Pivotal Bone Anchor Assembly With Expansion-Only Retainer
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Solution Overview
Problem
Existing polyaxial bone screw designs suffer from loose or floppy rotation of the receiver relative to the shank, making spinal surgery procedures difficult, and the contractile locking engagements are weak against pull-out forces.
Innovation Solution
A polyaxial bone screw assembly with a split retainer ring that provides expansion-only locking engagement, featuring a friction fit between the shank head and retainer, which is secured by a compression insert and a receiver, preventing disassembly and enhancing resistance to pull-out forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a polyaxial bone screw assembly allows rotation of the receiver relative to the shank, then the screw can be positioned at various angles, but the connection becomes loose or floppy making procedures difficult
Solution Approach 1:
The retainer ring is designed to be dynamically changeable between a contracted state during insertion and an expanded state during operation. The retainer ring can be compressed to allow the receiver to rotate freely during positioning, then expanded to provide stable locking engagement once the desired angle is achieved, resolving the contradiction between adaptability and stability.
Solution Approach 2:
The retainer ring's dimensional parameters change from a contracted diameter during insertion to an expanded diameter during operation. This parameter change allows the same component to provide both ease of insertion with rotation capability and stable locked position, eliminating the floppy connection problem.
2Strength
If a contractile locking engagement is used in the retainer, then the receiver can be secured to the shank, but the locking strength is weak against pull-out forces
Solution Approach 1:
Instead of using a contractile locking mechanism that relies on elastic recovery, this invention uses an expansion-only mechanism. The retainer ring expands outward to engage with the receiver and maintains that expanded state through a locking feature, inverting the conventional approach to provide superior pull-out resistance.
Solution Approach 2:
The retainer ring is divided into functional segments: an expansion portion that engages the receiver and a locking portion that maintains the expanded state. This segmentation allows each portion to be optimized for its specific function, with the locking portion providing enhanced resistance to pull-out forces.
3Stability of the object's composition
If the retainer ring is made resilient to allow expansion, then the shank can be retained in the receiver, but the retainer may still rotate or move relative to the receiver
Solution Approach 1:
The retainer ring is pre-formed with a locking feature that automatically engages when the expansion portion is compressed during assembly. This preliminary configuration of the locking mechanism ensures that once the retainer is expanded, it cannot rotate or move relative to the receiver, providing stable retention without complicating the assembly process.
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 offers a secure, non-floppy connection that resists pull-out forces effectively, allowing for easier and more stable spinal fixation procedures.
Implementation Method 1
a friction fit resilient expansion-only split retainer for capturing the shank head in the receiver lower cavity
Implementation Method 2
a friction fit resilient expansion-only split retainer
Implementation Method 3
a compression insert and a receiver, preventing disassembly and enhancing resistance to pull-out forces
Implementation Method 4
a threaded shank with a pair of parallel projecting branches or arms which form a yoke
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver defining a central bore with a circumferentially-extending horizontal interference structure located above an upper expansion region and a lower locking region, and a shank having an upper end portion up-loadable into the central bore through a bottom opening. The assembly also includes a retainer having an upper portion and a lower expansion portion pre-positioned within the upper expansion region and being operable to capture the upper end portion of the shank, and a compression insert postionable within the central bore having an upwardly-facing surface for engaging a rod, an exterior side surface for engaging the horizontal interference structure, and a lower recess for receiving the upper portion of the retaining structure in a side-to-side overlapping engagement. The compression insert is restrained from moving downwardly within the central bore until forcible downward displacement by direct engagement with tooling to drive the exterior side surface of the compression insert at least partially through the horizontal interference structure.