Recent advances in shoulder arthroplasty

Automated In-Vitro Testing of Orthopaedic Implants: A Case Study in Shoulder Joint Replacement

This investigation presents the design and preliminary validation of a single station simulator with biaxial motion and loading designed to mimic the kinematics of the glenohumeral joint during arm abduction in the scapular plane. Although the design of the glenoid holder allows the glenoid component to translate in all three axes, it is primarily loaded axially, which brings it into contact with the oscillating humeral head, but is also loaded superiorly to simulate common subluxation of the humeral head. Simulating arm abduction in the scapular plane simplifies component alignment and removes the need for anterior—posterior loading, thereby creating a stable joint without the need to simulate capsular constraints. In this more physiologically accurate simulator design, the load and motion profiles influence the contact kinematics, but the wear path is ultimately determined by the conformity and constraint designed into the bearing couple. The wear data are determined and correlated with clinically retrieved glenoid components, as well as previously reported in-vitro studies, thus verifying use of the simulator in testing alternative materials and designs. The key design features, as well as the improvements proposed through this study, can be incorporated into the design of test fixtures for any other orthopaedic implant such as the hip, knee, spine, elbow, and finger.

Loosening performance of cemented glenoid prosthesis design pairs

OBJECTIVE

The purpose of this pilot study (n=3) was to compare the loosening performance of glenoid prosthesis design pairs where only one design variable differed.

DESIGN

Glenoids were subjected to dynamic edge loading in a biaxial test setup.

BACKGROUND

Glenoid component loosening is the primary concern in total shoulder arthroplasty.

METHODS

After the humeral head was cycled 100,000 times to the superior and inferior edges of the glenoid, the tensile edge displacements were measured under superior and inferior off-center loading.

RESULTS AND CONCLUSIONS

Based on this study, a rough-backed design had dramatically better loosening performance than a smooth-backed; curved-backed was superior to flat-backed; a less-constrained articular surface was better than a more-constrained articular surface; pegs outperformed a keel; threaded pegs were marginally preferable to cylindrical pegs; and an all-polyethylene design rocked slightly less than a metal-mesh-backed design.

RELEVANCE

A comparison of the laboratory loosening behavior of glenoid prostheses may lead to improved designs, subsequently leading to a reduction in the incidence of clinical loosening.

Mechanical testing of shoulder prostheses and recommendations for glenoid design

Glenoid component loosening generates the greatest concern among surgeons performing total shoulder arthroplasty. Laboratory testing of glenoid prostheses may lead to improved design, subsequently leading to a reduction in the incidence of clinical loosening. The goals of this study were to develop a laboratory test method to address glenoid loosening and to apply this method to a variety of prosthesis designs. With use of a biaxial apparatus, glenoid components were cyclically subjected to superoinferior edge loading, mimicking the off-center rocking-horse phenomenon thought to contribute to glenoid loosening clinically. Before and after the rocking test was performed, compression and distraction of the superior and inferior edges were measured with the humeral head displaced to each edge. Rocking performance could not be predicted from initial measurements, indicating the necessity for dynamic loading to evaluate the likelihood of loosening. A roughened fixation surface for outperformed a smooth fixation surface, a curved backing showed almost half the distraction of a flat backing, and a nonconstrained prosthesis distracted less than a more constrained prosthesis.