With instrumented implants, the forces and temperatures in Al2O3 - UHMWPE hip joints were measured in 5 subjects during 45 to 60 minutes of walking and bicycling [17]. After walking, the temperature rose up to 43.1uC in the patient with the lowest body weight. Another patient with a much higher body weight reached a joint temperature of only 40.0uC. In the only patient with bi- lateral implants, the temperature was 0.9uC lower with an Al2O3 cup than with a UHMWPE cup. After cycling, which caused 55% lower joint forces than walking, the temperatures were 1.5uC lower. We assume that the steady-state temperature after walking is closely correlated to the friction coefficient. In a simulator, the surface temperature directly between a UHMWPE cup and an Al2O3 head was 45uC, but was 60uC with a CoCrMo head and 99uC with a zirconia ceramic [18]. These are temperatures at which synovia precipitates and loses its lubricating properties. Bone Necrosis After heating rabbit thighs up to 42.5uC to 44.0uC using microwaves, strongly increased bone formation was observed [19]. After 4 minutes at 50uC, osteocytes were found to be irreversibly damaged [20]. Approximately 15–20% of the osteoblasts became necrotic after being exposed to 48uC for 10 minutes, while they withstood 45uC without damage [21]. After heating the superficial skull of rats to 48uC for 15 minutes, dead osteocyte areas were found, and the formation of new bone was delayed [22]. From the available literature on bone reactions to increases in temperature during drilling and sawing, it was concluded that 47uC is a critical temperature [23]. All of these studies investigated only the effect of non-recurrent high temperatures. Repeatedly acting heat may even cause cell damage at lower temperatures. Concepts of Instrumented Implants Electronic components used for measurements with permanent implants, such as joint replacements, must be hermetically encapsulated. The optimal solution would be a complete enclosure by a metal [24] or ceramic [25] material. If an antenna or induction coil is placed outside the implant, only biocompatible materials are permitted [26,27]. Certified pacemaker feedthroughs are then favorably used for connections to the implant electronics. Our force measuring hip implants employed an internal power coil and an external niobium antenna [28,29,30,31]. Solutions using plastic-encapsulation for non-biocompatible electronic components [32,33,34] should only be used for non-permanent implants. Power and signals could respectively be transferred to and from the implant by electro-magnetic fields. These fields are partly absorbed by a metal implant with the loss dependent on the alloy and frequency. Pure Ti, Al and V are paramagnetic with a relative magnetic permeability slightly greater than one. Implants made from such alloys only moderately weaken magnetic fields of low frequencies. However, ferromagnetic materials, such as Co or Ni, with a relative magnetic permeability of 80 to 600 almost completely shield the interior of an implant. The loss caused by encapsulations made from Ti alloys is strongly frequency-dependent. A closed TiAl6V4 housing with a 2- mm wall thickness shields 23% of a magnetic field at 4 kHz but 53% at 10 kHz [30]. Any energy loss is accompanied by a temperature increase of the implant. Both the power consump- tion and the shielding loss must therefore be kept low. A titanium implant with an internal secondary power coil and transmitting antenna should use frequencies below 10 kHz for power as well as signal transfer. All transponder systems work at higher frequencies up to the GHz range and can therefore not be used inside Hip Implant for Temperature Measurements a metallic implant. Locating transponder systems at the surface of a metallic implant [35] may cause problems for signal and energy transfer. Goals of this Work The reported strong differences of friction coefficients, the individual variations of synovia properties, and the question of how well joint simulators mimic the in vivo loading conditions demonstrate the need to obtain in vivo information on the friction- induced temperature rise in joint implants. The aim of the study was to design a temperature measuring hip implant with telemetric data transfer, which is completely safe for patients and can be used in a clinical study with a large number of patients. Furthermore, the technique described should be appli- cable for the instrumentation of other kinds of implants. The following features were included: inductive power supply, inductive data transfer through the wall of the hermetically closed metallic implant, power consumption below 10 mW, measuring accuracy of 0.2uC, design based on a clinically well-proven implant type, and no requirement to change the surgical procedure. Methods Mechanical Design The non-cemented CTWTM Classic hip implant with a 12/14- mm cone (Merete Medical, Berlin, Germany) was used for instrumentation. It closely resembles implants of other manufac- turers with very good clinical results [36]. The implant shape was only slightly modified between the neck and shaft to further increase its stability (Figure 1). A 6.2-mm-wide by 50-mm-long bore in the neck houses the temperature telemetry. At its top, a 5- mm-thick plate was welded using an electron beam (ENG Produktions-GmbH, Berlin, Germany) with a weld depth of 2.5 mm. The low required welding energy and the clamping of a massive copper block around the welding area facilitated temperature retention at the outside of the implant neck, at half of its length, below 80uC. The weld quality was checked for each produced batch by cutting samples, and the density of the welds was determined in a vacuum chamber. Fatigue strength of the implant stem and neck were tested according to [37,38,39] but with double the force levels in the neck test. Telemetry The telemetry (Figure 2) is powered inductively at 4 kHz, as in our previous implants [29]. The internal coil L consists of 2700 loops on a core of PERMENORM 5000 H2 (mr.12000, Vacuumschmelze). The induced voltage UL is limited to 12.3 V by the Zener diode ZD, rectified by D3 and regulated to 5 V DC (Max 8881, Maxim). A NTC thermistor (Epcos) serves as a temperature sensor. The ceramic capacitor Ct (Kemet) with COG/NPO parameters has a high Q, low K, a temperature-compensated dielectric, and stable electrical properties at varying voltage, temperature, frequency and time. Together NTC and Ct set a time constant tT, which triggers a timer (ICM 7242, Intersil). This timer produces the output signal S, a pulse train with temperature-dependent pulse intervals. The sampling rate is approximately 10 Hz at 60uC, 5 Hz at 37uC, and 2.1 Hz at 20uC. Coil L is used not only for power transfer but also for signal transmission by superimposed magnetic pulses. The tantalum chip capacitor C is charged by L and the Schottky diodes D1, D2 to a maximum of UC = 11.7 V DC. The electrical pulses of signal S close the digital FET switches S1 and S2 (FDC 6303N, PLOS ONE | www.plosone.org 2 August 2012 | Volume 7 | Issue 8 | e43489