Helical Guide Flanges for Spinal Fixation Anti-Splay
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Solution Overview
Problem
Conventional open-headed bone screws with threaded plugs face issues of radial splaying and loosening due to high torques required for locking spinal fixation rods, leading to potential failure and increased implant bulk when trying to resist these forces.
Innovation Solution
The development of helical guide and advancement flanges with complementary anti-splay contours on inner and outer flanges, featuring radially outward and inward crests and roots, respectively, that resist splaying by engaging anti-splay surfaces when the inner member is torqued into the outer member, providing a secure locking mechanism without bulk increase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If threaded plugs are used to close open-headed bone screws, then the implant can secure the spinal fixation rod, but the high torque required causes radial splaying and loosening of the arms
Solution Approach 1:
The closure member is segmented into multiple arms that can independently engage with the bone screw arms, allowing distributed load bearing and preventing concentrated radial forces that cause splaying. The multi-arm structure divides the clamping function across several contact points.
Solution Approach 2:
The invention transitions from a simple radial threading mechanism to a three-dimensional helical engagement system. The helical flanges create engagement paths that extend along the longitudinal axis, converting purely radial forces into combined axial and helical force vectors that resist splaying more effectively.
2Strength
If the arms are strengthened to resist splaying, then the implant can withstand high torque, but the implant profile increases and becomes bulkier
Solution Approach 1:
Instead of increasing arm thickness radially, the invention utilizes the longitudinal dimension through helical flange engagement. The anti-splay surfaces extend along the helical path, providing increased engagement area and strength without increasing the radial profile of the implant components.
Solution Approach 2:
The arms are designed with localized helical flange features concentrated at the engagement zones, providing enhanced strength precisely where needed to resist splaying, while maintaining minimal profile in non-critical areas. The anti-splay surfaces are positioned specifically at the load-bearing interfaces.
3Reliability
If V-shaped threads are used in the threaded plug, then the rod can be clamped securely, but the camming action produces radial forces that splay the arms
Solution Approach 1:
Instead of using V-shaped threads that generate outward radial camming forces, the invention inverts the approach by using helical flanges with anti-splay surfaces that generate inward-directed or neutral radial forces. The helical engagement geometry reverses the harmful camming action into a beneficial clamping mechanism that prevents rather than causes splaying.
Solution Approach 2:
The invention converts the potentially harmful radial forces into beneficial clamping forces through the helical flange geometry. The helical angle and anti-splay surface orientation transform what would be outward splaying forces into inward-directed clamping forces that secure the rod while preventing arm separation.
Data Source
AI summary
A spinal fixation device combines an open-headed anchor member, such as a bone screw or a hook, with a closure member to thereby clamp a spinal fixation rod to the anchor member. The anchor member has spaced apart arms forming a rod receiving channel. The arms have arm extensions or tabs connected to main portions of the arms by weakened regions to enable the tabs to be broken-off or separated after the rod is clamped. The closure member and inner surfaces of the arms and tabs have helical anti-splay guide and advancement interlocking flanges formed thereon which cooperate to prevent splaying the arms and extensions as the closure member is advanced into the rod receiving channel. The flanges have anti-splay contours which can be formed on load flanks or stab flanks of the flanges. The load flanks can be oriented in such a manner as to aid in the anti-splay characteristics of the flanges or to control the proportioning of axial stresses between the flanges.