Impact of growth on spinal biomechanics and tether force in VBT: a simulation study

Vertebral Body Tethering (VBT) allows correction of scoliosis while preserving the spinal mobility. Despite an increasing availability of data on clinical experience and surgical techniques, there is still a lack of knowledge on the underlying biomechanics, specifically when considered in combination with growth. Therefore, the purpose of this study is to develop a growing flexible spine model to investigate spinal biomechanics after VBT surgery.

A multi-body simulation approach was chosen. A growing, flexible thoracolumbar VBT spine model was developed to analyze the spinal biomechanics during various physiological movements and growth. The model includes a flexible spine with a VBT device and a musculoskeletal system. Therefore, the resulting tether tension and intervertebral compression force can be calculated with respect to the human anatomy and material properties of the VBT device.

During growth, the tether and compression forces increase continuously with the highest forces between L1 and L2. The highest tether force is measured at 50° lateral bend at 200 N pre-tension. The compression forces in a tethered spine are during adolescence up to twice higher than in a healthy spine.

The simulated biomechanical data provides insight into the forces exerted on the spine during various physiological movements and the remaining growth. They are consistent with previously published clinical data and underlie the finding that age at surgery or residual growth doesn't greatly affect tether forces. During growth, however, intervertebral compression forces increase.