A biomechanical investigation of dual growing rods used for fusionless scoliosis correction

Quick, Mark; Grant, Caroline A.; Adam, Clayton J.; Askin, Geoffrey N.; Labrom, Robert D.; Pearcy, Mark J.

doi:10.1016/j.clinbiomech.2014.11.008

Cited by 9 publications

(9 citation statements)

References 37 publications

(47 reference statements)

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“…However, those data would help to understand the somehow contradictory biomechanical effects of spinal implants, which keep the spine under distraction over a long period of growth but at the same time immobilize the bridged section. Instrumented spinal distraction entails deprivation from axial loading and presumably from rotatory and bending forces [ 14 ]. Morphological and biomechanical changes gain importance in case of implant removal at the end of growth with subsequent re-exposure of the spine to natural forces.…”

Section: Discussionmentioning

confidence: 99%

“…However, this may be different after spanning multiple spinal segments with stiff implants over many years and concomitant degeneration of facet joints. The surgical concept of harnessing growth by growth modulating implants deprives the spine from axial loads and increases axial rotation stiffness in experimental biomechanical investigations in porcine spines [ 14 ]. Thereby, it seems to impact upon the biological integrity of the spine and to conflict with the underlying non-fusion strategy.…”

Section: Discussionmentioning

confidence: 99%

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Metamorphosis of human lumbar vertebrae induced by VEPTR growth modulation and stress shielding

Hasler

Studer

Büchler

2015

Journal of Children's Orthopaedics

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IntroductionDistraction-based spinal growth modulation by growing rods or vertical expandable prosthetic titanium ribs (VEPTRs) is the mainstay of instrumented operative strategies to correct early onset spinal deformities. In order to objectify the benefits, it has become common sense to measure the gain in spine height by assessing T1-S1 distance on anteroposterior (AP) radiographs. However, by ignoring growth changes on vertebral levels and by limiting measurement to one plane, valuable data is missed regarding the three-dimensional (3D) effects of growth modulation. This information might be interesting when it comes to final fusion or, even more so, when the protective growing implants are removed and the spine re-exposed to physiologic forces at the end of growth.MethodsThe goal of this retrospective radiographic study was to assess the growth modulating impact of year-long, distraction-based VEPTR treatment on the morphology of single vertebral bodies. We digitally measured lumbar vertebral body height (VBH) and upper endplate depth (VBD) at the time of the index procedure and at follow-up in nine patients with rib-to-ileum constructs (G1) spanning an anatomically normal lumbar spine. Nine patients with congenital thoracic scoliosis and VEPTR rib-to-rib constructs, but uninstrumented lumbar spines, served as controls (G2). All had undergone more than eight half-yearly VEPTR expansions. A Wilcoxon signed-rank test was used for statistical comparison of initial and follow-up VBH, VBD and height/depth (H/D) ratio (significance level 0.05).ResultsThe average age was 7.1 years (G1) and 5.2 year (G2, p > 0.05) at initial surgery; the average overall follow-up time was 5.5 years (p = 1). In both groups, VBH increased significantly without a significant intergroup difference. Group 1 did not show significant growth in depth, whereas VBD increased significantly in the control group. As a consequence, the H/D ratio increased significantly in group 1 whereas it remained unchanged in group 2. The growth rate for height in mm/year was 1.4 (group 1) and 1.1 (group 2, p = 0.45), and for depth, it was −0.3 and 1.1 (p < 0.05), respectively.ConclusionsVEPTR growth modulating treatment alters the geometry of vertebral bodies by increasing the H/D ratio. We hypothesize that the implant-related deprivation from axial loads (stress-shielding) impairs anteroposterior growth. The biomechanical consequence of such slender vertebrae when exposed to unprotected loads in case of definitive VEPTR removal at the end of growth is uncertain.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Metamorphosis of human lumbar vertebrae induced by VEPTR growth modulation and stress shielding

