Stress relaxation at the bone-pedicle screw interface in human bone

• Published in: Spine (Philadelphia, Pa. 1976) Pa. 1976), 2006-05, Vol.31 (12), p.1321-1326
• Main Authors: INCEOGLU, Serkan, AKBAY, Atilla, MCLAIN, Robert F
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Biomechanical Phenomena; Bone Screws; Cadaver; Cerebrospinal fluid. Meninges. Spinal cord; Drug toxicity and drugs side effects treatment; Elasticity; Human viral diseases; Humans; Infectious diseases; Lumbar Vertebrae - surgery; Medical sciences; Nervous system (semeiology, syndromes); Neurology; Pharmacology. Drug treatments; Stress, Mechanical; Tensile Strength; Toxicity: osteoarticular system; Viral diseases; Viral diseases of the lymphoid tissue and the blood. Aids
• ISSN: 0362-2436; 1528-1159
• DOI: 10.1097/01.brs.0000218478.70656.63

A biomechanical study using human lumbar vertebra. To assess the effects of stress relaxation on the pullout behavior of the pedicle screw in human bone. 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. 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. 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. 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.