Bone Fastener Thread Geometry for Multi-Axial Fixation
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Solution Overview
Problem
Traditional fastener thread designs fail to provide sufficient fixation and load sharing at bone/fastener interfaces under multi-axial and off-loading conditions, leading to loosening over time.
Innovation Solution
The development of fastening devices and systems with improved thread designs, including dual helical threads with concave and convex undercut surfaces, and manufacturing methods that involve specific cutting head configurations and milling processes to create these advanced thread geometries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional fastener thread designs are used, then the fastener structure is simple and easy to manufacture, but the bone fixation and load sharing are insufficient under multi-axial and off-loading conditions
Solution Approach 1:
The thread design is segmented into multiple functional zones including compressive load bearing zones, tensile load bearing zones, and shear load bearing zones. Each zone has specific geometric features optimized for different loading conditions, allowing the fastener to handle multi-axial and off-loading forces effectively while maintaining manufacturing feasibility through systematic zone division
Solution Approach 2:
The thread geometry incorporates asymmetric features such as non-circular cross-sections and uneven pitch distributions that provide superior mechanical interlocking and load distribution compared to traditional symmetric threads. This asymmetry enables better resistance to multi-axial loading while the manufacturing process uses computer-controlled milling to achieve these complex geometries efficiently
2Strength
If traditional fastener thread designs are used, then the manufacturing process is simple, but the load sharing between bone and fastener interface is insufficient
Solution Approach 1:
The thread design employs variable geometric parameters including changing pitch, varying depth, and non-uniform spacing along the fastener length. These parameter changes optimize load distribution across different zones to maximize bone-fastener interface strength. The manufacturing process uses computer-controlled milling with programmed tool paths to efficiently create these variable parameter geometries
Solution Approach 2:
The thread geometry incorporates curved and rounded features rather than sharp angular transitions, creating smooth stress distributions and improving load sharing. The concave and convex surfaces with specific radii of curvature enhance bone contact area and distribute stresses more evenly, while the manufacturing process uses ball-end mills and controlled feeding rates to achieve these curved geometries
3Reliability
If advanced thread designs with multiple cutting heads are used, then the bone fixation and load sharing are improved, but the device complexity and manufacturing complexity increase
Solution Approach 1:
Multiple cutting heads are combined into a single multi-functional milling tool that can create different thread zones in one operation. The tool integrates various cutting elements with different geometries and orientations, allowing the manufacturing of complex advanced thread designs without requiring multiple separate machining operations or tools, thereby reducing overall manufacturing complexity while achieving superior fastener fixation
Data Source
AI summary
A process of forming a fastener with improved threading to resist multi-axial forces and off-axis loading scenarios is provided. The process may include placing first and second mill tools adjacent a shaft of the fastener having a proximal end and a distal end, rotating the shaft and the first and second mill tools, and translating the first and second mill tools along at least part of a length of the shaft to form: a first concave undercut surface oriented toward the proximal end, a first convex undercut surface oriented toward the distal end, a second concave undercut surface oriented toward the distal end, and a second convex undercut surface oriented toward the proximal end.