Polyaxial Bone Screw Assembly Creep-Resistant Locking Mechanism
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Polyaxial bone screws with flexible connecting members, such as those made from polymers like PEEK, tend to exhibit creep over time, leading to loosening of the frictional engagement between the receiver and the shank, causing misalignment and stress issues due to the deformation of materials under stress.
Innovation Solution
A polyaxial bone screw assembly featuring a shank with a compression insert that engages a longitudinal connecting member, allowing for selective angular positioning and secure locking through a single-piece closure structure that deforms with the connecting member, maintaining stability even if the member undergoes creep, utilizing a combination of spherical and planar surfaces for enhanced frictional engagement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a closure top or plug is used to lock the rod in the receiver, then the rod and receiver are locked in place, but the frictional engagement between the receiver and shank loosens over time due to creep deformation of the rod material
Solution Approach 1:
The invention divides the locking function into two separate mechanisms: (1) the closure top locks the rod in the receiver, and (2) a独立的 locking mechanism (such as a set screw or separate locking component) maintains the frictional engagement between the receiver and shank. This segmentation ensures that creep deformation of the rod does not affect the shank-receiver engagement, as the locking function is distributed across independent components.
Solution Approach 2:
The invention introduces an intermediary locking component (such as a set screw, separate locking mechanism, or intermediate structure) that directly engages with the shank or receiver to maintain frictional engagement. This intermediary element acts as a mediator that compensates for the loosening effect caused by rod creep, ensuring continuous stable engagement without relying solely on the rod-closure top-receiver assembly.
2Device complexity
If a single-piece closure structure is used to engage both the connecting member and compression insert, then assembly is simplified, but the structure must accommodate deformation without losing locking capability
Solution Approach 1:
The single-piece closure structure is designed with deformable features that change their geometric parameters (such as thickness, curvature, or engagement surface area) in response to connecting member creep. For example, the closure may include a compliant section that deforms to maintain contact pressure, or engagement surfaces that adapt their geometry to preserve frictional engagement despite material deformation over time.
Solution Approach 2:
The closure structure incorporates dynamic characteristics, allowing it to adapt its configuration in response to deformation forces. The closure may include flexible elements, spring-loaded sections, or geometry that enables it to shift and re-engage automatically as the connecting member creeps, ensuring continuous locking capability without requiring multiple separate components.
3Reliability
If spherical and planar surfaces are used for frictional engagement, then engagement stability is enhanced, but manufacturing precision requirements increase
Solution Approach 1:
Instead of requiring high precision across entire spherical or planar surfaces, the invention applies precision only to critical local zones. For example, only a specific portion of the spherical surface (such as a small contact patch or equatorial band) requires high geometric precision to ensure stable frictional engagement, while other portions of the surface can be manufactured with lower tolerance. This local quality approach maintains engagement stability while reducing overall manufacturing complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The assembly ensures that all components remain securely locked, preventing unintentional disassembly and maintaining stability even with flexible connecting members, providing independent locking for the shank and connecting member, and resisting deformation-induced loosening, thus ensuring reliable bone implantation and support.
Implementation Method 1
The cooperating shapes of the shank upper portion external surface and the receiver inner surface enable selective angular positioning of the shank body with respect to the receiver
Implementation Method 2
a single-piece closure structure that deforms with the connecting member, maintaining stability even if the member undergoes creep
Implementation Method 3
utilizing a combination of spherical and planar surfaces for enhanced frictional engagement
Data Source
AI summary
A pivotal bone anchor assembly includes a shank having a shank head and an anchor portion, and a receiver having a first channel for receiving a rod and an axial bore for receiving the shank head, with the axial bore including a downwardly-facing abutment surface and opposed rotation blocking and alignment structures beneath a helically wound thread. The assembly also includes a pressure insert having a second channel and opposite outwardly-projecting flanges with notches formed therein. The pressure insert is top loaded into a first position within the axial bore with the second channel in a mal-aligned position relative to the first channel, with subsequent rotation of the pressure insert moving the second channel into alignment with the first channel, the opposite outwardly-projecting flanges under the downwardly-facing abutment surface, and the opposed rotation blocking and alignment structures into the notches formed into the opposite outwardly-projecting flanges.