George and Harry
A demonstration at Queensland University of Technology, in which a robot, George, manipulates a model humerus, Harry, while holding the centre of the humeral head at a fixed point in space.
Why this matters
Keeping the centre of the humeral head fixed while the arm moves is what a healthy shoulder does, and it is what a replacement or a repair is trying to restore. Being able to reproduce that movement mechanically is the first step towards assembling a shoulder model that can be used to test how different implants and repairs behave under load.
What makes the shoulder difficult to model
The shoulder is a ball and socket joint, but unlike the hip the socket is shallow and the ball is large in comparison with it. Very little of the stability comes from the shape of the bones. It comes instead from the ring of cartilage around the socket rim, from the ligaments of the joint capsule, and above all from the four rotator cuff tendons, which pull the ball into the socket while the larger muscles move the arm.
That arrangement is what gives the shoulder more range of movement than any other joint in the body, and it is also why it depends so heavily on soft tissue working correctly. For anyone trying to build a test model it means the bones alone are not the system. The muscle forces have to be reproduced in the right directions and in the right proportions, and the centre of rotation has to stay where it belongs throughout the movement, which is precisely what the demonstration shows being achieved mechanically.
What a working model is used for
Once movement can be reproduced reliably and loads can be measured, the rig can be used to compare things that cannot be compared in patients. Two ways of repairing the same tendon tear can be loaded until they fail, so the stronger construct can be identified. A replacement component can be implanted in different positions and the resulting range of movement measured, which informs how it should be placed in an operation. Forces through the socket can be measured directly rather than inferred.
This is laboratory work and it does not describe what will happen to any particular person, but it is the evidence that sits behind implant design and behind the choice between competing surgical techniques. Treatment for an individual patient is decided on their own clinical assessment. Individual results vary.
Related information
- Our post on the QUT joint biomechanics research centre
- Our shoulder anatomy page
- Research
- Anatomic total shoulder replacement