Multi-Pitch Bone Compression Screw With Continuous Thread Transition
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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 usability and manufacturing efficiency.
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 first cutting tool to withdraw while a second tool engages, enabling continuous thread formation, and a method using a CNC unit to control the cutting tools for precise thread shaping, reducing cross-threading and enabling a longer screw length without predrilling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If variable pitch bone screws are used to achieve bone compression, then favorable bone compression characteristics are obtained, but higher insertion torque is required
Solution Approach 1:
The thread is divided into multiple pitch regions with different pitch values along the longitudinal axis. The first pitch region has a first pitch and the second pitch region has a second pitch that is less than the first pitch. This segmentation allows different portions of the screw to have optimized thread characteristics for their specific functions, reducing overall insertion torque while maintaining effective bone compression.
Solution Approach 2:
Different regions of the screw body are given different thread properties. The proximal region (first pitch region) has a coarser pitch suitable for initial bone engagement and compression, while the distal region (second pitch region) has a finer pitch that reduces insertion torque. This local differentiation optimizes both compression effectiveness and insertion characteristics.
2Reliability
If variable pitch bone screws are used to achieve bone compression, then favorable bone compression characteristics are obtained, but pre-drilling is required
Solution Approach 1:
The screw design incorporates a ball tip at the distal end that performs preliminary bone engagement and guidance. This ball tip allows the screw to be inserted without pre-drilling by mechanically preparing the bone pathway, eliminating the need for separate pre-drilling steps while maintaining the variable pitch compression benefits.
3Ease of manufacture
If thread-forming tools are used to manufacture variable pitch screws, then thread formation is achieved, but cross-threading occurs
Solution Approach 1:
The manufacturing process uses dynamic control of the thread-forming tool, varying the rotational speed and feed rate during thread formation. By dynamically adjusting parameters during the threading process, the system can accommodate the variable pitch requirements while preventing cross-threading through real-time synchronization of tool motion with the desired thread geometry.
Solution Approach 2:
The thread pitch parameter changes along the longitudinal axis of the screw, with the first pitch region having a first pitch and the second pitch region having a second pitch. This controlled parameter variation is achieved through precision machining that adjusts the thread-forming tool's motion parameters to create the desired multi-pitch configuration without causing cross-threading.
4Ease of manufacture
If the distance between the whirling tool and the central axis of the screw increases, then thread formation capability is improved, but cross-threading occurs
Solution Approach 1:
A ball tip is formed at the distal end of the screw body before the threading process. This ball tip serves as a preliminary structural element that guides the thread-forming tool and maintains proper alignment, allowing the tool to operate at optimal distances from the central axis without causing cross-threading.
Solution Approach 2:
The ball tip acts as an intermediary element between the screw body and the thread-forming tool. It provides a stable reference point and mechanical guide that enables precise thread formation at the desired radial distance from the central axis while preventing the tool from deviating and causing cross-threading.
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.