Introduction: Mechanical complications following lumbar fixation are due to the combination of various factors related to morphology, pathology, and surgery. The aim of this study was to provide a patient-specific Finite Element Model of the lumbar spine for the simulation of surgical strategies, and to use it as a predictive tool aiming to detect and reduce preoperatively the risks of mechanical complications.

Materials & Methods: A pre-existing 3D personalized FEM of the lumbar spine was used. Posterior implants and main degenerative pathologies were also modelled.

After in vitro validation based on 24 specimens and 4 different instrumentations, the model was used to simulate real cases. Applied loads were based on patient characteristics (weight, imbalance). Simulation results included mechanical stresses in the discs and within the implants.

Clinical consistency of the simulations was tested through the gathering of clinical data for 66 patients instrumented with lumbo-sacral rigid screw-rod systems. Two subsets were considered: “mechanical successes” (53), and “mechanical failures” (13, including 11 screw breakage and 2 screw loosening). Blind comparison was then performed between these observed clinical outcomes and numerical simulations results.

Results & Discussion: Among the 66 patients, simulation results highlighted specific behaviours for 9 patients for which mechanical loads on implants were significantly higher. All of these 9 patients were actual “mechanical failures”. None of the actual “mechanical successes” were associated with “abnormal” simulation results.

Conclusion: This is the first time finite element simulations helped predicting 9 failures out of 13 observed among a total of 66 patients. This is a promising step towards the possibility to use FEM as a clinically relevant simulation tool for surgery planning.

Purpose of the study: The mean rotation center (MRC) characterizes the movement of two solids in relation to each other. This parameter has been proposed for the cervical spine to describe the motion of vertebral segments. Two lateral views (flexion and extension) are required to draw the necessary lines and establish the centers of rotation. The process is rigorous but time-consuming. We validated a computerized analysis system for automatic determination of the cervical MRC and study the localizations observed in healthy subjects.

Material and methods: Validation of the computerized system. Accurate angle measurements: nine cervical spines were harvested from anatomic specimens. A K-wire was inserted sagittally into each vertebra. Lateral images were obtain in flexion and extension. The measurements of mobility made by the software were compared with manual measurements. Reproducibility tests (intra- and interobserver): six pairs of flexion and extension views in healthy subjects. Two different observers made fifteen successive measurements of each MRC for each spinal segment. Frequently encountered positions of the MRC in healthy subjects: stress films were obtained in 51 healthy subjects aged 18–40 years. For each spinal segment, the MCR was determined with the computerized system.

Results: Accuracy of the angle measurements: the precision was 1.4° for a 95% interval of confidence. Reproducibility: variability of the position in X and Y for the MRC (expressed in percent of the size of the vertebral body) was: 19.6 and 24.5 for C2–C3; 112 and 15.3 for C3–C4; 7.7 and 9.4 for C4–C5; 9.1 and 9.4 for C5–C6; 13.1 and 11.8 for C6–C7. Positions frequently encountered in healthy subjects: the most frequent position of the MRC varied from one segment to another. There was a frequent position for each segment. These frequent positions were situated in the posterosuperior quadrant of the subjacent vertebra for C2–C3, C3–C4, C4–C5, and C5–C6. For C6–C7, the frequent positions for MRC were at the level of the intervertebral space, behind the center of the disc.

Discussion: The software tested here appeared to provide good measurements for cervical spine from C3 to C7. At these levels, the measures were accurate and reproducible, as were the coordinates for the MCR of each segment. The frequent positions of the MRC found in this study are the same as reported by other authors. This method is easy to apply in routine practice.

Purpose: It is widely agreed that fusion of a spinal segment modifies the mechanical behaviour of sub-jacent vertebrae. The mean centre of rotation (MCR) is defined to study changes in the mechanical behaviour at junctions. This parameter describes the relative movement of an object moving from one position to another. The purpose of this study was to describe changes in the position of the MCR after posterolateral lumbar spine fusion and to determine factors influencing these changes.

Material and methods: Fifty-one patients with posterolateral fusion with or without instrumentation of the lumbar spine limited to one or two levels were reviewed. Preoperative and last follow-up stress x-rays of the lumber spine were studied. The following parameters were determined with Spinview, a devoted software, at the level of fusion, at the three suprajacent levels, and when appropriate, at subjacent levels: disc height, intervetebral angular mobility, position of the MCR. Pre and postoperative positions of the MCR were compared with the Wilcoxon test for paired variables. Univariate and multivariate analyses were performed to search for factors influencing changes in the position of the MCR. Variables studied were: age, follow-up, extent of the fusion and its anatomic position, instrumentation, preoperative mobility of the zone to be fused, and quality of the arthrodesis at last follow-up.

Results: There were no significant changes in the position of the MCR of the first suprajacent level. Two variables exhibited significant correlation with these changes: pre and postoperative angular mobility of the future zone of fusion, and use of instrumentation. Instrumentation significantly increased variability in the position of the MCR. Postoperative mobility of the zone of fusion minimised this variability.

Discussion: Studying variations in the position of the MCR appears to reflect well changes in the mechanical behaviour of levels adjacent to the spinal fusion. Use of appropriate software should be helpful for routine applications. In our series, changes in the position of the MCR correlated well with significant increase in angular and anteroposterior mobility and also with decreased disc height at the first suprajacent level. These observations explain early degradation of junction zones observed after arthrodesis.