Residual Stresses in Surgical Growing Rods

Area of Science:

• Biomaterials Engineering
• Mechanical Engineering
• Orthopedic Surgery

Background:

• Surgical growing rods for early onset scoliosis have a high failure rate.
• Fatigue resistance can be enhanced by inducing compressive residual stresses.
• Understanding residual stress evolution is crucial for improving implant longevity.

Purpose of the Study:

• To investigate the evolution of residual stress profiles in Ti-6Al-4V growing rods.
• To correlate material properties and processing steps with residual stress generation.
• To provide guidelines for extending the lifetime of growing rods.

Main Methods:

• Digital image correlation (DIC) with microbeam ring-core milling by focused ion beam (FIB) was used for residual stress evaluation.
• Experimental stress profiles were obtained for shot-peened rods before and after bending.
• A finite element model (FEM) was developed and validated against experimental data.

Main Results:

• Compressive residual stresses were maintained on both concave and convex sides of the rods after bending.
• The combination of initial shot peening and backstress significantly influenced surface compressive stress generation.
• Parametric analysis identified initial yield strength and kinematic hardening as primary factors, with bending angle and shot peening depth being less critical.

Conclusions:

• Materials with high kinematic hardening and low yield strength are optimal for inducing compressive residual stresses.
• Optimizing shot peening intensity and material properties can enhance growing rod fatigue resistance.
• This research provides a foundation for designing more durable growing rods, reducing failure rates in scoliosis treatment.