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Multichannel Implantable Telemetry System
In 1988 a 4-channel telemetry transmitter was developed in our laboratory to measure the loads acting on a hip joint. Three semiconductor strain gages inside the hollow neck of the prosthesis enable the measurement of three force components. Channel number four transmits the implant temperature.
To measure also the moment components, what is necessary for other implants, the number of channels had to be extended. Integration of most components on a single chip (bipolar transistor array) results in an 8-channel telemetry transmitter, which was used for internal spinal fixators and temperature measurements in hip implants since 1991.
Further miniaturisation leads to our 9-channel telemetry transmitter with a programmable custom-made integrated circuit for the instrumentation of other implants.
For the measurements the power induction coil and receiver antenna are fixed near the patient´s implant. The implanted telemetry transmitter is powered inductively by the external magnetic field. The measured signals were transmitted to the external device TELEPORT for on-line force display on a notebook screen. The activities of the patient are videotaped and the data from the implants are recorded simultaneously on the audio tracks of the same cassette. Selected measurement sequences are stored as clips and available at our database OrthoLoad.
9-Channel Telemetry Transmitter
The 9-channel telemetry transmitter is made in double sided thick-film hybrid technology with dimensions of 12.5 x 6mm. Analog and digital parts are combined on a single custom-made chip in BICMOS technology with a structure of 0.8 µm. The total die size is only 2 x 2.6mm. This low-power circuit includes a 9-channel multiplexer, a programmable memory, a pulse interval modulator and a radio-frequency transmitter.
Channel no. 1 - 6 are used for semiconductor strain gages or temperature sensors. Channel no. 7 transmits the temperature of the hybrid circuit. The magnetic field is regulated depending on the power information transmitted by channel no. 8. Synchronisation of the pulse train is done by aid of channel no. 9.
A Zener diode PROM module stores all information of the calibration process to adapt each channel to its corresponding sensor resistance. After successful programming this part of the hybrid circuit is cut off and total size is reduced to 9.5 x 6mm.
The 9-channel telemetry transmitter was used since 2004 for measurements of forces and moments in shoulder endoprosteses, knee endoprostheses and vertebral body replacements, since 2010 also in hip endoprostheses (Hip III). [Graichen et al., 2007].
8-Channel Telemetry Transmitter
Double-sided thick film hybrid technology with a bipolar transistor array (4.2 x 5.0mm) and 17 passive components. Dimensions: 14 x 7mm. Connection pads for six external sensors.
The 8-channel telemetry transmitter was used for load measurements with internal spinal fixators and hip joint temperature measurements [Graichen et al., 1996], [Graichen et al., 1999].