Medical Screw With Inner Reverse Thread for Bone Extraction
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Solution Overview
Problem
Current medical screws face challenges such as high rotational force requirements for extraction due to callus formation, potential deviation during drilling, and the need for multiple screw lengths, leading to difficulties in extraction and efficient use.
Innovation Solution
A medical screw with a screw thread and an inner reverse thread, paired with a removal jig featuring an outer reverse thread, allowing for easy disengagement and extraction by reversing the rotational direction, and the ability to trim screws to desired lengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a conventional medical screw with a standard thread is used, then the screw can be securely fixed in the bone, but the extraction requires high rotational force due to callus formation
Solution Approach 1:
The patent applies the inversion principle by creating an inner reverse thread that rotates in the opposite direction to the outer screw thread. During extraction, the removal jig engages with this reverse thread and rotates counterclockwise, causing the inner and outer threads to unscrew from each other. This reverse threading mechanism reduces the rotational force required for extraction compared to conventional single-threaded screws that require breaking through callus formation.
Solution Approach 2:
The patent segments the threading function into two independent components: an outer screw thread for bone engagement and an inner reverse thread for controlled disengagement. This segmentation allows the screw to maintain strong fixation through the outer thread while enabling easy extraction through the inner thread mechanism, resolving the contradiction between fixation strength and extraction ease.
2Adaptability or versatility
If multiple screw lengths are prepared to accommodate different bone fracture positions, then various fracture locations can be treated, but inventory complexity and cost increase
Solution Approach 1:
The patent applies the dynamics principle by making the screw length adjustable rather than fixed. The tubular body can be trimmed to the required length using a cutting tool, and the threading mechanism remains functional after trimming. This dynamic adjustment capability allows a single screw design to adapt to various fracture positions and depth requirements, eliminating the need to maintain multiple screw length variants in inventory.
3Strength
If the screw head is buried under callus during extraction, then the screw remains securely fixed, but the removal jig cannot engage with the screw head
Solution Approach 1:
The patent introduces a guide pin as an intermediary tool during extraction. The guide pin is inserted through the bone and engages with the tubular body of the screw, providing a pathway to reach the inner reverse thread even when the screw head is buried under callus. This intermediary mechanism allows the removal jig to engage with the screw's internal threading without requiring direct access to the external screw head, thus resolving the contradiction between fixation security and head accessibility.
Data Source
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AI summary
Provided is a medical screw which can be readily extracted from bones after fusion of bone fractures or at replacement of the screw. The medical screw 11 has a screw thread 11b extending on the outer surface of a distal end portion of the screw 11 and an inner reverse thread extending on the inner surface of a proximal end portion 11d of the screw 11 and made in the opposite direction to the thread direction of the screw thread 11b. An outer reverse thread at a distal end portion of the removal jig 17 is engaged with the inner reverse thread of the screw 11, while being rotated in the opposite direction to the thread direction of the screw thread 11b. This engagement integrates the screw 11 with the removal jig 17. The removal jig 17 is then rotated in the opposite direction to the drilled direction of the screw 11 to detach the screw thread 11b of the screw 11 from the bone fractured region D.