Bone Anchor Closure Thread Form for Receiver Arm Splay Control
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Solution Overview
Problem
Existing bone anchor closures for spinal surgery, such as v-thread and square thread forms, experience outward splay and stress concentration due to bending moments, leading to loosening and deformation, especially in open-ended bone screw receivers with spaced arms.
Innovation Solution
A closure structure with dissimilar mating guide and advancement flange forms on the closure and receiver arms, featuring splay control surfaces that allow initial pivot-splay to reduce torque and then arrest further splay during tightening, using a combination of angled load flanks and splay control ramps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If v-thread closure forms are used in open bone anchor receivers, then the closure can be easily installed and advanced, but outward splay of the receiver arms occurs at all loading levels without limit
Solution Approach 1:
The patent changes the geometric parameters of the thread form by transitioning from conventional v-threads to reverse angle threads with specific crest configurations. The reverse angle thread form with a strong outer tip crest modifies the load distribution parameters, creating a neutral radial reaction that prevents outward splay while maintaining ease of installation through the same helical advancement mechanism.
Solution Approach 2:
The patent applies local quality by creating different structural characteristics at different locations of the thread form. The outer tip of the reverse angle thread is strengthened with a specific crest configuration that provides localized support against outward splay forces, while the rest of the thread structure maintains its original advancement function. This localized strengthening at the critical outer tip region prevents splay without compromising installation ease.
2Stability of the object's composition
If buttress thread forms are used to reduce outward splay, then radial reaction is neutralized, but the threads can still be bent, deformed or sheared off by forces during installation and post-operative activities
Solution Approach 1:
The patent employs asymmetry in the reverse angle thread form where the leading and trailing surfaces are not symmetrically configured. The trailing surface is angled upwardly toward the thread axis at a specific reverse angle, creating an asymmetric load path that directs forces away from the outer tip. This asymmetric geometry, combined with the strengthened crest, distributes stresses more evenly and prevents concentration at the outer tip, thereby preventing bending, deformation, or shearing while maintaining splay control.
Solution Approach 2:
The patent incorporates a preliminary strengthening action by designing the outer tip crest configuration before the closure is installed. The reverse angle thread form with its strengthened outer tip is pre-configured to resist forces that will be applied during installation and post-operative activities. This preliminary structural preparation ensures that when forces are applied, the thread is already positioned to resist bending, deformation, or shearing at the critical outer tip location.
3Stability of the object's composition
If reverse angle thread forms with linear loading surfaces are used, then outward splay is reduced, but stress concentration occurs on the upper outer loading flank portions near the crest under moderate to heavy loads
Solution Approach 1:
The patent introduces dynamics by allowing the contact point between the closure thread and receiver arm to migrate along the thread flank during loading. The reverse angle configuration enables the load to be carried at different locations depending on the load level - at lower loads the contact is on the upper outer flank, but as load increases the contact point moves downward along the reverse angled surface. This dynamic load redistribution prevents stress concentration from building up at the critical upper outer crest region, while still maintaining splay control through the reverse angle geometry.
Data Source
AI summary
Open implant closure structure including a helically wound thread form.