Abstract
Study Design.
Cadaveric laboratory study.
Objectives.
To characterize and compare intracranial, intradural (cervical, thoracic, lumbar), and lumbar epidural pressure changes during transforaminal and interlaminar endoscopic lumbar procedures under gravity and pump irrigation, with and without simulated outflow occlusion.
Summary of Background Data.
High-pressure irrigation improves visualization and hemostasis during endoscopic lumbar discectomy but may increase spinal canal and intracranial pressures and contribute to neurological complications if not well controlled.
Materials and Methods.
Three fresh-frozen human cadavers were instrumented with pressure probes in the lumbar epidural space (L3–4), intradural space at C5–6, T8–9, and L3–4, and intracranially. At L3–4, transforaminal and interlaminar uniportal approaches were performed using gravity irrigation (50, 100, 130 cm H2O) and two pump systems (30–120 mm Hg; arthroscopy pump and spine-certified pump). Each setting was tested with open outflow and with temporary outflow occlusion by blocking the working channel. Pressure readings were recorded as relative increases (delta mm Hg) from baseline.
Results.
Without occlusion, the interlaminar approach produced higher lumbar epidural pressure increases than the transforaminal approach across all irrigation modalities (up to 10 mm Hg). Differences at intradural levels and intracranially were small (<5 mm Hg). With outflow occlusion, the pressure differential narrowed or reversed, with transforaminal access producing higher lumbar epidural and intradural pressures at higher pump settings. Intracranial pressure differences remained modest, with only small increases under occlusion at higher pump settings.
Conclusions.
Irrigation generates compartment- and approach-specific pressure responses. Interlaminar access shows higher baseline lumbar epidural pressure, whereas transforaminal access is more susceptible to pressure surges during occlusion. Even small intracranial pressure increases may be clinically important in susceptible patients; careful irrigation pressure control and avoidance of prolonged outflow occlusion are recommended.
Key Words: endoscopic lumbar discectomy, irrigation pressure, epidural pressure, intradural pressure, intracranial pressure
Recent advancements in spinal endoscopic surgery, specifically for the treatment of lumbar disc herniation, have emphasized the need to understand the implications of irrigation pressure and associated neurological complications. The transition from traditional open surgeries to minimally invasive endoscopic techniques has brought well-established benefits, including smaller incisions, minimized tissue disruption, reduced bony resection, minimal blood loss, and faster recovery.1,2 Since the introduction of endoscopic spinal surgery in the 1980s, scientific attention remained limited until recent years, and publications have increased rapidly. Endoscopic spinal surgeries employ irrigation systems to maintain clear visualization of the surgical field. Initially, gravity-based systems were widely used, but modern techniques have shifted toward automated pump systems that offer more consistent fluid delivery and better control over surgical compartment distention. Regional preferences differ, with gravity irrigation still predominant in Asia. Automated pumps regulate pressure and adjust flow, improving visualization and hemostasis but posing risks if poorly regulated. The effect of continuous irrigation needed for adequate visibility during endoscopic procedures is mostly unknown. The use of high-pressure irrigation systems has been linked to a variety of postoperative neurological complications, including seizures, visual disturbances, and headaches. It is known that epidural fluid collections usually appear on MRI after endoscopic discectomy, mostly due to accumulation of irrigation fluid, and typically resolves over time without treatment.3 Improper regulation of pump pressure can lead to fluid accumulation in the epidural or intradural spaces through continuous irrigation, possibly due to an undetected dural tear. This fluid buildup can increase intracranial and intradural pressure, which in turn may cause the neurological symptoms mentioned above, along with more severe conditions such as autonomic dysreflexia, subarachnoid hemorrhage, hydrocephalus, cardiac arrhythmias, and cauda equina syndrome.4,5
Therefore, inadequate irrigation pressure control may lead to over-pressurization and complications. Transforaminal and interlaminar approaches are two commonly used techniques to access the epidural space during spinal endoscopy (Figure 1). Approach selection depends on the surgeon’s preference and the pathology location. Recently, a large systematic review and expert consensus proposed a decision-making algorithm for full-endoscopic lumbar discectomy.6 Their algorithm recommends the transforaminal approach primarily for foraminal and extraforaminal disc herniations at both upper and lower lumbar levels, and for central and subarticular herniations at upper lumbar levels. Conversely, the interlaminar approach is preferred for central and subarticular disc herniations at L4/L5 and L5/S1, particularly in the presence of a high iliac crest or high-grade disc migration. Despite widespread use, comparative knowledge of approach-specific epidural and intradural pressure dynamics remains limited.
