Why is the prototypical patient with a ruptured Achilles tendon about 40 years old? What are the biological and mechanical steps leading to tissue failure?
steps help answer the first question.)
As a person ages, there are changes in collagen cross-linking that result in increased stiffness and loss of elasticity. As the tendon stiffens, it becomes predisposed to rupture. Furthermore, habitual loading of the Achilles tendon with walking causes wear and tear. With age, this wear and tear is less perfectly repaired.
Thus, a person at age 40 is much more likely to suffer an Achilles tendon rupture than a person at 20, as the 40-year-old tendon is stiffer and weaker.
If the integrity of a tendon decreases with aging, you might assume that a person at age 60 is even more likely to suffer an Achilles tendon rupture than a 40-year-old. However, this is not what we see in clinical practice. The prototypical patient with an Achilles tendon rupture is typically under the age of 50.
To understand why, we must first review the anatomy shown in Figure 1.
Figure 1: Anatomy of the Achilles tendon
The two main posterior
calf muscles, the soleus and the gastrocnemius, coalesce to form the Achilles tendon,
which then inserts on the posterior aspect of the calcaneus (see Figure 1). This muscle complex then
powers flexion at the ankle.
If the Achilles contracts** with the foot in a neutral position without external forces applied to the foot, such as if the foot is dangling off a high chair, the ankle will plantar flex *** [downward].
When the ankle undergoes plantarflexion, the gastrocnemius and the soleus get shorter, thus bringing their point of insertion on the calcaneus closer to the shin. This combination of muscle-firing and muscle-shortening is known as a “concentric contraction.”
There is also “isometric contraction,” where the length of the muscle stays the same; for example, a person holding a weight in their hand with the elbow fixed at 90 degrees. And there is even “eccentric contraction,” when the muscles are working, yet actually getting longer. That occurs when there is an extrinsic force applied. An example of “eccentric contraction” would be a person holding a weight in their hand, trying to keep the elbow fixed at 90 degrees, but slowly losing the battle with the weight moving toward the ground.
The purpose of eccentric contraction of any muscle crossing a joint is decelerating motion that is powered by an external force. With regard to the Achilles (gastrocnemius/soleus complex), eccentric contraction decelerates passive dorsiflexion of the ankle, usually imposed by body weight when the foot strikes the ground.
When the forefoot lands on the ground during a normal stride, the ground applies a force to the foot that tries to dorsiflex the ankle. The Achilles undergoes eccentric contraction, resisting that motion and slowing it down.
Note in particular that landing from a jump places particularly high forces on the Achilles because the foot is otherwise forced upward, via dorsiflexion of the ankle, as the person lands. It is this large eccentric loading force that can cause the Achilles tendon to rupture. If the ankle were relaxed and pushed into dorsiflexion, nothing would be torn. However, when motion is resisted, the tissue might fail. Thus, some consider an Achilles tendon rupture to be a self-inflicted wound.
Returning to the 40-year-old patient in question: he is more likely to rupture his Achilles than a 20-year-old because he has accumulated tendon damage over the years. He is also more likely to rupture his Achilles than a 60-year-old because he is able to generate the power needed to actually tear the tissue.
There may be a psychological contribution as well. A 40-year-old is also less aware of how their body has aged. Indeed, the prototypical patient is a “weekend warrior” who plays basketball or tennis with the same zeal as their 20-year-old self, but with far less tolerance of excessive loads to their tendons.
** For simplicity, we will refer to actions as if the Achilles performs them (e.g. “the Achilles powers flexion”), although this is not literally true. The muscles, the gastrocnemius and the soleus, perform all of the actions, with the Achilles simply transmitting that action to the foot.
*** Of note, the motion of the ankle moving downward (e.g. when standing on tiptoe) is known as “plantar flexion.” It is not referred to simply as “flexion” because the term “extension” is not used at the ankle. The motion opposite plantar flexion is known as “dorsiflexion.”