Polyaxial Bone Screw Compression Insert for Creep Stability
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Solution Overview
Problem
Polyaxial bone screws used in spinal surgery face issues with materials exhibiting creep or viscoelastic behavior, leading to loosening of the frictional engagement between the receiver and shank, causing mis-alignment and stress due to deformation over time, especially with rods made from materials like PEEK or rubber.
Innovation Solution
A polyaxial bone screw assembly with a dual locking mechanism featuring an outer fastener and inner set screw, where the outer fastener engages a compression insert that exclusively presses on the shank upper portion, maintaining frictional contact and preventing loosening, even if the connecting member deforms, and includes features for secure bone implantation and easy assembly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a closure top or plug is used to capture the rod in the receiver, then the rod is locked in place and the bone screw shank is locked in a desired angular position, but the system becomes unreliable over time when the rod material exhibits creep or viscoelastic behavior
Solution Approach 1:
A compression insert is introduced as an intermediary component between the closure top and the bone screw shank. The compression insert exclusively engages the shank upper portion, transferring and maintaining compression force on the shank even when the rod deforms. This mediator isolates the shank from direct contact with the rod, preventing rod deformation from affecting the frictional engagement that locks the angular orientation.
2Ease of operation
If the closure top presses against the rod to lock it in place, then the rod is secured, but the frictional engagement between the receiver and shank loosens when the rod deforms
Solution Approach 1:
The locking function is segmented into two independent mechanisms: (1) the closure top locks the rod in place by pressing against it, and (2) the compression insert maintains frictional engagement with the shank by exclusively engaging the shank upper portion. This segmentation ensures that rod deformation does not affect the shank locking mechanism.
3Reliability
If a dual locking mechanism with outer fastener and inner set screw is used, then the shank and connecting member are independently secured, but the device complexity increases
Solution Approach 1:
The outer fastener and inner set screw are merged into a single integrated closure top component. The closure top performs both locking functions simultaneously: its outer surface presses against the compression insert to lock the shank, while its inner surface engages the connecting member to lock it in place. This merging reduces component count while maintaining independent securing of both elements.
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 solution ensures a secure and stable angular orientation of the shank relative to the receiver, maintaining fixation even with materials prone to creep, providing a lightweight, low-profile design that prevents unintentional disassembly and supports a wide range of angular orientations.
Implementation Method 1
maintaining frictional contact and preventing loosening
Implementation Method 2
compression insert that exclusively presses on the shank upper portion
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver having a receiver channel configured for receiving an elongate rod, an axial bore with inwardly protruding structures, and horizontally and circumferentially-extending tool engagement grooves formed into upper side outer surfaces of the receiver. The assembly also includes a shank having a proximal capture portion positionable into a lower portion of the axial bore below the inwardly protruding structures. The assembly further includes an insert positionable into the axial bore above the proximal capture portion of the shank, with the insert having an insert channel also configured for receiving the elongate rod, a bottom surface configured for frictional engagement with a partially spherical upper surface of the proximal capture portion, and vertically-elongate grooves formed into outer side surfaces of the insert and configured for receiving the inwardly protruding structures when the insert channel is co-aligned with the receiver channel.