Figure 1.
Schematic of interlaminar and transforaminal endoscopic lumbar discectomy showing the positions of epidural (red) and intradural (blue) pressure probes relative to each approach. Exact probe locations may vary due to their potential mobility within the epi- or intradural spaces.
Using an experimental model previously described by the authors, we aimed to measure epidural, intradural, and intracranial pressures during transforaminal and interlaminar lumbar endoscopy.7 We compare the resulting pressures during transforaminal and interlaminar endoscopy to propose guidelines regarding the irrigation pump pressures and gravity settings for these two different approaches. We compared pressure responses between approaches to derive guidance for irrigation pump and gravity settings. Given the increasing reliance on algorithm-based corridor selection, understanding approach-specific pressure dynamics is essential for optimizing safety and decision-making.
MATERIALS AND METHODS
The local ethics committee approved the study presented (KEK-ZH-Nr. 2023-01986)). It was conducted in accordance with the Declaration of Helsinki.
Experimental Setup and Endoscopic Technique
The experimental setup was arranged as previously described: pressure measurements were obtained using a multimodal setup combining fluid-filled catheters and solid-state sensors. Intradural pressures were recorded using lumbar catheters (Neuromedex Lumbalkatheter 4.5F; Neuromedex GmbH, Hamburg, Germany) connected to external transducer systems and monitoring units (RAUMEDIC AG, Helmbrechts, Germany). Epidural pressure was measured using a solid-state microsensor (Codman Microsensor, Codman Specialty Surgery; Integra LifeSciences Corp., Princeton, USA), while intracranial pressure was assessed using both an intraventricular catheter (TraumaCath Ventricular Catheter Set, Integra LifeSciences Corp.) and an intraparenchymal probe (Neurovent P, RAUMEDIC AG). All sensors were positioned under imaging guidance and connected to monitoring systems for continuous real-time data acquisition. Before measurement, transducers were zeroed to atmospheric pressure and aligned to the level of the respective sensor tip to ensure accurate hydrostatic referencing. Pressure signals were continuously recorded and digitally stored using a dedicated acquisition platform.7 Three fresh-frozen human cadaveric specimens, including the torso and head, were thawed and instrumented with pressure transducers placed in the lumbar epidural space (L3/4), lateral ventricles (intraventricular) of the brain, brain parenchyma (intraparenchymal), and intradural catheters advanced via a sacral approach to cervical (C5/6), thoracic (T8/9), and lumbar (L3/4) levels, as described above. Methods for data acquisition and sensor calibration followed previously validated protocols.8,9
Endoscopic procedures were performed using transforaminal and interlaminar uniportal approaches at the level L3/4. For the standard transforaminal approach, the endoscope was introduced through the Kambin triangle at the L3/4 level under fluoroscopic guidance, allowing access to the lateral recess and exiting nerve root. In the interlaminar approach, the endoscope was inserted between the laminae at the same level, with the ligamentum flavum split to enter the epidural space and access the central canal. Both approaches were performed under continuous irrigation and endoscopic visualization to ensure accurate placement and anatomic orientation. A senior endoscopic spine surgeon conducted each procedure. Pressure measurements in all five compartments were recorded under three different irrigation conditions: (1) Gravity-based irrigation using the Ringer solution elevated to 50, 100, and 130 cm above the dural sac; and (2) two electronically controlled endoscopic pump systems.
In the gravity setup, hydrostatic pressure was generated passively by positioning the fluid reservoir at predetermined heights corresponding approximately to 37, 74, and 96 mm Hg. Standard endoscopic tubing connected the reservoir to the working channel, with no external modulation of flow or pressure.
