Strain and force transducers used in human and veterinary tendon and ligament biomechanical studies

Biomechanical studies often aim at determining the contribution (in terms of load or strain) of a tendon or ligament in posture, gesture or locomotion. To this end, many transducers have been developed since 30 years. These devices implanted within or attached to the inside of the tendon or ligament must be compliant enough to measure in vivo the tissue load or strain without interfering with the movement of man or animals. They can be transducers with variation of electrical resistance (liquid metal strain gauge, buckle transducer, implantable force transducer and pressure transducer), variation of magnetic field (Hall effect transducer) and variation of light flow (optic fibre). Their use requires surgery in order to implant them and it is limited in time because of their invasive character and the development of fibrous healing reactions. Besides, the transducer dimensions and its position in the tendon can influence the transducer output signal. Moreover, the latter may not reflect the behaviour of the tendon as a whole but only locally. In addition, a calibration is required in order to convert the output signal into a strain or a force. In animals, this calibration is generally made by a post-mortem procedure on dissected anatomical specimens; in man, an indirect calibration procedure using inverse dynamic calculations is generally performed. However, the calibration conditions cannot reproduce exactly the in vivo conditions. So far, only invasive transducers have allowed to measure strain or force in tendons with all constraints and limits mentioned above.