Growing construct forces in early-onset scoliosis: How do TGR and VEPTR compare?

Growing construct expansion surgeries in the setting of early-onset scoliosis (EOS) involve surgeon's feel to determine the optimal amount of distraction force to maximize correction and spinal length gained without compromising anchor integrity. Growing construct designs may require different amounts of distraction force to lengthen the spine or chest wall depending on the device used and various patient factors. Understanding these variations could guide appropriate force levels for the rational design for next-generation remote/external distraction devices and ultimately patient/deformity-specific lengthening forces.

The purpose of this study was to determine distraction force levels for expansion events in traditional growing rods (TGRs) and vertical expandable prosthetic titanium rib (VEPTR) constructs across patients with various anchor attachment points and etiologies of EOS.

With IRB approval, two different distraction devices (VEPTR distractor and TGR/rod distractor) were outfitted with electronic strain gauges to measure distraction force. Force data were recorded at a rate of 40 Hz during each surgery via a laptop connected to a PowerLab 8/35 data acquisition system. After each surgery, the distractor was validated on a custom calibration device. The data collected were then analyzed to determine the maximum force and average force during every distraction.

From February 2019 to June 2023, a total of 146 in vivo distractions were collected on 52 patients with VEPTRs and 28 in vivo distractions were collected on 9 patients with traditional growing rods. TGR procedures required significantly higher forces than VEPTR (437.89 N vs 349.10 N, p < 0.001).

Modification of two different distraction devices with electronic strain gauges identified differences in distraction force between traditional growing rods and VEPTR constructs during expansion surgeries in children with EOS. Our findings highlight that implant type and potentially other factors may play significant roles in the amount of force required for lengthening the spine and chest wall. Current market implant technology (MCGR maximum force of 187 N) is likely underpowered compared to traditional technology across all construct types and disease classifications. These insights should inform the development of next-generation remote lengthening construct design.