De la hipótesis especifica 01

The most uncertain aspect about the use o f foil gauges is the drift rates that are likely to occur with time. Drift in this context is defined as the observed change in measured strain due to no change in applied strain. This effect becomes increasingly significant as its magnitude increases compared to the dynamic range o f strain over which the gauge normally operates. In the M kl instrumented prosthesis the longitudinal shaft gauges experience a strain change o f 40 microstrain per applied 70kg bodyweight. A drift rate o f 20 microstrain per year, typical o f foil gauges at high humidity (Freynik 1976) would therefore be measured as a change o f 0.5 bodyweight per year, which is an unacceptable error especially when accumulated over several years. In a laboratory experiment, it is often possible to correct for any drift in the strain gauges by removing the stress and then either nulling the instrumentation or recording the new 'zero' reading. However, in an implant the gauges are inaccessible and it is not possible to reliably induce a state of zero stress in order to reconstruct the 'zero' conditions. It is therefore essential to create conditions in which the drift is acceptably low, or if this is not possible, to have some knowledge o f the rate of drift likely to prevail. The parameters known to have an effect on gauge drift were temperature, humidity and applied strain. An experiment was set up in order to find out how much drift might be expected from foil gauges under various conditions of applied strain and humidity at 37°C. The experiment was originally conceived in the early months o f the project when it was thought likely that all the instrumentation might have to be housed outside the prosthesis, with the gauges on the external surface exposed to a wet environment. However, even after it had been decided to enclose the instrumentation within sealed cavities [#4.1.4], it was still unclear how dry the cavities needed to be to attain an acceptably low level o f drift; strain gauge

manufacturers are reluctant or unable to publish data on drift, and therefore the experiment was required.

Sets o f foil gauges were mounted on titanium strips and strained in tension using 3-point bending to 0, 500 and 2000 microstrain (pe). Subsets of gauges at each level o f applied strain were exposed to relative humidities of 0 and 75% relative humidity (RH) in a thermostatted cabinet held at 37°C. Each gauge was bonded according to the manufacturers instructions and wired as one quarter o f a full bridge. A reference gauge (unstrained and over silica gel) and two high-stability resistors completed each bridge. A single strain gauge amplifier was used for all gauges, and readings were logged at intervals of several days by manually switching each bridge at a time into circuit. Any amplifier offset drift was noted at each measurement. Three such bridges containing one test gauge were investigated under each set of conditions. Three full bridges o f the same high- stability resistors were used to determine the drift due to the resistors alone, to establish confidence limits for the measurements.

Results showed that the unstrained dry gauges exhibited no detectable drift rates compared to the all-resistor bridges; at 75%RH they exhibited between 12 and 44pe drift over the first 100 days. The dry gauges at 500ps tension exhibited no net drift. The dry gauges at 2000ps tension showed significant drift rates (varying between 40 and 11 Ope during the first 100 days), and a similar drift rate was found in 2000pe gauges at 75%RH.

These results suggest that i) above a certain level o f humidity (somewhere between diy and 75%RH) drift is induced even in unstrained gauges, ii) there may be a critical strain level, between 500 and 2000pe, above which drift starts to become apparent, even when dry. The experiment did not determine how dry the gauges need to be nor what maximum strain can be tolerated to ensure reasonable stability over long periods. A substantial amount of work would have been required to determine the mechanism of drift under various levels of applied strain and humidity, and the results were therefore used only as a guide.

Since the peak dynamic strains experienced at either gauged site are unlikely to be more than 500pe, it was concluded that serious drift of foil gauges would not occur if the cavities could be kept at a low relative humidity. Also, it would be possible to monitor the drift occurring in an identically instrumented non­ implanted prosthesis, and this would act as a control for monitoring drift in the implanted units.

A further experiment to determine the drift of various types of gauge during dynamic strain cycling was set up. A machine, nicknamed the 'Wobbler', was built to apply bidirectional bending at up to 4Hz to up to 80 thin metal strips having mounted or printed strain gauges, figure 4.4. Instrumentation was designed and built to measure the instantaneous resistance of each gauge at up to 100 times per machine cycle, and to interface with a computer for data logging. Due to problems in interfacing the software with the instrumentation, and to pressure of other work, this test has not yet been carried out.

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Figure 4.4 The ‘Wobbler’. Titanium strips having strain gauges bonded are cyclically strained, and the gauge resistance monitored.

In retrospect, experience with the first two Mkl patients has shown little evidence o f strain gauge drift. During resting, shaft readings have varied by no more than 0.25BW (lOpe) in either subject over a 3 year period. The variation is not cumulative, suggesting that it is due at least in part to varying muscle tension. This low level o f drift is most likely due to the low average strains experienced by the gauges and their relatively dry (<40% RH), constant temperature environment. Humidity sensors were used in the implanted devices, and the %RH values telemetered along with the strain and temperature data, to warn o f any change over time [#4.4.3.6]. No large RH changes were ever detected in any o f the 4 subjects.