Purpose: Pedicle screw loosening is a frequent complication in lumbar spine fixation, most commonly among patients with poor bone quality. Determining patients at high risk for insufficient implant stability would allow clinicians to adapt the treatment accordingly. The aim of this study was to develop a computational model for quantitative and reliable assessment of the risk of screw loosening. Methods: A cohort of patient vertebrae with diagnosed screw loosening was juxtaposed to a control group with stable fusion. Imaging data from the two cohorts were used to generate patient-specific biomechanical models of lumbar instrumented vertebral bodies. Single-level finite element models loading the screw in axial or caudo-cranial direction were generated. Further, multi-level models incorporating individualized joint loading were created. Results: The simulation results indicate that there is no association between screw pull-out strength and the manifestation of implant loosening (p = 0.8). For patient models incorporating multiple instrumented vertebrae, CT-values and stress in the bone were significantly different between loose screws and non-loose screws (p = 0.017 and p = 0.029, for CT-values and stress, respectively). However, very high distinction (p = 0.001) and predictability (R²_Pseudo = 0.358, AUC = 0.85) were achieved when considering the relationship between local bone strength and the predicted stress (loading factor). Screws surrounded by bone with a loading factor higher than 25% were likely to be loose, while the chances of screw loosening were close to 0 with a loading factor below 15%. Conclusion: The use of a biomechanics-based score for risk assessment of implant fixation failure might represent a paradigm shift in addressing screw loosening after spondylodesis surgery.

Purpose: The aim of this study was to elucidate segmental range of motion (ROM) before and after common decompression and fusion procedures on the lumbar spine. Methods: ROM of fourteen fresh-frozen human cadaver lumbar segments (L1/2: 4, L3/4: 5, L5/S1: 5) was evaluated in six loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC). ROM was tested with and without posterior instrumentation under the following conditions: 1) native 2) after unilateral laminotomy, 3) after midline decompression, and 4) after nucleotomy. Results: Median native ROM was FE 6.8°, LB 5.6°, and AR 1.7°, AS 1.8 mm, LS 1.4 mm, AC 0.3 mm. Unilateral laminotomy significantly increased ROM by 6% (FE), 3% (LB), 12% (AR), 11% (AS), and 8% (LS). Midline decompression significantly increased these numbers to 15%, 5%, 21%, 20%, and 19%, respectively. Nucleotomy further increased ROM in all directions, most substantially in AC of 153%. Pedicle screw fixation led to ROM decreases of 82% in FE, 72% in LB, 42% in AR, 31% in AS, and 17% in LS. In instrumented segments, decompression only irrelevantly affected ROM. Conclusions: The amount of posterior decompression significantly impacts ROM of the lumbar spine. The here performed biomechanical study allows creation of a simplified rule of thumb: Increases in segmental ROM of approximately 10%, 20%, and 50% can be expected after unilateral laminotomy, midline decompression, and nucleotomy, respectively. Instrumentation decreases ROM by approximately 80% in bending moments and accompanied decompression procedures only minorly destabilize the instrumentation construct.

Musculoskeletal modeling is a well-established method in spine biomechanics and generally employed for investigations concerning both the healthy and the pathological spine. It commonly involves inverse kinematics and optimization of muscle activity and provides detailed insight into joint loading. The aim of the present work was to develop and validate a procedure for the automatized generation of semi-subject-specific multi-rigid body models with an articulated lumbar spine. Individualization of the models was achieved with a novel approach incorporating information from annotated EOS images. The size and alignment of bony structures, as well as specific body weight distribution along the spine segments, were accurately reproduced in the 3D models. To ensure the pipeline’s robustness, models based on 145 EOS images of subjects with various weight distributions and spinopelvic parameters were generated. For validation, we performed kinematics-dependent and segment-dependent comparisons of the average joint loads obtained for our cohort with the outcome of various published in vivo and in situ studies. Overall, our results agreed well with literature data. The here described method is a promising tool for studying a variety of clinical questions, ranging from the evaluation of the effects of alignment variation on joint loading to the assessment of possible pathomechanisms involved in adjacent segment disease.

