Enthesopathy: What’s that?

Enthesopathy: What’s that?

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Enthesopathy is one of those terms in medicine that sounds more exciting than what it really is. In fact it is just a descriptive term, whereby it is composed of two parts; the first part entheso refers to a specific structure within the musculoskeletal system called the enthesis and the second part, pathy, signifies the presence of pathology. Therefore, in essence it is a fancy way of saying that the enthesis is not normal.

So what is an enthesis?

Well, for a long time the enthesis has simply been considered as the specialised area where tendons in the body join onto bone. It is often forgotten that is also refers to the site where ligaments, muscles, and even the lining of the joint (called the joint capsule) join onto bone.

However, more recently it has been shown that the enthesis is more complicated than previously thought, relating intimately to the important function of this structure. The complexity arises from the co-ordinated interplay of a number of structures that are in close proximity to the enthesis. These include the bursae (which are effectively fluid filled sacs, which lessen friction and provide cushioning at these sites), surrounding adipose tissue (in other words, fat, that allows a space for the supply of nutrients), as well as the bone itself (which at the enthesis is thinner and therefore more deformable).

As a result, this complex of functionally related structures is now referred to as the enthesis organ.

The structure of the enthesis organ is specifically designed to provide a strong and stable anchor for the transmission of force from one type of body tissue to another.

This force is either generated by muscles in order to promote wanted movement or is the consequence of the person’s interaction with the environment, which may not always be advantageous, and would therefore need to be resisted by the body’s ligaments. In order to achieve this effectively the enthesis organ must be able to facilitate the smooth transition of this force. Most important of all, this requires of it the ability to dissipate these forces appropriately.

Otherwise this area is prone to failure, which would manifest clinically as pain, swelling, and possibly loss of function or stability.

The way in which it is able to achieve this is predominantly by increasing the surface area of this interface. This would reduce pressure, given that pressure is inversely related to the area through which a force is imparted, ie P=F/A. There are a number of ways in which the enthesis organ does this, both at a macroscopic as well as microscopic level; such as the enthesis fanning out to join a number of bones or other entheses or at the microscopic level where the bone/tendon interface has a convoluted pattern.

The design of this structure is so effective towards its purpose that generally the enthesis organ is less commonly injured due to mechanical causes, such as overuse or a specific traumatic event, compared to the tendon itself or the bone underlying the enthesis.

However, in contrast, the enthesis organ is often a target for attack in a number of systemic inflammatory rheumatic diseases, most notably the sero-negative spondyloarthropathies, which are characterised by the inappropriate activation of the immune system against itself. The reasons why the immune system chooses to attack this structure and what the clinical consequences of the attack are will become the topic of future blogs.

Until then, I’m sure your enthesis organs will come to mind the next time you side-step the bank manager, sprint to your car, or jump in delight.

BJC Health established the Sydney Spondyloarthritis Centre in 2011. We raise the profile of these diseases, we provide a better pathway to diagnosis, provide education as well as world-class treatment. Most importantly, we care & we want to improve the lives of people suffering from these diseases. Read about it here.

For more patient-centred information regarding the autoimmune diseases causing enthesopathy, please read:

Psoriatic Arthritis

Spondyloarthritis 

Ankylosing Spondylitis 

 

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