The knee joint is the biggest joint in the human body. Since it has to carry the entire weight of the body, it is heavily loaded. In case of degenerative arthritis or rheumatoid arthritis, after fractures and other diseases the joint can be replaced by an endoprosthesis. More than 150.000 knee replacements are implanted in Germany every year (Source: Statistisches Bundesamt, data from 2011). With this routine operation the function and resilience of the joint can be reconstructed for the next 15 years or longer.
In exceptional cases complications can occur at an earlier time. One reason for these complications can be a high loading of the knee joint. So far only few realistic data about the loading of the knee joint is available. Better knowledge will allow testing the stability of new implants under realistic conditions and to improve them before their use in patients. It will furthermore help to improve the physiotherapeutic treatment to optimize the healing process and to give patient advices for a right behaviour after implantation. In general the knowledge of the loading helps to understand the biomechanics of the knee joint.
Instrumented Knee Joint
To obtain realistic loading data a knee implant with a 9-channel telemetry transmitter was developed witch enables six-component load measurements in a primary total knee replacement. Both forces in axial, medio-lateral and antero-posterior direction and flexion-extension, varus-valgus and internal-external moments can be measured.
The instrumented knee joint is a modification of the INNEXTM System, Type FIXUC (Zimmer GmbH, Winterthur, Switzerland). The standard femur component and tibial insert are used. Only the tibial component was modified to enable the integration of the electronic devices. During the modification of the tibial component the patients' safety was of special importance.
At the time of the operation the patients were between 60 and 75 years old and suffered from gonarthrosis. During the first weeks after the operation the patients are accompanied to the physiotherapy to measure the loading in the knee joint during the physiotherapeutic treatment. In the following months mainly activities of daily living like walking, stair climbing, standing up and sitting down are subject of the measurement.
In the following examples the measured forces are given in percent of the patient's body weight (%BW).
The first results show that the knee joint is heavily loaded. During walking the resultant force is typically about 250 %BW (Video 717KB). Going downstairs the knee joint is even loaded with up to 350 %BW (Video 463KB). Some acting moments are higher than expected. This knowledge will have a high influence on the preclinical testing of new implants.
Further data about the loading in the knee joint is available in our database OrthoLoad.This database will be constantly extended and will contain the data of all patients at the end of the research.