[Biomechanics of the bone-screw interface in transpedicular spinal instrumentation]

Implant loosening, catastrophic failure of the bone-screw interface, material migration, and loss of stability of the fixation component assembly constitute a serious complication in adult spinal surgery. The contribution of biomechanics is based on experimental measurement and simulation of transpedicular spinal fixations. The cortical insertion trajectory showed an increase in the resistance of the screw-bone interface with respect to the pedicle insertion trajectory, both for axial traction forces to the screw and for stress distribution in the vertebra. The double-threaded screws and standard pedicle screws had similar strength. Partially threaded screws with four-thread showed better resistance to fatigue in terms of a higher failure load and number of cycles to fail. Cement or hydroxyapatite augmented screws with also showed a better fatigue resistance in osteoporotic vertebrae. Rigid segment simulations confirmed the presence of higher stresses on the intervertebral discs causing damage to adjacent segments. The posterior body of the vertebra may be subjected to high stresses, in the bone-screw interface, being this bone region more susceptible to failure.