Hip Implant for Temperature Measurements Figure 5. Implant calibration. The implants were calibrated in a circulating water bath at temperatures between 23uC and 58uC. doi:10.1371/journal.pone.0043489.g005 potentiometer (AD5282, Analog Devices). The pulses are con- verted to 12-bit digital values (MAX197, Maxim Integrated Products). The microcontroller checks the received signals S for missing pulses, amplifies their peak values to 361 V, counts their temperature-dependent time intervals tT and sends S and tT to a Windows PC. Power Supply During the measurements the power coil is placed around the hip joint (Figure 3). The power oscillator generates a sinusoidal output voltage at 460.5 kHz. This frequency is permanently adapted to the resonance frequency of the coil. The oscillator output voltage and, thus, the magnetic field strength are controlled by the microcontroller. Primary and secondary power coils are fixed to the thigh and the femur, respectively. Except from soft tissue deformations they therefore move in the same way during walking and the induced supply voltage varies by not more than 5%. Using an induction frequency of 4 kHz with loose air-coupled coils a shift of the titanium implant is not detectable by the primary power coil. The voltage UC and, thus, the strength of the transmitted signal pulse directly depend on the magnetic field strength. For UC below 5 V, the circuit is unstable, and the pulses cease. Levels above 11.7 V are prevented using Zener diodes. The range of the internal voltage UC is controlled by analyzing the amplitude of the received pulses. Beginning with a high amplification, the magnetic field strength is reduced until the pulse height begins to decrease; UC is then at its upper limit of 11.7 V. When the pulses vanish, UC has reached its lower limit of 5 V. Based on these two values, UC is set to 7.5 V. The operating range of 6 V to 9 V is a compromise between sufficient signal strength and the minimal power dissipation. This range allows position changes between the signal source inside the implant and the power coil around the leg without endangering the power supply. If UC nevertheless exceeds one of its borders, UC is automatically re-adjusted. Data Processing The evaluation program is written in Visual Basic (Figure 4). Signal S is displayed, and its peak values are marked as well as the times used for counting t . The temperature is calculated from T previously obtained calibration data and charted for visual control. Calibration For calibration, 5 prostheses were placed in a circulating water bath at 10 different temperatures between 23uC and 58uC (Figure 5). The temperature was adjusted with an accuracy of 0.1uC and measured close to the implant with an accuracy of 0.01uC (9540, Guildline Instruments). From this data, an average polynomial temperature curve, Temperature = f(tT), was calculat- ed (Figure 6). Only an offset temperature at 37.5uC must later be determined for the implants used in patients. Results Implant Safety Only prostheses with neck lengths XS (extra-small), S (small), M (medium) and L (large) will be implanted. For fatigue tests, a longer neck length of XL (extra-large) was used. Under this more severe condition, the implant stem passed the fatigue tests [37]. Then, the neck was tested with a maximum load of 5.3 kN during 10 million cycles [38]. This load was further increased every 1 million cycles in steps of 1 kN, without failure, up to 13 kN. This load was more than 2 times higher than the standard force for testing implant necks. The mechanical tests were performed by EndoLab GmbH (Rosenheim, Germany), and the sterilization process was certified by Vanguard AG (Berlin, Germany). PLOS ONE | www.plosone.org 5 August 2012 | Volume 7 | Issue 8 | e43489