Threaded Megaprosthesis Stem for Bone Fixation
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Solution Overview
Problem
Current megaprosthesis stems face issues such as loosening over time, requiring long segments of native bone for fixation, precision challenges in sizing, stress-shielding leading to bone resorption, and difficulty in removal, resulting in high failure rates and complications in revision surgeries.
Innovation Solution
A threaded stem design with a shaft having threads on its outer surface, providing increased stability, tolerance for sizing mismatches, controlled insertion force, maintained compression, reduced pull-out risk, and ease of removal, while preserving bone stock and minimizing mechanical breakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If uncemented press-fit stems are used to achieve osteointegration, then long-term stability is improved, but early post-surgical micromotion increases leading to fixation failure
Solution Approach 1:
The stem is divided into multiple threaded segments along its length, each capable of independent engagement with the bone. This segmentation allows progressive threading into the bone canal, providing incremental stability while maintaining osteointegration potential. The threaded segments create multiple anchor points that distribute mechanical loads and reduce micromotion throughout the stem-bone interface.
Solution Approach 2:
Threads are pre-formed on the stem surface before implantation. During insertion, these pre-formed threads automatically engage with the bone canal wall, creating immediate mechanical interlocking. This preliminary threading action establishes early stability without requiring cement or extensive bone preparation, while the thread geometry is designed to allow subsequent bone ingrowth for long-term osteointegration.
2Strength
If stem length is increased to improve fixation stability, then fixation strength is improved, but the requirement for native bone segment length increases
Solution Approach 1:
The stem incorporates threads only at specific locations along its length rather than uniformly across the entire surface. Threaded segments are strategically positioned in regions where bone engagement provides maximum mechanical advantage, while leaving other regions smooth or porous to facilitate bone ingrowth. This localized threading approach maximizes fixation strength within shorter bone segments by concentrating mechanical engagement at optimal positions.
Solution Approach 2:
The stem combines different surface characteristics along its length - threaded regions for immediate mechanical fixation and smooth or porous-coated regions for osteointegration. This composite design allows the threaded portions to provide strong anchorage in limited bone segments while the non-threaded portions promote biological bonding, achieving both short-term and long-term stability without requiring extensive native bone length.
3Stability of the object's composition
If cemented stems are used to achieve immediate stability, then early post-surgical stability is improved, but long-term loosening occurs due to cement failure
Solution Approach 1:
The invention replaces the cement-based mechanical bonding system with a direct mechanical threading system. Instead of relying on cement to transfer loads between stem and bone, threaded engagements create direct mechanical interlocking through screw-thread mechanics. This substitution eliminates cement-related failure modes (debonding, fragmentation, loosening) while providing both immediate stability through thread engagement and long-term reliability through bone-on-threads osteointegration.
Solution Approach 2:
The threaded stem design allows the implant to create its own fixation mechanism during insertion without requiring external cement application. As the stem is threaded into the bone canal, the threads automatically engage with the bone wall, generating self-contained mechanical interlocking and compression forces. This self-service fixation eliminates dependence on cement, reducing the number of potential failure points while maintaining both early and long-term stability.
4Strength
If stem diameter is increased to improve fixation, then fixation stability is improved, but the risk of bone fracture during insertion increases
Solution Approach 1:
The stem incorporates adjustable or variable-diameter threading along its length, with thread depth and pitch varying to optimize fixation while minimizing bone stress. The threaded sections are designed with progressive engagement characteristics, allowing gradual bone compression and thread formation during insertion rather than sudden force application. This dynamic threading design provides strong fixation in proportion to the available bone diameter while reducing the risk of iatrogenic fractures during the insertion process.
Data Source
AI summary
The present invention relates to a prosthesis, such as a megaprosthesis, for a joint replacement or any segmental bone deficit. In particular, the present invention relates to a stem for a prosthesis having threads on at least part of an outer surface thereof. A prosthesis, such as a megaprothesis, is also provided. The prosthesis contains the threaded stem and a modular body engaged to the stem. The modular body contains a bone replace segment or a replacement joint, such as replacement knee joint, hip joint, shoulder joint, wrist joint, ankle joint, elbow joint, joints of the hand, joints of the foot, etc.