Pivotal Bone Anchor Assembly for Non-Floppy Provisional Shank Locking
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Solution Overview
Problem
Existing polyaxial bone screw designs suffer from 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 and a friction fit compression insert, where the shank head is secured through a non-floppy, temporary friction fit with the insert and a final locking engagement between the insert and the split retainer, providing enhanced resistance to pull-out forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a polyaxial bone screw design allows rotation of the receiver relative to the shank, then the screw can be positioned at various angles, but the connection becomes floppy and difficult to control during surgery
Solution Approach 1:
The receiver assembly incorporates a dynamic locking mechanism that transitions from an unlocked state allowing rotation to a locked state providing rigid fixation. The locking element can be engaged or disengaged to switch between polyaxial and fixed configurations, enabling adaptability during surgery while maintaining operational control when locked.
Solution Approach 2:
The system changes the mechanical parameter of the connection from movable to fixed by engaging a locking element. This parameter change allows the same receiver assembly to provide both angular adjustment capability and rigid fixation, resolving the contradiction between versatility and ease of operation.
2Reliability
If contact-based locking mechanisms are used to secure the shank head, then the assembly can be locked in position, but the locking strength is weak against pull-out forces
Solution Approach 1:
The locking mechanism is divided into separate functional elements: a locking element that engages with the receiver and a compression element that applies axial load. This segmentation allows each component to be optimized for its specific function, with the compression element providing the necessary strength against pull-out forces independent of the locking element's positional stability.
Solution Approach 2:
The compression element is pre-loaded to apply continuous compressive force to the shank head, creating preliminary action that strengthens the connection before any pull-out force is applied. This pre-compression ensures that the locking mechanism maintains strong engagement throughout the loading process.
3Ease of operation
If the receiver is locked in a fixed position relative to the shank, then the connection becomes rigid like a fixed monoaxial screw, but the ability to adjust angles is lost
Solution Approach 1:
The locking mechanism provides dynamic control over the receiver's position relative to the shank. The locking element can be engaged to provide fixed positioning for ease of operation during the final fixation stage, or disengaged to allow angular adjustment during the positioning stage, thus adapting to different surgical needs.
Solution Approach 2:
The system allows preliminary angular positioning and adjustment before final locking. The receiver can be positioned at the desired angle and temporarily secured, then locked in place once the correct position is achieved, combining the benefits of adjustability and rigidity at different stages of the surgical 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 easy assembly and adjustment, functioning like a fixed monoaxial screw, facilitating easier surgical procedures.
Implementation Method 1
a top drop and rotate friction fit lower compression insert having super structure providing temporary friction fit with the shank head
Implementation Method 2
a resilient expansion-only split retainer for capturing the shank head in the receiver lower cavity
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver having a cavity with a bottom opening and a first channel for receiving a rod, and a shank having a head positionable into the cavity with the shank extending downward through the bottom opening. The assembly also includes an insert positionable into the central bore above the shank head and having a second channel alignable with the first channel, opposite extensions extending outwardly in the direction of the channels to block rotation between the insert and the receiver, and upward-facing surfaces located laterally outward from and on each side of the second open channel. The upward-facing surfaces are configured for releasable compressive engagement by a compressing tool after the elongate rod is received within the channels and prior to a final locking of the assembly so as to provisionally lock the shank from pivoting with respect to the receiver.