1. 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

2. 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.

3. 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

4. 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

5. 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)

6. 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

7. Results: Example
Pre-implant
Pre-Explant
Explant

8. 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.

9. 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

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

11. 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

12. References ASTM International, Standard F1717, 2015
Agarwal et al. The Spine Journal 2015; 15:
Bess et al. J Bone Joint Surg Am. 2010; 92:
Folz et al. ORS 2016; 1697
Hill et al SRS 2015; 33
Jain, Crawford. The Growing Spine, Ed. 2, 2016; Ch. 16:
Nguyen et al. Proc. IMechE v. 225 Part H: J Engineering in Medicine 2010;
Ponnappan et al. The Spine Journal 2009; 9:
Shinohara et al. ORS 2016; 1702
Shorez et al. J ASTM 2010; 9:2 ID JAI103493; and ORS 2010; 1408
Singh et al Spine 38:
Slivka et al. Spine Deformity; :
Upasani et al. Spine Deformity 2016; 4:
Yang et al. J Child Orthop; 2009; 3: