Arthroplasty Implant Thread Geometry for Off-Axis Fixation
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Solution Overview
Problem
Joint arthroplasty devices face issues with loosening due to multi-axial forces and off-axis loading scenarios, as traditional thread designs and tapered stems fail to provide sufficient fixation, leading to instability over time.
Innovation Solution
The development of arthroplasty implants with improved thread designs, including dual helical threads with angled concave undercut surfaces and varying diameters, flange components, and articulating members for enhanced bone fixation and load sharing, particularly in shoulder arthroplasty.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional thread designs and tapered stems are used, then the device structure is simple and easy to manufacture, but the fixation strength is insufficient under multi-axial forces and off-axis loading
Solution Approach 1:
The patent applies local quality by varying the thread geometry along the length of the implant shaft. Different sections of the shaft have different thread characteristics (pitch, depth, orientation) optimized for specific loading conditions at different depths within the bone, thereby improving fixation strength without requiring complex overall design
Solution Approach 2:
The patent employs asymmetric thread designs where the thread profiles are not uniform around the shaft circumference or along the shaft length. This asymmetry allows the threads to better engage with bone in specific directions, providing enhanced resistance to multi-axial forces and off-axis loading while maintaining manufacturability
2Reliability
If traditional thread designs are used, then the manufacturing process is simple, but the device reliability deteriorates over time due to loosening under multi-axial forces
Solution Approach 1:
The patent changes critical geometric parameters of the threads including pitch, depth, width, and orientation angles along the length of the shaft. These parameter variations are designed to optimize load distribution and bone engagement at different depths, improving long-term stability while the parameters remain within manufacturable ranges
Solution Approach 2:
The patent divides the shaft into multiple segments along its length, with each segment having distinct thread characteristics. This segmentation allows each portion of the implant to be optimized for the specific mechanical environment at that depth, enhancing overall reliability without requiring the entire device to be overly complex
3Strength
If uniform thread design is used, then the manufacturing is easier, but the load sharing capability is insufficient under off-axis loading conditions
Solution Approach 1:
The patent implements local quality by designing different thread geometries for different sections of the shaft. proximal sections may have threads optimized for compressive loads while distal sections have threads optimized for tensile or shear loads, enabling better load sharing under off-axis conditions
Solution Approach 2:
The patent extends the thread design from simple cylindrical geometry to include conical or tapered sections, and potentially non-circular cross-sections. This dimensional variation allows the threads to engage bone more effectively in multiple directions, improving load sharing capacity while maintaining reasonable manufacturing complexity
Data Source
AI summary
A bone implant may include a shaft having a proximal end, a distal end, a longitudinal axis, a proximal shaft portion, and a distal shaft portion. The proximal shaft portion may include a first minor diameter, and a first helical thread disposed about the proximal shaft portion defining a first major diameter. The first helical thread may include a first concave undercut surface. The distal shaft portion may include a second minor diameter, and a second helical thread disposed about the distal shaft portion defining a second major diameter. The second helical thread may include a second concave undercut surface. The first and second concave undercut surfaces may be angled towards the distal end of the shaft. The second minor diameter may be smaller than the first minor diameter and the second major diameter may be smaller than the first major diameter.