The twisted structure of the Achilles tendon unraveled: A detailed quantitative and qualitative anatomical investigation

Abstract

The Achilles tendon (AT) consists of fibers originating from the soleus muscle (SOL), which lies deep, and the medial (GM) and lateral (GL) heads of the gastrocnemius muscle, which lie superficial. As the fibers descend toward the insertion of the AT, the individual subtendons twist around each other. The aim of this study was to investigate the twisted structure of the AT and its individual subtendons. Specimens of the AT, with preserved calcaneal bone and a fragment of the triceps surae muscle, were obtained from 53 fresh-frozen, male cadavers (n=106 lower limbs). The angle of torsion of each of the AT's subtendons was measured using a specially designed and 3D-printed tool. The mean distance between the most distal fibers of the triceps surae muscle and the superior border of the calcaneal bone was 60.77±14.15 mm. The largest component of the AT at the level of its insertion into the calcaneal bone is the subtendon from the GL (44.43%), followed by the subtendon from SOL (27.89%), and the subtendon from GM (27.68%). The fibers originating from the GM rotate on average 28.17±15.15°, while the fibers originating from the GL and SOL twist 135.98±33.58° and 128.58±29.63°, respectively. The torsion of superficial fibers (GM) comprising the AT is significantly lower than that of deeper fibers (GL and SOL). The cross-sectional area of the AT is smaller at the level of the musculo-tendinous junction than at the level of its insertion. This study illustrates the three types of the AT with differently twisting subtendons, as well as a generalized model of the AT. Types of AT torsion may potentially alter the biomechanical properties of the tendon, thus possibly influencing the pathophysiologic mechanisms leading to the development of various tendinopathies.

Non‐uniformity in pre‐insertional Achilles tendon is not influenced by changing knee angle during isometric contractions

“…This could cause a more distinct non‐uniform behavior at this level of the tendon. Towards the insertion, tendon rotation appears to be less present, corroborating a different morphology and mechanical behavior of this area . Secondly, the study of Slane et al evaluated local tendon behavior during an eccentric (dynamic) deformation, whereas this study used an isometric (static) contraction in a neutral ankle position.…”

“…Indeed, there is still uncertainty whether the tendinous portions arising from each muscle are still morphologically distinguishable in the free AT. Some papers indicate discernable sub‐tendons in the free AT, while others do not…”

“…Indeed, there is still uncertainty whether the tendinous portions arising from each muscle are still morphologically distinguishable in the free AT. Some papers indicate discernable sub-tendons in the free AT, 13 while others do not. 20 Importantly, there are a few differences in methods used to evaluate the non-uniform behavior between the current study and previous research.…”

“…9 This is hypothesized to be linked to proportionally more loading of the soleus muscle in the flexed knee position, with relatively less gastrocnemius contribution, following its origin proximal to the knee joint. 9 Given the fact that the middle and deep layers of the AT are considered to be originating from the soleus muscle, 13 relatively higher activation of the soleus muscle is hypothesized to result in relatively higher displacement in the middle and deep layers of the AT. 9 However, it is unclear to what extent these intratendinous changes occur due to the positional changes or due to the subsequent biomechanical advantages or disadvantages that arise.…”

Tendon — function-related structure, simple healing process and mysterious ageing

“…It may has an obvious further mechanical and clinical implications [45]. Osteotendinous junctions are common areas of overuse trauma, especially activity-related.…”