Polyaxial Bone Screw Assembly for Variable Rod Diameters
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Solution Overview
Problem
Current spinal fixation systems face challenges in accommodating spinal rods of varying diameters and materials, as existing bone screws are often specific to a particular rod configuration, making it difficult to change or use rods of different diameters during surgical procedures without removing previously implanted screws.
Innovation Solution
A bone screw assembly comprising a screw, outer housing, and inner housing with a V-shaped configuration and set screw, allowing for adjustable securement of rods between 3.5 mm to 7 mm diameters, with the ability to lock the assembly in place using a pin and set screw, and optionally using different materials for the screw and housings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bone screw is designed for a specific rod diameter, then the fixation is stable and reliable, but the adaptability to different rod diameters is reduced
Solution Approach 1:
The bone screw is divided into multiple functional segments: a polyaxial mechanism with movable inner and outer housings that can independently adjust, and a fixed threaded portion for bone anchoring. This segmentation allows the rod-receiving portion to adapt to different diameters while maintaining stable fixation through the segmented locking mechanism.
Solution Approach 2:
The bone screw incorporates dynamic elements including a movable inner housing that can shift axially within the outer housing, and a polyaxial mechanism that allows angular adjustment. These dynamic components enable the screw to adapt to various rod diameters and configurations while maintaining reliable fixation through controlled movement and locking.
2Adaptability or versatility
If a bone screw assembly is designed to accommodate multiple rod diameters, then the adaptability is improved, but the device complexity increases
Solution Approach 1:
The bone screw employs a nested structure where the inner housing is positioned within the outer housing, and the rod-receiving mechanism is nested within the screw body. This nesting approach allows multiple functional components to be integrated in a compact arrangement, accommodating various rod diameters without proportionally increasing overall device complexity.
Solution Approach 2:
The bone screw assembly is designed as a universal system that can accommodate rods of different diameters and materials through its polyaxial mechanism and adjustable housing. This multi-functionality is achieved through standardized interfaces and scalable geometric features that work across a range of rod specifications without requiring completely different screw designs.
3Ease of operation
If the bone screw allows polyaxial adjustment, then the ease of operation is improved, but the stability of the fixed configuration is reduced
Solution Approach 1:
The polyaxial mechanism is designed to perform preliminary adjustment functions during implantation, allowing the surgeon to optimize rod alignment and screw orientation before final locking. The mechanism enables preliminary positioning in multiple axes, which is then secured through subsequent locking actions that freeze the configuration.
Solution Approach 2:
The inner housing acts as an intermediary element between the outer housing and the rod, facilitating polyaxial adjustment while maintaining stability. This intermediary component allows controlled movement and positioning, then transmits and locks the final configuration through its interaction with the outer housing and rod-receiving surfaces.
Data Source
AI summary
A bone screw assembly includes a screw, an outer housing, and an inner housing. The screw includes a shank and a head. The outer housing has a proximal end, a distal end, and a passageway extending longitudinally therethrough. The outer housing defines a saddle at the proximal end thereof for receiving at least a portion of a rod therein. The inner housing is positionable within the passageway of the outer housing. The inner housing has a recessed distal portion for receiving the head of the screw therein. The inner housing also includes a proximal surface defining a substantially V-shaped configuration for receiving rods of different diameters therein.