Serkan Inceoğlu1, Atilla Akbay, Robert F McLain. 1. Spine Research Laboratory, The Cleveland Clinic Spine Institute, The Cleveland Clinic Foundation, Cleveland, OH 44195, USA.
Abstract
STUDY DESIGN: A biomechanical study using human lumbar vertebra. OBJECTIVES: To assess the effects of stress relaxation on the pullout behavior of the pedicle screw in human bone. SUMMARY OF BACKGROUND DATA: The standard pullout test is widely used to assess screw-holding power; however, the effects of incremental deformation on the pullout behavior of pedicle screws are not known in human bone. A recent study in bovine bone suggests that stress relaxation during incremental pullout significantly reduces failure loads seen in pullout testing. METHODS: Fourteen human lumbar vertebrae were instrumented with pedicle screws and tested with one of two pullout protocols: standard pullout model, a continuous withdrawal, and stress relaxation pullout model, an incremental withdrawal allowing stress relaxation. Peak loads and stiffness were calculated and relaxation data were assessed using the stretched exponential function. RESULTS: Pullout strength and stiffness were significantly lower in the stress relaxation model group (P < 0.05). The characteristic relaxation time decreased and relaxation ratio increased with the increasing deformation level. The pullout and stress relaxation parameters were found to correlate with the bone quality. CONCLUSIONS: The stress relaxation process significantly affects the mechanical and viscoelastic properties of the bone-screw interface in human cadaveric bone. Stress relaxation pullout model might be a better representative of in vivo conditions by introducing the time and incremental deformation into the in vitro biomechanical testing.
STUDY DESIGN: A biomechanical study using human lumbar vertebra. OBJECTIVES: To assess the effects of stress relaxation on the pullout behavior of the pedicle screw in human bone. SUMMARY OF BACKGROUND DATA: The standard pullout test is widely used to assess screw-holding power; however, the effects of incremental deformation on the pullout behavior of pedicle screws are not known in human bone. A recent study in bovine bone suggests that stress relaxation during incremental pullout significantly reduces failure loads seen in pullout testing. METHODS: Fourteen human lumbar vertebrae were instrumented with pedicle screws and tested with one of two pullout protocols: standard pullout model, a continuous withdrawal, and stress relaxation pullout model, an incremental withdrawal allowing stress relaxation. Peak loads and stiffness were calculated and relaxation data were assessed using the stretched exponential function. RESULTS: Pullout strength and stiffness were significantly lower in the stress relaxation model group (P < 0.05). The characteristic relaxation time decreased and relaxation ratio increased with the increasing deformation level. The pullout and stress relaxation parameters were found to correlate with the bone quality. CONCLUSIONS: The stress relaxation process significantly affects the mechanical and viscoelastic properties of the bone-screw interface in human cadaveric bone. Stress relaxation pullout model might be a better representative of in vivo conditions by introducing the time and incremental deformation into the in vitro biomechanical testing.
Authors: Michael Werner; Niels Hammer; Christian Rotsch; Isabell Berthold; Mario Leimert Journal: Med Biol Eng Comput Date: 2019-11-18 Impact factor: 2.602