Polyaxial Bone Screw With Spring Washer Angulation
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Solution Overview
Problem
Current polyaxial pedicle screws for posterior application in the cervico-thoracic spine face challenges in achieving high angulation and low profile while maintaining sufficient pull-out strength and retaining the screw yoke in a manipulable position during spinal fixation.
Innovation Solution
A polyaxial bone screw design featuring a threaded fastener with a yoke and screw support system, including a compressible spring washer and tri-lobe screw support, allows for high angulation and secure retention of the yoke in a manipulable position through a combination of frictional forces and articulation, enhancing stability and ease of use during spinal fixation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If polyaxial pedicle screws are designed for high angulation and low profile in posterior cervico-thoracic application, then the screw can be better positioned and angled for spinal fixation, but the pull-out strength of the screw is reduced
Solution Approach 1:
The screw assembly is divided into separate functional components: a threaded fastener for engagement, a yoke for positioning and rod attachment, a screw support for articulation, and a spring washer for retention. This segmentation allows each component to be optimized independently - the fastener maintains pull-out strength while the yoke and screw support enable high angulation and low profile positioning.
Solution Approach 2:
The spring washer acts as an intermediary element between the screw support and the yoke, providing frictional retention that allows the screw to maintain its high-angled position without sliding. This intermediary mechanism enables the screw to achieve both high angulation and sufficient retention strength.
2Ease of operation
If the screw yoke is retained in a manipulable position using friction, then the yoke can be easily positioned during surgery, but the yoke may shift position under load or during rod placement
Solution Approach 1:
The spring washer provides dynamic retention through elastic deformation - it compresses under the yoke's weight and during manipulation, then springs back to maintain frictional contact. This dynamic mechanism allows the yoke to remain stable during surgery while still being easily repositionable when needed.
Solution Approach 2:
The spring washer changes its physical parameters (compression, elasticity) based on the operational state. During insertion and manipulation, the washer compresses to provide strong frictional retention. During rod placement or repositioning, the washer expands to allow controlled movement, then returns to its compressed state to secure the yoke again.
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 design provides improved angulation and retention of the yoke, facilitating efficient spinal fixation by maintaining the screw's stability and allowing for precise orientation and secure attachment to the connecting rod, while maintaining sufficient pull-out strength for the desired application.
Implementation Method 1
provides sufficient friction to retain the bone screw yoke in a manipulable position relative to the fastener
Implementation Method 2
compressible spring washer
Data Source
AI summary
A polyaxial bone screw for attaching a rod to a vertebra comprises a fastener having a threaded shank and a head. A yoke has a rod receiving channel for receiving the rod. A screw support rotatably attached to the yoke has a cavity articulatingly and rotatably supporting the fastener head, a hole receiving the threaded fastener shank therethrough, and a plurality of slots for maximum screw angulation. A crown has an upper rod receiving surface, a lower surface in contact with the fastener head and a projecting surface in contact with the yoke. A biasing element between the screw support and the yoke applies biasing forces among the fastener, the crown, and the yoke to provide sufficient friction to retain the yoke in a manipulable position relative to the fastener. A set screw is supported by the yoke to overcome the bias and to rigidly secure the components.