An in vitro study to identify worst-case parameters for assessing mechanical performance of anterior vertebral body tethering (AVBT) systems.

PURPOSE: Anterior vertebral body tethering (AVBT) is a novel surgical option for scoliosis patients that preserves spinal mobility. However, tether-related failures such as tether breakage are common, and cause of these failures is unknown. Furthermore, there is a lack of data on how different design and surgical factors influence the subsequent in vivo performance of these AVBT systems. This study aims to examine the mechanical durability of a mock AVBT system under various mechanical loading scenarios resulting from important surgical factors.

METHODS

A mock AVBT system was designed using Titanium screws and Polyethylene Terephthalate (PET) tether to evaluate mechanical performance under various clinically relevant loading conditions. Static and dynamic tensile tests assessed the effects of screw angulation (0°/40°), tether gauge length (length between the vertebral anchors: 25/76 mm), setscrew torque (5/6/7 Nm), and retightening. Constructs were tested using Instron equipment; measuring stiffness, yield, and failure loads. Statistical analyses compared configurations to identify factors influencing AVBT durability.

RESULTS

For this mock AVBT system, failure primarily occurred at the tether/setscrew junction due to slippage. Greater screw angulation (40°) increased yield and runout loads compared to 0°. Changing the gauge length had minimal effect on failure loads. Furthermore, the higher setscrew torque (7 Nm) improved strength and fatigue resistance, while setscrew retightening also increased yield and runout loads.

CONCLUSIONS

Due to the infancy of this technology, there is a need for deeper understanding of the failure mechanics of AVBT systems. These findings identify testing parameters that affect mechanical performance and suggest future research directions for assessing the mechanical integrity of AVBT devices.