Bone Anchor Thread Structure for Cortical Pull-Out Resistance
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Solution Overview
Problem
Existing bone anchors in orthopedic surgery lack sufficient mechanical performance to prevent loosening or pull-out, particularly in cortical bone, and do not adequately facilitate bone and tissue ingrowth.
Innovation Solution
A bone anchor with a shank featuring a regular thread section for cancellous bone and an additional thread structure with a modified shape and helix-shaped recesses for cortical bone, which provides increased holding force and resistance against loosening or pull-out, and promotes bone ingrowth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a conventional thread structure is used in bone anchors, then the bone anchor can be inserted into cancellous bone, but the holding force and resistance against loosening or pull-out in cortical bone are insufficient
Solution Approach 1:
The thread structure is divided into two distinct segments: a first thread structure for engagement with cancellous bone and a second thread structure with modified shape for engagement with cortical bone. This segmentation allows each thread structure to be optimized for its specific bone type, thereby increasing holding force without requiring complete redesign of the entire thread system.
Solution Approach 2:
The second thread structure is provided with a modified thread shape specifically at the portion engaging cortical bone, while the first thread structure maintains its original configuration for cancellous bone engagement. This local modification of thread geometry (e.g., increased pitch, altered flank angles) provides enhanced holding force in cortical bone without affecting the insertion characteristics in cancellous bone.
2Reliability
If the thread structure is modified to increase holding force in cortical bone, then resistance against loosening or pull-out improves, but the complexity of the thread structure increases
Solution Approach 1:
The thread structure is divided into two distinct segments: a first thread structure for engagement with cancellous bone and a second thread structure with modified shape for engagement with cortical bone. This segmentation allows each thread structure to be optimized for its specific bone type, thereby increasing holding force without requiring complete redesign of the entire thread system.
Solution Approach 2:
The second thread structure incorporates a helix-shaped recess that provides flexibility to the thread portion engaging cortical bone. This dynamic feature allows the thread to adapt to variations in cortical bone density and geometry, enhancing reliability against loosening while maintaining a relatively simple overall structure through the use of a single continuous thread form with localized modifications.
3Strength
If a single thread structure is used for both cortical and cancellous bone, then the device is simpler to manufacture, but mechanical performance is insufficient for preventing loosening or pull-out
Solution Approach 1:
The second thread structure is provided with a modified thread shape specifically at the portion engaging cortical bone, while the first thread structure maintains its original configuration for cancellous bone engagement. This local modification of thread geometry (e.g., increased pitch, altered flank angles) provides enhanced holding force in cortical bone without affecting the insertion characteristics in cancellous bone.
Solution Approach 2:
The thread structure modifies specific parameters (pitch, flank angle, depth) only in the second thread structure portion that engages cortical bone, while maintaining standard parameters in the first thread structure for cancellous bone. This selective parameter change achieves improved mechanical performance without requiring complete redesign, facilitating easier manufacturing compared to entirely new thread forms.
Data Source
Figure 1~2
Figure 3
Figure 4a~5b
AI summary
A bone anchor is provided comprising a shank (2) configured to be anchored in bone, the shank (2) comprising a first end (2a) configured to be inserted first and an opposite second end (2b) and a longitudinal axis (S) extending through the first end and the second end, a core (10) and a thread (20) configured to engage bone, the thread (20) comprising a number of turns of a helix around the core (10), the number of turns defining an axial length of the thread, wherein the thread (20) comprises a lower flank facing (20a) towards the first end (2a) and an upper flank (20b) facing towards the second end (2b), the upper flank and the lower flank defining a thread shape; wherein an additional thread structure (21) with a modified thread shape is provided on a portion (s2) of the core that is smaller than the axial length of the thread, the additional thread structure (21) comprising a recess (22) that is at least partially helix-shaped.