Polyaxial Bone Anchor Assembly With Insert Locking Against Creep Loosening
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Solution Overview
Problem
Polyaxial bone screws with polyaxial motion capabilities face issues with components loosening due to creep deformation in materials like PEEK and rubber, leading to misalignment and stress, especially when using nested closure tops with thin walls that lack sufficient strength.
Innovation Solution
A polyaxial bone screw assembly with a shank and receiver design featuring a compression insert that applies pressure exclusively on the shank upper portion, using a dual locking mechanism with an outer fastener and inner set screw to maintain secure angular positioning, even with materials exhibiting creep deformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If nested closure tops with thin walls are used to maintain low profile, then device compactness is improved, but structural strength deteriorates leading to component loosening
Solution Approach 1:
The closure mechanism is divided into nested components: an outer closure top and an inner closure top. The outer closure top provides structural strength and engagement with the receiver, while the inner closure top engages with the shank. This segmentation allows each component to be optimized for its specific function without requiring the entire assembly to be bulky.
Solution Approach 2:
The inner closure top is nested within the outer closure top, with the inner closure top having a smaller diameter and being receivable within the outer closure top. This nested configuration maintains a low profile while distributing structural requirements across multiple components, preventing the need for a single bulky closure structure.
2Adaptability or versatility
If materials like PEEK and rubber are used for polyaxial motion capabilities, then adaptability is improved, but reliability deteriorates due to creep deformation causing loosening
Solution Approach 1:
Compression inserts are introduced as intermediary components between the closure tops and the shank/rod. These inserts engage with both the closure mechanism and the shank or rod, distributing loads and reducing direct stress on the polyaxial joint. This intermediary structure helps prevent creep-induced loosening while preserving polyaxial motion capabilities.
Solution Approach 2:
The compression inserts act as cushioning elements that are installed beforehand to distribute and reduce stresses on the polyaxial joint components. By providing this protective cushioning in advance, the design prevents creep deformation and loosening that would otherwise occur under sustained loading conditions.
3Stability of the object's composition
If compression force is applied to maintain locked position, then stability is improved, but stress on components increases leading to potential failure
Solution Approach 1:
The compression force application is segmented through multiple components: the outer closure top engages with the receiver, the inner closure top engages with the shank, and compression inserts are positioned between these components and the rod. This segmentation distributes the compressive load across multiple engagement points, reducing stress on any single component while maintaining stable locked positioning.
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver comprising a base portion defining a cavity and an upper portion with upright arms extending upward from the base portion to define an open channel for receiving a rod, with the cavity communicating with the open channel to define a central bore having opposed horizontally-extending insert abutment surfaces below a guide and advancement structure, and tool engagement recesses extending horizontally around side outer surfaces of the upright arms. The assembly also includes a shank comprising a proximal capture portion configured for positioning within the cavity and a distal anchor portion configured for attachment to the bone. The assembly further includes a pressure insert positionable within the central bore of the receiver, upon which top surfaces of the pressure insert engage the insert abutment and guide surfaces to prohibit upward movement of the pressure insert out of the receiver.