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This paper presents the design and characterization of a microelectromechanical system (MEMS)-based intraoperative monitoring system for improving lumbar surgery safety. A MEMS pressure-sensing module is designed and incorporated into a nerve root retractor tip to directly monitor pressure exerted on a nerve root during a lumbar surgery. Animal experiments are conducted for intraoperative pressure monitoring to investigate effects of nerve root retraction during a surgery. Amplitude and latency of electrophysiological response of a nerve root are measured during different time intervals after retraction under various retraction magnitude and duration conditions. Correlation between exerted pressure on a retracted nerve root and its electrophysiological response is investigated. The relationship between intraoperative pressure and alteration of neural tissue structure is analyzed by morphological observation. Experimental results indicate that a nerve root injury is strongly related to the magnitude and duration of its retraction. The prototype MEMS-based intraoperative monitoring system can potentially alert surgeons about risk factors associated with nerve root injury during a lumbar surgery as well as provide critical surgical guidelines. The system can also serve as a basis for implementing an intelligent robotically controlled closed-loop lumbar surgical operating system in the future.