Biomechanical Evaluation of 4 Different Foundation Constructs Commonly Used in Growing Spine Surgery: Are Rib Anchors Comparable to Spine Anchors?

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Abstract
In vitro animal model. To compare the strength of 4 different anchor constructs commonly used as foundations in growing spine surgery. Children with progressive early-onset scoliosis often require surgical intervention to control the deformity and allow continued growth. The foundation sites of growing spine constructs take a significant load and can fail. This study compares the strength of 4 commonly used constructs applying the same load in a porcine model. Forty immature porcine specimens including soft tissues (10 per group) were instrumented with 1 of 4 bilateral proximal anchors at T5-T6. The four groups were: screw-screw (SS), lamina hook-hook (HH), rib hook-hook (RR), and transverse process to lamina hook-hook (TPL). The entire specimen was kept intact except for surgical site exposure. A unique fixture was designed to brace the specimen and provide a counterforce. The ultimate load was identified as the greatest load recorded for a construct and analyzed by a set of 1-way analysis of variance using the SPSS 12.0 statistical package. All specimens eventually failed at the bone-anchor interface. No failures were observed in the instrumentation used. The means and standard deviations of ultimate loads were measured as RR (429 ± 133 N), SS (349 ± 89 N), HH (283 ± 48 N), and TPL (236 ± 60 N). There was no statistically significant difference between the following construct pairs: RR/SS, SS/HH, and HH/TPL. Young's modulus was calculated for each construct type and no statistically significant difference was determined. This study showed that RR and SS constructs had the greatest ultimate strength but also the greatest variability among the foundations tested. However, the HH and TPL constructs had lower ultimate strength but were less variable. Rib-based anchors may be considered as an alternative in upper foundation constructs in growing rod techniques.

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