Nonlinear stiffness profiles of external fixators constructed with composite rods

Area of Science:

• Biomaterials Engineering
• Orthopedic Biomechanics
• Medical Device Design

Background:

• External skeletal fixators (ESF) are crucial for fracture stabilization.
• Current ESF designs typically exhibit linear stiffness, which may not optimally support fracture healing.
• Nonlinear stiffness characteristics could offer improved control over the mechanical environment at the fracture site.

Purpose of the Study:

• To investigate whether composite connecting rods can impart nonlinear stiffness properties to unilateral and bilateral ESF.
• To evaluate these properties under axial loading and cranial-caudal bending conditions.

Main Methods:

• Six-pin external skeletal fixator (ESF) constructs were fabricated using composite (titanium and silicone) or solid titanium connecting rods.
• Mechanical testing was conducted in axial loading and 4-point cranial-caudal bending.
• Stiffness was measured at low and high load ranges to identify nonlinear behavior.

Main Results:

• Solid connecting rods resulted in linear stiffness across all configurations and loading conditions.
• Bilateral composite fixators demonstrated nonlinear increasing stiffness under both axial loading and bending.
• Unilateral composite fixators showed nonlinear increasing stiffness in axial loading but not in bending.

Conclusions:

• Composite connecting rods successfully introduced nonlinear increasing stiffness in bilateral ESF and in unilateral ESF under axial loading.
• This nonlinear stiffness characteristic offers potential for enhanced fracture healing by allowing surgeons to modulate the mechanical environment.
• The findings suggest a novel approach to ESF design for improved clinical outcomes.