Stemless Shoulder Implant Thread Design for Bone Fixation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing shoulder joint prostheses, particularly those with humeral components, face challenges in achieving secure fixation within the humerus with minimal bone removal and ensuring immediate stability without cementing, which complicates revision procedures and affects range of motion.
Innovation Solution
A humerus implant with a cup-shaped body featuring a thread design that includes inclined leading and undercut trailing surfaces, along with cutouts, to minimize insertion torque and maximize pull-out force, allowing secure fixation without cement and facilitating easy bone growth for increased stability over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a stem is used for fixation in the humerus, then fixation stability is improved, but bone removal is excessive and revision becomes complicated
Solution Approach 1:
The invention extracts the stem component from the prosthesis design, replacing it with a stemless cup that directly interfaces with the humeral head. This eliminates the need for extensive stem insertion and associated bone removal, while maintaining fixation stability through the threaded cup design that engages directly with the humeral bone.
Solution Approach 2:
Instead of inserting a stem into the humerus to provide stability, the invention inverts the approach by using a cup that threads directly into the humeral bone without requiring a stem. This reverses the traditional fixation methodology and achieves stable fixation with minimal bone removal.
2Reliability
If cementing is used for fixation, then immediate stability is achieved, but the joint cannot be used immediately and revision is complicated
Solution Approach 1:
The invention replaces the chemical fixation method (cementing) with a mechanical fixation system consisting of threads and fins that provide immediate mechanical interlocking with the bone. This mechanical engagement provides immediate stability without requiring cement curing time, allowing the joint to be used right away.
3Force
If a thread design with fins is used, then pull-out force is increased, but insertion torque increases
Solution Approach 1:
The fins are designed with varying depths and orientations at different locations around the cup. This local variation in fin geometry allows the thread design to engage bone effectively for high pull-out force while reducing overall insertion torque by distributing the engagement forces more efficiently around the cup perimeter.
4Power
If fins with inclined leading surfaces are used, then insertion torque is minimized, but fixation stability may be reduced
Solution Approach 1:
The fins exhibit asymmetric geometry with inclined leading surfaces that facilitate easy insertion with minimal torque, while the trailing surfaces and overall fin configuration provide robust engagement for high pull-out force. This asymmetric design allows the fin to cut into the bone efficiently during insertion while maintaining strong mechanical interlocking during use.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The thread design achieves high pull-out force and minimal insertion torque, enabling immediate use and enhanced stability, with bone growth further increasing removal torque, thus addressing the challenges of secure fixation and ease of revision.
Implementation Method 1
The fins are specifically designed to secure the body within the bone (30) even without cementing. Due to the radial anchoring in the bone material the fins cause a comparatively high pull out force of the implant. At least one fin, preferably a plurality of fins, most preferably all fins have an inclined leading surface (21) which helps in incising into the bone by rotating the body in an inward direction. By this measure the insertion torque is minimized.
Implementation Method 2
Furthermore at least one fin, preferably a plurality of fins, most preferably all fins have a forward oriented undercut trailing surface (25). This results in an angle of less than 90 degrees between a top surface (23) and the trailing surface (25), blocking rotation in the opposite, outward direction. Therefore the removal torque is maximized.
Implementation Method 3
Tests have proven that this thread design results in removal torques similar to and preferably at least 60% of insertion torques immediately after insertion and without any recovery period allowing further bone growth into the structure. Due to such bone growth the removal torque increases further with time.
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
A humerus implant for a reverse shoulder prosthesis has a cup shaped body and a thread at the outside of the body. The thread has a plurality of fins with cutouts between the fins. At least one fin has an inclined leading surface which helps in incising into the bone by rotating the body in an inward direction. Furthermore at least one fin has a forward oriented undercut trailing surface blocking rotation in the outward direction.