Pivotal Bone Anchor Assembly with Expansion-Only Split Retainer
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Solution Overview
Problem
Existing polyaxial bone screws with contractile locking engagements are weak against pull-out forces during spinal reduction procedures, and their modular designs often require complex configurations to prevent disassembly.
Innovation Solution
A polyaxial bone screw assembly featuring a resilient expansion-only split retainer that forms a sliding, pivoting relation with both the shank head and receiver, providing a stronger locking mechanism without contractile engagement, allowing for secure attachment and resistance to pull-out forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If contractile locking engagement is used in polyaxial bone screws, then assembly and disassembly is enabled, but the locking strength against pull-out forces is insufficient
Solution Approach 1:
Instead of using a contractile retainer that shrinks to lock the shank, this invention uses an expansion-only retainer that expands outward to lock the shank. The retainer expands into the receiver cavity to create a friction fit, inverting the conventional approach of using contraction for locking. This expansion mechanism provides superior locking strength while maintaining ease of assembly through a simple insertion motion.
Solution Approach 2:
The retainer's cross-sectional dimensions change from a compressed state during insertion to an expanded state during locking. By controlling the expansion parameter, the retainer transitions from a non-locking insertion phase to a strong locking phase, achieving both ease of assembly and high locking strength without requiring complex contractile mechanisms.
2Adaptability or versatility
If modular polyaxial screw design is used, then flexibility in positioning is improved, but device complexity increases due to multiple components
Solution Approach 1:
The receiver, retainer, and compression insert are combined into a single integrated assembly that functions as one unit. This merging reduces the number of separate components while maintaining the polyaxial flexibility and positioning capabilities, thereby reducing device complexity without sacrificing adaptability.
Solution Approach 2:
The integrated receiver-retainer-compression insert assembly performs multiple functions: it provides polyaxial movement capability, secures the shank head, and enables compression. This multi-functionality in a single assembly reduces the need for separate components, simplifying the overall device while maintaining versatility.
3Ease of operation
If contractile retainer ring is used, then shank head capture is enabled, but the structure becomes weak against pull-out forces
Solution Approach 1:
The retainer inverts the conventional contractile mechanism by using expansion-only design. The retainer expands outward against the receiver cavity walls to create a strong friction fit that resists pull-out forces, while still capturing the shank head effectively during insertion.
Solution Approach 2:
The retainer and receiver cavity utilize spherical or curved surfaces to create a conforming friction fit. The curved geometry allows the retainer to expand smoothly into the receiver cavity, maximizing contact area and friction-based resistance to pull-out forces while maintaining ease of insertion.
4Strength
If expansion-only split retainer is used, then locking strength is improved, but device complexity may increase
Solution Approach 1:
The retainer is designed as a split or segmented structure that can be inserted as a single unit and then expands to lock. This segmentation allows for a simplified manufacturing process while achieving the complex expansion-locking function, thereby improving locking strength without proportionally increasing device 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 design enhances the resistance to pull-out forces and provides a more secure attachment, preventing disassembly once assembled, while allowing for flexible angular adjustments and independent locking features for improved spinal correction techniques.
Implementation Method 1
a resilient expansion-only split retainer for capturing the shank head in the receiver lower cavity
Implementation Method 2
the retainer being slidingly engageable with both the shank head and a surface defining the receiver cavity
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver having a channel configured to receive an elongate rod and communicating with a bottom opening through a central bore, with the central bore having a lower portion configured to receive the capture portion of a bone anchor with the bone anchor being pivotal with respect to the receiver in a non-locked configuration. The assembly further includes a compression insert that is also receivable within the central bore with an upper surface configured for engagement with the elongate rod, a central opening, and upward-facing surfaces on tool engagement structures positioned radially outward from the central opening. The tool engagement structures are configured for direct engagement by tooling to receive a continuously-applied downwardly directed force that temporarily locks the position of the bone anchor with respect to the receiver prior to the elongate rod being secured within the channel in a locked arrangement.