Top-Loaded Snap-On Retainer for Polyaxial Bone Screw Assembly
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Solution Overview
Problem
Current polyaxial bone screws face challenges in securely and easily attaching to bones during spinal surgery, particularly in achieving the desired rotational position of the screw head relative to the shank, which can complicate the implantation of longitudinal connecting members.
Innovation Solution
A polyaxial bone screw assembly featuring a shank with a pivotable upper portion, a receiver with a compression insert, and a resilient retainer that 'snaps' into place, allowing for angular adjustments and secure locking, enabling easy assembly and alignment of the screw components before or after implantation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a fixed bone screw design is used, then the structure is simple, but the rod cannot be favorably positioned for insertion
Solution Approach 1:
The bone screw incorporates a polyaxial mechanism that allows the receiver head to pivot and rotate relative to the shank, transforming the fixed structure into a dynamic, adjustable one. This enables the rod to be favorably positioned at multiple angles before final locking, resolving the contradiction between operational flexibility and structural simplicity.
Solution Approach 2:
The screw is divided into separable components including the shank, receiver head, and retainer mechanism, allowing independent adjustment of the receiver relative to the shank. This segmentation enables favorable rod positioning while maintaining overall structural integrity through the locking mechanism.
2Ease of operation
If a polyaxial bone screw is used, then the rod can be favorably positioned, but the procedure becomes lengthy and difficult
Solution Approach 1:
The receiver and retainer are pre-assembled as a unit with the shank before implantation. This preliminary assembly allows the polyaxial mechanism to be pre-configured, reducing the time required during the surgical procedure while maintaining the ability to adjust rod positioning angles.
Solution Approach 2:
The receiver and retainer mechanisms are combined into an integrated assembly that functions as a single unit during implantation. This merging reduces the number of separate steps required, shortening the procedure time while preserving the polyaxial adjustment capabilities for easy rod insertion.
3Reliability
If the receiver is locked in a particular position, then the connection is secure, but adjustment during the procedure is limited
Solution Approach 1:
The retainer mechanism provides a dynamic locking system that allows the receiver to be adjusted to various angular positions during implantation and then securely locked in place. This dynamic capability maintains both adaptability for positioning and reliability for the final connection.
Solution Approach 2:
The system allows change in the angular parameter of the receiver relative to the shank during the procedure, then locks this parameter in a selected position. This parameter change capability enables both adjustment flexibility and connection security at the desired angle.
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
This design facilitates secure and flexible attachment of the screw to the bone, allowing for precise alignment and easy insertion of longitudinal connecting members, reducing procedural complexity and enhancing surgical efficiency.
Implementation Method 1
an open ring-like resilient retainer for capturing the shank upper portion in the receiver along a cylindrical, frusto-conical, curvate or combination interface, the upper portion and attached retainer thereafter being pivotable with respect to the receiver prior to locking of the shank into a desired configuration
Data Source
AI summary
A pivotal bone anchor assembly includes a shank having a capture structure with a radiused upper surface above a circumferential capture recess, and a receiver having an axial bore with a partial spherical seating surface adjacent a bottom opening and an expansion chamber with a downward-facing stop surface. The assembly also includes a retainer having an annular upper surface, a partial spherical outer surface, and an inner surface complementary with a recessed surface of the capture recess. Upon the capture structure being uploaded through the bottom opening, the upper surface of the retainer engages the stop surface of the expansion chamber as the radiused upper surface of the capture structure slidably engages the inner surface of the retainer to cause the retainer to expand within the expansion chamber, until the retainer snaps into the capture recess to couple the retainer to the capture structure and the outer surface of the retainer pivotably engages the seating surface of the axial bore.