Ask Dr. K: ‘Double-jointed’ people really hypermobile

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.

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.