Hasler

Studer

Büchler

2015

Journal of Children's Orthopaedics

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“…However, the porcine spines utilized were nondeformed and thus may represent substantially different biomechanical properties from spines with scoliotic curves 38 Additionally, the use of a 4 Nm moment target for testing may represent another potential limitation as this may vary from the physiologic forces seen in EOS. However, this value has been used previously in porcine biomechanical testing 25,26 . Our in vitro testing algorithm was to test the rigid rod construct before the TelGR construct; this process was chosen for ease of reproducibility but the failure to randomize may be a confounder to our results.…”

Section: Discussionmentioning

confidence: 99%

Novel use of telescoping growth rods in treatment of early onset scoliosis: An in vivo and in vitro study in a porcine model

et al. 2018

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Introduction Treatment of early‐onset scoliosis (EOS) can be difficult. Various forms of growing rods exist to correct deformity while delaying definitive spinal fusion. The disadvantage of traditional growing rods is need for repeated surgical lengthening procedures. Telescoping growth rods (TelGR) are a prototype new, guided growth technology with a rod mechanism that allows spontaneous longitudinal growth over time without manual lengthening. We hypothesized that the TelGR system will permit unrestricted growth with limited complications through 12 weeks in vivo , and that the range of motion (RoM) in each of three directions and stiffness of the TelGR system would not be significantly different than the rigid rod system in vitro . Materials and Methods In vivo : Six immature pigs were surgically implanted with TelGR with cephalad fixation at T6‐7 and caudal fixation at T14‐L1. Radiographs of the involved vertebral segments were measured postoperatively and after 12 weeks. In vitro : A robotic testing system was utilized for flexibility tests in flexion‐extension (FE), lateral bending (LB), and axial rotation (AR) of eight immature porcine specimens (T3‐T15). Testing was performed on both dual rigid rods and bilateral TelGR with instrumentation at T4‐5 and T13‐14. Results In vivo : Over the 12‐week period, the rod length of the TelGR increased an average of 65 mm. In vitro : TelGR demonstrated significantly increased motion in LB and AR RoM compared with rigid rods. No difference was noted in FE RoM. Discussion The in vivo results in this study showed expected skeletal growth with spines instrumented with TelGR. In vitro findings of increased RoM in AR and LB suggest that the TelGR system may be less rigid than traditional growing rods. Treatment with TelGR might, if proven efficacious in the clinical setting, decrease the need for repeated surgical intervention compared with traditional growing rods. This study adds to the limited body of biomechanical evidence examining guided growth technology.

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“…Rod material, number of rods, addition of cross connectors, and rod attachment types have all been shown to increase the stiffness and/or decrease the mobility of the spine. [17–20, 36, 37] Other studies have shown that a less rigid construct produces fewer adjacent level changes. [20] This study utilized a rod construct with a simulated rib-to-lumbar attachment, producing a comparably flexible system, which had not been tested previously.…”

Section: Discussionmentioning

confidence: 99%

“…Currently, there is little biomechanical data regarding in-plane range of motion, out-of-plane range of motion, stiffness, motion symmetry, or disc pressure within the thoracic spine under the constraint of a pediatric unilateral distraction rod. [17–20] Motion, stiffness, and pressure experienced at an intervertebral joint are all clinically rooted biomechanical measures that can be used to monitor the integrity of the joint and the spine as a system. [21–24] By evaluating the implant biomechanics more closely, it may be possible to improve clinical success and reduce complications and the need for multiple procedures.…”

Section: Introductionmentioning

confidence: 99%

Biomechanical Evaluation of a Growth-Friendly Rod Construct

et al. 2017

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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.

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A biomechanical investigation of dual growing rods used for fusionless scoliosis correction

Abstract: Based on these findings, the Semi-constrained growing rods were shown to not increase axial rotation stiffness compared with un-instrumented spines. This is thought to provide a more physiological environment for the growing spine compared to dual rigid rod constructs.

Cited by 9 publications

References 37 publications

Metamorphosis of human lumbar vertebrae induced by VEPTR growth modulation and stress shielding

Metamorphosis of human lumbar vertebrae induced by VEPTR growth modulation and stress shielding

Novel use of telescoping growth rods in treatment of early onset scoliosis: An in vivo and in vitro study in a porcine model

Biomechanical Evaluation of a Growth-Friendly Rod Construct

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