Multimodal intraoperative neuromonitoring during total hip arthroplasty in severe hip deformities: how do we do it and when do the alerts occur?

Total Hip Arthroplasty (THA) with leg lengthening can offer great outcome in patients with severe hip deformities but comes with higher risk of intra-operative nerve injury. The aim was to investigate the relationship between surgical steps and multimodal intraoperative neuromonitoring (IONM) events during THA in severe hip deformities. This cohort study of 27 cases with symptomatic severe hip deformities, such as Crowe 3 and 4 developmental dysplasia of the hip (DDH). All cases received THA with IONM between September 2019 and December 2023. The goal of surgery in all cases was to reestablish the original center of rotation. Monitored modalities were Transcranial Motor Evoked Potentials (TcMEPs), Somatosensory Evoked Potentials (SSEPs) and free-running Electromyography (EMG). Alarm criteria were a TcMEP amplitude loss of 80%, neurotonic discharges on the free-running EMG and/or significant SSEP amplitude loss. Follow-up took place during hospital stay and at the follow-up visit 8 weeks postoperatively. In 16 of 27 cases we found IONM signal alerts that correlated with several surgical events (in total 22 events): surgical exposure/hip dislocation (12 events), acetabular reconstruction/cementing (4 events) and trial reduction (6 events). Actions taken were removal or replacement of instruments, neutralization of the leg, screw replacement of the massive autograft (2 events) and extensive subtrochanteric osteotomy (4 events). In 7 cases the IONM alerts concerned solely sciatic innervated muscles, in 2 cases solely femoral and in 7 cases both sciatic and femoral innervated muscles. In 14 cases normalization of IONM signals occurred spontaneously or after corrective maneuvers, in 2 cases no normalization occurred during surgery. No case showed a postoperative neurological deficit at follow up. Multimodal IONM in THA for severe hip deformities revealed numerous specific surgical steps leading to IONM signal alerts. In a majority of cases adjustment of the surgical procedure could normalize the IONM signals. None of our patients experienced a postoperative neurological deficit.