Bone Anchor Receiver Rotation Stops for Creep-Resistant Locking
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Solution Overview
Problem
Polyaxial bone screws with flexible connecting members, such as those made from polymers like PEEK, experience creep deformation over time, leading to loosening of the frictional engagement between the receiver and shank, causing misalignment and stress due to body movement and stresses.
Innovation Solution
A polyaxial bone screw assembly with a shank, receiver, and compression insert that allows for selective angular positioning, featuring a compression insert with planar sides to receive a connecting member, and a closure structure that engages the insert for secure locking, even with creep deformation, using metal-to-metal frictional engagement.
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 and shank are locked in place, but the frictional engagement loosens over time due to creep deformation of the rod
Solution Approach 1:
A compression insert made of creep-resistant material is introduced as an intermediary component between the closure top and the rod. The compression insert maintains frictional engagement with the rod while being pressed by the closure top, preventing loosening due to rod creep deformation over time.
Solution Approach 2:
The material parameter of the compression insert is specifically selected to be creep-resistant, contrasting with the rod material that exhibits creep. This parameter change in material properties allows the compression insert to maintain dimensional stability and continuous frictional engagement despite rod deformation.
2Adaptability or versatility
If a polymer rod is used for flexibility, then adaptability is improved, but creep deformation occurs leading to loosening
Solution Approach 1:
The material parameter of the compression insert is changed to be creep-resistant while the rod maintains its polymer material for flexibility. This differential material selection allows the system to simultaneously achieve flexibility from the polymer rod and engagement stability from the creep-resistant compression insert.
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 maintains a locked position despite creep deformation, preventing unintentional disassembly and ensuring stable alignment of the shank with the receiver, providing a secure and flexible connection for spinal implants.
Implementation Method 1
metal-to-metal frictional engagement
Data Source
AI summary
A pivotal bone anchor assembly includes a shank having a shank head, 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 downwardly-facing abutment surfaces and opposed rotation blocking structures beneath a helically wound thread. The assembly also includes a pressure insert having a second channel and opposite outer projections. The pressure insert is loaded into the axial bore with the second channel in a mal-aligned position relative to the first channel, with subsequent rotation moving the second channel into alignment with the first channel and the opposite outer projections into an overlapping engagement with the downwardly-facing abutment surfaces to inhibit upward movement of the pressure insert. During rotation, the opposite outer projections are initially inhibited from further rotation by engagement with the opposed rotation blocking structures until a force is applied to the pressure insert with the tooling to further rotate the opposite outer projections at least partially past the opposed rotation blocking structures.