Polyaxial Bone Anchor Assembly With Anti-Floppy Receiver Locking
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Solution Overview
Problem
Existing polyaxial bone screws suffer from loose or floppy rotation of the receiver relative to the shank, making spinal surgery procedures difficult, and the contact surfaces in prior art designs are weak against pull-out forces.
Innovation Solution
A polyaxial bone screw assembly with a split retainer ring that provides a friction fit and expansion locking engagement, preventing the receiver from being disassembled once connected, and featuring a base portion that supports the shank head below the receiver, enhancing stability and 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 it provides flexibility for spinal alignment, but the connection becomes loose or floppy making procedures difficult
Solution Approach 1:
The patent implements a dynamic locking mechanism where the retainer ring can transition between expanded and contracted states. During insertion, the retainer is expanded to allow rotation and adjustment. Once positioned, a locking tool contracts the retainer to secure the receiver to the shank, providing both flexibility during procedure and stability during fixation.
Solution Approach 2:
The retainer is designed as a split ring with multiple segments that can be independently manipulated. This segmentation allows the retainer to be compressed to a small diameter for insertion through the bone, then expanded to capture the shank head, and finally locked in place, providing both adaptability and operational ease.
2Adaptability or versatility
If contact surfaces are designed for rotation and adjustment, then polyaxial functionality is achieved, but strength against pull-out forces is reduced
Solution Approach 1:
The retainer ring provides dynamic contact surfaces that transition from smooth and rounded during insertion to interlocking and friction-fit during locking. The expanded retainer contacts allow rotation and adjustment, while the locked retainer creates strong friction and mechanical interlocking to resist pull-out forces, thus providing both polyaxial functionality and strength.
Solution Approach 2:
The retainer ring is made from a resilient material that combines elasticity with friction-generating surfaces. This composite approach allows the retainer to deform elastically during insertion for easy passage, then provide high friction and mechanical interlocking when locked, achieving both polyaxial movement capability and strong pull-out resistance.
3Adaptability or versatility
If the receiver is made removable for adjustment, then adaptability is improved, but reliability of connection is reduced
Solution Approach 1:
The retainer ring provides a dynamic connection that can be easily adjusted during insertion and manipulation, then securely locked in place to maintain reliability. The split retainer design allows the receiver to be adjusted and repositioned while inserted, but once the locking tool contracts the retainer, the connection becomes stable and reliable, preventing accidental loosening during the procedure.
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 and allows for easier manipulation during spinal correction procedures, functioning like a fixed monoaxial screw with independent locking capabilities.
Implementation Method 1
a friction fit resilient expansion locking split retainer for capturing the shank head in the receiver lower cavity
Implementation Method 2
The retainer includes upwardly extending tangs that are deployed in the receiver cavity so that the retainer and captured shank head are stabilized and retained
Implementation Method 3
a top drop and turn in place lower compression insert... The shank is finally locked into a fixed position relative to the receiver by frictional engagement between the insert and a lower split ring-like portion of the retainer, as described previously, due to a downward force placed on the compression insert
Data Source
AI summary
A pivotal bone anchor assembly includes a bone anchor having a proximal capture portion and an anchor portion, and a receiver having a channel for receiving a rod, an internal cavity with upper expansion and lower locking regions, and a central bore with an inwardly-protruding circumferentially-extending interference engagement structure. The assembly also includes a pressure insert having an upper surface for engaging the rod and an exterior surface for engaging the interference engagement structure to restrict movement of the pressure insert within the central bore. After the proximal capture portion of the bone anchor is positioned within internal cavity, the pressure insert is downwardly movable within the central bore to force the exterior surface at least partially past the interference engagement structure to inhibit the pressure insert from moving back up within the central bore.