Parametric study of pedicle screw: Effect of stress transfer parameters on pullout strength

Abstract

A pedicle screw is an important implant used to fix and stabilize a cervical spine when spinal alignment is needed. The size and characteristics of pedicle screws affect pullout strength of the screws when inserted into the cervical spine. The pullout strength is greatly affected by the quality and duration of cervical spine treatment. To aid orthopedic surgeons in choosing a suitable pedicle screw for Thai cervical spine patients, the pullout strength of pedicle screws had to be considered. Since pedicle screws are different in design, size and characteristics, a middle ground parameter-Stress Transfer Parameter (STP) obtained through formula and finite element analysis, was used to compare the screws. Nine commonly-used pedicle screws were studied under the chosen parameters namely Outer Diameter (OD), Core Diameter (CD), Pitch (P), Proximal Root Radius (PRR), and Distal Root Radius (DRR). Each type of screw was inserted into C3 C4 C5 C6 and C7 of Thai cervical spines to bear the axial load in order to analyze their pullout conditions using Finite Element Analysis and to evaluate the STP. The maximum STP occurred on Y I pedicle screw as follows: 0.57 at C3, 0.48 at C4, 0.43 at C5, 0.48 at C6 and 0.59 at C7. The maximum STP resulted from the large OD and CD which could bear more axial load. The large Pitch decreased the stress concentration on each screw thread and the small PRR and DRR increased the slope of the blade to resist the pullout load.