Bone Screw Variable Z-Axis Translation Locking Mechanism
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Solution Overview
Problem
Current bone screw systems face challenges in achieving precise alignment and secure fixation due to anatomical variations and differences in screw placement techniques, often requiring undesirable anterior/posterior translation or manual assembly, which can lead to suboptimal alignment or screw pullout.
Innovation Solution
A modular bone fixation system featuring a bone fastener with an outer receiver, screw retainer, and inner receiver that allows for variable angle orientation and longitudinal translation, utilizing a spring mechanism and locking prongs to secure the connecting rod, enabling easy assembly and stable fixation without compromising the bone-screw interface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If anterior/posterior translation is used to align the tulip/receiver with the rod or plate, then alignment is improved, but the risk of vertebral body fractures and screw pullout increases
Solution Approach 1:
The system is divided into modular components: an outer receiver fixed to the vertebra, an inner receiver that can translate longitudinally within the outer receiver, and a lock mechanism. This segmentation allows the inner receiver to adjust position without requiring anterior/posterior translation of the entire screw assembly, thereby maintaining alignment precision while preserving screw fixation stability.
Solution Approach 2:
The inner receiver is designed to be movable relative to the outer receiver in the longitudinal direction, providing dynamic adjustment capability. This allows the system to adapt to anatomical variations and achieve proper alignment through controlled translation of the inner receiver only, rather than forcing the entire screw assembly to translate, thus avoiding vertebral body fractures and screw pullout.
2Manufacturing precision
If the screw is partially backed out to allow z-axis adjustment, then alignment is improved, but the bone-screw interface strength is reduced
Solution Approach 1:
The separation of the inner and outer receivers allows z-axis adjustment to be achieved by translating the inner receiver relative to the outer receiver, rather than backing out the entire screw. This ensures that the screw threads remain fully engaged in the bone, maintaining bone-screw interface strength while still allowing necessary alignment adjustments.
3Adaptability or versatility
If manual assembly of the screw and head construct is required, then adaptability is improved, but the complexity of the procedure increases
Solution Approach 1:
The inner receiver is nested within the outer receiver, forming a compact integrated assembly that simplifies the overall structure. This nested configuration allows the components to be pre-assembled and sterilized together, reducing the need for complex manual assembly during surgery while maintaining the adaptability of the modular design for various anatomical configurations.
4Adaptability or versatility
If polyaxial screws with articulating tulip or receiver are used, then adaptability is improved, but the device complexity increases
Solution Approach 1:
The articulating mechanism is segmented into the outer receiver (fixed to vertebra) and the inner receiver (movable within outer receiver). This segmentation simplifies the overall structure by allowing the inner receiver to handle the complexity of angulation adjustment through its ability to translate and rotate independently, while the outer receiver remains a simple fixed component.
Solution Approach 2:
The inner receiver is designed with dynamic movement capabilities, allowing it to translate longitudinally and rotate relative to the outer receiver. This dynamic design provides polyaxial adjustment functionality without requiring a complex articulated mechanism in the outer receiver, thereby reducing overall device complexity while maintaining adaptability.
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 system provides secure and adjustable fixation by allowing for precise alignment and angulation of the bone fastener, reducing the risk of screw pullout and enhancing the stability of the bone-screw interface, while facilitating easy assembly and adjustment during surgery.
Implementation Method 1
utilizing a spring mechanism and locking prongs to secure the connecting rod
Data Source
AI summary
A bone fixation system with variable z-axis translation is provided. The system includes an outer tulip coupled to a bone fastener via a screw retainer. An inner tulip is coupled to the outer tulip such that the inner tulip is longitudinally movably relative to the outer tulip. The inner tulip includes a lock that provides a seat for a connecting rod. The inner tulip together with a seated rod is permitted to translate along the z-axis inside the outer tulip when in an unlocked position. Also in the unlocked position, the bone fastener is free to angulate relative to the outer tulip. The z-axis position of the inner tulip and rod relative to the outer tulip is fixed in a locked position. Also, in the locked position, the bone fastener is locked with respect to the outer tulip. The system may be adjusted between the locked and unlocked positions by way of a set screw.