Abstract
Introduction The complex movement of scapula is significant for the support of the arm and the stability of the shoulder joint. Recent investigations showed an adaptation in scapula rhythm after total shoulder replacement with a big variability within subjects. The latter can change the loading pattern in the glenohumeral contact forces and affect the performance of shoulder prosthesis.
Methods In this study, Newcastle shoulder model was used to simulate a total shoulder arthroplasty and investigate joint stability. The model describes the DELTA ® prosthesis; a reverse anatomy design with a socket component attached to the humeral head and a hemi-ball to the glenoid. Scapula kinematics data of 6 shoulders were recorded using a palpating technique. The subjects had a total shoulder replacement after severe rotator cuff damage. Standard and daily activities were then analysed.
Results and Discussion Scapula kinematics data show an increased scapular lateral rotation, which influences the joint contact forces. Comparing contact forces on the Glenohumeral joint, results indicate that the scapula rhythm adaptation reduces the compressive forces and shifts the shear component more superiorly to the glenoid. The scapula rhythm data used in this study show a large variability, which also affect the loading results. This effect is more significant in “reaching tasks”, where high humeral elevation is required and joint contact loads are maximum. The anterior shear forces in these tasks can be as great as 19% of body weight
Conclusions The adaptation in scapulohumeral rhythm after a shoulder joint replacement has already been reported. The reason for this adaptation cannot be explained yet and may be pain related or due to muscle adaptation that takes place after the arthroplasty. This change in kinematics influences the loading pattern of the glenohumeral joint. In particular the increased shear forces must be taken into considered in prosthetic design.
Correspondence should be addressed to The Secretary, British Elbow and Shoulder Society, The Royal College of Surgeons of England, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
