The shoulder joint is the one with the largest range of motion within all human joints. In fact it is not only the glenohumeral joint, commonly known as the shoulder joint, but also the accompanying acromioclavicular and the sternoclavicular joint and the gliding motion of the scapula that realize this huge range of motion. It is more than other joints like the hip or the knee controlled by muscles and tendons. It is less restricted by bony structures than other joints like the hip and mainly controlled by muscles and tendons.
Although it is not involved in the locomotion process, the shoulder joint is heavily loaded during everyday life. This combined with fragile structure increase the risk of injury. On the other hand the shoulder joint is rather seldom affected by osteoarthritis because the high loads occur not continuously like in the hip or knee joint. Therefore approximately 10 000 shoulder joint replacements face more than 10 times more knee and hip replacements every year in Germany. A reason for a replacement of the shoulder beside osteoarthritis can be rheumatoid arthritis or necrosis in the humeral head.
Today the loosening rate of shoulder implants is still higher than at the hip or knee joint. To improve the implants and their fixation in the bone, exact knowledge about the magnitude and direction of shoulder joint load is an essential factor.
Measurements of the shoulder joint loads can furthermore improve preclinical test scenarios like fatigue testing of new implant prototypes. Last but not least the results of these measurements can lead to specific advices for patients and medical staff which activities can be done or should be avoided immediately after shoulder surgery.
Instrumented Shoulder Implant
The picture shows an instrumented shoulder implant capable to measure forces, moments and additionally the temperature acting in the glenohumeral joint. It was developed in the Biomechanics Lab of the Charité and contains a measuring unit with 6 semiconductor strain gages and a 9-channel telemetry transmitter. Each strain gage requires one channel of the telemetry while the remaining three channels are used for transmitting the temperature, the current supply voltage and a synchronising signal. At the lower end an inductive coil ensures the power supply. The measuring signals are leaded with a pacemaker feedthrough to the antenna protected by a cap of PEEK which transmits the signals to the external measuring unit.
The first measurements show that even easy looking activities of daily living like the lifting of a coffee pot (VIDEO 1068KB) can lead to shoulder joint loads that exceed the own body weight.
On the other hand the typical motions during physiotherapy shwoed only low forces on the joint (VIDEO 1057KB).
A big influence on the loads in the joints has long lever arm and/or additional external weights. This can be seen at the examples of abduction (VIDEO 1064KB) and elevation (VIDEO 1352KB) with and without 2 kg.
A very interesting insight in his professional career as a cartoonist for newspapers, books and calendars was given by Patient S1R during one measurement. The video (VIDEO 645KB) shows that even painting can be a "hard job" for the shoulder joint with loads higher than 50% of the own bodyweight.
All results of the study will be summarized together with data from the other projects of in vivo load measuring in a free online database OrthoLoad. Some examples are already available in the database which will be constantly expanded.