Variable-Pitch Bone Compression Screw Threading Without Cross-Threading
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Solution Overview
Problem
Variable pitch bone screws require higher insertion torque and present manufacturing challenges, such as cross-threading, due to limitations in current thread-forming tools, which restrict screw design and length, and lack a defined head, affecting their operational efficiency and manufacturing feasibility.
Innovation Solution
A bone screw design with a continuous, multi-pitch external thread and a defined head, featuring a thread transition zone that allows for a smooth transition in pitch, enabling the use of automated CNC machines to form the thread without cross-threading, and a method for programming CNC units to control cutting tools for precise thread formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If variable pitch threads are used to achieve bone compression, then compression effectiveness is improved, but insertion torque increases and manufacturing complexity increases
Solution Approach 1:
The thread is divided into multiple pitch regions (first pitch region with larger pitch, second pitch region with smaller pitch) separated by a transition zone. This segmentation allows different thread sections to perform different functions: the first pitch region provides easier insertion, while the second pitch region provides better compression, resolving the contradiction between insertion ease and compression effectiveness.
Solution Approach 2:
Different pitch values are applied to different locations along the thread length. The proximal portion has a larger pitch for easier insertion, while the distal portion has a smaller pitch for better compression. This local differentiation resolves the contradiction by optimizing each region for its specific function rather than using a uniform pitch.
2Reliability
If variable pitch threads are used to achieve bone compression, then compression effectiveness is improved, but manufacturing precision deteriorates due to cross-threading
Solution Approach 1:
A thread transition zone is introduced as an intermediary region between the first pitch region and the second pitch region. This transition zone with intermediate pitch values serves as a buffer that allows smooth pitch change without causing cross-threading, thereby maintaining manufacturing precision while achieving variable pitch compression.
Solution Approach 2:
The pitch is made dynamic rather than static, transitioning from a larger pitch value to a smaller pitch value along the thread length. This dynamic pitch adjustment allows the thread to adapt to different functional requirements at different locations, improving both compression effectiveness and manufacturability.
3Productivity
If whirling tools are used for thread forming, then manufacturing speed is improved, but thread pitch control deteriorates leading to cross-threading
Solution Approach 1:
The thread forming process is segmented into multiple stages corresponding to different pitch regions. Each pitch region is formed with appropriate tool parameters, and the transition zone provides a buffer that prevents cross-threading. This segmentation allows high-speed whirling tools to be used while maintaining pitch control accuracy.
Solution Approach 2:
The pitch parameter is changed along the thread length rather than remaining constant. By implementing a variable pitch design with distinct pitch regions and a transition zone, the patent enables whirling tools to form threads at high speed while maintaining accurate pitch control through parameter variation along the thread length.
4Ease of operation
If the minor thread diameter is increased to provide more space for drive socket, then ease of operation is improved, but manufacturing precision deteriorates due to cross-threading
Solution Approach 1:
The thread design applies local quality by having different pitch values at different locations. The proximal portion with larger pitch provides better drive socket access, while the distal portion with smaller pitch maintains manufacturing precision. This local differentiation resolves the contradiction between ease of operation and manufacturing precision.
Data Source
AI summary
A method of shaping a bone screw with an automated device having a CNC unit includes rotating a blank coupled to a spindle about an axis defined by a coordinate system of the CNC unit, advancing a cutting tool proximally through an exterior of the blank at a speed to form a helical thread along a shaft, and reducing the speed to provide a variable, proximally decreasing pitch along at least a portion of the shaft. A first cutting tool is automatically transitioned toward disengagement from the blank when 1) a relative axial position between the first cutting tool and the blank coincides with a first coordinate of a predetermined location, and 2) a relative rotational position between the first cutting tool and the blank coincides with a second coordinate of the predetermined location. The first coordinate is along the axis, the second coordinate is an angular position about the axis, and the first and second coordinates are defined by the coordinate system. A second cutting tool is moved into engagement with the exterior of the blank so that the second cutting tool engages the blank substantially at the predetermined location, in a manner enabling continuation of the thread.