Relevant biomechanics to growth modulation

Tricia St Hilaire, Brian Snyder, Robert M. Campbell

Research output: Chapter in Book/Report/Conference proceedingChapter

1 Scopus citations

Abstract

When surgical correction is indicated in patients with early onset scoliosis (EOS), the goals are to maximize correction, preserve thoracic growth, and avoid complications such as device dislodgement, fatigue fractures, infection, and other soft tissue problems. The aims of this chapter are to (1) assist physicians in understanding the basic biomechanical principles to avoid failure, correct spinal deformity, and maintain growth in children with complex deformity and (2) establish the groundwork of potential performance criteria for new growth modulation devices. Basic biomechanical properties that should be taken into account when selecting the appropriate treatment for a growing child include the strength of material, type of material, and structural geometry of the device. Each rod used to correct a deformity is able to withstand a certain amount of load as determined by the shape and composition of the rod. The load is the force the deformed spine places on the rod as well as any additional forces from the child’s play activities. In scoliosis, a force is applied to correct the deformity. After the initial correction, a constant force remains on the spine. Over time, the progressive correction of the deformity that occurs is due to creep. In posterior-based systems, bending rigidity and the unsupported working length between anchor points dictate the stability of the system. Specifically, the longer the unsupported working length of rod is, the less rigid the system and the larger the deflection. The rod diameter in posterior systems should be large enough to have the structural rigidity capable of withstanding biomechanical loads seen over time. The most common mechanism of rod breakage is fatigue failure. Fatigue failure is like repetitively bending a coat hanger wire and eventually having it break. The fatigue life of a system depends on the fluctuation of stress, the mean stress level, and the way it varies over time. In an active child, small degrees of rod bending and torsion with normal activities can fracture a rod in time. Fatigue failure can be accelerated by stress concentrations, stress fluctuations, corrosion, and surface stress. In anterior-based systems, correction can be immediate by tensioning the tether which applies an initial corrective moment in compression, followed by correction that is passive, so that compression is generated as longitudinal growth is inhibited by the staple or the tether over time. When using anterior systems that inhibit growth, the treating physician should also have an idea of the expected growth remaining in each spine segment to be able to predict expected correction over time and avoid overcorrection. The system, the application, and its use over time should be evaluated as a whole to make the best choice for the patient. Treatment objectives are often attained by customizing the structural properties of the device, the mode of fixation, and loading conditions to the individual patient’s needs. Successful growth modulation system should avoid sharp transitions in the structural geometry of the system that could lead to stress risers and minimize the contact of dissimilar materials to decrease potential for corrosion. In the development of new systems for the growing child, there is a need for sound performance criteria based on these principles. Preclinical testing is essential to ensure that devices are able to withstand both the corrective and superimposed forces seen during distraction and over time. Mechanical testing would ideally reproduce the same stress levels and fluctuations that are seen in vivo. New guidelines and procedures developed specifically for pediatric patients are needed so that the biomechanical properties of growth modulation systems can be compared in a consistent manner.

Original languageEnglish (US)
Title of host publicationThe Growing Spine
Subtitle of host publicationManagement of Spinal Disorders in Young Children, Second Edition
PublisherSpringer Berlin Heidelberg
Pages31-45
Number of pages15
ISBN (Electronic)9783662482841
ISBN (Print)9783662482834
DOIs
StatePublished - Jan 1 2015
Externally publishedYes

Keywords

  • Biomechanics
  • Deformity
  • Failure
  • Growth modulation
  • Load
  • Material
  • Strength
  • Testing

ASJC Scopus subject areas

  • Medicine(all)

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