The growth modulating effects of tether tension on vertebral growth are biphasic: a study of posterior vertebral body tethering (pVBT) in a novel kyphotic porcine model.

Abstract

PURPOSE: To measure the effects of posterior vertebral tethering (pVBT) on disc pressure and the effect of tether tension on growth modulation in the hyperkyphotic swine model, and to use computational modeling to predict growth modulation in scenarios unable to be tested in the animal model. METHODS: Swine were divided into non-operative control, single-level apical pVBT, or multi-level posterior pVBT groups. Pulsed fluorochrome labeling was used to measure regional vertebral growth rates, digital radiographs to assess changes in vertebral alignment, and pressure transducers to measure intervertebral disc pressures. Finite element analysis (FEA) was performed to simulate tether-mediated disc space correction. RESULTS: Kyphotic swine had significantly greater angular kyphosis than control swine at 11- and 13-weeks, and deformities increased from 2 to 5 months of age. At 2-weeks post-operative, high-tension single level tethering resulted in significantly greater growth modulation than low-tension (53 ± 43% vs - 1 ± 15%, p = 0.03) or non-operative controls (p = 0.01), however, at 2-4 weeks, growth modulation was normalized between the low and high tensioned cohorts (14 ± 11% vs 10 ± 10%, p = 0.6). The FEA predicted that growth plate stress distributions worsen as the average disc height post-realignment is decreased. CONCLUSION: Increased tether tension results in more effective early growth modulation in the young flexible spine without increasing disc pressure, however, these tension-related benefits are transitory as growth modulation becomes load-independent with time. Computational modeling predicted that in the less flexible spine, vertebral growth may be arrested rather than modulated.