Understanding femoral nerve injuries during extreme lateral lumbar interbody fusion using transabdominal muscle action potential and an event-based protocol

OBJECTIVE: Extreme lateral lumbar interbody fusion (XLIF) carries a significant risk of femoral nerve injury, particularly at L4-5, where neural structures are close to the transpsoas corridor. While neuromonitoring techniques such as triggered electromyography and somatosensory evoked potentials are widely used, they lack the real-time sensitivity and specificity necessary for reliably detecting nerve compromise. The absence of real-time neuromonitoring data in XLIF has limited the understanding of when and how femoral nerve injuries occur, leaving surgeons without a clear strategy to mitigate these injuries intraoperatively. The aim of this study was to evaluate the reliability of detecting nerve injury during XLIF using transabdominal muscle action potential (TMAP) monitoring with an event-based protocol. METHODS: A retrospective analysis of consecutive patients who underwent single-position prone XLIF from 2020 to 2024 was conducted. Patients with lateral implants placed in the femoral nerve distribution (L2-5) were included. TMAP monitoring was performed intraoperatively at predefined procedural steps to monitor nerve integrity. TMAP changes were correlated with postoperative quadriceps motor deficits to determine the timing of injury. Sensitivity, specificity, and predictive thresholds of TMAP were also analyzed. RESULTS: One hundred sixty-one patients (mean age 67.9 ± 9.8 years, BMI 30.7 ± 5.6 kg/m2) were included; 63.4% of patients were female, and 82.6% underwent XLIF at L4-5. Uninterrupted TMAP monitoring was achieved in all cases. Postoperative quadriceps muscle weakness (≤ 3/5 strength) occurred in 4 patients (2.5%), with no significant difference in retractor times between injured and uninjured patients. TMAP monitoring detected all injuries intraoperatively, demonstrating 100% sensitivity. Threshold analysis identified a change in TMAPs of 400-500 mA as clinically significant for the development of a postoperative neurological change (p < 0.01). Specificity at this threshold change ranged from 83.4% to 89.8% while maintaining 100% sensitivity. The event-based protocol revealed that injuries were linked to specific procedural steps, with the docking phase posing a high risk for femoral nerve injury. CONCLUSIONS: This study provides new insights into femoral nerve injuries incurred during XLIF, demonstrating that nerve compromise is more strongly associated with specific procedural steps rather than prolonged retractor time. TMAP monitoring, integrated with an event-based protocol, enabled real-time identification of nerve compromise, revealing that direct mechanical trauma was the primary mechanism of femoral nerve injury in XLIF.