Stiff yet Soft: Osteoporosis in Ankylosing Spondylitis

By Dr Irwin Lim, Rheumatologist

Stiffening of the spine. That's characteristic of ankylosing spondylitis. And yet, it's been known for years that the bone is also less dense. Less dense bones mean an increased fragility, and an increased ability to break. Fracture. Osteoporosis.

At first glance, this seems contradictory.

Why should patients with ankylosing spondylitis (AS) where overgrowth of bone is typical, also have a more fragile skeleton?

Well, in AS, the inflammation occurs in bone, and typically where ligaments and tendons attach to the bone (enthesitis).

In AS, this localised inflammation, if unchecked, will lead to loss of bone mineral, resulting in reduced bone density and therefore, fragile bones.

In addition to this, chronic systemic inflammation affects the balance of a variety of chemicals which are important in the process of bone remodelling.

In normal bone, bony remodelling takes place constantly. A process where bone formation is closely coupled to bone resorption or break down. With chronic inflammation, this process is uncoupled.

And, at different sites, the response can be different. So, we see both excessive bone formation and excessive bone resorption.

How this happens exactly still needs to be worked out but progress is being made.

The bottom line:

Patients with ankylosing spondylitis are at much higher risk of developing fractures of the spine and at other sites.

Making matters worse, the AS spine is stiff (not flexible) and therefore more likely to fracture when subjected to a significant force.

Osteoporosis is a silent disease until the fracture (read: You won't know until you break). This means it's easily forgotten and the diagnosis not made.

Given osteoporosis is potentially preventable and eminently treatable, we need to highlight this complication.

If you have ankylosing spondylitis, were you aware of the risk of osteoporosis? Have you been tested?

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.