Intervertebral Disc Health Preservation After Six Months of Spinal Growth Modulation

Abstract

Spinal growth modulation has been proposed as a non-fusion strategy for treatment of idiopathic scoliosis, although the effect of this treatment modality on intervertebral disc health has not been evaluated in detail. The objectives of this in vivo study were to assess the creation of three-dimensional spinal deformity during six months of growth modulation compared with that in sham-surgery controls, and to compare, with use of magnetic resonance imaging (MRI), gross morphological, histological, and biochemical analyses, disc health between control animals and animals treated with a spinal tether. Six immature Yucatan mini-pigs underwent anterior spinal instrumentation with vertebral screws connected by a polyethylene tether over four consecutive thoracic vertebrae (T8-T11). An additional six animals underwent sham surgery (screw placement only [the control group]). Radiographs were obtained preoperatively, postoperatively, and monthly thereafter during six months of growth. Computed tomography (CT) and MRI studies were performed ex vivo, and the spines were sectioned for histological and biochemical analyses. Multivariate analysis of variance (MANOVA) was used to compare six-month postoperative data between the control and tethered animals, with the alpha level of significance set at 0.05. Radiographs and CT images demonstrated the creation of significant three-dimensional deformity (p < 0.013) in the tethered animals compared with the controls. Macroscopic, MRI, and histological evaluation revealed no signs of disc degeneration, with a bulging gelatinous nucleus pulposus, discrete fibrous anular lamellae, and uniformly hyperintense T2-signal intensity within the nuclei pulposi. Biochemical analysis demonstrated no significant difference in the nuclei pulposus between the tethered and control vertebrae; however, the water content (p < 0.001) of both sides of the anulus fibrosus and the glycosaminoglycan content (p < 0.001) of the left side of the anulus fibrosus differed significantly between the two groups. Six months of spinal growth modulation created significant spinal deformity in all three planes compared with what was found in the sham-surgery controls. Although disc health was qualitatively maintained, quantitative changes in the anulus fibrosus water content and the disc height were observed on the side opposite to the tether. These changes likely represent metabolic responses of the discs to compressive loads generated by the flexible tether.