Expandable pedicle screw may have better fixation than normal pedicle screw: preclinical investigation on instrumented L4-L5 vertebrae based on various physiological movements

Abstract

Lumbar spine fracture is typically treated by means of screw fixation, the primary aim of which is to reduce fracture by achieving bony union such that the spinal anatomy is restored. Pedicle screw fixation has certain advantages over conventional vertebral screws, e.g. 3-column fixation and improved surgical alignment. However, expandable pedicle screws have been reported to impart better anchorage as compared to conventional pedicle screws, both in case of healthy and osteoporotic bone. The clinical studies notwithstanding, there is a paucity of preclinical investigations on expandable pedicle screws used on lumbar vertebrae. By employing anatomically viable FE models, the present study intended to estimate stress-strain fields of a functional spinal unit (FSU) of intact L4-L5 vertebra and to further compare the same with FSUs instrumented with normal and expandable pedicle screws under different physiological loading condition. The various physiological loading regimes appeared to have significant influence on the overall load transfer in the L4-L5 vertebrae. The expandable pedicle screw predicted marginally improved anchorage as compared to the normal pedicle screws, with more contact area with the bone resulting in higher stresses (~ 1.6 MPa) and high strain at the contact sites. This is indicative of improved stability albeit having marginally greater risk of screw pullout. Greater area (15-80%) with peak stresses at the bone-screw interfaces also indicates lesser degree of stress shielding. Thus, stability aside, one may expect to have lower loosening issues too with the use of expandable pedicle screws.

Keywords: Biomechanics; Finite element analysis; Fracture fixation; Pedicle screw; Spine.