Achilles Tendon
The Achilles tendon (TA) is the strongest and thickest tendon in the human body, and is formed at the confluence of the gastrocnemius and soleus muscles. The tendon is 15cm long and approximately 6 millimetres in thickness, in which between 80-95% of the collagen fibres are type 1, accounting for the mechanical strength of the tendon (Rompe et al, 2008; Pierre-Jerome et al, 2010). The two gastrocnemius heads and the soleus fuse approximately 5-6cm from its insertion into the calcaneus. The TA inserts into the posterior calcaneus, distal to the posterosuperior calcaneal tuberosity. Potentially the TA would be at risk of wear and tear due to friction between it and the calcaneus. However the calcaneal bursa, situated between the TA and the calcaneus, helps to cushion the area and reduce potential risk of friction damage (Ham et al 2015). Between fusing and inserting, the TA rotates 90 degrees (medial fibres rotate posteriorly and posterior fibres rotate laterally). This rotation is believed to support the elastic recoil of the TA, as the spiralisation of the fibres leads to an area of concentrated stress and can be seen as a mechanically advantageous feature (Pierre-Jerome et al, 2010; Doral et al, 2009). During stance phase of gait, 2.5 times the body weight is transmitted through the TA and during running this figure increases to 6-8 times body weight through the TA (Pierre-Jerome et al, 2010).
Although the TA lacks a true paratendinous sheath, it is encapsulated by the paratenon; which is continuous with the perimysium and periosteum of the calf muscle and calcaneus (Brukner & Kahn, 2007). The paratenon is a richly vascularised structure which accounts for most of the blood supply to the TA. TA blood supply also originates from the musculotendinous and osteotendinous junctions (Rompe et al, 2008; Brukner & Kahn, 2007). The tendon is relatively avascular, particularly around the region of the confluence of gastrocnemius and soleus, 2-7cm from the insertion to the calcaneus and as such, this is the point at which it is most susceptible to injuries and ruptures (Pierre-Jerome et al, 2010; Brukner & Kahn, 2007). This 5cm portion of the TA also represents the thinnest cross sectional area of the tendon, ranging from .4-1.4cm2 (Magnusson and Kjaer, 2003)
This avascularity has been identified through angiographic injection techniques (Paavola et al, 2002). The blood supply to the tendon is through branches of the peroneal and posterior tibial arteries. Blood supply to the proximal parts of the tendon is through vessels connecting from the gastrocnemius and soleus muscles (musculotendinous), and to the distal aspects from an arterial plexus at the posterior part of the calcaneal bone (osteotendinous). The distal supply starts at the insertion point and extends proximally for around 2cm (Bjur, 2010). As previously mentioned, due to the limited blood supply to this tendon, the oxygen consumption and metabolism are both low compared to muscles (Maffuli et al, 2011).
Nerve supply to the TA branches from the surrounding musculature and cutaneous nerves. These nerve endings usually terminate on the surface of the paratenon (Tan and Chan 2008). Myelinated nerve fibres are mechanoreceptors (detecting pressure and temperature), while the unmyelinated fibres are nociceptive (detecting pain) (Tan and Chan 2008).
The gastrocnemius muscle consists of 2 heads, with the medial head arising from the medial popliteal surface of the femur, posterior to the medial supracondylar line and the adductor tubercle (Doral et al, 2010). The lateral head originates from the posterolateral aspect of the femoral condyle, from a region extending from proximal and posterior to the lateral epicondyle to the most distal aspect of the linea aspera. The gastrocnemius muscles acts as both a knee flexor and a plantarflexor of the ankle, and is activated when jumping and running (Doral et al, 2010). As such it contains predominantly type II muscle fibres for this explosive powerful performance (Bjur, 2010). The medial head of gastrocnemius supplies fibres to the posterior and lateral portions of the TA, whereas the fibres of the lateral head predominantly supply the anterior tendon layer. The remaining anteromedial portion of the TA is supplied by fibres from the soleus muscle (Wyndow et al, 2010). The soleus muscle is a large flat muscle that lies deep to the gastrocnemius. Together with the two heads of gastrocnemius, it forms the triceps surae, which acts to plantarflex the ankle. The soleus muscle originates from the medial border of the middle third of the tibia along the soleal line as well as from the proximal third of the posterior fibula, and it inserts into the posterior calcaneus through the TA (Doral et al, 2010). The soleus muscle has a stabilizing effect on the foot and is continuously active during erect standing (Gravare Sibernagel, 2006).
