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 ProperOes of Rod + Construct Right Rod for the Right Job 4. Material Proper6es 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 ProperOes Purpose The purpose of this study was to use a four point bending apparatus to define the elasOc and plasOc deformaon properOes of a spectrum of rod materials including work hardened stainless steel, Otanium 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 deflec6on: 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 moOon capture system Rod Proper6es Measured 1. IniOal linear deflecOon (elasOc) 2. Yield Point (ElasOc PlasOc) 3. UlOmate Load Rod Proper6es Measured Four Point Bending to 20 Degrees Mean load deformaon graphs generated Comparisons made between: UlOmate load, Yield point, Bending SOffness, Angle of Deformaon at Yield (α) One-way ANOVA (p<0.05) & Bonferroni post-hoc test mulOple 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 CorrecOon 1. Titanium Alloy 2. Ultra High Strength Steel Take Home Points New OpOon: Cobalt Chromium Rod + Titanium Screws “Feel” like HS Steel but plasOcally 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 magneOc 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 .