Bicortical S1 Screw Fixation May Obviate the Need for Iliac Screws in Minimally Invasive Surgery for Adult Spinal Deformity

Abstract

Introduction Minimally invasive surgery (MIS) allows for the maintenance of the spine's soft-tissue envelope, preserving many of its stabilizing structures. Lumbosacral stability increases when S1 screws are placed in a bicortical fashion. MIS techniques, along with bicortical S1 screw placement, may obviate the need for iliac screw fixation in the treatment of adult spinal deformity (ASD). Methods A retrospective review from 2009 to 2014 included all patients treated for ASD using MIS techniques. Lateral interbody fusion (LIF) was performed for all lumbar interbody fusion levels L1–L5. Transforaminal lumbar interbody fusion (TLIF) was performed at all L5–S1 levels. Patients were divided into two groups. Group 1 (Hybrid) had open pedicle screws with bilateral iliac screw fixation. Group 2 (MIS) had percutaneous pedicle screws without iliac screws. All S1 pedicle screws were placed in a bicortical fashion in both groups. CT scans were obtained at 1-year postoperation. Two independent board-certified radiologists assessed fusion. Results Twenty-seven patients met the inclusion criteria and were at least 1 year out from surgery. Average follow-up was 2 years. Group 1 included 11 patients (38 levels); Group 2 included 16 patients (52 levels). There was no difference in age, height, weight, BMI, number of lateral levels fused, and pre- or postoperative Cobb angles between the two groups. There was a statistical difference in the mean length of the posterior construct between Group 1 and Group 2 (10 vs. 6 levels, p < 0.05), mean blood loss (1,727 vs. 465 mL, p < 0.001), mean operative times (505 vs. 329 minutes, p < 0.001), number of complications (8 [72%] vs. 5 [31%] patients, p = 0.03), and mean length of stay (14 vs. 7.6 days, p = < 0.01). CT scans were available for 25 of the 27 patients and both radiologists agreed there was solid fusion at all interbody levels including L5–S1 (100% fusion rate). Two patients, one from each group, had full-length scoliosis X-rays that demonstrated fusion, without evidence of hardware failure, radiolucency, migration or sacral fracture. Conclusion MIS techniques preserve many of the spine's stabilizing structures. Additional stability is observed when S1 screws are placed bicortically and may obviate the need for iliac screw fixation. These techniques resulted in a 100% interbody fusion rate without failure, thus questioning the dogma that iliac screw fixation is required in long ASD MIS constructs.