Real-Time Water Pressure Monitoring in Unilateral Biportal Endoscopic Spine Surgery
Article information
Abstract
Objective
Unilateral biportal endoscopic (UBE) spine surgery is a minimally invasive technique that uses continuous irrigation to improve visualization and control bleeding. Effective water pressure management is crucial for patient safety, particularly at the cervical and thoracic levels where spinal cord injury risk is higher. However, real-time pressure monitoring remains underexplored. This study evaluates the impact of real-time water pressure monitoring on safety during UBE surgery.
Methods
A prospective study was conducted involving 20 patients undergoing UBE lumbar spine surgery. Patients were divided into 2 groups based on the irrigation system: gravity-based or infusion pump. Real-time water pressure was monitored using a digital sensor throughout surgery. Each procedure was categorized into 3 phases: phase I, working space preparation; phase II, laminectomy; phase III, flavectomy, dura exposure, and discectomy. Data was analyzed according to the type of irrigation system and surgical phase.
Results
The mean water pressure in the surgical field during UBE spine surgery was 17.98±8.07 mmHg, with no significant differences between surgical phases. However, the infusion pump system maintained significantly lower mean pressure (12.10±3.51 mmHg) compared to the gravity-based system (23.86±6.97 mmHg, p=0.001). The infusion pump system consistently maintained a significantly lower mean water pressure compared to the gravity-based system.
Conclusion
Real-time water pressure monitoring during UBE surgery enhances safety by enabling improved control of pressure within the surgical field. Both the gravity-based and infusion pump systems safely maintained working space pressure, with the pump system showing significantly lower pressure levels.
INTRODUCTION
Unilateral biportal endoscopic (UBE) spine surgery was first introduced by Dr. De Antoni in 1996 [1]. Since the 2010s, UBE spine surgery has undergone significant advancements and has become widely adopted. Unlike conventional microscopic open spinal surgery, UBE used continuous saline irrigation to maintain a clear surgical field and control bleeding. Thus, UBE spine surgery is considered a water-based procedure. Effective water pressure management is critical for successful surgery, as it maintains visibility by clearing blood and debris. However, without proper maintenance of outflow, water pressure within the surgical field can rise excessively, potentially leading to complications such as muscle edema, ascites, and compressive neural injury [2,3].
Despite the recognized importance of water pressure in UBE spine surgery, the real-time measurement of working space pressure has yet to be precisely evaluated and reported. Hong et al. [4] conducted an in vivo study to evaluate water dynamics during UBE spine surgery; however, they did not measure real-time working space pressure. Other studies have indirectly estimated irrigation pressure or have focused on changes in cerebrospinal fluid (CSF) pressure rather than on the actual working space environment. However, understanding real-time pressure dynamics is crucial for optimizing surgical safety, particularly as UBE applications expand into the cervical and thoracic spine, where the spinal cord occupies most of the intradural space and is more susceptible to external compression.
The aim of this study is to evaluate real-time working space pressure during UBE spine surgery, with the goal of generating evidence that can enhance surgical safety. By investigating pressure dynamics in real time, we hope to establish a reliable method for maintaining optimal working space conditions, safeguarding the spinal cord and surrounding neural structures, and minimizing the risk of pressure-related complications during surgery.
MATERIALS AND METHODS
This study was approved by the Institutional Review Board of the Yeungnam University Hospital (YUMC 2022-06-004), and informed consent was obtained from all participants.
1. Study Design
This study was conducted prospectively. Twenty consecutive patients who underwent one-level UBE surgery performed by a single surgeon at a university hospital spine center between June 2022 and June 2023 were enrolled. The enrolled patients underwent 1-level unilateral laminectomy and bilateral decompression or discectomy using the UBE technique. Two irrigation systems were used: a gravity-based system and an infusion pump system. The gravity-based irrigation system consisted of 3,000 mL of sodium chloride solution suspended 50 cm above the operative field. This height setting was based on standard recommendations described in UBE surgical textbook [5]. The irrigation fluid management system 10K (ConMed, USA) utilized a pressurecontrolled automated pump, with the inflow pressure set at 30 mmHg (Fig. 1). The 2 systems were alternated between patients to ensure equal distribution among groups. We used a tubular working cannula and a semitubular retractor at the working portal to facilitate fluid outflow.
