Resorbable Magnesium Alloy Bone Screw with Torsional Pre-Stressing
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Solution Overview
Problem
Magnesium alloy bone screws face challenges with lower strength compared to titanium and high-grade steel, leading to potential over-tightening and breakage when used in self-tapping applications, especially in small-scale or cannulated designs, due to torque exceeding torsional strength, and issues with hydrogen release during degradation.
Innovation Solution
The bone screw incorporates pre-stressing with torsional stress opposite the screw-in direction, reduced cross-section and mass, and surface functionalization to lower torque requirements and hydrogen release, using a magnesium alloy with micro-abrasives and microcapsules for enhanced biocompatibility and miniaturization, along with a self-tapping and self-lubricating thread profile.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If magnesium alloy is used for bone screws, then biocompatibility and resorption capability are improved, but torsional strength is reduced leading to potential breakage
Solution Approach 1:
The patent applies preliminary torsional pre-stressing to the magnesium alloy screw during manufacturing. This pre-stressing creates compressive residual stresses that counteract the tensile stresses generated during screw-in operation, preventing the screw from exceeding its torsional strength limit and breaking, while preserving the magnesium alloy's biocompatible and resorbable properties
Solution Approach 2:
The patent changes the physical state of the screw by introducing controlled residual stresses through pre-stressing. This parameter change (from stress-free to pre-stressed state) fundamentally alters the screw's mechanical response to loading, enabling magnesium alloy to achieve sufficient torsional strength for self-tapping applications while maintaining its inherent biocompatibility
2Volume of moving object
If screw cross-section is reduced for miniaturization, then application flexibility is improved, but torsional strength is reduced increasing breakage risk
Solution Approach 1:
The patent applies preliminary torsional pre-stressing to miniaturized screws, creating residual compressive stresses that compensate for the reduced cross-sectional area. This allows the screw to maintain adequate torsional strength despite the reduced mass, enabling safe miniaturization for small-scale bone applications
Solution Approach 2:
The patent changes the stress state parameter of the miniaturized screw by introducing pre-stressing. This parameter change enables screws with reduced cross-sections (outer diameter 1.5-5 mm) to achieve the necessary torsional strength by transforming the stress distribution within the material
3Duration of action of moving object
If magnesium alloy is used for bone screws, then biodegradability is improved, but degradation time is excessive and hydrogen release occurs
Solution Approach 1:
The patent applies torsional pre-stressing that induces plastic deformation and microstructural changes in the magnesium alloy during manufacturing. These parameter changes in the material structure accelerate the controlled degradation process, reducing degradation time from potentially months/years to a more clinically appropriate timeframe, while the pre-stress also influences hydrogen evolution patterns
4Force
If surface functionalization is applied to reduce friction, then torque requirement is reduced, but manufacturing complexity increases
Solution Approach 1:
The patent modifies the surface parameters of the screw by applying functional coatings or treatments that alter friction characteristics. These parameter changes in surface properties enable reduced torque requirements for screw insertion, with the manufacturing complexity managed through established surface treatment technologies
Data Source
AI summary
A bone screw formed primarily of a magnesium alloy has a self-tapping thread, and is prestressed with a retained torsional stress oriented oppositely to the direction in which the screw is screwed into bone. The pre-stressing opposes the torsional stresses arising from screwing in the screw, thereby raising the effective strength of the screw and/or allowing for a reduction in its size/mass.