Variable Angle Bone Screw Hardened Head Design
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Solution Overview
Problem
Existing variable-angle locking screws face challenges in maintaining high cantilever strength and fatigue resistance, particularly when the hardness of the screw head is increased, which can lead to reduced shaft fatigue strength, especially in titanium screws.
Innovation Solution
A two-piece variable-angle bone screw design is introduced, where the threaded head has a greater material hardness than the shaft, allowing for enhanced cantilever strength without compromising the fatigue strength of the shaft.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the hardness of the screw head is increased to enhance cantilever strength, then the cantilever strength is improved, but the fatigue strength of the shaft is reduced
Solution Approach 1:
The bone screw is divided into two separate portions: a first portion forming the head with hardened threads, and a second portion forming the shaft with non-hardened threads. This segmentation allows each portion to have optimized material properties - the head portion achieves high hardness for cantilever strength while the shaft portion maintains ductility for fatigue resistance.
Solution Approach 2:
The hardening treatment is applied locally only to the first portion (head) of the bone screw, while the second portion (shaft) remains non-hardened. This local quality differentiation ensures that the head has the required hardness for cantilever strength without compromising the shaft's fatigue strength through selective material property modification.
2Device complexity
If a single-piece design is used for simplicity, then device complexity is reduced, but it becomes difficult to achieve both high head hardness and high shaft fatigue strength
Solution Approach 1:
The bone screw is constructed as two separate portions that are attached to each other. The first portion (head) and second portion (shaft) can be manufactured independently with different material treatments, then joined together. This segmentation enables optimization of each portion's mechanical properties without the constraints of a monolithic design.
Solution Approach 2:
The bone screw functions as a composite structure where the first portion and second portion have different material properties (hardened vs. non-hardened). This composite approach allows the assembly to exhibit both high head hardness and high shaft fatigue strength, combining the advantages of different material states in a single functional unit.
Data Source
Figure 1A~1B
Figure 1C
Figure 2A~2B
AI summary
A variable angle bone screw includes a first portion and a second portion that is separate from the first portion and configured to be attached to the first portion. The first portion defines at least a portion up to an entirety of the threaded head of the variable angle bone screw. The second portion defines at least a portion up to an entirety of the threaded shaft of the variable angle bone screw. The threaded head can have a first material hardness, and the threaded shaft can have a second material hardness that is less than the first material hardness.