Biomechanical analysis of reverse anatomy shoulder prosthesis

Abstract

This study uses adaptation of an established 3-D biomechanical shoulder model (Newcastle Shoulder Model) to investigate the biomechanical properties of reverse shoulder replacements that have become popular for severe rotator cuff arthropathy. The prosthetic model describes the DELTA® III geometry and can predict muscle and joint contact forces for given motion. A custom contact detection algorithm was developed to investigate the impingement problem. Results showed that the reverse design increases deltoid function by providing sufficient moment arm (42% increase compared to normal anatomy) and restores joint stability by reversing the envelope of joint contact forces. The data showed a good agreement with other biomechanical models. Further in this study scapula and arm kinematics of a group of DELTA III prosthetic subjects were recorded and compared with normal shoulder activity. The scapula kinematics showed increased lateral rotation and even if it is highly variable within the subjects (range:1.2-1.8 times the normal). there is a trend showing that good recovery shoulders have small change in their scapula rhythm and vice versa. The arm kinematics showed that even if the prosthetic subjects were able to complete most activities there was a variable range of humeral movement. Compared to the normal group the average elevation values were high but the internal/external humeral rotation was significantly smaller. The kinematic data were further used and analysed with the model and the results showed large differences in glenoid loading compared to normal shoulders. where there is an increase in superior (range:12%-52% bodyweight) and antero-posterior shear forces (range:8%-39% bodyweight). Impingement results predicted scapula bone notches similar in shape and volume with the literature which was impossible to eliminate without design modifications. The adapted prosthetic model was successfully used to analyse the biomechanics of a reverse design and provide a useful dataset that can be further used for design optimisation.