The first endoscopic pump system, a multifunctional arthroscopy pump, enabled manual adjustment of inflow pressure, which was tested at intervals from 30 to 120 mm Hg. The selected pressure ranges included both commonly used intraoperative settings and higher values to simulate worst-case conditions, such as impaired outflow or transient occlusion. This system provided continuous, nonpulsatile irrigation but was used only in inflow mode during experiments. The second pump, developed specifically for spinal endoscopy, offered a narrower adjustable pressure range from 30 to 100 mm Hg and likewise operated solely in inflow mode during testing.
For each pressure level and system, two irrigation conditions were tested: (1) Continuous flow with an open outflow channel; and (2) maximal occlusion achieved by manually sealing the outflow to simulate obstructed fluid egress. This occlusion maneuver allowed for controlled hydrostatic pressure buildup and the assessment of pressure differentials under impaired drainage conditions.
After stabilization of the irrigation system, baseline pressures were recorded in all compartments without fluid flow to eliminate transient fluctuations. Relative pressure changes (Δ pressure, mm Hg) induced by each irrigation modality were then measured. All protocols were repeated using both the transforaminal and interlaminar approaches to allow direct comparison of pressure dynamics between the two access routes.
All pressure readings under irrigation were reported as relative changes (Δ pressure, mm Hg) from the respective baseline. These values reflect the net pressure increase induced by each system and irrigation condition. Both transforaminal and interlaminar access routes were tested, with and without occlusion, to assess pressure differentials between the foraminal and intralaminar (Δ interlaminar–transforaminal) endoscopy in each configuration. Absolute pressures were recorded in all compartments. However, because baseline values varied between cadaveric specimens and compartments, intercondition comparisons were primarily performed using relative pressure changes (Δ pressure), allowing normalization across specimens and a more robust comparison between approaches.
RESULTS
All reported values represent pressure differences (Δ interlaminar–transforaminal) rather than absolute pressures.
Effects on Spinal Compartment Pressures Between Approaches
The comparison of irrigation pressure differences between the interlaminar and transforaminal approaches (Δ interlaminar–transforaminal) revealed approach-dependent variations that were influenced by spinal level, irrigation system, and pressure setting (Figure 2). Overall, the interlaminar approach resulted in generally higher pressures than the transforaminal approach, with the most pronounced differences observed at the lumbar epidural level. Differences at intradural and cranial levels remained low.
Figure 2.
Difference between interlaminar and transforaminal approach (Δ interlaminar–transforaminal): lumbar epidural, lumbar intradural, thoracic, and cervical absolute pressure differences across irrigation systems (positive values mean higher interlaminar pressure, negative values mean higher transforaminal pressure).
Gravity-Based Irrigation
Under gravity-assisted irrigation, the interlaminar approach produced consistently positive Δ values, indicating higher pressures compared with the transforaminal approach. The figure explicitly displays the difference (Δ), with pronounced deviations (Δ>5 mm Hg) occurring only at the L3–4 epidural level, reaching up to ~10 mm Hg. At cervical (C5–6), thoracic (T8–9), and lumbar intradural (L3–4) sites, differences remained minimal (<5 mm Hg).
System I (Arthroscopic Pump-Controlled Irrigation)
Across all tested pressures (30–120 mm Hg), Δ values remained consistently higher at all spinal levels. Clinically relevant differences were confined to the L3–4 epidural level, showing elevations up to 10 mm Hg. At other sites, including the cervical, thoracic, and intradural lumbar levels, Δ values remained below 5 mm Hg.
System II (Spine-Certified Pump-Controlled Irrigation)
System II yielded similar findings compared with system I. The interlaminar approach consistently generated higher Δ values than the transforaminal approach. Relevant differences were limited to the L3–4 epidural site, with median pressure elevations of up to 8 mm Hg across the pressure range (30–120 mm Hg). At the other levels, including C5–6, T8–9, and L3–4 intradural, pressure differences remained small.
Effects of Occlusion Between Approaches
The pressure differential between interlaminar and transforaminal approaches was significantly influenced by the presence or absence of occlusion during irrigation. This effect was most pronounced at the lumbar (L3–4) epidural site and became increasingly evident with rising irrigation pressures across all systems (Figure 3).
Figure 3.
Difference between interlaminar and transforaminal approach (Δ interlaminar–transforaminal) with and without occlusion: effects on lumbar epidural, lumbar intradural, thoracic, cervical, and intracranial absolute pressure differences.
