Adjustable Bone Screw Assembly with Segmented Rod Seat
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Solution Overview
Problem
Existing spinal fixation systems, including polyaxial and monoaxial screws, face challenges in controlling movement during vertebral body rotation and are prone to loosening and breakage due to high moments applied by flexible rods in non-fusion stabilization systems, where the distance between the rod and vertebral body surface is large.
Innovation Solution
An adjustable bone screw assembly that allows controlled pivoting of a spinal rod about two selected axes, reducing the distance between the rod and vertebral bone surface, thereby minimizing the applied moment and enhancing stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If polyaxial screws are used to allow pivoting in all directions, then adaptability for positioning bone anchors is improved, but control during vertebral body rotation deteriorates due to undesirable movement in unwanted planes
Solution Approach 1:
The rod seat is divided into multiple components: a first lower rod seat element with a cylindrical bottom surface and a second lower rod seat element with a conical bottom surface. This segmentation allows independent control of pivoting movements about different axes while preventing movement in unwanted planes, resolving the contradiction between positioning flexibility and control stability.
2Adaptability or versatility
If the distance between the rod and vertebral body surface is large, then adaptability for rod placement is improved, but the moment applied to the screw increases leading to loosening and breakage
Solution Approach 1:
The invention introduces controlled pivoting movement about two selected axes as an additional degree of freedom. This allows the rod to be positioned at optimal angles relative to the vertebral body surface, reducing the perpendicular distance and thereby minimizing the moment applied to the screw while maintaining placement flexibility.
Solution Approach 2:
The rod seat allows changing the angular parameters of rod placement by enabling pivoting about two selected axes. This parameter adjustment optimizes the rod's position to minimize the moment arm, directly reducing the applied moment while preserving adaptability for different vertebral anatomies.
3Ease of operation
If monoaxial or polyaxial screws are used, then ease of operation for screw insertion is improved, but reliability deteriorates due to loosening and breakage from high moments
Solution Approach 1:
The rod seat incorporates dynamic pivoting capability about two selected axes, allowing the surgeon to adjust the rod angle during insertion to optimize the moment distribution. This dynamic adjustment maintains ease of operation while improving reliability by preventing excessive moments that cause loosening and breakage.
Data Source
AI summary
A bone screw assembly includes a screw body, including anchor portion and rod-receiving portion, and a rod seat movably mounted in the screw body to allow for controlled angulation between a spinal connection element disposed in the rod seat and the screw body. The rod seat is pivotable in one or more selected directions about one or more axes relative to the screw body. The rod seat may include a first lower rod seat element disposed in a recess of the screw body having a cylindrical bottom surface to facilitate pivoting in a first direction. A second lower rod seat element is stacked on the first lower rod seat element and has a conical bottom surface abutting a top surface of the first lower rod seat element to facilitate pivoting in a second direction.