The pressure-controlled infusion pump system is used in unilateral biportal endoscopic spine surgery. The system maintains a constant inflow pressure set at 30 mmHg, ensuring controlled irrigation during the procedure.
2. Real-Time Water Pressure Monitoring
Real-time water pressure within the working space was measured using a digital pressure monitoring system. A Codman Microsensor intracranial pressure (ICP) transducer, originally designed for measuring ICP, was employed for this purpose (Fig. 2). The Codman Microsensor is a well-established tool, with numerous studies confirming its reliability, stability, and accuracy in ICP monitoring [6]. The Codman Microsensor ICP transducer consists of a miniature strain gauge pressure sensor encased in titanium at the tip of a 100 cm, 3F flexible nylon tube. For real-time pressure monitoring, the nylon tube was inserted through the endoscopic portal alongside the endoscope (Fig. 3). The sensor tip of the Codman Microsensor ICP transducer was positioned within the working space to measure pressure changes in real time (Fig. 4). The Microsensor was moved together with the endoscope during surgery, maintaining proximity to the working space. This setup enabled continuous monitoring of working space water pressure throughout the surgical procedure (Supplementary Video Clip 1). The procedure was divided into 3 distinct phases based on surgical workflow: phase I involved creating surgical portals and detaching the paraspinal muscles to establish the working space; phase II included performing a laminectomy using a high-speed drill and Kerrison punch; and phase III focused on flavectomy and discectomy, culminating in exposure of the dura.
The Codman Microsensor intracranial pressure (ICP) transducer used for pressure measurement. This device features a miniature strain gauge pressure sensor encased in titanium at the tip of a 100 cm, 3F flexible nylon tube, originally designed for ICP monitoring.
Real-time pressure monitoring using the Codman Microsensor intracranial pressure transducer. The nylon tube was inserted through the endoscopic portal alongside the endoscope to measure intraoperative pressure during unilateral biportal endoscopic spine surgery.
Intraoperative view showing the tip of the Codman Microsensor intracranial pressure transducer positioned within the working space.
All surgeries in this study were performed by a single surgeon with experience in more than 100 cases of UBE spine surgery, ensuring consistency in surgical technique and intraoperative conditions.
3. Statistical Analysis
The mean and standard deviation were calculated for continuous variables, and comparisons were performed using the paired t-test. Differences between phases were assessed using 1-way analysis of variance (ANOVA). Statistical analysis was conducted using IBM SPSS Statistics ver. 29.0 (IBM Co., USA), and a p-value of less than 0.05 was considered statistically significant.
RESULTS
1. Demographics
The demographic and clinical characteristics of the study population are summarized in Table 1. A total of 20 patients were included, with a mean age of 53.35±19.10 years (range, 19–87 years). The cohort consisted of 14 males and 6 females, with a mean body mass index of 24.47±4.07 kg/m². Primary diagnoses included herniated lumbar disc in 10 patients, spinal stenosis in 6 patients, epidural abscess in 3 patients, and facet cyst in 1 patient. The most frequently treated level was L4–5 (14 patients), followed by L5–S1 (4 patients) and L3–4 (2 patients). The mean operation time was 64.5±7.05 minutes. Of the procedures performed, 13 were conducted on the left side and 7 on the right. There were no intraoperative complications related to real-time pressure monitoring, and most patients demonstrated favorable postoperative outcomes.
Demographic and clinical characteristics of the study population
2. Water Pressure in Working Space
The mean real-time water pressure in the working space during the procedures was 17.98±8.07 mmHg (Table 2). When analyzed according to surgical phase, the mean pressure was 19.77±9.55 mmHg during phase I (primary working space preparation), 18.15±8.65 mmHg during phase II (laminectomy), and 17.43±7.88 mmHg during phase III (flavectomy, discectomy, and dura exposure). ANOVA revealed no statistically significant differences in water pressure among the 3 phases (F=0.378, p=0.687), indicating consistent pressure control throughout the surgical workflow (Table 3).
