Nitinol Memory Rods Versus Titanium Rods: A Biomechanical Comparison of Posterior Spinal Instrumentation in a...

Area of Science:

• Biomedical Engineering
• Materials Science
• Orthopedic Surgery

Background:

• Spinal instrumentation commonly utilizes titanium rods.
• Nitinol (nickel-titanium) memory metal offers unique properties like superelasticity and shape memory.
• Evaluating novel materials like nitinol is crucial for advancing spinal fusion techniques.

Purpose of the Study:

• To compare the biomechanical performance of nitinol and titanium rods in posterior spinal instrumentation.
• To assess compression, bending, and torsional properties of nitinol versus titanium constructs.
• To investigate the fatigue resistance of nitinol and titanium spinal implants.

Main Methods:

• Biomechanical testing of 19 spinal constructs using ultra-high-molecular-weight polyethylene blocks.
• Comparison of 5.5-mm nitinol rods with 5.5-mm titanium rods.
• Static compression, bending, and axial rotation testing at 37°C.

Main Results:

• Nitinol and titanium constructs showed comparable stiffness under compression (P=.83) and similar torsional stiffness (P=.91).
• Nitinol constructs exhibited a significantly higher torsional yield point (P=.004) and greater torsional toughness (P=.0006).
• No significant difference in fatigue cycles until failure was observed between nitinol and titanium groups (P=.22).

Conclusions:

• Nitinol rods are biomechanically comparable to titanium rods in posterior spinal constructs regarding compression.
• Nitinol demonstrates superior torsional failure load and toughness compared to titanium.
• While nitinol showed a trend towards better fatigue resistance, further investigation may be warranted.