High-Tech Hip Implant for Wireless Temperature Measurements In Vivo Georg Bergmann, Friedmar Graichen*, Jo¨ rn Dymke, Antonius Rohlmann, Georg N. Duda, Philipp Damm Julius Wolff Institute, Charite´ – Universita¨tsmedizin Berlin, Berlin, Germany Abstract When walking long distances, hip prostheses heat up due to friction. The influence of articulating materials and lubricating properties of synovia on the final temperatures, as well as any potential biological consequences, are unknown. Such knowledge is essential for optimizing implant materials, identifying patients who are possibly at risk of implant loosening, and proving the concepts of current joint simulators. An instrumented hip implant with telemetric data transfer was developed to measure the implant temperatures in vivo. A clinical study with 100 patients is planned to measure the implant temperatures for different combinations of head and cup materials during walking. This study will answer the question of whether patients with synovia with poor lubricating properties may be at risk for thermally induced bone necrosis and subsequent implant failure. The study will also deliver the different friction properties of various implant materials and prove the significance of wear simulator tests. A clinically successful titanium hip endoprosthesis was modified to house the electronics inside its hollow neck. The electronics are powered by an external induction coil fixed around the joint. A temperature sensor inside the implant triggers a timer circuit, which produces an inductive pulse train with temperature-dependent intervals. This signal is detected by a giant magnetoresistive sensor fixed near the external energy coil. The implant temperature is measured with an accuracy of 0.1uC in a range between 20uC and 58uC and at a sampling rate of 2–10 Hz. This rate could be considerably increased for measuring other data, such as implant strain or vibration. The employed technique of transmitting data from inside of a closed titanium implant by low frequency magnetic pulses eliminates the need to use an electrical feedthrough and an antenna outside of the implant. It enables the design of mechanically safe and simple instrumented implants. Citation: Bergmann G, Graichen F, Dymke J, Rohlmann A, Duda GN, et al. (2012) High-Tech Hip Implant for Wireless Temperature Measurements In Vivo. PLoS ONE 7(8): e43489. doi:10.1371/journal.pone.0043489 Editor: Hani A. Awad, University of Rochester, United States of America Received March 7, 2012; Accepted July 20, 2012; Published August 22, 2012 Copyright: 2012 Bergmann et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: friedmar.graichen@charite.de Introduction Friction of Implant Materials High friction in joint implants and subsequent temperature rise during continuous activities, such as walking, may cause increased polyethylene wear, decreased polyethylene strength, or loosening of the cup in hip implants due to high frictional torque [1]. The natural cartilage has a coefficient of friction of 0.02 to 0.04 [2]. Articulating materials used for total joint replacement have higher friction. Coefficients reported in the literature are as follows: 0.04 to 0.05 for the combination Al2O3 - Al2O3, 0.05 to 0.055 for Al2O3 - UHMWPE, 0.06 to 0.07 for CoCrMo - UHMWPE, and 0.10 to 0.20 for CoCrMo - CoCrMo [3,4,5]. Moreover, a strong influence of the protein concentration in the synovia on friction was reported [5], especially for CoCrMo - CoCrMo. Synovia Properties After joint replacement, a pseudo-synovial membrane is formed, which produces hyaluronic acid (HA), similar to the natural membrane. The synovia volume is small. In the hip joint, volumes of 2.7 ml in asymptomatic hips and 6.1 ml in fractured hips were reported [6]. The properties of synovia vary considerably, and synovia can lose its lubricating properties at high temperatures [7]. Synovia viscosity in natural joints depends on the type of joint disease [8]. Differences of at least a factor of 10 were determined between subjects and between healthy and osteoarthritic joints [9]. The most decisive factor for lubrication is the protein content in the synovia [10]. The lubrication ability of synovia from degenerative knee joints was worse than that of bovine serum [11], which may indicate that joint simulators do not actually mimic the real situation in hip or knee implants if a ‘standardized’ bovine serum is used [12,13], especially if the temperature is kept constant at 37uC. The cited literature indicates that individually varying synovia properties may strongly influence wear and temperature increases in replaced hip and knee joints during long-lasting activities, such as walking. Temperature in Hip and Knee Joints In vitro temperature measurements on two intact human hip joints delivered a temperature increase of 2.5uC during simulated walking [14]. In vivo measurements of temperatures in the natural knee joint showed a 1uC increase in temperature after 20 minutes and 2uC after 40 minutes of walking [15]. Depending on the implant type and articulating materials, this increase was observed up to 7uC for a rotating hinge implant (CoCrMo - UHMWPE). In an analytical study, validated by simulator data, temperatures up to 51uC were found in CoCrMo - UHMWPE hip implants [16]. PLOS ONE | www.plosone.org 1 August 2012 | Volume 7 | Issue 8 | e43489