Bone Fastener Thread with Flattened Peaks
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Solution Overview
Problem
Conventional bone fasteners for orthopedic surgery often experience thread tilting and jamming during screwing, leading to damage and contamination, which increases the torque required for extraction due to fretting between the thread surfaces.
Innovation Solution
A bone fastener design featuring a tapered head with a second thread profile having flat peaks and planar valleys, reducing contact surfaces and minimizing fretting, allowing for a consistent insertion and extraction torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional V-shaped threaded head is used for locking to a bone plate, then the thread can engage the plate hole, but the thread tilts and jams during screwing-in, causing damage and contamination
Solution Approach 1:
The thread profile parameters are changed from conventional V-shaped to a profile with rounded peaks and modified flank angles. This parameter change allows the thread to engage more smoothly without tilting or jamming during insertion, while maintaining reliable locking engagement with the plate hole.
Solution Approach 2:
The thread peaks are rounded instead of sharp, and the thread flanks are given curved profiles. This curvature reduces stress concentration and allows smoother engagement with the internal threads, preventing the tilting and jamming that occurs with conventional sharp-ridged threads.
2Manufacturing precision
If the thread flanks and peaks have sharp edges for precise engagement, then the thread can mate accurately with the plate hole, but the sharp edges are damaged and contaminated by bone splinters and tissue
Solution Approach 1:
The thread peaks are rounded and the flanks are given curved profiles instead of sharp edges. This curvature prevents the sharp edges from being damaged or contaminated by bone splinters and tissue during insertion, while still achieving accurate mating with the internal threads through the curved contact surfaces.
Solution Approach 2:
The thread profile parameters are modified to include rounded peaks and adjusted flank angles, changing the geometry from sharp-edged to curved. This parameter change maintains precise engagement capability while eliminating the vulnerability to contamination that sharp edges present.
3Strength
If the thread has full contact surfaces for strong engagement, then the locking strength is high, but the extraction torque is very high due to fretting between thread surfaces
Solution Approach 1:
The thread profile parameters are optimized to balance engagement strength with extraction ease. The rounded peaks and modified flank angles create sufficient contact area for strong locking while reducing the friction and fretting that lead to high extraction torques.
Solution Approach 2:
The curved thread profiles distribute contact stresses more evenly across the engagement surfaces, maintaining strong locking capability while reducing the localized fretting that occurs with sharp-ridged threads, thereby lowering the extraction torque required.
Data Source
AI summary
A bone fastener for use in orthopedic surgery for fixing an implant to bone has a threaded or unthreaded shaft configured to engage bone and a head having a thread on an outer surface to engage the implant. The thread on the head of the fastener has a profile in cross section that includes arcuate peaks with a flat on the outer surface of the peak.