Water Dynamics in Unilateral Biportal Endoscopic Spine Surgery and Its Related Factors: An In Vivo Proportional Regression and Proficiency-Matched Study

Stable water dynamics during endoscopic spine surgery improves the surgeon's comfort and patient's outcomes. We aimed to measure the water dynamics during spinal surgery and identify the factors that facilitate stable water dynamics. This open-label, prospective, proficiency-matched, in vivo study included patients with single-level degenerative spinal disease. After assessing their heights and balancing the matched instrument, we measured the irrigation fluid pressure in various situations. We performed multiple regression analysis based on odds ratio (OR), confidence interval (CI), and relationships (proficiency-matched) with possible instrumental and physical characteristics. The instrumental factors were the presence and length of a rigid cannulation, and the physical characteristics were body mass index (body mass index [BMI]), skin-to-dura depth, height (interaction with BMI), and body weight (interaction with BMI). Of the 36 patients, 29 were included. The mean pressure of the operation cavity was 16.66 ± 9.12 cm H2O (12.25 ± 6.71 mm Hg). Water pressure with the rigid cannulation (9.41 ± 2.94 cm H2O [6.92 ± 2.16 mm Hg]) was significantly lower than that without cannulation (23.43 ± 7.57 cm H2O [17.26 ± 5.57 mm Hg], P < 0.01). Water pressure correlated with cannular length (OR = −1.08, CI = −1.79, −0.37, P < 0.01) and BMI (OR = 0.56, CI = 0.12, 0.99, P < 0.01). BMI showed a proportional relationship (r = 0.84, P < 0.01). During biportal endoscopy, we suggest maintaining water pressure between 4.41 cm H2O (2.41 mm Hg) and 31.00 cm H2O (22.83 mm Hg). Compared to physical characteristics, placement of the cannula and appropriate cannula length are important factors that affect water dynamics.