Bone Anchor Pressure Insert Assembly for Pull-Out Resistant Locking
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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 surgical procedures difficult, and the contact-based locking mechanisms 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, ensuring secure fixation without contraction, and includes a compression insert for final locking, mimicking a monoaxial screw function.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a contact-based locking mechanism is used to secure the receiver to the shank, then the assembly achieves fixation, but the locking mechanism is weak against pull-out forces
Solution Approach 1:
The locking mechanism is divided into two independent components: an expansion retainer that provides radial expansion locking and a compression insert that provides axial compression locking. This segmentation allows each component to specialize in one locking function, with the expansion retainer handling radial forces and the compression insert handling axial pull-out forces, thereby resolving the contradiction between locking strength and reliability against pull-out forces.
Solution Approach 2:
The compression insert acts as an intermediary element between the rod and the shank head. It transfers and distributes the compressive load from the rod to the shank head, creating a reliable load path that significantly enhances resistance to pull-out forces while maintaining the overall locking mechanism's strength.
2Stability of the object's composition
If a fixed receiver design is used to prevent rotation, then stability is improved, but manipulation and adjustment during surgery become difficult
Solution Approach 1:
The receiver assembly transitions from a dynamic state during insertion and positioning, where the expansion retainer can be compressed and the components can move relative to each other for easy manipulation, to a static locked state after assembly, where the retainer expands and the compression insert secures the shank, providing stability. This dynamic capability allows the system to satisfy both ease of operation during surgery and stability after fixation.
Solution Approach 2:
The expansion retainer is pre-compressed during assembly to store elastic energy, which is then released to provide the locking force that secures the receiver to the shank. This preliminary compression action allows for easy insertion and manipulation during surgery, while the subsequent expansion provides stable fixation, resolving the contradiction between ease of operation and stability.
3Strength
If a compression insert is added to lock the shank into a fixed position, then fixation strength is improved, but device complexity increases
Solution Approach 1:
The compression insert is integrated with the rod and receiver assembly as a unified locking mechanism. The insert combines radial expansion locking (through the retainer) and axial compression locking (through the insert itself) into a single assembly operation, thereby providing enhanced fixation strength without proportionally increasing device complexity. The components work together as a coordinated system rather than separate mechanisms.
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 enhanced stability and resistance to pull-out forces, providing a secure and stable connection between the shank and receiver, allowing for easier surgical manipulation and improved spinal correction techniques.
Implementation Method 1
a friction fit resilient expansion-only split retainer to capture the shank head
Implementation Method 2
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 base, as described previously, due to a downward force placed on the compression insert by a closure top pressing on a rod
Implementation Method 3
a friction fit between the shank head and retainer, ensuring secure fixation
Data Source
AI summary
A method of assembling a bone anchor assembly includes positioning a pressure insert into a central bore of a receiver. The receiver includes an open channel for receiving a rod and the central bore extending upwardly through the open channel to a top of the receiver and flanked on both sides by opposed inwardly-facing parallel planar surfaces extending between the central bore and front and back sides of the receiver defined by the open channel. The central bore also includes a horizontal inwardly-facing arcuate interference wedging surface having an upward-facing surface and extending between the opposed inwardly-facing parallel planar surfaces. The insert includes with an outer abutment surface having an upper-most surface portion and a lower-most surface portion. The upper-most surface portion of the pressure insert is engageable with the upward-facing surface of the arcuate interference wedging surface to inhibit downward movement of the pressure insert in the central bore without an exterior downwardly-directed pushing force being applied on the pressure insert.