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Mechanical Properties of Bioresorbable Self-reinforced Posterior Cervical Rods

Savage, Katherine BA*; Sardar, Zeeshan M. MDCM, MSc; Pohjonen, Timo MSc; Sidhu, Gursukhman S. MBBS*; Eachus, Benjamin D. BA*; Vaccaro, Alexander MD, PhD*

Journal of Spinal Disorders & Techniques: April 2014 - Volume 27 - Issue 2 - p E66–E71
doi: 10.1097/BSD.0b013e318299c6d8
Original Articles

Study Design: A biomechanical study.

Objective: To test the mechanical and physical properties of self-reinforced copolymer bioresorbable posterior cervical rods and compare their mechanical properties to commonly used Irene titanium alloy rods.

Summary of Background Data: Bioresorbable instrumentation is becoming increasingly common in surgical spine procedures. Compared with metallic implants, bioresorbable implants are gradually reabsorbed as the bone heals, transferring the load from the instrumentation to bone, eliminating the need for hardware removal. In addition, bioresorbable implants produce less stress shielding due to a more physiological modulus of elasticity.

Methods: Three types of rods were used: (1) 5.5 mm copolymer rods and (2) 3.5 mm and (3) 5.5 mm titanium alloy rods. Four tests were used on each rod: (1) 3-point bending test, (2) 4-point bending test, (3) shear test, and (4) differential scanning calorimeter test. The outcomes were recorded: Young modulus (E), stiffness, maximum load, deflection at maximum load, load at 1.0% strain of the rod’s outer surface, and maximum bending stress.

Results: The Young modulus (E) for the copolymer rods (mean range, 6.4–6.8 GPa) was significantly lower than the 3.5 mm titanium rods (106 GPa) and the 5.5 mm titanium rods (95 GPa). The stiffness of the copolymer rods (mean range, 16.6–21.4 N/mm) was also significantly lower than the 3.5 mm titanium alloy rods (43.6 N/mm) and the 5.5 mm titanium alloy rods (239.6 N/mm). The mean maximum shear load of the copolymer rods was 2735 N and they had significantly lower mean maximum loads than the titanium rods.

Conclusions: Copolymer rods have adequate shear resistance, but less load resistance and stiffness compared with titanium rods. Their stiffness is closer to that of bone, causing less stress shielding and better gradual dynamic loading. Their use in semirigid posterior stabilization of the cervical spine may be considered.

*Rothman Institute of Orthopedics, Thomas Jefferson University, Philadelphia, PA

Division of Orthopaedic Surgery, McGill University Health Centre, Montreal, QC, Canada

Inion Oy, Tampere, Finland

A.R.V. serves as a consultant to Inion Inc. (Weston, FL).

The authors declare no conflict of interest.

Reprints: Alexander Vaccaro, MD, PhD, Rothman Institute of Orthopedics, Thomas Jefferson University, 925 Chestnut Street, 5th Floor, Philadelphia, PA 19107 (e-mail:

Received January 22, 2013

Accepted April 21, 2013

© 2014 by Lippincott Williams & Wilkins, Inc.