Individual variation in Achilles tendon morphology and geometry changes susceptibility to injury

Abstract

The unique structure of the Achilles tendon, combining three smaller sub-tendons, enhances movement efficiency by allowing individual control from connected muscles. This requires compliant interfaces between sub-tendons, but compliance decreases with age and may account for increased injury frequency. Current understanding of sub-tendon sliding and its role in the whole Achilles tendon function is limited. Here we show changing the degree of sliding greatly affects the tendon mechanical behaviour. Our in vitro testing discovered distinct sub-tendon mechanical properties in keeping with their mechanical demands. In silico study based on measured properties, subject-specific tendon geometry, and modified sliding capacity demonstrated age-related displacement reduction similar to our in vivo ultrasonography measurements. Peak stress magnitude and distribution within the whole Achilles tendon are affected by individual tendon geometries, the sliding capacity between sub-tendons, and different muscle loading conditions. These results suggest clinical possibilities to identify patients at risk and design personalised rehabilitation protocols.

Keywords: Achilles tendon; computational biology; human; injury; mechanical property; medicine; systems biology.

Figure 1.. Three-dimensional computer models of Achilles sub-tendons.

Figure 2.. Mean displacement of the proximal soleus face of three models with different friction contacts when each sub-tendon was loaded in isolation.

Figure 2—figure supplement 1.. Mean transverse plane displacements (in mm) of the proximal soleus face of the three models with different friction contacts (x-axes) when each sub-tendon was loaded in isolation.

Large variations in displacements and directions were noted between different models under different loading conditions. The scale of the y-axes differs to allow better visualisation of differences in the data.

Figure 3.. Normalised (to frictionless, coefficient = 0) proximal soleus face displacement of different friction contacts when each sub-tendon was loaded in isolation.

Note the scale of the y-axes differs to allow better visualisation of differences in the data.

Figure 4.. Mean normalised (upper row) and peak (lower row) von Mises stress of the whole Achilles tendon for different friction coefficients when each sub-tendon was loaded in isolation.

Note the scale of the y-axes differs to allow better visualisation of differences in the data.

Figure 5.. Change in peak stress location when shifting from frictionless (upper row) to frictional (lower row) contact is structure dependent.

LG: lateral gastrocnemius, MG: medial gastrocnemius, S: soleus sub-tendon. View planes: Ant.: anterior, Pos.: posterior, Lat.: lateral, Med.: medial, Int.: internal view with the covering sub-tendon removed.

Figure 6.. Group differences in normalised soleus junction displacement during different stimulation trials.

Figure 7.. Simplified sagittal plane view of triceps surae muscle-tendon units and corresponding ultrasound measurement locations (left) and sequences of measurement during each stimulation trial (right).

MG/LG: medial or lateral gastrocnemius, SOL: soleus. MTJ: musculotendinous junction.