Analysis of glenoid fixation features under simulated off-centre loading

Abstract

Loosening of glenoid component is one of the major concerns in shoulder replacement surgery. There are many factors that can cause glenoid component loosening, one of which is arguably due to failure of the implant to achieve stability after surgery. During physiological movement of the joint, the implant may be loaded off-centre, causing the so-called 'rocking-horse' phenomenon. The design must therefore have fixation features that enhance the stability of the implant when being loaded off-centre. To study the stability due to eccentric load transfer on the implanted glenoid cavity, three dimensional model of the scapula was constructed from CT images. Four glenoid components were modelled - a glenoid with parallel pegs, another with slanted pegs, the third with keel and the fourth model without any protrusion as control. The glenoid cavities were virtually prepared using surface mesh of the implants. The implants were then inserted into their respective glenoid cavities and load simulating the off-centre loading was applied in the anterior-posterior direction. The material properties of the bone were assigned with cortical values. Results showed that glenoid component with pegs produced lower interface micromotion compared to the one with keel and the control model. The magnitude of micromotion and its distribution was found to be similar for the parallel and slanted pegs. There were little differences in terms of the stability of the glenoid with slanted pegs compared with parallel pegs. Preference is therefore given to glenoid component with parallel pegs due to the ease of registering the implant into the cavity.