Real-time water pressure (mmHg) in working space
Analysis of variance results comparing 3 phases
The infusion pump system demonstrated significantly lower pressure than the gravity-based irrigation system during all procedural phases. The mean pressure with the infusion pump was 12.10±3.51 mmHg, significantly lower than the pressure of 23.86±6.97 mmHg observed with gravity-based irrigation (p=0.001). When analyzed by surgical phases, the infusion pump consistently maintained lower pressure values. In phase I, the mean pressure was 12.20±3.11 mmHg, compared to 27.35±7.44 mmHg with gravity-based irrigation (p=0.001). In phase II, the pressure with the infusion pump was 12.13±4.39 mmHg, significantly lower than 24.17±7.62 mmHg with gravity-based irrigation (p=0.001). Similarly, in phase III, the infusion pump maintained a mean pressure of 11.96±3.53 mmHg, compared to 22.91±7.20 mmHg with gravity-based irrigation (p=0.001) (Table 4).
Comparison of water pressure among different infusion types
DISCUSSION
This study investigated real-time working space pressure during UBE spine surgery, providing direct measurements across different surgical phases. Several previous studies have assessed water pressure indirectly or examined cervical epidural pressure; however, none have directly measured working space pressure during UBE spine surgery [4,7,8]. Our findings show that the mean working space pressure was 17.98±8.07 mmHg, with no significant differences observed among the 3 surgical phases. Additionally, the infusion pump system maintained significantly lower pressure levels compared to gravity-based irrigation, providing improved irrigation control throughout the procedure. This study provides valuable insights into the pressure dynamics within the working space and their variability across surgical phases. Furthermore, our results suggest that real-time monitoring of water pressure may be a reliable indicator of safe surgical conditions, particularly for cervical and thoracic level UBE spine surgeries, where precise pressure control is essential to prevent complications. These findings contribute to an improved understanding of pressure management in UBE spine surgery and the potential for expanded indications while maintaining procedural safety.
As UBE surgical techniques continue to advance, their application is expanding to include cervical and thoracic spine lesions. Unlike the lumbar region, these areas are more susceptible to external pressure due to the spinal cord occupying most of the intradural space, necessitating careful pressure management to avoid complications. In our study, the average working space pressure was approximately 18 mmHg, independent of the input system used. Hong et al. [4] reported a mean working space pressure of approximately 12 mmHg, measured using a water column manometer and gravity-based irrigation system during UBE spine surgery. This value closely aligns with our findings, further supporting the consistency and reproducibility of water pressure dynamics associated with this surgical approach. While the water column manometer system is widely regarded as the gold standard for pressure measurement due to its high absolute accuracy, reliable zeroing capability, and minimal measurement drift, it is limited in its ability to provide continuous real-time data during surgery. In contrast, microtransducer-based systems such as the Codman Microsensor offer the distinct advantage of continuous real-time pressure monitoring, despite minor drift over time. Given the intraoperative context of this study, where real-time pressure changes are a critical checkpoint, the Codman Microsensor was selected as the most appropriate monitoring tool. This choice enabled dynamic assessment of pressure variations throughout the procedure something that would not have been possible with a conventional manometer system. The 95% reference interval for lumbar CSF opening pressure is 10 to 25 cm CSF (3.67–18.38 mmHg) [9]. Considering these established values, the water pressure experienced during UBE spine surgery does not exceed physiological intradural pressures, suggesting a minimal risk of significant compression or associated adverse effects during the procedure.
In this study, there was no significant variation in working space pressure across the 3 surgical phases. Notably, despite employing a high-speed drill in phase II, no significant elevation in working space pressure was observed. During UBE surgery, effective regulation of pressure outflow is crucial to maintaining a stable working environment. In our procedures, we used a tubular working cannula during phases I and II to prevent surrounding soft tissues from interfering with or wrapping around the drill. In phase III, we transitioned to a semitubular retractor, which allowed effective nerve root retraction while maintaining controlled irrigation and stable pressure outflow. Similarly, Hong et al. [4] emphasized the critical role of using a rigid cannula during endoscopic spine surgery to ensure efficient outflow of irrigation fluid and maintain stable water pressure dynamics. Their findings showed that the presence of a rigid cannula significantly reduced intraprocedural water pressure compared to procedures without a cannula, thereby underscoring its importance in achieving optimal surgical conditions. Our study emphasizes the necessity of employing devices that allow consistent outflow and controlled water pressure in the operative space. The choice of a specific device, such as a tubular cannula or semitubular retractor, can be adapted to personal preference and surgical technique, providing flexibility while ensuring optimal surgical conditions.
