Evaluation of the efficacy of intraoperative neurophysiological monitoring in determining cage height in transforaminal lumbar interbody fusion surgery

Background and aim: Transforaminal lumbar interbody fusion (TLIF) is a commonly used fusion technique in the treatment of degenerative spine diseases with less traction of neural structures, good clinical outcome and low complication rates. Motor-evoked potentials (MEPs) from intraoperative neurophysiological monitoring (IONM) modalities provide instant information on changes in the corticospinal pathway and have been used for a long time in spine surgery. Although cage height used in TLIF plays a key role in determining disc height, objective criteria for determining cage height have not been determined. It is not always possible to determine the disc height or to reach the desired disc height. Achieving the ideal disc height is important for ensuring the width of the lumbar lordosis and foraminal area which are the main targets of TLIF. We think that changes in MEP values during surgery can be used to determine the adequacy of the width of the foraminal area. Our aim is to determine the changes in radiomorphological measurements after IONM-guided TLIF operations and to investigate the relationship between changes in MEP amplitudes and foraminal area, height and cage height. Patients and methods: Between 2017 and 2019, 111 patients who underwent TLIF operation in the Orthopedics and Traumatology Department of Istanbul University Faculty of Medicine were identified. Among the 65 patients who were applied IONM, 22 patients with IONM recordings, preoperative and postoperative radiographs including lumbosacral computed tomography (CT) / magnetic resonance imaging (MRI) including IONM recordings, preoperatively and postoperatively were included in the study. The age, sex, diagnosis, TLIF applied levels, MEP amplitudes before the incision at the beginning of the operation, after insertion of pedicle screws during operation, and after the placement of the cage with TLIF were recorded and changes were calculated using correction factors such as age, sex, weight and duration of anesthesia. Preoperative and postoperative anterior-middle-posterior disc heights, foraminal area and heights of the decompressed side and opposite sides, and lumbar lordosis, segmental lordosis, segmental disc, sacral slope, pelvic incidence, pelvic tilt angles, and GAP scores were calculated. The changes in radiomorphological parameters after TLIF, the relationship between the cage heights and the changes in radiomorphological parameters with the changes in MEP amplitudes were analyzed with appropriate statistical methods. Results: A total of 22 patients, 13.6% (n=3) male and 86.3% (n=19) female, were included in the study. The mean age of the patients was 61,9±8,89 (37-78) years. TLIF was performed from one level in 10 patients and from 2 levels in 12 patients, most of them from L4-L5 (n=19). The mean heights of the 34 cages were 10.26±1.37 (8-14) mm. The change in MEP values (between measurements after the pedicle screws were placed and after the cage was placed) was 27.27±11.09% (15%-50%). TLIF increased anterior-middle and posterior disc heights of 2.02±1.55 mm, 2.73±1.68 mm and 1.67±1.25 mm, respectively. The increase in the middle disc height was found to be significantly higher (p<0.05) and the posterior disc height increased relatively less. Foraminal areas and heights were increased in both sides of the decompression and opposite sides. There was no correlation between cage heights or disc heights and MEP, but there was a positive correlation between MEP change and foraminal area change of decompressed side (r=0.501; p: 0.007; p <0.01). According to the calculated GAP scores (mean 3.36±3.39), the spinopelvic status of 13 patients was evaluated as proportional, 4 patients as moderately disproportionate and 5 patients as severely disproportionate. According to the GAP score, the targeted LLA mean of the patients was calculated as 63.96°±7.51 °, whereas the mean LLA measured postoperatively was 55.22°±9.63°. Conclusion and Discussion: General characteristics of our patients such as age, gender, reason and levels of TLIF application are consistent with the literature. The main objectives of TLIF are; foraminal area width, disc height, fusion between adjacent vertebrae, contributing to the provision of lumbar and segmental lordosis, and reducing the loading on posterior implants. Ensuring disc heights directly contributes to the correction of lumbar lordosis. In all our patients, anterior-middle and posterior disc heights were increased, and the relatively less increase in posterior disc height contributed to lumbar lordosis. There was no correlation between cage and disc heights and MEP change, however there was a positive correlation between postoperative foraminal area width and MEP change. MEP values can provide information about the adequacy of the width of the foraminal area during the operation. Calculation of preoperative spinopelvic parameters (PI, SS, PT) in TLIF is important to reach the desired LLA during the operation. The goal is to establish a balance (±10°) between LLA and PI. In our study, LLA, SLA, SDA and SS increased and PT decreased. There was a negligible change in PI, and technical errors were attributed. Although the balance between PI and LLA is generally achieved, there is a difference from the ideal LLA calculated according to GAP scores.