Eric S. Varley, DO 1,2 Thomas N. Nunn, BS 1,2 Arnel Aguinaldo, MS, ATC 1 Peter O. Newton, MD 1,2

1 Rady Children’s Hospital, San Diego, CA 2 University of California, San Diego, San Diego, CA Rod Biomechanics

1. Bone Quality (AIS vs. NMS) Balance 4 2. Rigidity of the Curve Factors

3. Structural Properes of Rod + Construct Right Rod for the Right Job 4. Material Properes of the rod Available Rod Materials • Stainless Steel – Work Hardening Process 3 diff rod strengths – (+): Experience, Cost, fague life1 – (-): Imaging Compability2, Corrosion3 • Titanium Alloys – (+): Imaging Compability2, Corrosion resistance3 – (-): Fague Life1 • ? Cobalt Chromium – (+): Imaging Compability2, Corrosion resistance3 – (-): ? Comparave Material Properes Purpose

The purpose of this study was to use a four point bending apparatus to define the elasc and plasc deformaon properes of a spectrum of rod materials including work hardened stainless steel, tanium alloy, and cobalt chromium alloy Methods  Five 5.5mm Rod Materials: 1. Standard Strength Steel (SS) 2. High Strength Steel (HS) 3. Ultra High Strength Steel (UHS) 4. Titanium Alloy (Ti) 5. Cobalt Chromium (CoCrMo)

 Four Point Bending to specific angle of deflecon:  3˚, 6˚, 9˚, 12˚, 15˚, 20˚, 30˚, 40˚, 60˚  One sample per angle Four Point Bending

α

 MTS Frame: 200mm samples 40mm of space between each point contact (Rate = 0.1 mm/s)  Sampled displacement (mm) & force (N)  Rod deformaon angle (α) measured with 3-camera moon capture system Rod Properes Measured

1. Inial linear deflecon (elasc) 2. Yield Point (Elasc  Plasc) 3. Ulmate Load Rod Properes Measured

Four Point Bending to 20 Degrees

 Mean load deformaon graphs generated  Comparisons made between:  Ulmate load, Yield point, Bending Sffness, Angle of Deformaon at Yield (α)  One-way ANOVA (p<0.05) & Bonferroni post-hoc test mulple comparisons (p<0.01) Results

Material Bending Yield Load (N) Ulmate Load Angle of Sffness (N) Deform @ (N*m^2) Yield (deg) CoCrMo 13 ± 2 ✔ 920 ± 183 1912 ± 27 5 ± 1 ✔ Ti Alloy 7 ± 2 1265 ± 24 1658 ± 38 12 ± 0 SS Steel 10 ± 4 700 ± 184 1512 ± 7 5 ± 1 ✔ HS Steel 12 ± 1 825 ± 283 1821 ± 30 7 ± 2 UHS Steel 10 ± 1 1394 ± 84 2016 ± 24 9 ± 1

* Each parameter significant across all rod materials (p<0.01) Discussion

Material Sffness Strength Ease of In Situ Bending Titanium + +++ + SS Steel ++ + +++ UHS Steel +++ +++ + CoCrMo +++ ++ +++

 In Situ Bending 1. Standard Strength Steel 2. Cobalt Chromium

 Rod Strength for Spinal Deformity Correcon 1. Titanium Alloy 2. Ultra High Strength Steel Take Home Points

 New Opon: Cobalt Chromium Rod + Titanium Screws

 “Feel” like HS Steel but plascally deforms at a much lesser angle

 Ideal choice for In Situ bending when imaging capability is important References

1. Lindsey C, et al. The effects of rod contouring on spinal construct fague strength. Spine 2006

2. Scuderi GJ, et al. A biomechanical evaluaon of magnec resonance imaging-compable wire in cervical spine fixaon. Spine 1993

3. Jacobs JJ, et al. Corrosion of metal orthopaedic implants. J Bone Joint Surg Am 1998