Frusto-Conical Washer for Bone Screw Fixation
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Solution Overview
Problem
Existing bone fixation systems fail to effectively join and stabilize bone fragments when the proximal bone fragment lacks structural integrity or when the bone screw threads are stripped, leading to inadequate compression and fixation.
Innovation Solution
A frusto-conical washer with a central bore and inwardly extending flange is used to accommodate the bone screw, allowing for proper engagement and countersinking, even in structurally compromised bone fragments, and can be used with both threaded and non-threaded bone screws, providing compression force for fragment stabilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a compression bone screw is used to engage the proximal bone fragment, then compression force is generated to join bone fragments, but the fixation fails when the proximal bone fragment lacks structural integrity or when threads are stripped
Solution Approach 1:
The washer acts as an intermediary component between the bone screw and the compromised proximal bone fragment. The washer's larger surface area distributes the compression force over a broader area of the bone fragment, reducing stress concentration on weakened bone structures. This mediator approach allows the fixation system to function reliably even when the bone fragment cannot directly support the full thread engagement loads.
Solution Approach 2:
The fixation system is segmented into distinct functional components: the bone screw provides compression force generation, while the washer provides force distribution and structural support. This segmentation allows each component to optimize its specific function, with the washer compensating for deficiencies in the bone fragment's structural integrity that would otherwise compromise the entire fixation system.
2Force
If the threaded proximal end of the bone screw directly engages the bone fragment, then compression force is applied, but the threads may strip or fail to engage properly in compromised bone
Solution Approach 1:
The washer is positioned and secured to the proximal bone fragment before the bone screw is fully tightened. This preliminary placement establishes a stable base that will distribute subsequent compression forces, preventing thread stripping during the tightening process. The washer essentially prepares the interface in advance to handle the impending compressive loads.
Solution Approach 2:
The washer changes the physical parameters of the interface between the screw and bone by increasing the surface area over which forces are distributed. This parameter change from a small threaded contact area to a larger washer surface area reduces pressure and stress concentration, allowing compression forces to be applied reliably without stripping the threads in compromised bone.
3Ease of operation
If the bone screw head passes completely through the washer, then the washer cannot provide proper seating and compression, but preventing passage limits screw engagement
Solution Approach 1:
The washer features an asymmetric central bore design with a tapered configuration that is larger at the proximal end and smaller at the distal end. This asymmetric geometry allows the bone screw to be inserted easily through the larger proximal opening while the smaller distal portion prevents the screw head from passing completely through. The asymmetry thus simultaneously enables easy insertion and provides proper seating prevention.
Solution Approach 2:
The tapered central bore of the washer utilizes a curved, conical geometry rather than a straight cylindrical bore. This curvature creates a gradual transition in diameter that guides the bone screw into proper alignment and positioning. The tapered shape naturally funnels the screw into the correct orientation while the varying diameter prevents complete passage, combining ease of insertion with precise positioning control.
Data Source
AI summary
A bone fixation system includes a compression bone screw and a washer for use with the bone screw in the reduction and fixation of bone fragments. The washer includes a frusto-conical body having a leading end, a trailing end, an exterior surface, an interior surface, and a central bore defined by the interior surface for receiving the bone screw. The interior surface is tapered resulting in the central bore's diameter being larger at the trailing end than at the leading end and the exterior surface has a greater taper angle than the taper angle of the interior surface.