Orthopedic Anchor Segmentation for Cancellous Bone Shear Resistance
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Solution Overview
Problem
Conventional orthopedic anchoring methods, such as screws, often fail to securely anchor in both cortical and cancellous bone due to the porous nature of cancellous bone, leading to compromised pull-out strength and hindered bone regrowth, as they tend to shear out the bone material.
Innovation Solution
The development of anchors with helical splines that have a long pitch and are driven rather than screwed, featuring a central aperture or channel along a curved axis, allowing for axial malletion and providing a larger surface area for engagement with cancellous bone, reducing the risk of shearing and promoting bone growth through porosity and textured surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional screws with threads are used for anchoring, then the anchoring mechanism is simple and easy to install, but the pull-out strength is compromised due to shearing of cancellous bone material
Solution Approach 1:
The anchor is divided into multiple splines that are separated from each other, allowing them to independently engage with the bone tissue. This segmentation prevents the concentrated shearing force of a single threaded screw, distributing the load across multiple discrete spline elements that can flex and adapt to the cancellous bone structure without causing catastrophic failure.
Solution Approach 2:
The anchor incorporates porous or latticed structures within its construction, allowing bone tissue to grow through and around the anchor elements. This porosity increases the surface area for bone-to-implant contact, enhances mechanical interlocking, and allows the bone to integrate with the anchor structure, significantly improving pull-out resistance while reducing stress concentration that causes shearing.
2Strength
If screws with standard thread pitch are used, then the anchoring depth is sufficient, but the actual purchase (physical engagement) is much less in cancellous bone due to its porous nature
Solution Approach 1:
The anchor design transitions from conventional linear threading to a three-dimensional spline configuration that engages bone tissue in multiple spatial dimensions. The splines extend radially outward from a central axis and can be oriented at various angles, creating engagement in radial, axial, and circumferential dimensions simultaneously, thereby maximizing physical contact with the limited cancellous bone material available.
Solution Approach 2:
The anchor employs composite construction combining dense cortical engagement zones with porous cancellous engagement zones. The structure integrates solid spline elements for cortical bone anchoring with porous or latticed regions for cancellous bone interlocking, optimizing mechanical engagement across the heterogeneous bone architecture and maximizing purchase in both bone types.
3Strength
If conventional screws are used for anchoring, then the installation process is straightforward, but bone regrowth or through growth around the anchor is hindered
Solution Approach 1:
The anchor incorporates extensive porous structures, lattices, and apertures throughout its construction that serve as conduits and scaffolds for bone tissue ingrowth. These porous regions allow osteogenic cells to migrate through the anchor structure, deposit bone matrix, and establish mechanical interlocking, transforming the anchor from a foreign object into an integrated part of the bone architecture that enhances both security and biological stability.
Solution Approach 2:
The anchor design enables self-reinforcement through biological processes. As bone tissue grows through and around the porous anchor structures, the biological system itself strengthens the anchoring without requiring additional mechanical fastening or adjustment. The growing bone actively creates its own fixation by infiltrating the anchor's porous architecture and forming direct structural connections.
4Strength
If anchors with large surface area are used to reduce shearing, then the pull-out resistance improves, but the device complexity increases
Solution Approach 1:
The complex surface area requirement is achieved through segmentation into multiple discrete spline elements rather than a single complex surface. Each spline is a relatively simple geometric form, but collectively they provide extensive surface area for bone engagement. This modular segmentation maintains manufacturing simplicity while achieving the necessary complexity for enhanced pull-out resistance.
Solution Approach 2:
The splines incorporate curved and helical geometries that naturally increase surface area compared to straight linear elements. The curved surfaces provide greater contact area with the bone tissue while maintaining smooth continuous forms that are easy to manufacture. The helical configuration of some splines adds circumferential engagement without requiring additional discrete components, achieving complexity through elegant geometric forms rather than assembly of multiple parts.
Data Source
AI summary
An anchor has a straight centerline and straight splines extending radially while running parallel thereto. Splines extend radially from a core penetrated by a channel to fit over a K-wire guide, and axially along the length of the elongate, cylindrical core. The center line passes along the center of a lumen or channel of the hollow core, while the splines extend radially outward from, and axially along the length of the core except along a conical taper near a leading or point end of the anchor. Installation may be from a posterior access or a lateral-posterior access, each anchor fixing two bones in at least one, but usually two, degrees of freedom. Multiple anchors may fix in all six degrees of freedom (three degrees of freedom in translation along three mutually orthogonal axes, and three degrees of freedom of rotation about those same axes).