Olson EM, Sturm PF, Jain VV, Schultz LR, Glos DL, Bylski-Austrow DI Early Onset Scoliosis Patient-Based Biomechanical Test for Traditional Growing Rods Constructs Olson EM, Sturm PF, Jain VV, Schultz LR, Glos DL, Bylski-Austrow DI

Complications of spine-based distraction Junctional kyphosis Infection Spontaneous fusion Anchor failure Rod fracture Most common biomechanical complication Rate ≥ 15% Yang JCO 2009 Bess JBJS 2010 Upasani Spine Deformity 2016 Jain, Lykissas, Crawford, The Growing Spine, Ch 16, 2016 There are several complications associated with the growing rods, but our project focused on characterizing the most common biomechanical complication, which is rod fracture. Again, we limited our preliminary study to traditional growing rods, but fractures and other complications occur in magnetically controlled growing rods as well.

Growing rod (GR) explants Previous study Contributed explants to GSSG / FDA study of explanted GRs Metallurgical analysis Failure initiation sites, modes Hill et al, GSSG SRS 2015 Continuing to collect explants for fatigue testing Current standards for testing spinal instrumentation, eg, vertebrectomy models, do not well simulate biomechanical conditions of growing rod constructs Anchor numbers, active length, sagittal plane moment arm

Purpose Specific aims Hypothesis: Preliminary clinical Toward determining if fatigue properties of explanted growing rods differ from those of new rods as assembled in configuration at pre-explant radiographs Define patient and instrumentation parameters relevant to biomechanical tests of GR constructs Compare to test standards and prior biomechanical studies Design biomechanical tests which simulate clinical construct conditions Hypothesis: Preliminary clinical Active length and thoracic spine length (T1-T12) increase over the implantation period

Methods Prospective Patient & instrumentation variables Longitudinal clinical and radiographic study IRB approved, parental consent Biomechanical test development Inclusion criteria EOS patients with GR construct removed for any reason Exclusion criteria Trauma prior to explant Explants altered after removal Patient & instrumentation variables Height, weight, gender, age Instrumentation components and assembly Primary curve Sagittal moment arm Active length of GR Locations of set screws, connectors, anchors Rod curvatures, visible defects Preliminary statistics (n=7) Active length (La) & T1-T12 length (Lt) Paired, one-tailed t-tests Bonferroni (α=0.05/2=0.025)

Results 7 patients enrolled At explant, age 8.9 yrs (± 2.2) 5 males, 2 females Curve corrected from 64° (±16) to 47° (±16) Rods: All 4.5 mm dual rods 4 Ti, 2 CoCr, 1 SS Anchors 5 side-by-side, 2 tandem 1 rod fracture At explant, age 8.9 yrs (± 2.2) Implant duration 3.0 years (±0.8), height 118cm (±14), weight 22.8kg (±4.8), moment arm 73 mm (±25). La increased from 19.8cm (± 3.4) to 21.8cm (± 3.0) (p<0.01) Lt increased from 15.63cm (±1.8) to 18.5cm (±1.8) (p<0.01) Rod fracture

Results: Example Pre-implant Pre-Explant Explant

Results: Spine length increased First 7 patients Active length increased 2 cm (range 0.5 – 4.5) * T1-T12 length increased 3 cm (range 0.0 – 6.3) * * p<0.025 After defining our design limitations, we outlined the controls and unknowns. Some of the variables will be defined and controlled across all tests, while others will be controlled to match the in vivo configuration for that particular construct. We plan to measure the fatigue life (or number of cycles until rod failure) and the displacement per loading cycle for each construct. Next, we decided on the values that will be controlled across all constructs by considering ASTM standards, the goals of our test, and our findings in documenting metrics on the initial seven subjects. Finally, we sketched a draft of the test set up and built a prototype to test.

Biomechanical test design Fatigue, cyclic compression, flexion-extension Cyclic loading until failure or run-out to maximum cycles Vertebrectomy model Single rod All constructs Anchors: 2 pedicle screws per level, 2 levels per foundation Number of screws per anchor Moment arm ≥50 mm Alignment of anchors in coronal and sagittal planes Load: frequency 1 Hz; magnitude 100N / 10 N Number of cycles Primary outcome variable Cycles to failure, or Displacement at run-out

Methods: Biomechanical test design Glos DL, Olson EM, 2016 Moment arm 50mm Cyclic loading 100N / 10N Test frequency 1 Hz

Conclusions Biomechanical test that better simulates in-vivo GR conditions was defined from EOS patients and current standards Explanted and new, explant-matched, GRs will be tested Results may be expected to help clarify role of configuration vs implantation in rod failures Significance Verified increased thoracic spine growth Continuing need to reduce failures & complications of spine distraction constructs

References ASTM International, Standard F1717, 2015 Agarwal et al. The Spine Journal 2015; 15:1848-1855 Bess et al. J Bone Joint Surg Am. 2010; 92:2533-43 Folz et al. ORS 2016; 1697 Hill et al. 2015 SRS 2015; 33 Jain, Crawford. The Growing Spine, Ed. 2, 2016; Ch. 16: 265-292 Nguyen et al. Proc. IMechE v. 225 Part H: J Engineering in Medicine 2010; 194-198 Ponnappan et al. The Spine Journal 2009; 9:263-267 Shinohara et al. ORS 2016; 1702 Shorez et al. J ASTM 2010; 9:2 ID JAI103493; and ORS 2010; 1408 Singh et al. 2013 Spine 38:1546-53 Slivka et al. Spine Deformity; 2013 1:395-400 Upasani et al. Spine Deformity 2016; 4:344-350 Yang et al. J Child Orthop; 2009; 3:145-168