Joint Loading & Musculoskeletal Analysis
The loads acting in joint prostheses and other orthopaedic implants is still partially unknown. The acting loads are required for different purposes.
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Hip Joint Temperature
Healthy hip joints have very low friction and are well lubricated. In all artificial hip joints friction is higher. This friction produces heat and it must be assumed that hip implants warm more up than natural joints during long lasting activities, associated with high joint loads. Bones and soft tissue could potentially be damaged by temperatures which are too high. Therefore it must be investigated which temperatures may develop in total hip replacements.
Higher temperatures can be expected when walking long distances, because the joint forces are then high, typically at about two and a half times body weight. The most common materials used for hip implants are metal or ceramic for the implant head and plastic, metal or ceramic for the cup. It must be assumed that the implant materials influence warming of the implant.
Instrumented Hip Implants for Force and Temperature Measurements
For investigating the temperature distribution in hip implants, measuring electronics are mounted inside the hollow prostheses (Hip II). Three strain gauges for force measurements are arranged in the neck and eight temperature sensors along the whole length of the implant shaft (Link zu Hüftgelenk, Temperaturmessprothese). Two 8-channel telemetry transmitters transmit a total of 16 measurement signals. Energy supply is achieved by an external magnetic field and an induction coil in the middle of the implant shaft [Graichen et al., 1999].
The implant shaft is made from titanium, the head with the inner transmitting antenna consists of ceramic. The artificial cup is made from polyethylene. In one patient both hip joints were replaced and in one of them a ceramic instead of a plastic cup was used.
Investigations
Measurements were taken on seven implants in five patients (EBL/EBR, KWL/KWR, HSR, PFL, RHR). Additionally to the implant temperature and the joint forces, the body temperature and the muscle temperature in the hip area were recorded (photo). Measured were the initial values and the temperature increase during an hour of walking at normal speed. Furthermore the temperatures after an hour cycling on an ergometer at the same metabolic power were determined, as joint loading and thus friction in the joint are much lower during cycling than walking.
In a computer simulation it was calculated how the friction induced warming in the implant is distributed in the adjacent bone and soft tissues.
Results
The diagram shows the end temperatures from all patients after an hour of walking. The temperatures were highest in the implant head (left, blue arrow) [Bergmann et al., 2001A]. This is the location where the heat is produced due to friction between head and cup of the prosthesis. Towards the tip of the stem the temperatures decrease. In one of the patients the temperature rose to more than 43 degree Celsius (green dashed curve), while it stayed much lower in other subjects. The computer simulation delivered as the most plausible explanation for the individually very different temperatures, that the lubrication properties of the synovial fluid vary widely between the patients [Bergmann et al., 2001B].
During cycling the temperatures are much lower than during walking. It became obvious that an artificial cup, made of ceramic instead of plastic (purple curve), causes much lower temperatures in the implant. It can therefore be assumed that a metal implant instead of ceramic head, paired with a polyethylene cup, will even cause temperatures higher than 43 degree Centigrade in some patients. If this is really the case and if the observed temperatures may have adverse effects on the longevity of the implants is goal of further own research activities.