Stress relaxation at the bone-pedicle screw interface in human bone.
METHODS
A biomechanical study using human lumbar vertebra.


OBJECTIVE
To assess the effects of stress relaxation on the pullout behavior of the pedicle screw in human bone.


BACKGROUND
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.
Inceoğlu
Serkan
S
Spine Research Laboratory, The Cleveland Clinic Spine Institute, The Cleveland Clinic Foundation, Cleveland, OH 44195, USA.
Akbay
Atilla
A
McLain
Robert F
RF
D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D004548:Elasticity; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery*; D013314:Stress, Mechanical; D013718:Tensile Strength