Auxetic behaviour of healthy tendons

A study by published in the September 2015 issue of Acta Biomaterialia reported that human tendons exhibits the very unusual auxetic property, meaning that they get fatter rather than thinner when stretched under normal range of motion.

Tendons are visco-elastic structures that connect bones to muscles and perform the basic function of force transfer to and from the skeleton. They are essential for positioning as well as energy storing when involved in more abrupt movements such as jumping. But they are also prone to damage, and when injuries occur, they may have serious consequences or irreparable. Injuries of tendons such as Achilles tendinopathies, which are common in athletes, are difficult to treat.

Understanding the viscoelastic behaviour of tendons is important, which may protect the tissue from damage. Tendons display a wave-form or crimped structure when relaxed. Stretching tendons at low strains results in the disappearance of this crimping. Further stretching will caused the fibres and fascicles slide against each other, eventually returning to their original shape when the load is released. Stretching beyond this range results in permanent deformation.

In vivo (live human), tendons usually deform within the low strain regions, with the exception of the energy-storing tendons, which are known to undergo higher strains. This is one of the reasons for the low incidence of injuries to positional tendons when compared to energy storing tendons.

A measure of the change in size of a material is called the Poisson’s ratio, which is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. As most materials get thinner when stretched in a direction, the Poisson’s ratio is usually a positive number. However, the Poisson’s ratio can also be negative, meaning that the material gets wider rather than thinner when stretched (also called auxetic behaviour).

The researchers from Malta tested ex vivo human Achilles and Peroneus brevis tendons, and the deep flexor tendon from pig and sheep. In vivo studies were also carried out on the Achilles tendons of two individuals for plantar flexion. The researchers showed that tendons are highly anisotropic (meaning that their elasticity has a different value when measured in different directions) and behave in a very unconventional manner when stretched. Tendons when stretched up to 2% along their length, i.e. within their normal range of motion, has negative Poisson’s ratio in some planes. This property is not limited to the human Achilles tendon, as it was also found in tendons taken from sheep and pigs.

Auxetic materials usually have high energy absorption and fracture resistance, and it was hypothesised that such a property may be lost if the tendons are damaged. This new information about tendons can form the scientific basis for a test for tendon health as well as enable the design of better tendon prosthesis which could replace damaged tendons.

It needs to be stressed that this behaviour was only observed in small strain deformations of tendons, i.e. ones which are less than 2% and which correspond to what one normally achieves in normal everyday activities, and in specific planes.