Multi-Pitch Bone Compression Screw for Lower Insertion Torque
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Solution Overview
Problem
Variable pitch bone screws require high insertion torque, necessitate pre-drilling, and are limited by manufacturing machines like Swiss Lathes, which can cause cross-threading and restrict design features such as screw length and head size.
Innovation Solution
A bone screw design with a continuous, multi-pitch thread that includes a thread transition zone, allowing a first cutting tool to withdraw and a second tool to engage seamlessly, enabling a defined head and reducing torque requirements.
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 pre-drilling is required
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 region facilitates easier insertion with lower torque, while the second region provides effective bone compression, thus resolving the contradiction between insertion ease and compression effectiveness.
Solution Approach 2:
Different pitch values are applied to different regions of the thread. The first pitch region has a larger pitch optimized for insertion, while the second pitch region has a smaller pitch optimized for compression. This local differentiation of thread properties allows each region to optimize for its specific function, reducing overall insertion torque while maintaining compression effectiveness.
2Ease of manufacture
If multi-axis Swiss Lathes with whirling tools are used for manufacturing, then thread forming capability is improved, but cross-threading occurs and design flexibility is limited
Solution Approach 1:
A thread transition zone is introduced as an intermediary region between the first and second pitch regions. This transition zone serves as a buffer that allows the whirling tool to smoothly change pitch without causing cross-threading, while still enabling the formation of both pitch regions. The transition zone mediates between the conflicting requirements of maintaining manufacturing simplicity and avoiding cross-threading defects.
3Volume of moving object
If whirling tool distance from central axis is increased to form threads, then thread diameter is improved, but cross-threading increases and head space is limited
Solution Approach 1:
The pitch of the thread is made dynamic rather than constant, with the pitch value changing along the length of the thread from the first pitch region through the transition zone to the second pitch region. This dynamic pitch allows the thread to accommodate variations in whirling tool position without causing cross-threading, while still achieving adequate thread diameter and head space.
4Reliability
If single-lead variable pitch threads are used, then compression characteristics are improved, but screw length is limited
Solution Approach 1:
The single-lead thread is segmented into multiple pitch regions with different pitch values. This segmentation allows the thread to maintain effective compression characteristics through the smaller pitch second region while accommodating longer screw lengths by using the larger pitch first region for insertion and the transition zone for smooth pitch change, thus resolving the limitation on screw length.
Data Source
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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.