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

Area of Science:

• Biomechanics
• Spinal Surgery
• Orthopedic Research

Background:

• Standard pullout tests assess pedicle screw holding power but do not account for incremental deformation.
• The impact of stress relaxation on pedicle screw pullout in human bone remains largely unknown.
• Prior studies in bovine bone indicate stress relaxation reduces failure loads.

Purpose of the Study:

• To evaluate the influence of stress relaxation on the pullout biomechanics of pedicle screws in human lumbar vertebrae.
• To compare the standard continuous pullout test with a stress relaxation incremental pullout model.

Main Methods:

• A biomechanical study utilizing fourteen human lumbar vertebrae.
• Pedicle screws were tested using either a standard continuous pullout or a stress relaxation incremental pullout protocol.
• Peak loads, stiffness, and relaxation data were quantified and analyzed.

Main Results:

• Pullout strength and stiffness were significantly lower (P < 0.05) in the stress relaxation group compared to the standard group.
• Characteristic relaxation time decreased while the relaxation ratio increased with higher deformation levels.
• Both pullout and stress relaxation parameters demonstrated a correlation with bone quality.

Conclusions:

• The stress relaxation phenomenon substantially impacts the mechanical and viscoelastic properties at the bone-screw interface in human cadaveric bone.
• A stress relaxation pullout model may offer a more accurate in vitro representation of in vivo conditions by incorporating time-dependent deformation.
• This research highlights the importance of considering viscoelastic effects in biomechanical testing of spinal implants.