Polyaxial Bone Screw Split Retainer Ring Assembly
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Solution Overview
Problem
Existing polyaxial bone screw designs with contractile locking engagements are weak against pull-out forces during spinal reduction procedures and can be difficult to assemble, as they often require the screw head and receiver to be inserted as a single unit, which can be cumbersome and lead to floppy rotation issues.
Innovation Solution
A polyaxial bone screw assembly featuring a split retainer ring with an expansion-only locking mechanism, where the retainer ring base is positioned below the shank head and engages with the receiver's horizontal and vertical loading surfaces, providing a stronger and more secure fixation without the need for disassembly, and allowing for independent locking and adjustment of the screw mechanism.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If contractile locking engagement is used to secure the shank head in the receiver, then the assembly can be disassembled for adjustment, but the structure becomes weak against pull-out forces during spinal reduction procedures
Solution Approach 1:
The locking mechanism is divided into two independent parts: an expansion-only split retainer ring that provides strong pull-out resistance, and a separate compression insert that enables adjustability. This segmentation allows each component to specialize in one function without compromising the other.
Solution Approach 2:
The contractile locking structure is completely removed from the design. Instead, an expansion-only retainer ring is used that remains engaged with the shank head, providing continuous strong retention while a separate compression insert handles the locking and adjustment function.
2Stability of the object's composition
If the screw head and receiver are inserted as a single preassembled unit, then the connection is stable, but the assembly becomes difficult and cumbersome to implant
Solution Approach 1:
The system is divided into three separable components: the shank with head, the receiver, and the expansion-only split retainer ring. The retainer ring is preattached to the shank head, but the shank and receiver can be implanted separately, combining ease of surgery with post-implant stability.
Solution Approach 2:
The expansion-only split retainer ring is preattached to the shank head before surgery, ensuring proper positioning and stability. This preliminary action eliminates the need for complex intraoperative assembly while maintaining connection stability.
3Adaptability or versatility
If a contractile retainer ring is used to lock the shank head, then the assembly can be disassembled, but the structure becomes difficult to assemble and may exhibit floppy rotation
Solution Approach 1:
Instead of using a contractile (compressible) retainer ring that requires complex assembly, the invention uses an expansion-only retainer ring that naturally expands to engage the shank head. This inversion of the locking mechanism simplifies assembly while preventing floppy rotation through continuous engagement.
4Adaptability or versatility
If the retainer ring can be removed from the receiver, then the assembly can be disassembled for adjustment, but the pull-out resistance is compromised
Solution Approach 1:
The system separates the retention function (expansion-only retainer ring) from the locking function (compression insert). The retainer ring remains permanently engaged with the shank head to maintain pull-out resistance, while the compression insert provides the locking mechanism that allows controlled disassembly for adjustment.
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 enhances pull-out resistance and stability by preventing the receiver from being removed from the shank head, providing a non-floppy connection, and allowing for easier assembly and adjustment, effectively addressing the limitations of prior art in terms of strength and usability.
Implementation Method 1
a friction fit resilient expansion-only split retainer ring for capturing the bone screw shank head in the receiver member assembly
Implementation Method 2
a friction fit resilient expansion-only split retainer ring for capturing the bone screw shank head in the receiver member assembly
Data Source
AI summary
A polyaxial bone screw assembly includes a threaded shank body having an integral upper portion receivable in an integral receiver, the receiver having an upper channel for receiving a longitudinal connecting member and a lower cavity cooperating with a lower opening. The receiver can have crimp tabs, but is devoid of spring tabs and collet-like flexible structures. A down-loadable compression insert (some with lock and release feature), a down-loadable friction fit split retaining ring having super structure for temporary friction fit with an up-loadable shank upper portion cooperate to provide for pop- or snap-on assembly of the shank with the receiver either prior to or after implantation of the shank into a vertebra. The shank and receiver once assembled cannot be disassembled.