Under no occlusion conditions (solid-colored bars), interlaminar approaches consistently produced higher pressures than transforaminal approaches across all measured compartments and spinal levels. This difference was most prominent in the L3–4 epidural space, where Δ pressure reached up to ~10 mm Hg, particularly under high irrigation pressures (130 cm H2O in gravity; 120 mm Hg in systems I and 100 mm Hg in system II). Cervical (C5–6) and thoracic (T8–9) levels, and intracranial pressure (ICP), showed smaller and more consistent Δ values, generally in the range of 2 to 5 mm Hg.
In contrast, under occlusion conditions (bars with white fill), the pressure differential was substantially reduced or even reversed. Specifically, at higher irrigation pressures in systems I and II, the L3–4 epidural and intradural compartments exhibited negative Δ pressure values, indicating higher pressures during transforaminal access compared with interlaminar access. This inversion was especially apparent at 100 and 120 mm Hg, where L3–4 Δ pressure values dropped below −10 mm Hg in some specimens. Similar but less pronounced reversals were observed in thoracic and cervical measurements.
Intracranial pressure (ICP) remained relatively stable, showing minimal differences between approaches under both occlusion conditions.
Intracranial Pressure Differences With and Without Occlusion
In this analysis of ICP changes during spinal endoscopy, pressure differences (Δ interlaminar–transforaminal) were examined under various irrigation systems, pressures of irrigation fluid, and conditions (occlusion vs. no occlusion) (Figure 4). Across most settings, pressure differences were small (<5 mm Hg), with no consistent or significant elevation favoring either the interlaminar or transforaminal approach, except under specific conditions described below.
Figure 4.
Difference between interlaminar and transforaminal approach (Δ interlaminar–transforaminal): Effects on absolute intracranial pressure differences with and without occlusion across irrigation systems.
Gravity-Based Irrigation
Under gravity-driven irrigation, the pressure difference remained below 5 mm Hg and was not relevant, regardless of occlusion status.
System I
With system I, the interlaminar and transforaminal approaches generated nearly identical ICPs across all tested pressures (30–120 mm Hg). Across both occlusion and nonocclusion conditions, no significant pressure differences were observed at any pressure level.
System II
System II revealed greater variability in pressure differences when irrigation was performed at higher pressure, particularly under occlusion. Especially at higher pressures (80–100 mm Hg), the transforaminal approach resulted in higher ICP than the interlaminar approach, with negative Δ values. These differences approached but did not exceed 5 mm Hg. Under nonocclusion conditions, no consistent trend was observed.
DISCUSSION
The present study investigated the pressure dynamics associated with two approaches in lumbar endoscopic spine surgery—interlaminar and transforaminal—under varying irrigation modalities and pressure conditions, with a focus on pressure translation across spinal and intracranial compartments. Our results provide novel insights into how approach, irrigation system, and occlusion influence compartmental pressures, including potentially clinically relevant elevations in the lumbar epidural space and ICP. The observed pressure changes should be interpreted in the context of known physiological conditions. While intracranial and intradural pressure alterations in this cadaveric model remained limited, more pronounced changes were observed in the epidural compartment under certain irrigation conditions. However, direct translation of these findings to clinical thresholds is limited by the absence of physiological regulation, including cerebrospinal fluid dynamics and autoregulatory mechanisms. Therefore, the present study focuses on relative pressure changes, which provide a more robust basis for comparison across experimental conditions than absolute values in a nonphysiological model.
Approach-Dependent Pressure Differences
Our findings show that pressure differences (Δ interlaminar–transforaminal) are consistently positive, indicating higher pressures during the interlaminar approach, particularly at the epidural level of surgery. This held true across all irrigation systems and pressure settings, including gravity-fed and electronically regulated pump systems. These differences, while generally modest (<10 mm Hg), were reproducible and approach-dependent, suggesting that anatomic and procedural characteristics intrinsic to each approach significantly influence local pressure dynamics. This may be due to differences in outflow resistance, surgical exposure geometry, or anatomic constraints that affect the dispersion and egress of irrigation.
One possible explanation for the higher epidural pressures observed during the interlaminar approach is that this technique may allow for a more centralized and symmetrical distribution of irrigation fluid within the spinal canal, resulting in more uniform and effective pressure propagation across both sides of the epidural space. In addition, the interlaminar access point is topographically closer to the midline and dorsal epidural space, which could further facilitate pressure accumulation in this region.
