Effect of implant stiffness on spinal growth in the pig spine

Introduction According to the ‘vicious cycle’ hypothesis proposed by Dr Stokes, sensitivity to load has been implicated in the progression of spinal deformity during growth due to the reaction of vertebrae to mechanical loads on their growth plate. Mechanical loads on the vertebrae are altered by the mechanical stiffness of the spinal implant. Unfortunately, the relationship between the implant stiffness and the modulation of spinal growth is not quantified. In vitro and finite element studies involving multiple pig spines and segments (T1–T4, T5–T8, and T9–T12) were used. Springs of varying length (320 N/m stiffness) and a metal link (64.5 × 106 N/m stiffness) were attached to adjacent vertebrae and the spines distracted to model growth. This review aims to analyse the study investigating the effect of implant stiffness on this growth using experimental and finite element techniques. Discussion It is shown that the addition of an implant to the spinal column will increase the stiffness of the spine. Furthermore, as the stiffness increases, the distraction of the spine decreases. In addition, asymmetric placement of the implant leads to rotation of the spine segment during distraction. Conclusion Spinal devices with different mechanical properties yield variable stiffness of the spine segments, as well as displacement and rotations, which will further affect the longitudinal growth of the spine. Introduction Much debate and research in the study of scoliosis in the past decade has focused on the ‘vicious cycle’ hypothesis. This hypothesis maintains that loads on the spine are asymmetric and involved in curve progression through changes in bone and disc growth and hence to vertebral body and disc wedging1. Axial and longitudinal loading of vertebral bodies has been shown to modulate the growth rate of the body relative to the control vertebrae1–4. Growth is modulated by increased compressive forces. As mechanical loads on the vertebrae are altered by the mechanical stiffness of the spinal implant, it is possible that the implant stiffness will modulate growth. Thus, the appropriate goal of surgical treatment for early onset scoliosis is to correct the progression of the spinal curvature while allowing growth of the spine and its adjacent structures. This has led to the introduction of fusionless implant devices, multiple level staples, vertical expanding prosthesis and a dual growing rod technique5,6. In a study using the dual growing rod technique, Akbarnia et al.7 reported an improvement of scoliosis from 82° to 38°, while T1-S1 length increased from 23 cm to 32.7 cm at the last follow-up. Unlike traditional instrumentations, such as Cotrel–Dubousset, which are very rigid and affect spine growth, these fusionless devices are more compliant. Nevertheless, the introduction of any implant to the spinal column will alter the mechanical behaviour of the spinal column and affect spinal growth. Glos et al.8 investigated the in vivo effect of a staple-like implant on the baseline disc stress in a pig spine. It was found that the implant does affect the stress increasing the mean stress between 0.1 and 0.2 MPa. Unfortunately, the authors performed the study for only one type of implant, so the relationship between growth modulation and the variation of implant stiffness is unknown. Thus, a study was undertaken to investigate different implant stiffness, spinal linear and angular displacements using experimental and finite element techniques. The aim of this review is to assess this study and the results obtained.