Polyaxial Bone Anchor Coupling Assembly with Snap-Over Retainer
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Solution Overview
Problem
Existing polyaxial bone anchors face challenges in achieving a safe and efficient connection between the bone anchoring element and the coupling assembly, requiring high retention force with low insertion force and minimal axial travel.
Innovation Solution
A coupling assembly with a receiving part, a retainer element featuring a compressible spring portion, and a pressure element that locks the bone anchoring element in a specific angular position, allowing for a snap-over mechanism that ensures secure retention with minimal insertion force.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a resilient spring means is used to bias the retainer towards the lower end of the housing, then the bone anchoring element can be retained securely, but the insertion force required to overcome the spring biasing force is high
Solution Approach 1:
The retainer element is pre-positioned in the accommodation space with the spring portion already biased, ready to engage the head upon insertion. This preliminary positioning ensures that when the head is inserted, the retainer element is already in the optimal position to provide retention force, reducing the force needed during the actual insertion process.
Solution Approach 2:
The coupling assembly is divided into distinct functional components: the receiving part with accommodation space, the retainer element with spring portion, and the head of the bone anchoring element. This segmentation allows each component to be optimized independently - the spring portion can be designed with specific mechanical properties to provide high retention force while requiring minimal insertion force to compress.
2Strength
If the head of the bone anchoring element is pushed against the spring force to achieve secure connection, then high retention force is achieved, but the axial travel required for insertion is large
Solution Approach 1:
The retainer element is designed to move not only axially but also radially as it engages the head. The spring portion provides both axial biasing force and radial constriction onto the head, creating a multi-dimensional engagement that achieves secure retention with minimal axial travel. The retainer element can clamp onto the head from multiple directions simultaneously.
Solution Approach 2:
The retainer element transitions from a static pre-biased position to a dynamic engaged state with minimal axial movement. The spring portion is designed to convert small axial compression distances into significant retention forces through its mechanical leverage and geometric configuration, allowing the system to achieve high retention force without requiring large axial travel.
3Reliability
If the retainer element is positioned into the bore of the housing with spring means, then the bone anchoring element can be retained, but the structure becomes complex
Solution Approach 1:
The retainer element and spring portion are merged into a single integrated component rather than separate parts. The spring portion is formed as an integral part of the retainer element, eliminating the need for separate spring assemblies and reducing the number of parts that need to be assembled and positioned within the housing bore.
Solution Approach 2:
The retainer element serves multiple functions simultaneously: it provides the retaining force through the spring portion, acts as a mechanical constraint on the head, and guides the insertion process. This multi-functionality reduces the need for additional specialized components, simplifying the overall coupling assembly structure while maintaining high retention reliability.
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 solution provides a secure connection with high retention force while requiring low insertion force and minimal axial travel, reducing the risk of inadvertently pulling out the bone anchoring element and minimizing complications such as milling or sticking out of the head from the bone.
Implementation Method 1
The retainer element has a structure that includes a spring portion that is biased in such a manner that the retainer element snaps automatically onto a head of the bone anchoring element when the head is being inserted
Implementation Method 2
The retainer element includes at least one spring portion that is compressible in an axial direction
Data Source
AI summary
A coupling assembly for coupling a rod to a bone anchoring element includes a receiving part having a first end, a second end, a recess having a bottom for receiving the rod, and an accommodation space having an opening at the second end of the receiving part for accommodating a head of the bone anchoring element, and a retainer element configured to be inserted into the receiving part from the first end and to hold at least part of the head, the retainer element having a first portion and a spring portion compressible in an axial direction attached to the first portion. When the retainer element is in the accommodation space in a first position, the spring portion extends in the axial direction from the first portion of the retainer element to an axial position between the first end of the receiving part and the bottom of the recess.