Effects of Sustained Tensile Distraction on Vertebrae and Intervertebral Disc Growth

Background: Directed growth modulation is commonly utilized as a surgical treatment for early-onset scoliosis. Growing rods are instrumented on the spine and apply sustained tension on the immature spine for a substantial amount of time, with the clinical goal of accommodating axial expansion of the spine. Despite the use of growing rods in humans, the mechanobiology of the spinal tissues under tensile loading remains relatively unknown. To bridge this knowledge gap, we developed a preclinical mouse model that allows for mechanistic investigations of sustained tension on the spine. Methods: Using custom 3D-printed washers and tunable springs, we distracted across the seventh and ninth caudal vertebrae of adolescent and young adult C57BL/6 female mice with forces that were approximately 2 times the body mass of the animal. The springs were replaced weekly to maintain tension for the duration of the experiment. A set of 6-week-old animals were first instrumented for 10 weeks to evaluate the feasibility and tolerability. Subsequently, the 6- and 12-week-old experimental animals were instrumented until they were 20 weeks of age in order to evaluate the effects of tension until adulthood. The spines were monitored using digital radiography and micro-computed tomography (µCT), and the intervertebral discs (IVDs) were evaluated using mechanical testing and compositional assays. Results: The device was well tolerated and caused no notable complications. The tensile forces lengthened the vertebrae in the 6-week-old animals that were instrumented for 14 weeks and in the 12-week-old animals that were instrumented for 8 weeks. Increased IVD heights were observed in the 6-week-old animals but not in the 12-week-old animals. The porosity of the vertebral end plates increased following instrumentation in all groups but progressively recovered over time. Conclusions: Distraction accelerated the lengthening of the vertebrae and the heightening of the IVD, with no observable degeneration or decline in the mechanical performance of the IVDs for these distraction conditions. Clinical Relevance: This model will be useful for investigating how spinal tissues adapt to directed growth modulation with maturation and aging.