While epidural pressure differences between approaches reached clinically relevant thresholds in some configurations, intradural and intracranial pressures remained largely comparable between both accesses, with differences <5 mm Hg. This suggests that, in the absence of dural compromise, such as a dural tear or compression of the dural sac caused by spinal canal stenosis, intrathecal compartments may be relatively well protected from the impact of irrigation-induced pressure changes during spinal endoscopy, regardless of the approach.
Importantly, the epidural pressure measured in cadaveric specimens may underestimate in vivo values. In living patients, the epidural venous plexus is engorged with blood and occupies more space within the epidural compartment. This vascular fullness can reduce the compliance of the epidural space, potentially amplifying the pressure rise during irrigation. Therefore, pressures encountered in clinical settings may exceed those recorded in this study.
Effect of Irrigation System and Pressure Settings
Both pump systems tested (nonspine-specified system I and spine-certified system II) produced progressive increases in compartmental pressure with rising irrigation pressure, as expected. However, system II demonstrated greater variability in pressure profiles under occlusion, especially in the ICP domain, raising concerns about predictability and regulation of pressure transmission in certain pump designs.
Gravity-fed systems, by contrast, provided more consistent and generally lower pressure differences, respectively elevations, albeit potentially at the cost of less effective visualization in clinical scenarios requiring robust hemostasis or tissue distension.
Influence of Occlusion
Occlusion of the endoscope’s outflow port, simulating impaired irrigation regress, had a profound impact on pressure transmission, reversing the usual pressure hierarchy between interlaminar and transforaminal access. Under these conditions, transforaminal access generated higher lumbar epidural and even intradural pressures, particularly at high pump settings. This effect likely reflects localized fluid entrapment and pressure buildup due to outflow restriction.
The presence of occlusion flattened or inverted pressure gradients not only locally but also at distant levels along the spinal axis. This suggests a potential for retrograde pressure transmission when fluid dynamics are dysregulated, particularly under high irrigation flow and obstructed outflow. The finding that pressure inversion occurred predominantly at lumbar intradural and epidural compartments under occlusion underscores the critical need for pressure monitoring and controlled irrigation, particularly during transforaminal procedures in anatomically narrow or obstructed foramina. The occlusion condition represents a controlled worst-case scenario rather than a routine intraoperative situation. However, transient or partial obstruction of outflow may occur during surgical manipulation or hemostasis, making these findings clinically relevant.
Intracranial Pressure and Safety Considerations
ICP remained relatively stable across most conditions and did not exhibit approach-dependent elevation under normal irrigation settings. However, system II under occlusion and high-pressure conditions induced a difference in ICP of up to 6 mm Hg between the interlaminar and transforaminal approaches, with higher values during transforaminal access. Although modest, such increases—especially if a dural tear is invisible or masked by fluid irrigation—may be clinically significant, particularly in vulnerable populations (e.g., patients with impaired cerebrospinal fluid dynamics, hydrocephalus, or reduced intracranial compliance). Importantly, these experiments were conducted in the absence of known dural compromise, such as a dural tear or spinal canal stenosis, suggesting that inadvertent or unrecognized dural tears in vivo could exacerbate these pressure elevations even further. A cadaveric study found that ICP rises above 20 mm Hg, which is known as clinically relevantly elevated, in the presence of a dural tear in lumbar spinal endoscopy in the case of occlusion, even at a pump pressure as low as 40 mm Hg.10
Given that even subclinical elevations in ICP have been associated with postoperative complications such as headaches, visual disturbances, or seizures,5,11,12 our findings support the routine use of moderate irrigation pressures and vigilant monitoring during endoscopic spinal surgery, especially when using occlusive instruments or operating in anatomically constrained environments.
Clinical Implications and Recommendations
Although approach-dependent differences were observed, their magnitude remained modest compared with the overall pressure elevations induced by irrigation and occlusion. Therefore, the present findings primarily improve understanding of irrigation-related pressure dynamics rather than providing definitive guidance on surgical approach selection. Nonetheless, some clinical implications and recommendations can be formulated regarding the surgical approach.
