1. Universitätsklinikum Carl Gustav Carus an der Technischen Universität Dresden Preoperative simulation reduces surgical time and radiation exposure for the staff in navigation-guided minimal-invasive Hybrid-LIF B. Rieger, D. Podlesek, W. Polanski, H. Jiang, G. Schackert MIS-HLIF in degenerative listhesis (a, b) is a hybrid of the PLIF- and the TLIF-procedure. The approach utilizes a standard unilateral midline approach for decompression, but bilateral (ipsilateral and contralateral crossover) facetotomy and discectomy (Fig. 2). The ipsilateral facetotomy is later utilized to implant a special designed cage (Fig. 5). Using navigation, pedicle screws are placed in a vertical (dorsoventral) vector ipsilaterally (Fig 3 -  ) and in conventional percutaneous dorsolateral technique on the contralateral side (Fig 3 -  ). This study reports that preoperative simulation accelerates MIS-HLIF and reduces the radiation exposure for the staff. Developing the SOP (Fig. 1-6) for MIS-HLIF, 163 patients with single-level degenerative lumbar spondylolisthesis underwent treatment so far. Previous quality assurance study of MIS-HLIF, Numeric rating scale (NRS), Oswestry disability index (ODI) and core outcome measures index (COMI) were assessed for the first 28 patients preoperatively as well as 6 weeks, 3 months, 6 months and 1 year after surgery (Tbl. 1). For these patients, segmental realignment was statistically objectified using Boxall`s method. Furthermore, the surgery time as well as radiation exposure for the staff were evaluated and compared to in-house conventional open surgery and MIS-TLIF. The radiation dose for the 3D-scan was subtracted from the cumulative dose to get the staff`s radiation exposure. P<0.05 was considered as significant. Previous outcome evaluation in the first 28 patients showed significant improvement of NRS, COMI and ODI scores at all postoperative follow-up time points (p<0.05). Further, postoperative statistical evaluation of x-ray data from these patients showed a significant reduction of listhesis from an average of 22% preoperatively to 9% postoperatively (p<0.01). The average blood loss was less than 500 ml. After establishing a preoperative simulation in the SOP, mean surgical time (by one surgeon) decreased from 180 (SD 27) to 140 minutes (SD 30) and mean radiation exposure for staff was reduced from 570 (SD 110) to 350 cGy·cm 2 (SD 50) significantly in the last 33 cases, compared with in house patients after MIS-TLIF: (720 cGy·cm 2, SD 90) and after open TLIF: (420 cGy·cm 2, SD 30) as a function of the BMI. 1. Preoperative Simulation 2. Decompression and Release 3. Navigation 4. Realignment MIS-HLIF combines advantages of the PLIF and TLIF procedure and its outcome is not inferior. The preoperative software- assisted simulation of the cage concerning the sagittal balance reduces the surgical time and radiation exposure for the staff. Based on preoperative imaging data, this simulation suggests the optimal height of the device so it has not to be defined intraoperatively via x-ray. CONCLUSION preoperativ6 weeks3 months6 months1 year NRS back 7,2 ± 0,43,5 ± 0,5*3,3 ± 0,5*3,9 ± 0,5* 3,6 ± 0,5* NRS leg 7,1 ± 0,53,4 ± 0,6*3,0 ± 0,5*3,2 ± 0,5* COMI 8,4 ± 1,35,4 ± 2,5*5,0 ± 2,8*5,1 ± 3,0* 4,9 ± 3,3* ODI 50 ± 1638 ± 16*34 ± 18*35 ± 19*31 ± 17* Tbl. 1: Clinical data (n=28) with preoperative, 6 weeks, 3, 6 months and 1 year postoperative NRS, COMI & ODI scores. Significant (*p<0,05) improvement. R E S U L T S Reference: Reinshagen C, Ruess D, Walcott BP, et al. A novel minimally invasive technique for lumbar decompression, realignment, and navigated interbody fusion. J Clinical Neuroscience 2015;22:1484-90. 5. Cage-Implantation6. Ipsilateral Screw Placement and Compression Fig. 2: A 30-40 mm posterior midline skin incision is used for decompression and contralateral undercutting. To maintain maximal stability, only the medial parts of the facet joint are resected. The contralateral facet is opened for better release and fast ossification. Afterwards the same approach served for nucleotomy. Fig 3: Insertion of guide-wires under navigation control (e) in standard fashion vis-à-vis . Ipsilaterally guide-wires are introduced using cortical bone trajectories . Navigation avoids the violation of the upper facets. Fig 5: With an anterior and posterior element linked by a joint, the cage is easiliy inserted through the space gained from facetotmy (f). Fig. 4: Contralaterally, pedicle-screws are implanted. They were then used to achieve optimal distraction and realignment (g) of the segment under fluoroscopic control. To secure the result, a rod is implanted and the screws are tightened. Fig 6: The second rod is implanted and the screw-towers are used to achieve optimal compression on the cage. Usually, a fluorsocopic scan is obtained before closing. The figure illustrates the position of tubular retractors in regard to muscle layers. HybridLIF leads to considerable reduction of paraspinal muscle and soft-tissue injury in comparison to standard TLIF techniques. Fig. 1: For restoring sagittal balance preoperative analysis is done by radiographs and the Vertaplan TM software (c). Position and range of motion of all lumbar segments are shown. An optimal cage type/length, ventral/dorsal height and angulation is proposed by the software’s cage database. After virtual placement (d) into the segment, the position and range of motion of every lumbar segment may again be simulated under dynamic conditions. M E T H O D S INTRODUCTION a b d c e f g h  