New means in spinal pedicle hook fixation. A biomechanical evaluation

Area of Science:

• Biomechanical Engineering
• Spinal Surgery
• Orthopedic Devices

Background:

• Pedicle hooks are crucial for posterior spinal instrumentation, but can experience significant three-dimensional forces.
• Screws enhance pedicle hook fixation, leading to the development of novel screw-augmented devices.
• Existing devices include the Universal Spine System (USS) and Cotrel-Dubousset (CD) hooks, with varying screw fixation methods.

Purpose of the Study:

• To experimentally determine the failure loads and mechanisms of three posterior spinal instrumentation devices.
• To compare the performance of a prototype two-screw pedicle hook against established USS and CD hooks, and a standard pedicle screw.

Main Methods:

• Failure loads were tested on human thoracic vertebrae using a matched pair experimental design.
• Implants evaluated included a Universal Spine System (USS) pedicle hook with one screw, a prototype two-screw pedicle hook, and a Cotrel-Dubousset (CD) pedicle hook with screw.
• Constrained and rotationally unconstrained posterior pull-out tests were conducted to simulate physiological loading conditions.

Main Results:

• In constrained tests, the 5-mm pedicle screw showed the highest pull-out force (1650 N), followed closely by the prototype two-screw hook (1530 N).
• The USS hook (910 N) and CD hook (740 N) had significantly lower failure loads in constrained tests.
• In unconstrained tests, the prototype two-screw hook remained the strongest (1617 N), while the USS hook (792 N) significantly outperformed the CD hook (464 N).

Conclusions:

• The prototype two-screw pedicle hook offers superior biomechanical fixation strength compared to USS and CD hooks.
• Screw augmentation significantly enhances the pull-out resistance of pedicle hooks for spinal instrumentation.
• Pedicular fracture was a common failure mode for the USS hook, indicating potential limitations in fixation integrity.