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

Abstract

Introduction: Distraction-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.

Methods: The 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).

Results: The 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.

Conclusions: VEPTR 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.

Fig. 1

7-year-old girl with a Goldenhar syndrome and a congenital 90° thoracolumbar kyphosis. a Prior to the index procedure at the age of 7 years, b 6 years after the index procedure and following eleven half-yearly distractions. Tremendous osseous remodelling occurred at the apical level T12/L1, and the vertebral bodies L2 to L5 significantly changed geometry: the height/depth ratio increased by one-third. During the same time the intervertebral discs lost height and the endplates became sclerotic

Fig. 2

4-year-old boy with congenital thoracic scoliosis treated with classic VEPTR-based expansive thoracostomy. The normally segmented lumbar spine was left alone. The patient, therefore, serves as a comparative control case with naturally developing geometry of the vertebral bodies from prior to the index procedure (a) to the follow-up (b). The height/depth ratio and the disc heights remained the same during the 4.5 years of observation

Fig. 3

a VBH*, VBD** and H/D ratio in group 1, b VBH*, VBD** and H/D ratio in group 2 (control group), c growth velocities for VBH* and VBD**. *Anterior vertebral body height, **vertebral body upper endplate depth (VBD) [1] in the sagittal plane