gms | German Medical Science

Objective: Loosening of pedicle screws at the bone-screw interface is a frequent complication, especially in the aging osteoporotic population. To increase fixation, augmentation of pedicle screws with cement injection is widely used. The aim of this study is to introduce a new pedicle screw concept with integrated shape memory alloy (SMA) actuator components for an improved anchorage in cancellous bone as well as to determine the fixation strength of the new screws.

Method: First, different shapes and surface structures of the actuator were tested in bone substitutes. Thereafter, the actuators were designed using finite element simulation and structurally integrated into 65 mm titanium pedicle screws. In the next step, ten screws were produced by selective laser beam melting for the experiments. The screws were inserted according to standard ASTM F543 into different bone substitutes and alcohol fixated cervical and thoracic vertebrae.

Results: The structure of the actuator concept demonstrates a compromise between resistance against rotational and translational movements and the complexity of the shape. Two SMA sheets were integrated along the screw axis. These actuators expand against the bone when warmed up to body temperature. In bone substitutes the pullout strength increased remarkably in bone substitutes from 432 N to 542 N and in human vertebrae from 240 N up to 293 N.

Conclusions: SMA actuators increase the pullout strength of pedicle screws out of bone substitutes and human vertebrae. In reference to the compressing force of the actuator sheets, some adjustments have to be done considering the boundary conditions for bone growth and bone degeneration. Additional experiments will focus on testing the screw prototype with biological specimens and thereafter in a clinical trial. The simulated parameters, e.g. activation force, of the SMA elements match the experimental data in good approximation.