The Strength and Stiffness of Thoracic Implant Anchors in Osteoporotic Spines

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Abstract
To gain a better understanding of factors affecting dorsal implant pull-out strength and stiffness so thoracic spinal implant fixation can be improved. Posterior pull-out testing of six thoracic implant anchor types was performed on 54 osteoporotic human vertebra, T2-10, with an average bone mineral density of 0.67 g/cm2. The mean loads at neural arch failure were similar (296 to 382 N). Pedicle strength was greatest at T2 and least at T3. Hook claws were stiffer than sublaminar wires (121 N/mm and 180 N/mm versus 42 N/mm and 52 N/mm, respectively). Facet hook and pedicle hook claws were comparable, as were subpars wires. Bone mineral density of the vertebral bodies correlated with the stiffness of the sublaminar wire anchors only. To optimize the strength and stiffness of implant constructs designed to resist forward flexion, this study supports using hook-claws rather than sublaminar wires and extension of fixation to T2 when clinically realistic.

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