gms | German Medical Science

Objective: Preclinical evaluation of spinal implants is a mandatory step to ensure their reliability and safety before clinical use and implantation. The American Society of Testing and Materials reapproved a standard for the evaluation and comparison of mechanical properties of posterior spinal fixators [F1717-2013], which reproduces a vertebrectomy model and assumes that the anterior column is totally compromised from supporting the load (Figure a). In this frame-work polyethylene blocks are used to mimic vertebral bodies (Figure b). Even if the overall test set-up (distances and angles) should be representative of the specific clinical use of the implant in a worst-case condition, some parameters are unclear and a direct comparison with anatomical/biomechanical data, when available, is difficult. This investigation aims at investigating whether the experimental set-up correctly describe the physiological anatomy of a stabilized segment and how each parameter affects the stress arising in the device.

Method: Anatomical parameters depending on the spinal level, were compared to available literature data or directly measured on biplanar X-ray images of 14 patients.

Others mechanical variables describing the design of the implant were considered and all parameters were investigated by means of a comparative numerical analysis. Stress values were calculated either considering the combination of the average values for each parameter and their worst case combination depending on the spinal level.

Results and conclusion: The experimental configuration represents quite well the anatomy of an average thoracolumbar segment. The stress on the spinal fixator is significantly influenced by the lever arm of the applied load, the position of the center of rotation of the functional spine unit and the pedicular inclination with respect to the sagittal plane. The worst case combination of anatomical parameters demonstrates that a posterior spinal fixator implanted below T5 could potentially undergo higher stresses than according to standard configuration, with a maximum increase of 15.2% at L1 level. Considering a polyaxial screw having its head tilted cranially, this value raises to 22.2%.

A revision of F1717 standard is suggested in order to take into account for the worst case condition found at L1 level: this will guarantee a higher reliability and safety of posterior fixator for a wider population of patients.