Analysis of the Biomechanical Effects of Vertebral Body Tethering with Apical Fusion.

Study design: Biomechanical study by using a multi-body simulation approach.

Objective: Objectification of spinal biomechanics after Vertebral Body Tethering with and without Apical Fusion.

Summary of background data: Vertebral body tethering, a motion preserving surgical technique for correction of adolescent idiopathic scoliosis, is increasingly being used for thoracolumbar curves. However, tether breakage remains a common problem with breakage rates up to 60% for TL curves. Therefore, surgeons have begun to adapt their surgical technique by fusing the apex. The short-term clinical studies show a significant reduction of the tether breakage rate to 10%, but little is known about the biomechanical reasons. Therefore, this study analyzes the intervertebral compression and tether force in a tethered spine without apical fusion and in a tethered spine with apical fusion between L1/2.

Methods: A multi-body simulation approach was chosen to analyze the biomechanical effects of two surgical techniques during different physiological movements. The tether and intervertebral compression forces in the different instrumented segments are once analyzed for a T10-L3 tethered spine and once for a T10-L3 tethered with additional L1/2 fusion using an intervertebral cage.

Results: VBT with apical fusion reduces the prevailing tether forces not only at the fused level by nearly 861 N, but also at the adjacent spinal segments by around 100 N. However, a significant increase in intervertebral compression force of approximately 706 N can be observed, especially at the adjacent spinal segments.

Conclusion: L1/2 fusion in a tethered spine reduces tether forces in adjacent segments and thus might decrease the rate of tether breakage. However, fusion results in increased intervertebral compression forces by up to 31% compared to an unfused spine. Long-term clinical studies are needed to further analyze and evaluate the biomechanical consequences.