It is the longest and thickest tendon in the body.
Taller and heavier people have thicker tendons than their smaller contemporaries.
The tendon is significantly bigger in your dominant leg if you play sports regularly.
It gets bigger as you get towards 50 years old.

Where is the Achilles tendon?

Well it’s the long cord that attaches your calf muscles to the heel at the back of your lower leg.

It comes in all shapes and sizes – from long and thin to short and thick but they all do the same job – transfer forces from the calf muscles to the foot and vice versa.
I say muscles because you have two muscle groups in the calf and they both perform slightly differing functions.

Gastrocnemius Muscle

Gastrocenmius muscle gives the calf its distinct shape.
This muscle itself has two distinct parts (heads) to it – medial and lateral.
These heads are attached to either side of the back of the knee and actually cross the knee joint to be attached to the lower end of the thigh bone (femur) by strong flat tendons.
The medial head is larger than the lateral which can sometimes be absent although I don’t think I’ve ever seen it. 
The terms medial and lateral are explained in Medical Terms Explained.

Soleus Muscle

The two heads come together about half way down the lower leg and blend with a sheet of fibrous tissue (aponeurosis) which joins onto the tendon of the third muscle –Soleus.
Soleus is attached to the upper ends of two lower leg bones namely the tibia and fibula and so is deep to the gastrocnemius muscle.
From here its fibres pass posteriorly to its tendon which blends with the gastrocnemius and other fibres extend further down the leg to attach directly to the Achilles tendon almost to its lower end.

Achilles tendon (or more correctly tendocalcaneus) is the thickest and strongest human tendon.
It begins near the middle of the lower leg and reaches up to 15cm long before attaching to the heel bone (calcaneus).

This diagram shows the back of the leg with the gastrocnemius muscle cut away to show the deeper Soleus muscle in its entirety.

The Gastrocnemius muscle sits over the upper part of the Soleus.
There is another muscle which is small and this called Plantaris (this can be absent in many people).
Plantaris is not shown in the diagrams but it starts on the lateral side of the femur and passes obliquely down the leg to attach to the medial side of the Achilles tendon or directly onto the medial aspect of the heel bone (calcaneus).

Structure Of Achilles Tendon

The tendon consists of bundles of connective tissue fibres called type-I collagen fibres.
Collagen is the main structural protein in the body which provides the strength to the tendon. 
Type-I Collagen is present in bone, tendons, cartilage and scar tissue and is a very strong protein when it is linked to adjacent protein molecules.

The tendon ranges in thickness between 4mm to over 7mm. Taller people have thicker tendons than shorter people.
There are differences between the average thickness of the Japanese population compared to western populations.
Heavier people have tendons with an increase in cross-sectional area (slightly thicker and wider) to help deal the the increased forces in walking and running.
Take a look at the MRI photograph on the right. I will explain what you are looking at.
Imagine cutting through the right ankle joint parallel to the floor and lifting the leg and upper part of the ankle away.
you would be looking down on the lower part of the sub-talar joint – the Calcaneum bone.
At the back of the heel you see the black structure – this is the Achilles tendon.

The tendon has a high tensile strength and is able to stretch up to 4% without any damage to the structure of the fibres. However if the stretch is 8% then the tendon ruptures.

During running the Achilles tendon can sustain peak forces of around 6 to 8 times bodyweight and it can apparently handle forces up to 12.5 times bodyweight.
By my calculations this is over a ton for someone weighing just under 13 stones!!

The fibres of the tendon twist by anything from 30° to 150° prior to its insertion in the Calcaneum, with most of the rotation occurring in the last 5-6 cm.
This rotation is thought to aid in elastic recoil of the tendon which allows it to store energy and then release it back to the foot to aid walking and running. Imagine pulling on both ends of a length of thick wool or yarn. As you pull there is some unwinding of the loose spiral in the wool and it lengthens slightly. When you release your pull is springs back to its original length. Obviously the collagen fibres in the Achilles are not as loosely arranged but you can now perhaps understand how this phenomenon works.
This ‘untwisting’ of the tendon allows greater instantaneous muscle power and speed of reaction than could be generated by the calf muscles acting alone.

The tendon is covered by a tissue known as the paratenon. This is a sheath of synovial cells which produce a kind of lubricant. The inner layer is in contact with the tendon itself, and the outer layer is continuous with the other surrounding tissues and their rich blood supply.
See picure on the left from www.thefootandankleclinic.com which shows how the inflamed parartenon can be stripped away from the underlying tendon.
The paratenon can become inflamed and enlarged, usually in runners because of the repetitive nature of running with each foot hitting the ground the same way every stride 300-400 times a mile. This thickening in the paratenon produces the ‘creaking’ sensation of crepitus which can be felt over a swelling in the tendon. This swelling can be from the paratenon or from degenerative changes in the tendon itself.

This degeneration and resultant thickening is why the tendon has a greater cross-sectional area in the over 50′s.
There is also a response to training in the tendon because it has been shown that the tendons of the dominant leg in sportspeople have a significantly greater cross-sectional area than people who do not take part in sport.

The blood supply to the Achilles tendon comes mainly from vessels derived from the posterior tibial artery around where the muscles join up with the tendon. Branches from this area pass down the tendon and through the paratenon to supply the tendon itself. There is also a blood supply which supplies the tendon as it inserts into the Calcaneum and branches pass upwards from here. Where these two supplies meet is known as the “watershed zone”. This is an area 2-6 cm proximal to the Calcaneum, in which the blood supply is less abundant. This is the region where most degeneration and therefore rupture of the Achilles tendon occurs. This ‘dodgy’ blood supply gets even sparser with age which is why age is a factor in Achilles tendon ruptures.

