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

Abstract

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.

Smaller Intervertebral Disc Volume and More Disc Degeneration after Spinal Distraction in Scoliotic Children

“…GFSI induced changes in the bony spine, i.e., the vertebrae, have been hypothesized for long but there are very few reports on actual measurements of spine morphology under GFSI treatment. Hasler et al [ 6 ] have proposed an increase in vertebral height during treatment, which was measured on plain radiographs and is therefore susceptible to errors. Our own group has published data on reduced vertebral body volume in depth, below the level of T7, comparing untreated versus pre-treated children with MCGRs [ 8 ].…”

“…GFSI is frequently associated with heterotopic ossification and autofusion of the spine [ 3 , 4 , 5 ]. One of the concerns raised in the discussion is the effect of GFSI on the morphology of vertebrae and discs [ 6 ]. The evaluation of such changes is somewhat difficult because of several factors, such as a heterogeneous EOS population with many underlying diseases, genetic predispositions, multiple additional problems and disabilities.…”

“…In addition, measurements of morphologic changes, i.e., vertebral or disc shape, or even estimation of biomechanical properties like stiffness of the spine, have mostly been based on plain X-rays, with all the limitations that two-dimensional imaging have when attempting the evaluation of three-dimensional phenomena [ 7 ]. Hasler et al [ 6 ] have described some of the changes seen in lumbar vertebrae on radiographs after GFSI treatment and Lippross et al [ 8 ] reported reduced vertebral body volume using magnetically controlled growing rods (MCGR). To our knowledge, few data have been published on intervertebral disc changes after GFSI treatment [ 9 ] and no data after MCGRs, despite disc mobility ultimately determining spinal flexibility.…”

Radiographic Outcome and Complication Rate of 34 Graduates After Treatment With Vertical Expandable Prosthetic Titanium Rib (VEPTR): A Single Center Report

“…25 In contrast to "late-onset" pediatric scoliosis surgery with generally very good deformity correction, the ability to further correct the spine at the end of growth sparing surgery is limited. Altered vertebral anatomy with poor bone quality because of year-long implant-related stressshielding, 26 in combination with poorly perfused scar tissue and stiff high-grade deformities are the consequences of repetitive surgical lengthening procedures, making final instrumented fusion surgery in these patients highly demanding. In 62% of the growing rod graduates reported by Flynn et al 12 the spine was described as completely stiff, allowing for > 50% deformity correction in only 15% of patients.…”

A biomechanical study on the effect of lengthening magnitude on spine off-loading in magnetically controlled growing rod surgery: Implications on lengthening frequency

“…[21][22][23][24] Previous studies on vertical expandable prosthetic titanium rib have observed changes in morphology of the vertebral body and pedicles after distraction, stating that stress shielding may be a consequence of distraction forces. 25,26 If severe, it could affect instrumentation and ability to correct the curvature further during final fusion. [27][28][29] The optimal frequency and magnitude of MCGR distraction are subject to debate, with variations ranging from monthly to 6-monthly distractions.…”