Abstract
Introduction
Varying degrees of posterior glenoid bone loss occurs in patients with end stage osteoarthritis and can result in increased glenoid retroversion. The excessive retroversion can affect implant stability, eccentric glenoid loading, and fixation stresses. Ultimately, the goal is to correct retroversion to restore normal biomechanics of the glenohumeral joint. The objective of this study was to identify the optimal augmented glenoid design based on finite element analysis (FEA) modeling which will provide key insights into implant loosening mechanisms and stability.
Materials and Methods
Two different augmented glenoid designs, posterior wedge and posterior step- were created as a computer model by a computer aided design software (CAD). These implant CAD models were created per precise manufacturers dimensions and sizes of the augmented implant designs. These implants were virtually implanted to correct 20° glenoid retroversion and the different mechanical parameters were calculated including: the glenohumeral subluxation force, relative micromotion at the bone-cement interface the glenoid, implant and cement mantle stress levels. The FEA model was then utilized to make measurements while the simulating abduction with the different implant designs. The biomechanical response parameters were compared between the models at comparable retroversion correction.
Results
The model prediction of force ratio for the augmented wedge design was 0.56 and for the augmented step design was 0.87. The step design had higher force ratio than the wedge one at similar conformity settings. Micromotion was defined as a combination of three components based on different directions. The distraction measured for the wedge design was 0.05 mm and for the step component, 0.14 mm. Both implants showed a similar pattern translation wise. The greatest difference between the two implants was from the compression standpoint, where the step component showed almost three times more movement than the wedge design implant. Overall, the step design registered greater micromotion than the wedge one during abduction physiologic loading. The level of stress generated during abduction on the glenoid vault was 1.65 MPa for the wedge design and 3.78 MPa for the step one. All stress levels were found below the determined bone failure limit for the bone and polyethylene (10–20 MPa). Concerning implant stress, the results measured on the backside of the wedge and step components were 6.62 MPa and 13.25 MPa, respectively. Both components showed high level of stress level measured on the cement mantle, which exceeded the endurance limit for cement fracture (4 MPa).
Discussion
The augmented glenoid is a novel surgical implant for use in with severe glenohumeral osteoarthritis. Unlike standard glenoid prosthetics, the augmented glenoid is better suited for correcting moderate to severe retroversion. Whereas a step design might provide higher glenohumeral stability, the tradeoff is higher glenoid vault, implant and cement mantle stress levels, and micromotion, indicating higher risks of implant loosening, failure or fracture over time, leading to poorer clinical outcomes and higher revision rates.
