Polyaxial Bone Screw With Spherical Capture And Compression Insert
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Solution Overview
Problem
Existing polyaxial bone screws face challenges in aligning and securing rods within the bone screw receiver, particularly during lengthy and difficult spinal surgery procedures, where components may not remain properly aligned, and the fixed head screws make it difficult to position the rod correctly.
Innovation Solution
A polyaxial bone screw assembly with a resilient structure extending from the receiver to bias against a compression insert, allowing for top or bottom loading and preventing rotational movement of the insert, which includes a retainer and compression insert that can be pivotally engaged with the shank and receiver, enabling flexible articulation and secure locking of the rod.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a fixed head bone screw is used, then the structure is simple, but the rod cannot be favorably positioned and it is difficult or impossible to place the rod within the receiver head
Solution Approach 1:
The bone screw employs a polyaxial mechanism that allows the receiver head to rotate and articulate relative to the shank, transforming a fixed structure into a dynamic one. This enables the rod to be inserted at various angles (typically up to 30 degrees) before the components lock into place, solving the positioning flexibility problem while maintaining structural integrity
2Ease of operation
If polyaxial bone screws with open-ended receivers are used, then rod insertion is facilitated, but components may not remain properly aligned during lengthy surgical procedures
Solution Approach 1:
The bone screw incorporates pre-configured alignment features including guide surfaces, engagement protrusions, and locking mechanisms that are prepared in advance. These features automatically guide and maintain component alignment during the surgical procedure, ensuring that the rod, receiver, and shank remain properly positioned throughout the operation without requiring continuous adjustment
Solution Approach 2:
The invention introduces intermediary alignment structures such as guide surfaces and engagement features that mediate between the rod and the shank. These intermediary elements facilitate proper alignment by providing physical guidance and constraint surfaces, ensuring that components connect in the correct orientation while allowing for polyaxial movement during insertion
3Strength
If compression inserts are designed to engage rod surfaces, then secure engagement is achieved, but the inserts may rotate or become misaligned within the receiver
Solution Approach 1:
The compression insert features asymmetric geometry with non-circular cross-sections, flat surfaces, or keyed configurations that prevent rotation. This asymmetric design ensures that the insert can only be positioned in one specific orientation within the receiver, maintaining stable alignment while still providing secure engagement with the rod surface through optimized contact surfaces
Solution Approach 2:
Alignment features such as guide surfaces, engagement protrusions, and locking mechanisms act as intermediaries between the compression insert and the receiver. These intermediary elements prevent rotational movement and maintain proper orientation of the insert within the receiver, ensuring stable alignment while allowing the insert to fulfill its compression and engagement function
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 assembly provides a low-profile, easy-to-use spinal implant with aligned components that are cost-effective to produce, ensuring secure and flexible engagement of the rod with the bone screw, facilitating precise alignment and stabilization during spinal surgery.
Implementation Method 1
a resilient structure extending from the receiver and biasing against the compression insert at a depression formed in a surface of the insert, the resilient structure prohibiting rotational movement of the compression insert within the receiver
Data Source
Figure 1
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AI summary
A polyaxial bone screw assembly includes a receiver, a shank, an articulation structure for retaining the shank in the receiver and a compression insert for engagement with a longitudinal connecting member such as a rod. The articulation structure includes substantially spherical convex and concave surfaces that slidably engage both shank and receiver surfaces to provide compound articulation between the receiver and the shank. The receiver includes inwardly directed spring tabs engaging the insert and prohibiting rotation of the insert within the receiver.