Rod-to-Bone Coupling With Wave Spring Retention
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Solution Overview
Problem
Existing polyaxial bone anchors face challenges in providing a safe connection with low insertion force, high retention force, and minimal axial travel, while also being easy to manufacture and assemble.
Innovation Solution
A coupling assembly with a receiving part, retainer element, and spring element, including a wave spring that facilitates snap-over of the retainer onto the bone anchoring element's head, allowing for a low insertion force and high retention force, while minimizing axial displacement, and a pressure element for angular positioning, enabling a modular design with various anchoring elements.
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 with high force, but the insertion force required to overcome the spring biasing is increased
Solution Approach 1:
The retainer is pre-positioned in a retracted position within the housing, and the spring is pre-compressed to a controlled extent, so that when the bone anchoring element is inserted, the retainer is already in optimal position to engage the head with minimal additional force
Solution Approach 2:
The retainer is designed as a dynamic component that can move axially within the housing, allowing it to flex and adjust during insertion to reduce the peak force required, while still providing high retention force when engaged
2Reliability
If the head of the bone anchoring element is pushed against the spring force for insertion, then the connection can be secure, but the axial travel or displacement required for insertion is increased
Solution Approach 1:
The retainer is nested within the housing and the spring is nested within the retainer, creating a compact arrangement where the spring's active length is minimized while still providing sufficient biasing force, thereby reducing the axial travel required for insertion
Solution Approach 2:
The spring's physical parameters (wire diameter, coil diameter, number of active coils) are optimized to provide the required retention force within a compressed space, reducing the axial distance the head must travel during insertion while maintaining connection safety
3Ease of manufacture
If a modular design with separate retainer and spring components is used, then the device can be easily manufactured and assembled, but the device complexity increases
Solution Approach 1:
The retainer and spring are combined into a single integrated component where the spring is formed as an integral part of the retainer structure, eliminating the need for separate assembly steps while maintaining the modular benefits of standardized components
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, low-profile connection with reduced risk of milling or sticking out, allowing for easy assembly in situ and compatibility with diverse bone anchoring elements, ensuring stability and ease of manufacturing.
Implementation Method 1
a spring element (7) configured to be arranged in the receiving part (5)
Implementation Method 2
the spring element is a wave spring element, which can generate a higher spring force on a given axial length compared to other spring elements
Data Source
AI summary
A coupling assembly for coupling a rod to a bone anchoring element, including a receiving part having first and second ends, a recess at the first end for the rod, and an accommodation space for a head of the bone anchoring element, the accommodation space having an opening at the second end to permit insertion of the head into the receiving part, a retainer element configured to be arranged at least partially in the accommodation space and to hold at least part of the head, and a spring element configured to be arranged at least partially in the accommodation space and to be compressed in an axial direction. The retainer element and the spring element are separate parts. When the retainer element and the spring element are in the accommodation space of the receiving part in a first position, the spring element extends into the recess of the receiving part.