Open Bone Anchor Closure With Dual-Start Flanges for Splay Control
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Solution Overview
Problem
Existing bone anchor closures, such as v-thread and square thread forms, experience significant splay issues when subjected to loads, leading to deformation and instability in spinal fixation systems.
Innovation Solution
A closure structure with balanced mating guide and advancement flange forms on both the closure and bone anchor, featuring a dual-start helically wound design, controls splay by ensuring axial loading and clearance, using a 'boot'-shaped flange form with oblique splay control ramps to stabilize the receiver arms during torquing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If v-thread closure is used, then easy advancement and installation is achieved, but outward splay of receiver arms occurs at all loading levels
Solution Approach 1:
The patent applies different thread form characteristics to different portions of the closure. The leading surface uses a shallower angle for easy advancement, while the trailing surface uses a steeper angle for splay control. This local differentiation allows the closure to simultaneously achieve ease of installation and stability under load by optimizing each surface's geometric properties for its specific function.
Solution Approach 2:
The closure employs asymmetric thread geometry where the leading and trailing surfaces have different angles relative to the thread axis. The leading surface angle is shallower than the trailing surface angle, creating an asymmetric profile that generates differential forces during advancement. This asymmetry produces a net inward radial force that counteracts outward splay while maintaining smooth installation.
2Stability of the object's composition
If buttress thread form is used, then outward splay is reduced, but threads can still be bent and deformed by installation forces
Solution Approach 1:
The patent modifies the geometric parameters of the thread form, specifically the angles of the leading and trailing surfaces relative to the thread axis. By optimizing these angular parameters, the closure achieves a balance where the thread structure resists both splay and deformation during installation. The parameter optimization ensures that forces during installation are distributed more evenly across the thread profile.
Solution Approach 2:
The closure design incorporates a splay control mechanism that actively counteracts outward splay during the advancement process itself. The asymmetric thread geometry creates radial inward forces that prevent splay before it can occur, rather than attempting to correct splay after it has developed. This preliminary action maintains thread integrity throughout the installation process.
3Force
If square thread closure is used, then forces are directed axially, but marginal splay occurs under moderate load and considerable splay under heavy load
Solution Approach 1:
The patent applies different angular characteristics to different surfaces of the thread. The leading surface has a shallower angle that facilitates axial force transmission, while the trailing surface has a steeper angle that provides stronger radial containment. This local quality differentiation allows the thread to simultaneously achieve axial force direction and effective splay control across various loading conditions.
Solution Approach 2:
The asymmetric thread geometry creates a imbalance in force distribution where the steeper trailing surface generates stronger radial inward forces compared to the shallower leading surface. This asymmetric force distribution maintains axial force direction while providing superior splay control, especially under heavy loads where symmetric threads would fail to contain the arms effectively.
Data Source
AI summary
Open receiver implant and closure structures having helically wound guide and advancement splay control surfaces. Thread form heights, thicknesses, and other geometries are described.