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Latent Risk of Visceral Injury in Upper Lumbar Endoscopic Surgery: A Prone-Position Computed Tomography Analysis
Takayuki Kitahara1,2, Takafumi Ohshima1, Naoto Ono1
1Department of Orthopedics, Tokushima University Graduate School, Institute of Biomedical Sciences, Tokushima, Japan.
Background:
Transforaminal full-endoscopic spine surgery (TF-FESS) in the upper lumbar spine poses a risk of visceral injury. Standard supine imaging frequently overestimates the safety margin due to gravity-dependent visceral mobility. This study aimed to quantify the positional discrepancy between supine and prone imaging and to identify predictors of a narrow corridor using prone-position computed tomography (CT).
Methods:
Preoperative prone-position CT images of 57 consecutive patients who underwent TF-FESS at L1/L2 or L2/L3 were retrospectively analyzed. We measured the visceral limit and the nerve root limit. Additionally, a subanalysis of 23 patients compared the safety margins between supine and prone positions. Multivariable linear regression analysis was performed to identify independent predictors determining the width of the safety margin.
Results:
Supine imaging significantly overestimated the safety margin compared with prone imaging (mean difference: 18.0 mm, P < 0.001). Anatomically, the nerve root was located 21.2 mm medial to the visceral organs (P < 0.001), indicating that it is the first obstacle encountered; however, the visceral limit constitutes an inviolable boundary. A critically narrow safety margin (≤50 mm) was observed in 24.6% (14/57) of patients. Multivariable analysis identified lower body mass index (P < 0.001) and a larger entry angle (P = 0.006) as significant independent predictors of a narrow margin.
Conclusions:
Relying on supine imaging poses a significant latent risk because it overestimates the safety corridor by approximately 2 cm. Although the nerve root is the primary anatomical constraint, the retroperitoneal organs represent an absolute danger zone that cannot be manipulated. Preoperative evaluation using prone-position CT is essential to accurately visualize the "actual" proximity of retroperitoneal organs, particularly in lean patients.
Clinical Relevance:
This study underscores the necessity of prone-position imaging for surgical planning in upper lumbar TF-FESS, particularly for patients with low body mass index, to avoid catastrophic visceral injury.
