Shoulder Arthroplasty Stem Geometry for Metaphyseal Loading
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Solution Overview
Problem
Current shoulder arthroplasty implants face issues such as over-tensioning of the joint, bone loss due to stress shielding, and the need for multiple sizes and offsets to accommodate varying patient anatomy, leading to complications like decreased range of motion and increased fracture risk.
Innovation Solution
A convertible shoulder arthroplasty system with a stem design that loads the metaphysis to prevent stress shielding, uses a statistical shape model for optimal sizing, and includes fins for rotational stability, allowing conversion between anatomic and reverse configurations without an intermediate tray, and minimizes bone removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If adaptor trays are used in onlay designs to convert from anatomic to reverse configuration, then the spherical head can be exchanged, but the joint becomes over-tensioned leading to decreased range of motion and acromial fractures
Solution Approach 1:
The cup component is designed with a nested structure where the articulating surface is positioned within the stem's intramedullary canal rather than on top of it. The cup's concave surface nests within the stem, eliminating the need for external adaptor trays and preventing over-tensioning of the joint soft tissues.
Solution Approach 2:
The cup component serves as an intermediary element that directly couples the spherical head to the stem through its articulating surface, eliminating the need for intermediate adaptor trays. This direct connection prevents the accumulation of thickness that would otherwise over-tension the joint.
2Reliability
If press-fit stem designs achieve fixation in the diaphysis, then secure fixation is obtained, but stress shielding occurs in the proximal metadiaphyseal and metaphyseal regions leading to bone loss
Solution Approach 1:
The stem is designed with varying cross-sectional geometries along its length, with larger dimensions in the proximal metaphyseal region and smaller dimensions distally. This local variation in stem geometry distributes mechanical loads more evenly, providing secure fixation while maintaining physiologic stress distribution to prevent bone loss in the proximal region.
Solution Approach 2:
The stem's tapered geometry creates a dynamic load distribution system where the proximal metaphyseal region bears more load during insertion and function, naturally transferring stresses to preserve bone density in the proximal metadiaphyseal and metaphyseal regions while maintaining secure fixation.
3Reliability
If cylindrical stem designs are used to achieve fixation in the shaft, then secure fixation is obtained, but a large number of sizes and offsets are required to address varying patient anatomy
Solution Approach 1:
The stem is designed with asymmetric cross-sectional geometry featuring flat surfaces and keyed configurations that provide rotational stability and anatomical alignment. This universal design allows a single stem type to accommodate varying patient anatomy through selective orientation and positioning rather than requiring multiple sized variants.
Solution Approach 2:
The stem employs asymmetric cross-sectional geometry with flat surfaces and keyed features that naturally align with the anatomical orientation of the humeral canal. This asymmetry provides inherent rotational stability and anatomical matching, eliminating the need for multiple offset variations while maintaining secure fixation across different patient anatomies.
4Ease of manufacture
If onlay designs are used for reverse arthroplasty, then the reverse poly cup can be placed on the resection plane, but additional thickness is added to the joint creating over-tensioning
Solution Approach 1:
The cup component is nested within the stem's intramedullary canal, with the articulating surface positioned inside rather than outside the stem structure. This nested configuration eliminates the need for onlay placement on the resection plane, preventing the addition of excessive thickness that would cause joint over-tensioning while maintaining ease of implantation.
Data Source
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AI summary
Shoulder arthroplasty system comprising an implant and an articular cup (2) coupled to the implant, the articular cup including an articular surface with an apex (122), the apex being configured to be disposed distal to a resection plane in a humerus bone; the implant includes a stem (1) having a proximal portion (7), a distal portion (12), an anterior portion, a posterior portion, a medial portion (14), and a lateral portion (13), wherein the stem has a size and shape for insertion into an intramedullary canal of the humerus bone, the humerus having a metaphysis and a diaphysis, wherein the proximal portion of the stem comprises a concave taper (10) decreasing in size in a direction extending from the proximal portion toward the distal portion, and wherein distal portion comprises a distal taper (11) decreasing in size in a direction extending from the proximal portion toward the distal portion, and wherein the distal taper comprises a taper in a direction extending between the anterior portion and the posterior portion, and wherein the distal taper comprises a taper in a direction extending between the medial portion and the lateral portion, and wherein the shape of the stem is configured to load the metaphysis with a load greater than a load on the diaphysis.