Do people who are “double-jointed” have twice as many joints?

DEAR DOCTOR K:

This may be a silly question, but here goes: Do people who are “double-jointed” have twice as many joints?

DEAR READER:

I can see how you might think that. If the question is silly it’s because the medical term we use is imprecise and misleading. The term makes it sound as though “double-jointed” people have two joints in places that other people have only one, or that they have twice the normal amount of motion. Neither is true of people who are double-jointed. In fact, the vast majority of humans have the same number of bones and joints.

So who are the people that are called double-jointed? Remember that kid in grade school who could bend his thumb backward until it touched his wrist? Or the girl who could wrap her legs behind her head? They might have called themselves double-jointed.

The correct terminology for people with greater than normal flexibility of their joints is “hypermobility.” That simply means the joints (and surrounding structures, including ligaments and tendons) are able to bend farther than average. In most cases, it’s not clear why a person has this extra mobility.

Most people with hypermobility are otherwise normal and healthy. Hypermobility can cause problems in people who dislocate their shoulder, hip or kneecap — but most people who have these dislocations are not hypermobile.

Hypermobility is usually harmless. However, there are two uncommon diseases that cause hypermobility and other more serious problems. The first, Marfan’s syndrome, leads to abnormal connective tissue including tendons, ligaments, blood vessels, bones and cartilage. People with Marfan’s syndrome can have problems involving the heart and major blood vessels, sudden collapse of the lungs, arthritis, and problems with the lens and retina of the eyes. The other, Ehlers-Danlos syndrome, can also cause weakening of the walls of major blood vessels.

Unusual hypermobility of the joints has been described in people (particularly children) with chronic fatigue syndrome. It also has been reported in people who inherit, or who develop later in life, a disorder of the mitochondria, the little “battery packs” that provide energy inside every cell in our body.

In most people, flexibility tends to decline with age. A few things may explain this age-related decrease:

  • Tiny injuries in tendons and ligaments that occur throughout life;
  • Diminished muscle mass and tone;
  • Biochemical changes that occur in connective tissues as we age.

That’s not to say that the loss of flexibility is inevitable in all people as they age. Just look at some older yoga masters.

But as for those kids who could wrap their legs around their heads when they were in grade school? Chances are, if you asked them today, they wouldn’t find that trick quite so easy — unless, for some reason, they had been practicing doing it every day.

Thanks for asking a “silly” question. I think many people are confused by the term “double-jointed,” so I hope I’ve cleared things up.