Fig 1.2: Diagrammatic representation of lower limb musculature and TA (Ham et al 2015).
The TA is composed mainly of collagen fibres (65-80%), which are the smallest unit of the tendon that are subject to mechanical strain (Kannus, 2000). In a healthy tendon, up to 95% of the collagen fibres are type I, though through the processes associated with AT and aging, some of these are replaced by type III fibres (Pierre-Jerome et al, 2010). These are predominantly longitudinally orientated fibres but they also run transversally and horizontally (Maffuli et al, 2011). The tight parallel orientation of the collagen fibres transfer great tensile forces from the muscles to the bone, but withstand shear forces less well, and provide little resistance to compressive forces (Freedman et al, 2014; Brukner & Kahn, 2007). Cross linking takes place between each collagen fibre (Harries et al, 2000). The cross-linking feature of tendons contributes to the tendons ability to transmit great forces, and it also provides protection against enzymatic, mechanical or chemical breakdown of the collagen molecules.
In a healthy TA, the tendon transfers forces which are generated by the gastrocnemius and soleus to move the ankle joint. It also utilises elastic energy to minimise the energetic costs during walking (Wang et al, 2012). By utilising this elastic mechanism, the Achilles acts as an energy provider during locomotion, as it uses its natural elasticity and its spiral orientation to execute a stretch-recoil cycle to release energy during late stance phase. This increases performance and decreases the dependence on the gastrocnemius and soleus for energy production (van Ginkel et al, 2009). The overall elasticity of the tendon is quite low, and only stretches by between 3-8% of its length (Bjur, 2010). Elastin in the tendon is the substance responsible for the flexibility and elasticity of the tendon, and accounts for only 1-2% of its dry weight (Rompe et al, 2008).
Collagen fibrils also play a role in elastic energy utilization as they are said to retain a crimp (Tan and Chan 2008). This is essentially a wavy formation of the fibrils (Fig. 1.1). This crimp, or wavy formation, continues to exist until a strain of greater than 2% is applied to the tendon. This represents the “Toe” portion of the stretch strain curve (Fig 1.3). The initial response to lengthening of the tendon is elongation and straightening of these fibres, as well as elastin fibres providing additional elasticity to the tendon (Paavola et al, 2002). After this point the stiffness of the tendon is increased and if too much strain is applied (>4%) then collagen fibre damage can occur. Ultimate strain of the tendon is approximately 8%, at which point rupture is likely to occur (Tan and Chan 2008).
Along its length the composition of the TA changes, with proximal tendon fibres also containing muscle fibres near the musculotendinous junction. The mid-portion of the tendon contains true tendon tissue, and towards its insertion into the calcaneus, fibres change from tendon tissue to a fibrocartilage mixture, and finally into lamellar bone (Gravare Sibernagel, 2006).
As the collagen accounts for approximately 80% of the tendons mass, and the elastin makes up 1-2%, the remainder of the tendon consists of a proteoglycan-water matrix, within which the collagen and elastin are embedded (Kannus, 2000). This proteoglycan complex is referred to as ground substance, and one of its functions is to help collagen fibres adhere to each other, while also providing lubrication to allow them to slide past each other (Harries et al, 2000). Within the tendon, collagen fibres are tightly packed, and these tightly packed fibres progressively aggregate into larger structures and bundles of fibres, called primary, secondary and tertiary fibre bundles, and amalgamation of these tertiary bundles form the tendon proper (Bjur, 2010). As already mentioned, the TA is surrounded by a peritendinous sheath (paratenon) to reduce friction, rather than a true synovial sheath (Kannus, 2000). This paratenon is a richly vascularised fibrillar elastic sleeve which permits free movement of the TA against surrounding tissues. The paratenon consists of a combination of type I & III collagen fibres, elastic fibrils and synovial cells for lubrication. Beneath the paratenon is a layer of epitenon, beneath which is the endotenon, whose function is to provide structure in the binding of primary, secondary and tertiary fibre bundles (Kannus, 2010).
