Biomechanical Evaluation of a Growth-Friendly Rod Construct

Abstract

Background Distraction type rods mechanically stabilize the thorax and improve lung growth and function by applying distraction forces at the rib, spine, pelvis, or a combination of locations. However, the amount of stability the rods provide and the amount the thorax needs is unknown. Methods Five freshly frozen and thawed cadaveric thoracic spine specimens were tested lateral bending, flexion/extension, and axial rotation in displacement control (1°/sec) to a load limit of ± 5 Nm for five cycles after which a growth-friendly unilateral rod was placed in a simulated rib-to-lumbar attachment along the right side. The specimens were tested again the same modes of bending. From the seven Optotrak Orthopedic Research Pin markers (Northern Digital Inc., Waterloo, ON, Canada) inserted into the top potting to denote T1, and the right pedicles at T2, T4, T5, T8, T9, and T11 and the Standard Needle Tip Pressure Transducers (Gaeltech, Isle of Skye, Scotland) inserted into the T4/T5 and T8/T9 discs, motion, stiffness, and pressure data were calculated. Parameters from the third cycle of the intact case and the construct case were compared using two-tailed paired t-tests with 0.05 as the level of significance. Results With the construct attached, the T1–T4 segment showed a 30% increase in NZS during extension (p = 0.001); the T8–T12 segment experienced a 63% reduction in the in-plane ROM during flexion (p = 0.04); and the T8/T9 spinal motion unit had a significant decrease of 24% in EZS during left axial rotation (p = 0.04). Conclusions It’s clear the device as tested here does not produce large biomechanical changes, but the balance between providing desired changes while preventing complications remains difficult. Clinical Relevance Investigating the biomechanical effect growth-friendly rods have on the thoracic spine could lead to better understanding of treatment outcomes, both positive and negative.

The rib cage reduces intervertebral disc pressures in cadaveric thoracic spines by sharing loading under applied dynamic moments

“…Eight fresh-frozen human cadaveric thoracic spines (T1-T12) with the rib cage intact were obtained (4 female, 4 male, age range 61–71). Data from experimental testing of these specimens has been previously reported (Anderson et al, 2016; Galvis et al, 2017; Sis et al, 2016), but the disc pressures that are the focus of the current analysis have not been previously examined. Specimens were potted at T1 and T12, and positioned upright in a testing machine (Applied Test Systems, Butler, PA) that can apply pure unconstrained moments at the T1 level.…”

Serum Titanium Levels Remain Elevated But Urine Titanium is Undetectable in Children With Early Onset Scoliosis Undergoing Growth-Friendly Surgical Treatment: A Prospective Study

“…Although growth-friendly constructs contain fewer implants compared with instrumented spinal fusion, the implants sustain higher loads and are more frequently manipulated. Excessive or increased intersegmental motion from sustained load or repeated lengthenings may cause continued generation of metal wear debris 23,24 . Our VEPTR group had the most total number of lengthenings, which may account for the higher levels of serum titanium.…”

Mechanical Analysis of 3 Posterior Fusion Assemblies Intended to Cross the Cervicothoracic Junction

“…Customized MATLAB (MathWorks, Natick, MA) code was used to translate the optical motion tracking data into rotation of the specimens using Euler decomposition techniques. [8][9][10] The range of motion (ROM) and stiffness were then calculated for each specimen (Fig. 4).…”