ASTM F1717 standard for the preclinical evaluation of posterior spinal fixators: Can we improve it?

Abstract

Preclinical evaluation of spinal implants is a necessary step to ensure their reliability and safety before implantation. The American Society for Testing and Materials reapproved F1717 standard for the assessment of mechanical properties of posterior spinal fixators, which simulates a vertebrectomy model and recommends mimicking vertebral bodies using polyethylene blocks. This set-up should represent the clinical use, but available data in the literature are few. Anatomical parameters depending on the spinal level were compared to published data or measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing implant design were considered, and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by considering either the combination of the average values for each parameter or their worst-case combination depending on the spinal level. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The stress on the pedicular screw is significantly influenced by the lever arm of the applied load, the unsupported screw length, the position of the centre of rotation of the functional spine unit and the pedicular inclination with respect to the sagittal plane. The worst-case combination of parameters demonstrates that devices implanted below T5 could potentially undergo higher stresses than those described in the standard suggestions (maximum increase of 22.2% at L1). We propose to revise F1717 in order to describe the anatomical worst case condition we found at L1 level: this will guarantee higher safety of the implant for a wider population of patients.

Preclinical evaluation of posterior spine stabilization devices: can the current standards represent basic everyday life activities?

“…A simplified implant design was assumed [18]. Screws were positioned within the pedicles according to their orientation with respect to the anatomical planes; moreover, the spinal rod shape was drawn to follow the natural curvature of the lordotic segment.…”

“…A simplified implant design was assumed [18] and the pedicle screws were introduced within the test blocks (Fig. 1).…”

A Study of the Dynamic Anterior Cervical Plate Implant by Computer Aided Virtual Engineering Method

“…Nevertheless, the experiment outcomes cannot predict the actual situation of in vivo application of the modified DACBP system. In the literature, the complications concerning the front cervical plate are restricted to releasing or breaking of the equipment and tracheoesophageal or neurovascular auxiliary wounds [16]. Different sorts of complications that may prompt to clinical indications are overlooked, and the emphasis is fundamentally made on unconstrained plates [17].…”

“…It is found that the slackening of rod-screws occurs through a cross-split screw head that can be fastened into the biomaterial plate [1,2]. Vertebral compression in the back of the body causes 1.4 million fractures in the musculoskeletal system worldwide [3]. In this study, a most negative scenario of vertebrectomy situation was applied, where the implant inserts are embedded in vertebral body-like pieces and need to bear presso-flexion.…”

Parametric and Structural Analysis of Dynamic Anterior Cervical Biomaterial Plate Implant by Computer Aided Virtual Engineering

“…Vertebral compression in the back of the body causes 1.4 million fractures in the musculoskeletal system worldwide [3].…”