You now know:-

• a fairly detailed anatomy of the Tendo-Achilles
• how it links the calf muscles to the Calcaneum.
• how the tendon is constructed
• how its’ spiral construction aids propulsion
• its’ role in transferring forces from the foot to the leg and vice versa.
• blood supply to the Achilles
• what the ‘watershed area’ is
• significance of the ‘watershed area’ in degeneration

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In the past the patient was put in a plaster cast with the foot pointing down and it was hoped that the tendon healed well enough to provide function. Surgery was considered less suitable because there was an increase risk of infection particularly as the wound in the leg needed to be 20 cm long to get proper access to the tendon. The surgery can now be carried out through very small incisions with a much lower risk of infection.

There is a body of opinion that surgical correction of the ruptured tendon is almost always necessary. This is performed in order to regain the maximum strength of the Achilles, as well as the normal pushing-off strength of the foot. The strength of the muscle depends on the correct tension between the muscle and the tendon. The only way that the correct tension on the tendon can set is by accurately repairing the tendon ends.

The Achilles Tendon generally tears about 5 cm above the heel attachment. The ends separate from each other a bit like cutting the ends of an elastic band.
These ends are pulled together at the surgery and stitched with strong sutures.

If the rupture has been present for many months, there is no urgency to do the surgery, but one really should not wait too long, since the Achilles tendon continues to pull up into the leg and the gap between the tendon ends gets larger, making the surgery more difficult.

Acute vs Chronic Tear

An Achilles tear can be considered acute if it is less than a few weeks since the injury occurred. Ideally it should be operated on quickly. Sometimes however, especially with amateur athletes and the so-called ‘weekend-warriors’ over the age of 30, the problem is sometimes missed and it is only after several weeks that a diagnosis of ruptured Achilles is made. These chronic tears almost always require surgery as the ends have separated and the muscle has shortened which means that conservative management is not likely to produce a satisfactory result.
The type of surgery performed depends on the size of the gap between the tendon ends and the extent of separation that is present.
In an acute injury the separation is minimal, then the tendon ends can be stitched together. If the separation is more significant, then other procedures need to be performed.
As the gap gets bigger, the options then range from using a strip of the lining of the existing Achilles, using another tendon as a tendon transfer, or even using an Achilles tendon graft which comes from the tissue bank.

A tendon transfer is only used when there is a massive gap present, or the Achilles has been ripped off its’ attachment on the heel bone. When it comes to using a different tendon to substitute for the Achilles there are a few tendons which can theoretically be used, but the tendon transfer which is preferred uses the second strongest muscle in the leg, the muscle to the big toe (the flexor hallucis longus).
The disadvantage of this operation is that the flexor hallucis muscle is not as strong as the Gastrocnemius which powers the leg. Nonetheless, patients are able to push off fairly comfortably with the tendon transfer and can even participate in some sporting activities. This operation can be performed through a very small incision on the back of the ankle although there are risks of damage to a nerve and local blood vessels with this technique.

After Surgery

The individual surgeon has his own preferences for how quickly the patient is allowed to put weight through the leg. In the past the leg could be immobilised post op in a plaster cast for a few weeks but that does not always happen nowadays. Current thinking is that early weight bearing encourages a better healing of the repair with much less muscle wasting and therefore better function. So walking is begun in a removable boot which controls the range of motion at the ankle.

Here is a quote from the surgeon who operated on David Beckham – Dr Orava said: “Today we allow patients to do motion earlier than before. In this case we try to get motion back as soon as possible. The operation takes less than one hour. [After the] first month, there can gradually be more and more motion and muscle contractions, and very gradually [shifting] from light training to harder training in the second month. After that, one is usually able to walk and maybe start running lightly after two months if everything goes fine. All this depends on the type of tear.”
“Top athletes usually heal well but it’s a few months until you are back at the same level as you were before. I don’t think it is a big difference to be [aged] 25 or 35, it will be three, four, five months. Most of these patients come back at the same level, even in a top soccer player.”

Japanes surgeon Yotsumoto and his colleagues have applied a side-locking loop technique of their own design for the core suture, using braided polyblend suture thread, with peripheral cross-stitches added. Their patients started active and passive ankle mobilization from the next day, partial weightbearing walking from 1 week, full-load walking from 4 weeks, and double-legged heel raises from 6 weeks after surgery. The results showed range of motion recovery equal to the intact side averaged 3.2 weeks. Double-legged heel raises and 20 continuous single-legged heel raise exercises were possible at an average of 6.3 weeks and 9.9 weeks, respectively. The patients resumed sports activities or heavy labor at an average of 14.4 weeks. There were no complications.

Rehab post op

Early Rehab is all about maintaining and regaining range of movement at the ankle especially dorsiflexion (the foot up towards the front of the leg)
Rehab is then about recovering strength in the tendon and calf muscles while not putting the repair at risk.
As mentioned earlier walking is started in the boot very quickly and this helps to maintain the normal muscle control.
Patients work with elasticated bands as resistance to increase strength and can use static bicycles to maintain aerobic fitness.
Balance exercises are used to recover normal neurological patterns and improve proprioception.
Swimming helps aerobic fitness and allows weight bearing to take place with some of the bodyweight supported by the buoyancy of the water.
Eccentric exercises (lowering down from up on toes) are essential because this is how the calf and Achilles tendon function normally.
After the boot is removed walking is progressed to fast walking and then gentle running.
The speed and intensity of the running is gradually increased.
Sports specific drills and training are introduced as soon as possible – this make it easier to return to sport quickly.

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