Bone Anchor Closure Thread Geometry for Receiver Arm Splay Limiting
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Solution Overview
Problem
Existing bone anchor closures, such as v-thread and square thread forms, suffer from splay issues, where the receiver arms of open-ended bone screws flex outwardly under load, 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 arms, 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 lock the receiver arms in place.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If v-thread closure is used, then the closure can be easily advanced into the receiver, but the receiver arms splay outwardly without limit under load
Solution Approach 1:
The closure is divided into multiple functional surfaces: a first surface with a first angle for initial engagement and advancement, and a second surface with a second angle for controlling splay. This segmentation allows different portions of the closure to perform different functions - advancement versus splay control - resolving the contradiction between easy advancement and arm stability.
Solution Approach 2:
Different portions of the closure have different angular characteristics. The first surface has a steeper angle for effective advancement, while the second surface has a shallower angle to control splay. This local differentiation of geometric properties allows the single closure component to simultaneously achieve easy advancement and prevent arm splay under load.
2Stability of the object's composition
If buttress thread form is used, then outward splay of receiver arms is reduced, but the threads can still be bent and deformed by forces during installation
Solution Approach 1:
The closure is segmented into multiple angular surfaces that work together: the first angled surface handles advancement forces, while the second angled surface specifically counteracts splay forces. This segmentation distributes the mechanical loads across different surfaces, preventing any single surface from being overloaded and deformed during installation.
Solution Approach 2:
The solution moves from a single-angle thread form (2D profile) to a multi-surface closure geometry (3D structure with varying angles). By adding the dimension of multiple angular surfaces, the closure can simultaneously resist advancement forces and splay forces, increasing overall strength and deformation resistance.
3Force
If square thread closure is used, then forces are directed axially, but splay occurs under moderate to heavy loads
Solution Approach 1:
Different surfaces of the closure have different angular properties optimized for different functions. The first surface with its steeper angle maintains axial force directionality, while the second surface with its shallower angle specifically addresses splay control under load. This local quality differentiation allows the closure to simultaneously achieve axial force direction and splay prevention.
Solution Approach 2:
The closure geometry is designed to dynamically respond to loading conditions. Under advancement forces, the first surface engages to maintain axial direction. Under splay forces, the second surface engages to control arm separation. This dynamic engagement of different surfaces based on applied loads allows the closure to maintain both axial force direction and arm stability across varying load conditions.
Data Source
AI summary
A bone anchor assembly featuring a receiver and a closure. The receiver includes a base portion with a bottom surface and an upper portion with a pair of opposed upwardly extending arms. An open rod-receiving channel is situated between these arms, each arm having an interior surface with a helically wound thread form. A closure is designed to be threadably positioned within the channel. The closure includes a cylindrical body with a central axis and an outer surface featuring a mating helically wound thread form. This thread form includes an outer crest portion, a root portion, a thread pitch, an upper linear thrust surface, and a lower linear clearance surface. The height of the outer crest portion is slightly less than or equal to the height of the root portion.