An evolving understanding of the enthesis

Historical view of inflammation at the enthesis

In the past doctors regarded inflammation or degeneration of the enthesis as interesting but a comparably unusual problem.

It was also considered that diseases of the enthesis were easy to recognise and diagnose.

For example heel pain and associated tenderness or swelling at the point of the Achilles tendon attachment to the bone is easy to recognise.

Other common sites where doctors could diagnose enthesis inflammation were the knee cap or elbow.

Diseases of Enthesis were viewed as very focal or occurring just at the exact point of tendon or ligament attachment to bone.

Historically doctors viewed inflammation at attachment sites as a focal disorder. That is, it was thought that the problem was right at the exact point of attachment of ligaments or tendons to the bone. Doctors focussed down on this precise location like the crosshairs on a telescopic site might be used to focus on a target.

It is now recognised that diseases of the enthesis can cause pain and inflammation over a wide area.

This is because this working enthesis comprises many more parts that just the attachment point to bone.

The actual point of tendon or ligament attachment to bone lacks blood vessels so may not be the site of maximal inflammation in diseases of the enthesis.

This is best explained by the "eye of the hurricane" analogy below.

Historically doctors viewed inflammation at attachment sites as a focal disorder at the centre of a target zone like the crosshairs on a telescopic site. However, the problems associated with the enthesis especially when it is inflamed spread over a wide area. The actual point of tendon attachment may be spared or relatively spared when the enthesis is diseased. This scenario is analogous to the eye of the hurricane that may be comparatively quiet whilst the storm rages nearby. This image captures how the whole concept of enthesis related disorders has changed a great deal. This is vital for understanding why people get joint and skeletal pain in a wide range of settings.

Entheses are very common and often very small

In the past doctors overlooked just how common enthesis disorders were for 3 main reasons.

  • Many attachments sites are deep within the body and cannot be seen or examined directly.
  • Many attachment sites are minute or form complex micro-machines that were too small to appreciate.
  • Attachment site disease triggered swelling and fluid accumulation in the nearby joint. So the inflammation that was starting in attachment sites was not appreciated and wrongly attributed to joint lining inflammation. You can't start a fire without a spark!

How many entheses are there in the body?

According to Wikipedia there are about 640 muscles in the body. The majority of these have at least two attachment sites making approximately 1280 tendon attachments.

According to Wikipedia there are about 206 bones in the adult human. The majority of these are attached to each other by a tough tissue called a capsule which is present on both sites of joints making about another 400 insertions.

Also each joint has at least two ligaments with a combined number of attachments sites of at least 4.

Some joints like the knee have at least 4 major ligaments and numerous smaller ones.

This puts the number of insertions in the body at well over 2000!

However, the diversity and number of entheses was not appreciated but pain and disability can arise from diseases of many of these structures.

To summarise the traditional thinking was that disorders of the enthesis were not that common and when they were present the disease was right at the attachment point. Both of these suppositions were incorrect.