Uni-Planar Bone Fixation Assembly for Rod Alignment Without Screw Depth Shift
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Solution Overview
Problem
Conventional bone fixation elements, such as pedicle screws, allow limited motion of the bone anchor to a single plane, requiring adjustment of the anchor seat to align with the rod slot, which can lead to advancement or withdrawal of the screw from the bone surface.
Innovation Solution
A bone fixation subassembly with an anchor seat and collet that permits the bone anchor to rotate and pivot freely in a desired plane, preventing movement in other planes, allowing the screw to maintain depth without adjusting its position relative to the bone surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the bone anchor is permitted to rotate freely about the axis of rotation relative to the anchor seat, then the flexibility and adaptability of the fixation assembly is improved, but the control over the plane of motion becomes difficult to manage
Solution Approach 1:
The motion control is segmented into two independent components: rotation about the axis of rotation (unrestricted) and pivoting in a specific plane (controlled by guide walls). This segmentation allows the bone anchor to rotate freely for adaptability while the guide walls segment the pivoting motion to a single plane, preventing complexity from arising from uncontrolled multi-directional motion.
Solution Approach 2:
The guide walls act as an intermediary structure between the bone anchor and the anchor seat. These guide walls mediate the motion by permitting rotation while constraining pivoting to a single plane, thus resolving the contradiction between freedom of rotation and controlled motion planes without requiring complex mechanical mechanisms.
2Adaptability or versatility
If the rod slot is not in line with the trajectory of the rod, then the adaptability of the assembly to different trajectories is improved, but the anchor seat must be turned which results in advancing or withdrawing the screw toward and away from the bone surface
Solution Approach 1:
The bone anchor assembly is made dynamic by allowing the bone anchor to rotate freely about its axis and pivot within a controlled plane. This dynamic capability enables the assembly to adapt to different rod trajectories without requiring manual adjustment of the anchor seat orientation, thereby maintaining precise screw depth positioning while achieving adaptability to various trajectories.
Solution Approach 2:
The solution moves from a fixed, single-plane alignment requirement to a multi-dimensional motion capability. By allowing rotation about the axis of rotation and controlled pivoting in one plane, the system gains freedom in multiple dimensions, enabling adaptation to different rod trajectories without compromising the precision of screw depth positioning.
3Stability of the object's composition
If the bone anchor is constrained to pivot in only one plane, then the stability of the fixation is improved, but the ability to align with various rod trajectories is limited
Solution Approach 1:
The motion degrees of freedom are segmented into rotation about the axis (unrestricted, providing adaptability) and pivoting in a single plane (controlled by guide walls, providing stability). This segmentation resolves the contradiction by allowing stable single-plane pivoting for fixation stability while enabling rotation for trajectory alignment adaptability.
Solution Approach 2:
The system changes the parameter of motion freedom by allowing rotation about the axis of rotation while maintaining constrained pivoting in one plane. This parameter change enables the bone anchor to achieve both stability (through controlled single-plane pivoting) and adaptability (through free rotation for trajectory alignment).
Data Source
AI summary
A bone fixation assembly includes a plurality of bone fixation elements that each include a bone anchor configured to be implanted into underlying bone, such as a vertebra. Each bone anchor is received in an anchor seat, and the anchor seats are joined by a fixation rod so as to operatively couple and fix the position and orientation of the vertebrae relative to each other. The bone anchor is free to rotate relative to the anchor seat, and is also free to pivot in a desired direction relative to the anchor seat.