In our study, the infusion pump system maintained significantly lower water pressures compared to the gravity-based irrigation system across all phases of UBE spine surgery. This result highlights the advantage of using an infusion pump system for intraoperative management of water pressure. The observed stability in the infusion pump system is likely attributable to its capacity to adjust flow rates dynamically in response to changes in working space resistance. Specifically, when the infusion pump system is programmed to a target pressure, such as 30 mmHg, it adjusts flow to maintain that pressure by responding to resistance within the working space. When resistance increases, the pump slows the flow rate. This passive response contributes to a more stable and controlled intraoperative environment. In contrast, the gravity-based irrigation system lacks this adaptive control capability, relying solely on the fixed height of the saline bag to generate constant pressure. Consequently, even when resistance is encountered in the surgical field, the gravity system delivers saline at an unchanged rate, potentially causing fluctuating or elevated pressures. Thus, the ability of the infusion pump system to maintain lower water pressure could help mitigate risks associated with excessive or fluctuating pressures during endoscopic spine procedures.
Although several studies have examined the effects and safety of working space pressure on cervical epidural pressure changes during UBE spine surgery, no studies have assessed real-time pressure changes in the working space during actual surgical conditions. Monitoring real-time pressure in the working space during UBE spine surgery can greatly enhance surgical stability and safety. If the working space pressure increases intraoperatively, the outflow status can be verified and the pressure appropriately adjusted by reducing inflow. Our intention was not to assert the superiority of infusion pump systems over gravitybased irrigation systems, but rather to evaluate the pressure dynamics of both under typical surgical conditions. While a comparison between the 2 irrigation methods was included, the primary aim of this study was to characterize the working space pressure during UBE spine surgery using real-time monitoring—regardless of the irrigation method employed. The results demonstrated that both irrigation systems can effectively maintain safe and stable pressure within the surgical field when used appropriately. The infusion pump system showed slightly better numerical consistency and lower working space pressure; however, both systems remained within a clinically acceptable and safe range. These findings support the feasibility and safety of UBE spine surgery even for cervical and thoracic lesions at the spinal cord level, as the working pressures did not significantly exceed physiological intradural pressure levels.
This study had several limitations. First, the sample size included in the analysis was relatively small, potentially limiting the generalizability of the findings. A larger sample size would be needed to validate these results across diverse patient populations and various surgical conditions. Second, the height of the gravity-based irrigation system (50 cmH₂O≒37 mmHg) was numerically higher than the set pressure of the infusion pump (30 mmHg), which may have influenced the results. Nevertheless, our findings showed that the infusion pump system provided slightly more stable and consistent pressure readings, with lower peak fluctuations compared to the gravity system. Third, the study did not account for certain variables that could influence working space pressure, including patient-related factors, specific surgical techniques, and surgeon experience. For example, we did not specifically measure the vertical distance from the skin to the lamina in each patient, which may influence outflow resistance and pressure dynamics. However, all surgeries were performed with careful attention to maintaining adequate water outflow, and efforts were made to minimize variability by using a tubular cannula and a semitubular retractor. Future research should incorporate these factors to achieve a more comprehensive understanding of the relationship between working space pressure and surgical outcomes.
CONCLUSION
In this study, we performed real-time pressure monitoring during UBE spine surgery and presented the results. Both the gravity-based irrigation system and the infusion pump system were able to safely maintain the working space pressure. The infusion pump system demonstrated significantly lower working space pressures compared to the gravity-based irrigation system. Its ability to dynamically adjust flow rates according to working space resistance helps maintain consistent pressure, thereby reducing the risk of sudden pressure fluctuations that may compromise surgical safety.
Supplementary Material
Supplementary Video Clip 1 is available at https://doi.org/10.14245/ns.2550648.324.
Real-time pressure monitoring setup displaying continuous measurement of the working space water pressure alongside the surgical monitor throughout the procedure.
Notes
Conflict of Interest
The authors have nothing to disclose.
Funding/Support
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No 2021R1G1A1092922).
Author Contribution
Conceptualization: DY, IJ, SWK; Data curation: DY; Formal analysis: DY; Funding acquisition: DY; Methodology: DY, IJ; Project administration: IJ, SWK; Visualization: DY; Writing – original draft: DY; Writing – review & editing: DY, SWK.