Interlaminar approaches may inherently carry a higher risk for local pressure buildup; thus, irrigation pressure should be carefully titrated, especially when operating near dural margins or in patients with compromised CSF circulation.
Transforaminal approaches, while generally associated with lower pressures under standard flow, may pose a higher risk for pressure elevation under occlusion, particularly when high-pressure pump settings are used. Instruments that obstruct the outflow should be used with caution, and irrigation pressure should be kept as low as possible.
Patient anatomy may warrant a tailored decision-making process regarding the surgical approach. Selecting the transforaminal route in patients with already compromised epidural compliance could mitigate the risk of pressure-related complications, although this must be balanced against the technical requirements of the decompression and surgical target.
Further, pump systems specifically designed for spinal endoscopy may require additional safety features or pressure-limiting algorithms to prevent unanticipated ICP elevation.
The minimal pressure transmission observed in the cranial compartment under most conditions suggests a degree of protective compartmentalization, though vigilance remains warranted under extreme or prolonged irrigation conditions.
Limitations
Our study is limited by its cadaveric design, which, although controlled and reproducible, does not account for dynamic CSF circulation, tissue compliance, and autoregulatory mechanisms present in vivo. Measurements were made in the absence of dural tears; extrapolation to such cases should be made with caution. The order of procedures may have introduced a systematic bias: All specimens underwent the transforaminal approach before the interlaminar approach. This sequencing could potentially confound the results, for example, by altering residual pressure conditions within the epidural space.
Another methodological limitation lies in the different types of pressure measurement systems used. The epidural pressure was measured using a pressure inducer, whereas the intradural pressure was assessed using a lumbar catheter with a fluid-filled column. This discrepancy introduces a nonlinear relationship between epidural and intradural pressure measurements. Finally, the small sample size, though sufficient for trend identification, precludes definitive statistical comparisons across all permutations of pressure, system, and occlusion state, and limits generalizability and increases susceptibility to inter-specimen variability.
CONCLUSION
This study demonstrates that endoscopic approach, irrigation modality, and occlusion status significantly influence spinal and intracranial pressure dynamics during lumbar endoscopy.
The interlaminar approach consistently produces higher epidural pressure differences, whereas occlusion can shift pressure elevation toward the transforaminal access, particularly at high irrigation pressures. Although intracranial pressure changes remain modest, vigilance is warranted, especially when high-pressure irrigation is combined with occlusive instruments. These findings support rational irrigation protocols, pressure monitoring, and approach-specific safety strategies to reduce neurological risk during endoscopic spine surgery.
Key Points
Irrigation during endoscopic lumbar surgery produces approach-specific pressure changes within spinal compartments.
The interlaminar approach generates higher baseline lumbar epidural pressures than the transforaminal approach.
Outflow occlusion can reverse pressure gradients and induce higher pressures during transforaminal access.
Intradural and intracranial pressure changes remain small under most conditions.
Careful irrigation pressure control is essential to minimize neurological risk.
Footnotes
The device(s)/drug(s) is/are FDA-approved or approved by the corresponding national agency for this indication.
V.H.and M.F. contributed equally and share senior authorship.
M.F. reports consulting fees, stock ownership, and/or advisory roles with the following companies: Arthrex, Medacta, and Zurimed (Balgrist University Startup), Incremed (Balgrist University Startup), Moving Spine (Balgrist University Startup), Artonis (Balgrist University Startup), and DePuy Synthes. J.W. reports stock ownership in Moving Spine. V.H. consults for Arthrex and Nexon. The remaining authors declare no conflicts of interest.
Contributor Information
Alexandra Stauffer, Email: alexandra.v.stauffer@gmail.com.
Jana F. Schader, Email: jana.schader@balgrist.ch.
Carl M. Zipser, Email: carl.zipser@balgrist.ch.
Najmeh Kheram, Email: Najmeh.Kheram@balgrist.ch.
Marie-Rosa Fasser, Email: marie-rosa.fasser@hest.ethz.ch.
José M. Spirig, Email: jose.spirig@balgrist.ch.
Jonas Widmer, Email: jonas.widmer@balgrist.ch.
Vincent Hagel, Email: vhagel@yahoo.de.
Mazda Farshad, Email: mazda.farshad@balgrist.ch.
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