simultaneous laminotomy and medial facetectomy (Table 5) (56). However, the existing biomechanical studies do not account for the patient-level variability in PI anatomy.

The role of degenerative changes on native and post-decompression spinal mechanics

Divergent outcomes have emerged from studies probing the relationship between degeneration severity and lumbar ROM and stiffness, with some reporting reduced ROM across all planes as degeneration worsens and others describing an incremental rise in ROM culminating in a plateau phase, resembling the Kirkaldy-Wilis model (57,58). Spinal stiffness is often found to decrease during early degeneration stages, followed by an increase during advanced degeneration due to proteoglycan loss, desiccation, decreased viscoelastic properties, osteophyte formation, and ligament hypertrophy and ossification (25). However, in select patient populations the altered load distribution associated disc and facet degeneration may lead to segmental hypermobility and overloading of adjacent segments (59). Furthermore, the effect of degenerative changes on spinal mobility may depend on the axis of loading where spinal stiffness was shown to increase during flexion, extension, and lateral bending but may not change notably or can even decline during axial rotation (24,35,57).

Degeneration is often modeled in FEA studies by reduction of disc height (40–80% based on severity), alteration of nucleus area, and modifying the elasticity modulus (2:1 ratio) and Poisson’s ratio (0.48) of the nucleus pulposus and annulus to simulate reduced compressibility (34,60-63). Specifically, in moderate disc degeneration models flexion/extension and lateral bending motion reduced four and two degrees, respectively, compared to analogous non-degenerative models (34,60). These effects regarding increased segmental mobility and increased shear and flexural strength were further pronounced in severe degeneration (64,65). However, the baseline annular stresses were larger for all loading conditions in degenerative relative to non-degenerative intact spinal models (maximum 78% increase in spinal flexion) (Table 6) (24,34,35,60). Of note, the evidence from these degenerative FEA models is limited by factors not accounted for in the simulation including, such as changes in fluid dynamics, biochemical composition within the disc, patient-specific variability, and biomechanical adaptations and along with their validation against limited experimental data.

In four FEA studies directly comparing the changes in post-laminectomy spinal stability across degenerative and non-degenerative models, the magnitude of changes in ROM were consistently lower in degenerative models (24,34,35,60). In simulated bilateral facetectomy there was a maximum 20% difference in postoperative rotational mobility changes between degenerative and non-degenerative models (degenerative: 80% change, non-degenerative: 100% change) (35). Likewise, in a bilateral laminectomy model there was up to a 45% difference in magnitude of post-decompression flexion changes (severe degenerative: 15% change, non-degenerative: 60% change) (34). Overall, the post-decompression change in annular stress was also decreased in degenerative versus non-degenerative models with a 15–20% lower magnitude post-laminectomy change in spinal flexion (34,60). However, after bilateral facetectomy the increase in annular stress during only axial rotation was approximately 10% more in the degenerative models (degenerative: ~50% change, non-degenerative: ~40% change) (Table 6) (35).

Summary

Laminectomy resulted in significant ROM increases in all directions, which were generally maximized in flexion and axial rotation and increased with the number of procedural levels. Conversely, both unilateral and bilateral laminotomy models exhibited minimal effects on spinal stiffness and spinal motion in flexion, extension, and lateral bending. Laminotomy did have a notable effect on rotational mobility, however there was a 100% increase between post-laminotomy, and post-laminectomy ROM. In single-level partial facetectomy, 50–75% medial facet resection was found to be the threshold for substantial increase in rotation and translation of the disc and SAP. Whereas in multi-level partial facetectomy, medial facet resection of 45% notably increased segmental mobility and intradiscal pressures. Considering PI resection, 25% pars removal during simulated laminectomy significantly increased postoperative pars stress and 75% pars removal approached the yield stress of cortical bone. Lastly, while the effects of spinal degeneration on segmental mobility are complex, the percentage increase in post-decompression spinal instability was generally decreased in degenerative relative to non-degenerative lumbar models.

Limitations

This review, while comprehensive in its analysis of biomechanical changes following lumbar laminectomy, laminotomy, and facetectomy, does encounter several limitations inherent in the studies analyzed. One of the primary limitations is the heterogeneity in model characteristics and loading conditions. The biomechanical studies utilized a variety of models, including cadaveric specimens and FEA models. This diversity leads to a wide range of results and complicates direct comparisons and generalizations. Additionally, the variability in surgical techniques across the studies might have impacted the outcomes, limiting the ability to draw uniform conclusions about the effects of specific surgical procedures on spinal kinematics.

