This paper proposes a novel expandable-retractable pedicle screw and analyzes its functionality. A specially designed pedicle screw is described which has the ability to expand and retract using nitinol elements. The screw is designed to expand in body temperature and retract by cooling the screw. This expansion-retraction function is verified in an experiment designed in larger scale using a nitinol antagonistic assembly. The results of this experiment are compared to the results of a finite element model developed in Abaqus in combination with a user material subroutine (UMAT). This code has been developed to analyze the nonlinear thermomechanical behavior of shape memory alloy materials. The functionality of the proposed screw is evaluated with simulation and experimentation in a pullout test as well. The pullout force of a normal screw inserted in a normal bone was simulated, and the result is compared with the results of the expandable screw in osteoporotic bone. Lastly, strength of the designed pedicle screw in a foam block is also verified with experiment. The reported finite element simulations and experiments are the proof for the concept of nitinol expandable-retractable elements on a pedicle screw which validate the functionality in a pullout test. 1. Introduction Bone screws for various spinal treatments and fixations have been used for about 70 years . Pedicle screws are used as bone anchoring elements to firmly grip the bone to facilitate attachment to the spinal implants. Using the pedicle screws’ connection rod, surgeons can fixate the spinal segments together for spinal fusion. The pedicular fixation system (which consists of a minimum of four pedicle screws and the rod) can resist high loads and stabilize a fractured spine. Medical applications of pedicle screws show that tolerating the applied forces is possible for pedicle screws inside a healthy bone. When the bone is not healthy, poor screw purchase becomes the main concern . Osteoporosis is a common bone disease in which the bone mineral density (BMD) reduces. Osteoporosis decreases the bone strength which causes an increased risk of fracture in the bony structures of the patients. This disease is very common in elderly people and steeply increases with age. The main concern of surgeons performing the pedicle screw fixation surgery on patients suffering from osteoporosis is the probability of loosening or pullout failure of the screw during or after surgery . To overcome the drawbacks of osteoporosis in pedicle screw fixations, several methods have been used. Increasing
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