This study set out to evaluate an E-type force transducer for the measurement of dynamic force on surgical sutures. During calibration using a materials testing machine the transducer showed linear behavior across the force range anticipated in our ultimate application and at a physiological frequency of 2 Hz. Minimal hysteresis between transducer output during the rise and fall of force and good correlation between actual and transducer-determined forces were seen and a mean percentage error of less than 5% was measured. While this error value is clearly not as low as can be achieved with a load cell, it is within our expected error and allows this technique to be further investigated for potential in vivo application. The technique is particularly suited to studies that aim to measure a change in force under dynamic conditions, rather than to determine absolute one-off forces.
The frequency response of the system was not definitively determined by performing calibration at a variety of loading frequencies. Although the authors anticipated that transducer output would be uniform across a range of low frequencies, the data presented here () and limited preliminary testing at higher frequencies (> 5 Hz) suggest that transducer deformation and recoil may lag behind the force on the suture, yielding less linear results. The actual error during linear changes in force is likely to be much lower than the 4.4% presented here. The frequency response of the system is considered appropriate for measuring the physiological frequency of respiration at exercise (~2 Hz).
To test our transducer in a realistic environment before in vivo implantation, we employed an in vitro model capable of generating physiological laryngeal airflows. This model was able to reproduce one component of the force that is likely to be exerted on laryngoplasty sutures in vivo, namely the force due to changes in intra-luminal pressure associated with respiration. The other likely contributors to cyclic force on sutures in vivo are coughing and swallowing. These activities would be difficult, if not impossible, to mimic convincingly in vitro. Although our validation was therefore limited to cyclic airflows, the results of these experiments demonstrate adequate stability of transducer response and can be extrapolated to other circumstances. Translaryngeal pressure and suture force were significantly correlated, providing evidence that changes in intra-luminal pressure are likely to cause changes in loading of the suture in vivo.
Previous in vitro experiments have measured the cyclic forces on laryngoplasty sutures associated with changing intra-luminal pressure (21
). Those experiments used load cells mounted in series with the suture on cadaveric larynges and are a useful comparison in terms of the accuracy of our transducers. In the study by Ducharme, the oscillating force generated by cyclic airflow was determined to be 0.17 ± 0.24 N, which is similar to the force determined here (21
Since the technique relies on changes in electrical resistance of the strain gauge, fluctuations in environmental temperature often play a role in absolute strain gauge outputs during in vitro or materials testing applications. Attempts were made to minimize this effect by using a 3-wire 1/4 Wheatstone bridge. Changes in temperature are of less concern in the in vivo application of strain gauges in endothermic species. Ultimately, changes in force in response to a given action or activity of relatively short duration are often of greater interest than absolute values, and acute temperature fluctuations are unlikely to be encountered. At the start of all strain gauge experiments, the bridge is balanced and the output zeroed. Differences in intercept do not play a large role in transducer accuracy.
In conclusion, this paper has shown that E-type buckle force transducers are sensitive and accurate enough for measuring the force on surgical sutures. In vitro and in an ex vivo equine laryngeal model they demonstrated a highly linear response over the anticipated range of forces and at a physiological frequency.