Anti-splay Bone Anchor Locking Cap Design
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Solution Overview
Problem
Current pedicle screw systems suffer from splay issues due to threaded locking caps, leading to insecure grip, cross-threading, thread breakage, and potential loosening, posing safety risks and requiring excessive time for engagement.
Innovation Solution
An anti-splay bone anchor design featuring a screw shank, screw head, and locking cap with axial interference profiles and a set screw, eliminating threading on the outer surface and utilizing radial slots for secure engagement without friction-based locking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If threaded locking caps are used for engagement with screw heads, then the locking mechanism can be implemented, but splay occurs causing insecure grip and safety concerns
Solution Approach 1:
The patent removes the threading feature from the locking cap, extracting the harmful threaded engagement mechanism that causes splay. The locking cap becomes a smooth, non-threaded component that interfaces with the screw head through a different mechanism (bearing surface engagement), thereby eliminating the splay problem while maintaining locking functionality.
Solution Approach 2:
Instead of using external threading on the locking cap to achieve engagement, the patent inverts the approach by using internal threading on the screw head and having the locking cap engage through a smooth bearing surface. This reversal of the threading location eliminates the splay-causing external threads on the cap while preserving the locking function.
2Reliability
If threaded locking caps are used, then locking can be achieved, but engagement takes excessive time and causes cross-threading
Solution Approach 1:
The patent extracts the threading mechanism from the locking cap, replacing it with a smooth bearing surface that allows for faster, more reliable engagement. This eliminates the time-consuming process of threading the cap onto the screw head and prevents cross-threading issues entirely.
Solution Approach 2:
The patent substitutes the threaded mechanical engagement system with a bearing surface-based mechanical system. The locking cap engages with the screw head through a precision-machined bearing surface that allows for quick, accurate positioning and secure locking without the complexities of threading.
3Reliability
If friction-based locking is used, then locking function is achieved, but the cap may back out due to body jerks and twists
Solution Approach 1:
The patent incorporates a preliminary action by designing the locking cap with a specific engagement geometry that pre-positions the cap securely on the screw head. The bearing surface and engagement features are designed to maintain stable positioning from the outset, preventing back-out due to body movements without relying solely on friction.
Solution Approach 2:
The patent employs composite design principles by combining multiple engagement features (bearing surfaces, interference fits, and geometric constraints) within the locking cap and screw head assembly. This composite approach creates a multi-mechanism locking system that provides both friction-based holding and geometric prevention of back-out.
4Reliability
If threaded locking caps are used, then locking is achieved, but thread breakage and loosening occur
Solution Approach 1:
The patent extracts the threading feature from the locking cap, eliminating the source of thread breakage and loosening problems. By removing the external threads on the cap that are susceptible to cross-threading and thread stripping, the design achieves reliable locking without compromising thread integrity.
Solution Approach 2:
The patent inverts the threading arrangement by placing internal threads on the screw head rather than external threads on the locking cap. This inversion protects the threads from the damaging effects of cap installation and removal, as the screw head threads are better protected and less prone to breakage.
Data Source
AI summary
An anti-splay bone anchor is provided. The anti-splay bone anchor comprises a screw shank, a screw head and a locking cap member as the primary components. The screw head comprises a first opposing member having a first profile and a second opposing member having a second profile; a through hole for receiving the screw shank there-through. The locking cap member comprises a body, a through hole, a set screw and a first and a second tab. The through hole has threads on the inner surface which is adapted to threadably receive the set screw having threads on the outer surface. The first tab and the second tab, each extending radially outward from the locking cap member, have a third and a fourth profile which are complimentary to the first profile and the second profile, respectively.