Hip Implant for Temperature Measurements Figure 6. Standard temperature curve. The temperature-dependent signals of 5 implants are plotted. doi:10.1371/journal.pone.0043489.g006 The telemetry system described was approved by BerlinCERT (Berlin, Germany) using standards of both EU Directive 90/385/ EWG (AIMDD, Active Implantable Medical Device Directive) and EU Directive 93/42/EWG (MDD, Medical Device Di- rective). A planned clinical study with 100 patients will begin after receiving approval from our Ethics Committee. Measuring Accuracy The measuring accuracies were determined at 37.5uC, 43uC, and 50uC. The measured temperatures were compared with readings from a thermometer with an accuracy of 0.01uC. The error of the calibrated implants was always below 0.1uC. To determine how fast the instrumented implants are able to detect temperature changes, a prototype was placed in cold circulating water. Then, boiling water was quickly added such that the temperature rose from 23uC to approximately 51uC. This temperature increase was recorded by the implant within 70 s (Figure 4). Discussion The clinical study on temperature rise in hip implants is planned with 100 patients. The shape of the original implant was unchanged by the instrumentation, and thus, current surgical procedures can be used. Therefore, the same good clinical success, such as with other Spotorno-like implants, can be expected. Investigations will be performed with 4 different combinations of head and cup materials and 2 different head sizes. This data will allow us to answer the following questions: a) Are certain material combinations producing such high temperatures that implant fixation of patients with badly lubricating synovia is endangered? b) How much do the lubrication properties of synovia differ individually? c) Can patients at risk be identified by intra-operative synovia tests? d) Do joint simulators deliver realistic results for friction and wear? Our previous instrumented joint implants (www.OrthoLoad. com), with multi-channel telemetries for load measurements, required higher sampling rates and a signal transmission by radiofrequency. Because such signals are shielded by the metallic implant, electrical feedthroughs and an antenna outside of the implant were needed, which had to be biocompatible and protected mechanically. The low frequency magnetic pulses, now used for data transfer, are only marginally weakened by titanium implants and can therefore be transmitted through the metallic wall. This enables the design of mechanically safe and simple instrumented orthopedic implants. In addition, the de- scribed technique is also well suited for measuring strains or detecting implant loosening by frequency analyses. The low power consumption of 7 mW prevents a temperature increase by the inductive power supply. The transmission rate of 5 Hz is sufficiently high for measuring temperatures, and the measuring error of 0.1uC is lower than expected. The transmission rate can be increased, for example, up to 50 Hz, with good accuracy. For measuring fast changing signals this rate may still be too low. Sampling can be accelerated, however, if the signals are not transmitted in real-time. Instead, they can be measured at a high rate, stored temporarily in the PLOS ONE | www.plosone.org 6 August 2012 | Volume 7 | Issue 8 | e43489