Physeal effects of posterior VBT are not uniform throughout a multi-tether construct in the kyphotic swine model

To assess if vertebral location within posterior VBT construct affected the overall vertebral growth rate, % growth modulation, and to determine if the proximal and distal physes within each tethered disk space responded similarly to tethering.

Six hyper-kyphotic swine underwent multi-level posterior compressive tethering. Pulsed fluorochrome labeling was performed. Growth rates, % growth modulation, physeal zonal thicknesses, vertebral epiphyseal ossification, location of central nucleus pulposis, and vertebral shape by location within tether construct were measured.

Mean growth rates were similar throughout the vertebral levels studied and no significant difference was found between tethered or adjacent levels. However, the mean thickness of tethered physes was thinner than the adjacent uninstrumented physes (621 ± 36 μm vs 728 ± 52 μm, p = 0.001, adj p = 0.004) and this difference appeared to be primarily due to differences in the proliferative zone. The most proximal instrumented vertebral level, that corresponded anatomically to the 2nd and 3rd most distal thoracic level), had the greatest % growth modulation (55 ± 17%) compared to other instrumented levels (p = 0.0001, adj p value 0.004). No significant differences in growth rate, physeal thickness, or % growth modulation were found between the distal and proximal vertebral physes within each tethered disk. Adjacent level junctional kyphosis was observed through the significant reversal (negative) growth modulation at levels outside the construct (p < 0.0001, adj p value 0.004).

Mean vertebral growth was not significantly inhibited by a posterior tether. Despite uniform tensioning throughout each construct, the most proximal tethered level experienced the most growth modulation indicating that growth may not be modulated the same at each level within a construct. Additionally, asymmetric vertebral growth may contribute to junctional kyphosis in the growing spine.