Objective: Endoscopic spine surgery is an emerging technique of minimally invasive spine surgery. However, headache, seizure, and autonomic dysreflexia are possible irrigation-related complications following full-endoscopic lumbar discectomy (FELD). Pressure elevation through fluid irrigation may contribute to these adverse events. A validated experimental model to investigate parameters for guideline definition is lacking. This study aimed to create an experimental setting for FELD with pressure assessments to prove the concept of repeatable and sensitive measurement of intracranial, intra- and epidural pressures during spine endoscopy.Methods: To measure intradural pressure, catheters were introduced through a sacral approach and advanced to lumbar, thoracic, and cervical levels in human cadavers. Similarly, lumbar epidural and intracranial probes were placed. The dural sac was filled with Ringer solution to a physiologic pressure of 15 cmH2O. Lumbar endoscopy was performed on 3 human cadavers at the L3–4 level. Pressure changes were measured continuously at all sites and the effects of backflow-occlusion were monitored.Results: Reproducibility of the experimental model was validated with catheters at the correct locations and stable compartmental pressure baselines at all levels for 3 specimens (mean±standard deviation: 1.3±2.9 mmHg, 9.0±2.0 mmHg, 6.0±1.2 mmHg, respectively). Pressure increase could be detected sensitively by closing the system with backflow-occlusion.Conclusion: An experimental setup for feasible, repeatable, and precise pressure measurement during FELD in a human cadaveric setup has been developed. This allows investigation of the effects of endoscopic techniques and pump pressures on intra-, epidural and intracranial pressure and enables ranges of safe pump pressures per clinical situations.

Posterior and transforaminal lumbar interbody fusion (PLIF, TLIF) are well-established procedures for spinal fusion. However, little is known about load sharing between cage, dorsal construct, and biological tissue within the instrumented lumbar spine. The aim of this study was to quantify the forces acting on cages under axial compression force with and without posterior instrumentation. Biomechanical cadaveric study. Ten lumbar spinal segments were tested under uniaxial compression using load cell instrumented intervertebral cages. The force was increased in 100N increments to 1000N or a force greater than 500N on one load cell. Each specimen was tested after unilateral PLIF (uPLIF), bilateral PLIF (bPLIF) and TLIF each with/without posterior instrumentation. Dorsal instrumentation was performed with 55N of compression per side. Cage insertion resulted in median cage preloads of 16N, 29N and 35N for uPLIF, bPLIF, and TLIF. The addition of compressed dorsal instrumentation increased the median preload to 224N, 328N, and 317N, respectively. With posterior instrumentation, the percentage of the external load acting on the intervertebral cage was less than 25% at 100N (uPLIF: 14.2%; bPLIF: 16%; TLIF: 11%), less than 45% at 500N (uPLIF: 31.8%; bPLIF: 41.1%; TLIF: 37.9%) and less than 50% at 1000N (uPLIF: 40.3%; bPLIF: 49.7%; TLIF: 43.4%). Without posterior instrumentation, the percentage of external load on the cages was significantly higher with values above 50% at 100N (uPLIF: 55.6%; bPLIF: 75.5%; TLIF: 66.8%), 500N (uPLIF: 71.7%; bPLIF: 79.2%; TLIF: 65.4%), and 1000N external load (uPLIF: 73%; bPLIF: 80.5%; TLIF: 66.1%). For absolute loads, preloads and external loads must be added together. Without posterior instrumentation, the intervertebral cages absorb more than 50% of the axial load and the load distribution is largely independent of the loading amplitude. With posterior instrumentation, the external load acting on the cages is significantly lower and the load distribution becomes load amplitude dependent, with a higher proportion of the load transferred by the cages at high loads. The bPLIF cages tend to absorb more force than the other two cage configurations. Cage instrumentation allows some of the compression force to be transmitted through the cage to the screws below, better distributing and reducing the overall force on the pedicle screws at the end of the construct and on the rods.

Planning of surgical procedures for spinal fusion is performed on standing radiographs, neglecting the fact that patients are mostly in the sitting position during daily life. The awareness about the differences in the standing and sitting configuration of the spine has increased during the last years. The purpose was to provide an overview of studies related to seated imaging for spinal fusion surgery, identify knowledge gaps and evaluate future research questions. A literature search according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) extension for Scoping Reviews (PRISMASc) was performed to identify reports related to seated imaging for spinal deformity surgery. A summary of the finding is presented for healthy individuals as well as patients with a spinal disorder and/or surgery. The systematic search identified 30 original studies reporting on 1) the pre- and postoperative use of seated imaging of the spine (n=12), 2) seated imaging of the spine for non – surgical evaluation (n=7) and 3) seated imaging of the spine among healthy individuals (12). The summarized evidence illuminates that sitting leads to a straightening of the spine decreasing thoracic kyphosis (TK), lumbar lordosis (LL), the sacral slope (SS). Further, the postural change between standing and sitting is more significant on the lower segments of the spine. Also, the adjacent segment compensates the needed postural change of the lumbar spine while sitting with hyperkyphosis. The spine has a different configuration in standing and sitting. This systematic review summarizes the current knowledge about such differences and reveals that there is minimal evidence about their consideration for surgical planning of spinal fusion surgery. Further, it identifies gaps in knowledge and areas of further research.