Another significant limitation is the replication of complex spinal dynamics in these models. While cadaveric and FEA models are invaluable for understanding biomechanical principles, they may not fully replicate the dynamic physiological conditions of the whole living human spine. Factors such as muscle activity, live neurological feedback, and individual variations in spinal anatomy and pathology are difficult to simulate. This limitation underscores the challenge in correlating biomechanical findings with clinical outcomes, as the studies focus primarily on mechanical aspects without fully accounting for patient-specific factors such as pain, functional status, or long-term clinical stability.

Furthermore, many biomechanical models do not account for variables such as changes in fluid dynamics, biochemical composition within the disc, and patient-specific variability in spinal anatomy. These omissions can limit the applicability of the findings to the general population. In the context of facetectomy studies, there is a lack of high-quality evidence supporting specific thresholds for facet resection. Factors like facet size, orientation, and degenerative changes are often not comprehensively addressed in the reviewed studies.

The studies on PI resection also come with inherent limitations. They often do not consider individual variability in anatomy, which could limit their applicability to a wider patient population. Moreover, while some studies incorporate models of degeneration, there remains a need for more nuanced understanding of how degenerative changes impact post-decompression spinal mechanics. This is particularly important given the variability in degenerative patterns among individuals.

A crucial point to consider is that these studies predominantly focus on relative rather than absolute instability. This means they examine changes in spinal stability compared to a preoperative or baseline state, rather than assessing a clinical state of spinal failure under physiological loads that leads to symptoms such as pain or neurological dysfunction. This focus on relative instability might not fully capture the clinical implications of spinal decompressive procedures, emphasizing the need for careful interpretation of these biomechanical changes in the context of clinical practice.

Lastly, our review methodology, including preferences for human cadaveric models and exclusion of cervical or thoracic spinal models, introduces its own set of limitations, such as potential selection bias, and limits the scope of our review. This exclusion might overlook some relevant biomechanical dynamics that could be applicable to the lumbar spine. Overall, while the findings of the studies reviewed provide valuable insights into the biomechanical impacts of various decompressive procedures, the limitations outlined above highlight the need for cautious interpretation when applying these results to clinical scenarios.

Conclusions

Biomechanical simulation of lumbar spinal decompression without fusion demonstrates greater post-procedural instability in laminectomy relative to laminotomy, bilateral compared to unilateral surgeries, multi-level compared to single-level surgeries, medial facet resection of 45–50% or more, and pars resection of 25% or more. In the future, patient-specific FEA models that are individualized to facet and pars anatomy, spinal alignment, and degenerative changes may enable more reliable preoperative prediction of post-decompression spinal stability and identify cases with increased likelihood of iatrogenic spondylolisthesis that would benefit from concomitant fusion. Furthermore, subsequent research should aim to determine the biomechanical thresholds of increased segmental motion, intradiscal pressure, and bone stresses that are predictive clinically relevant instability.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Mark Lambrechts and Brian Karamian) for the series “Degenerative Spine Disease” published in AME Medical Journal. The article has undergone external peer review.

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://amj.amegroups.com/article/view/10.21037/amj-23-148/rc

Peer Review File: Available at https://amj.amegroups.com/article/view/10.21037/amj-23-148/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://amj.amegroups.com/article/view/10.21037/amj-23-148/coif). The series “Degenerative Spine Disease” was commissioned by the editorial office without any funding or sponsorship. A.N.N. reports no conflicts of interest relevant to the manuscript content (disclose leadership of fiduciary role in AO spine, CSRS, LSRS, AOA, SRS, and Techniques in Orthopaedics; receipt of materials from Pfizer, premia, and AO spine). B.A.F. reports no conflicts of interest relevant to the manuscript content (disclose consulting fees from synthes, theradaptive, Medtronic, and kuros; payment or Honoria from AO spine; Stock or stock options from clear choice and neuroinnovations; and receipts of materials from ankasa). A.S.S. reports no conflicts of interest relevant to the manuscript content (disclose royalities or licensces with CTL Amedica and Jaypee; consulting fees from cerapedics, Depuy Synthes/J&J). The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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