Bottom-Loaded Bone Anchor Assembly for Precise Polyaxial Alignment
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Solution Overview
Problem
Existing polyaxial bone screws face challenges in achieving precise angular alignment of the receiver relative to the shank, leading to floppy rotations and difficulty in positioning longitudinal connecting members, especially in spinal surgery applications.
Innovation Solution
A polyaxial bone anchor assembly with a receiver, shank, and expandable retainer that allows for a non-tapered locking engagement and friction fit between the shank and retainer, enabling easy alignment and secure fixation of the receiver relative to the shank, using compression inserts with optional lock and release features.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a fixed head or receiver is used relative to the shank, then the structure is simple and manufacturing is easy, but the rod cannot be favorably positioned and alignment is difficult or impossible
Solution Approach 1:
The patent applies a polyaxial mechanism that allows the receiver to pivot and rotate relative to the shank, transforming a fixed structure into a dynamic one. This enables the receiver to be positioned at multiple angles and orientations, allowing favorable positioning of rods while maintaining manufacturing simplicity through a modular design with a shank, receiver, and retainer assembly.
2Ease of operation
If polyaxial bone screws allow for loose or floppy rotation of the head, then the rod can be favorably positioned, but precise angular alignment is difficult to achieve
Solution Approach 1:
The patent replaces the traditional mechanical locking mechanism with a friction-fit system. The retainer provides frictional engagement with the shank, allowing controlled movement during positioning while maintaining stable angular alignment once positioned. This friction-based system eliminates floppy rotations and achieves precise angular alignment without complex mechanical locks.
Solution Approach 2:
The patent changes the engagement parameter from rigid mechanical connection to friction-based connection. By controlling the friction coefficient and normal force between the retainer and shank, the system achieves both ease of positioning during installation and precise angular alignment during operation, resolving the contradiction between flexibility and precision.
3Reliability
If a non-tapered locking engagement is used, then the retainer provides stable fixation, but assembly and disassembly may be more difficult
Solution Approach 1:
The patent uses a dynamic friction-fit mechanism where the retainer can be easily assembled by pushing the shank through the retainer body, and disassembled by applying axial force to overcome the friction. This dynamic system provides stable non-tapered locking engagement during operation while maintaining ease of assembly and disassembly through controlled friction, resolving the contradiction between reliability and ease of operation.
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 provides a stable, easy-to-use, and cost-effective mechanism for securing longitudinal connecting members, ensuring precise angular alignment and secure fixation without floppy rotations, enhancing the effectiveness of spinal surgery procedures.
Implementation Method 1
The retainer is in a non-tapered locking engagement with the shank upper portion when the shank is in a locked orientation with respect to the receiver
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver; an insert, an expandable open ring retainer, and a shank. During assembly, the insert and the open ring retainer are uploaded into a central bore and an inner cavity of a lower portion of the receiver, respectively, through a bottom opening. The shank head of the shank is then uploaded through the bottom opening and through the open ring retainer, causing the open ring retainer to expand to an expanded configuration as the shank head is uploaded therethrough and thereafter return to a non-expanded configuration and engage an interior seating surface of the inner cavity to capture the shank head within the lower portion of the receiver.