Nested Locking Assembly for Compact Spinal Rod Fixation
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Solution Overview
Problem
Existing bone anchoring devices have larger upper portion dimensions due to the size of the locking assembly, which is not suitable for applications in limited spaces such as the cervical spine, and lack a tool for independent fixation of locking elements.
Innovation Solution
A locking assembly with a smaller outer diameter and nested locking elements, along with a tool for simultaneous but independent fixation, utilizing a flat thread to prevent leg splaying and allowing for compact design without an outer ring, and a tool that engages with the locking elements for precise torque application.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional locking assembly with external thread and nut member is used, then reliable locking of the rod is achieved, but the outer diameter and overall dimensions of the bone anchoring device become too large for limited spaces such as the cervical spine
Solution Approach 1:
The locking assembly employs nested locking elements where a first locking element with external thread is positioned within a second locking element that has an internal thread. This nesting arrangement allows both locking functions to be integrated in a compact configuration, reducing the overall outer diameter while maintaining reliable locking of the rod through the cooperative action of both elements.
Solution Approach 2:
The locking assembly is divided into separate locking elements (first locking element with external thread, second locking element with internal thread) that can be independently manufactured and then assembled. This segmentation allows for optimized design of each component and facilitates the nested configuration that reduces overall device size while preserving locking reliability.
2Volume of moving object
If the locking assembly is reduced in size for compact design, then suitability for cervical spine application is improved, but the tightening torque capacity and thread form requirements become more constrained
Solution Approach 1:
The nested configuration of locking elements allows the torque transmission path to be optimized within the reduced diameter. The external thread of the first locking element engages with the internal thread of the second locking element, creating an efficient torque transmission mechanism that maintains adequate tightening capacity despite the compact overall dimensions.
Solution Approach 2:
The thread parameters (pitch, diameter, profile) of both the external and internal threads are specifically optimized for the reduced-size locking assembly. By adjusting thread parameters, the design achieves sufficient tightening torque capacity within the constrained outer diameter, balancing size reduction with force transmission requirements.
3Manufacturing precision
If independent locking of rod and head is required, then positioning precision is improved, but additional locking elements increase device complexity
Solution Approach 1:
The locking function is segmented into two independent elements: the first locking element secures the rod through its external thread engagement, while the second locking element secures the head through its internal thread engagement. This segmentation enables independent adjustment and locking of each component, achieving precise positioning while keeping the overall device complexity manageable through the nested integration of these elements.
Data Source
AI summary
A locking assembly for securing a rod in a receiving part of a bone anchoring device includes a first locking element having a first end and a second end and a longitudinal axis of rotation and an outer surface provided with an external thread, a coaxial bore passing entirely through said first locking element and an internal thread provided at said bore, a second locking element having a longitudinal axis of rotation and an outer surface with an external thread cooperating with the internal thread of said first locking element. The first locking element has a recess between the first end and the second end, that defines a circumferentially closed wall portion. The interior of the wall portion has a longitudinally extending structure for engagement with a tool. Furthermore, a tool is provided which has sections which can be independently engaged with the first and second locking element, respectively.