Summary

The objective of this study was to address the stability and micromotion of symmetry and asymmetry onlay-inset prostheses designs with different curvature and variable peg length cemented and uncemented using finite element analysis.

Abstract

Objective. The objective of this study was to address the stability and micromotion of symmetry and asymmetry onlay-inset prostheses designs with different curvature and variable peg length cemented and uncemented using finite element analysis.
Background. Glenoid component failure is a significant postoperative complication that usually leads to revision surgery. This study evaluated the different prostheses design for patient specific bone loss and the contribution of design factors on initial fixation. Glenoid component loosening due to eccentric compressive loads is the primary concern in total shoulder arthroplasty. This micromotion between the implant and the bone interface is commonly used as an indicator for implant stability. Minimization of micromotion is desirable for osseointegration. Effect of the following on micromotion have not been under scrutiny in current literature: thickness, implant depth, curvature symmetricity, process of implantation, forces acting on different areas of the implant.
Methods. Variations in the numbers of cross section conforming to glenoid bone geometry resulted in symmetry and asymmetry between implant designs were analysed using FE method. Two types of designs were investigated: Onlay – Based on size of the glenoid of the patient. Entire glenoid cavity is shaved off; Inset – Novel design based on cases wherein a portion of glenoid is shaved instead of complete glenoid. Each type is further classified into symmetric and asymmetric curvatures, and different thicknesses.
The model was built using shoulder CT and mimics. Bone density was assigned through a correlation of previous experiments in out lab and pixel resolution factors. A load of 500-750-and 1000 N was applied at three different glenoid positions (SA: superior–anterior; SP: superior–posterior; C: central) to mimic off-centre and central loadings during activities of daily living. A total of 13 different designs were analysed under different bone cuts and loading conditions.
Results. Based on this study, asymmetry was shown to play a crucial role in fixation, and stability. The onlay with symmetry in terms of cross section areas and curved spline designs had a higher loosening at the opposite edges of the loading points. A cemented fully immersed prosthesis was better than a uncemented articular surface; longer peg lengths (18mm) outperformed the shorter pegs (10-12mm); and all pegs were assumed cylindrical and all were made of high density polyethylene.
Conclusion. For our study, Curve/Flat type of onlay was preferred as it is more widely used than other mentioned types. It is important to mention that in practice, a Curve/Curve type of implant would result in lesser amount of glenoid bone to be shaved off than a Curve/Flat implant. This in-depth study addressed loosening behavior of glenoid prostheses and patient specific improved designs, which may lead to a reduction in the incidence of clinical loosening.
Keywords: Micromotion, glenoid, loosening, prosthesis design, TSA.