• Burt Yaszay, Joshua D Doan, Kevin C Parvaresh, and Christine L Farnsworth.
• Division of Orthopedics, Rady Children's Hospital-San Diego, San Diego, CA.
• Spine. 2017 Apr 1; 42 (7): 443-449.

Study DesignBiomechanical evaluation using porcine spines.ObjectiveCompare the fixation strength of two currently used fusionless adolescent idiopathic scoliosis correction techniques following cyclic loading using porcine spines.Summary Of Background DataThe ability of fusionless implants to control or correct scoliosis in a growing patient requires such implants to maintain spinal fixation. Because they cross the disc, motion may weaken fixation over time.MethodsEight pig spines were divided into cycled segments (T10-T13) and uncycled segments (T7-T8, L2-L3). Initial range of motion (ROM) was determined in torsion, flexion-extension, and lateral bending (0.5°/s to 1.75 N·m).Staple group (n = 4): 6 mm parallel staples were inserted on the right anterolateral spine across each intervertebral disc. Cycled segments received six staples (three adjacent discs) and uncycled segments received four staples (two separate discs).Tether group (n = 4): 5.35 × 35 mm right anterolateral vertebral body screws were placed into each vertebra. Cycled segments received four screws and uncycled segments received four screws. Screws in cycled segments were connected with a flexible tether tensioned to straight alignment.ROM of instrumented cycled segments was measured, and then segments were loaded to the measured ROM in flexion-extension (2000 cycles), lateral bending (1000 cycles), and axial rotation (2000 cycles). Implants were axially loaded to failure. Parametric tests compared pre- to postimplant ROM; nonparametric tests compared staple to screw pullout strength; P < 0.05 was significant.ResultsThere were no differences in ROM before instrumentation between groups. ROM was not changed except tethers decreased left lateral bending (-6.2°). Although staple pullout was less than screw pullout for cycled and uncycled segments (P < 0.05 and P = 0.057, respectively), there was no difference in pullout strength with and without cyclic loading for either group (P = 0.4).ConclusionTethers decreased lateral bending away from the tether. Screws had almost five times greater load to failure than staples. Five thousand cycles did not result in loosening of either staple or tether screws.Level Of EvidenceN/A.

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