Because bending your thumb backwards is only half of it.
First things first, the phrase « double-jointed » is inaccurate — it actually has to do with loose ligaments.
The medical term is hypermobility or laxity (looseness) of the joint, which means the joint can move beyond a normal range of motion, says Dr. Jonathan Vigdorchik, professor of orthopedic surgery at NYU Langone Medical Center.
Three things determine a joint's range of motion: bone stability (how the joint sockets fit together), ligaments and tendons (the connective tissue wrapping around the joint to hold it in place), and the surrounding muscles (that keep your joint aligned when you move).
Hypermobility can be associated with a range of medical conditions (more on some of those in a sec) or it can just be something random that your body does occasionally. It's usually caused by loose ligaments and tendons which don't properly connect the joint, says Dr. Jennifer Hand, medical geneticist and dermatologist at the Mayo Clinic. That looseness can be from a defect in connective tissue, or from repeated extension and contortion of the body in a way that stretches the ligaments.
freefaller25 / Wikimedia Commons / Via commons.wikimedia.org
It’s totally normal to have a hypermobile joint here and there, especially in the hands.
Having a bendy finger or two is super common since the hand is full of joints and ligaments. Something like that or a random backwards-bending elbow is called localized hypermobility, meaning it's specific to one joint. This happens when specific ligaments happen to be loose or the bone is abnormal, says Vigdorchik.
You might also develop hypermobile joints from putting unique stress on them at young age when those bones are still forming (like a baseball pitcher's throwing shoulder) or if an injury causes them to heal in a weird way.
But some people have generalized joint hypermobility, which means they’re bendy and « double jointed » all over.
Generalized hypermobility means that many joints in the body can move beyond the normal range of motion due to an overall looseness in ligaments. Exactly which and how many major joints which are affected — the hips, shoulders, or knees — and the severity of associated joint problems varies by person, Vigdorchik says.
« A lot of people with generalized hypermobility don't even notice until they're adults and they start to have joint pain, » he says. It's more common in women than men, says Hand, but doctors don't have enough research to explain why gender is such a big factor.
Freefaller25 / Via commons.wikimedia.org
A doctor can actually diagnose this with special flexibility tests.
You don't need to see a doctor if you are hypermobile, but if you've had dislocations and joint pain or you're super bendy and just want to know what's up, you can see a doctor to test for it. They'll perform series of tests to assess the mobility of the joints on a 9-point scale called Beighton's Score.
« You get one point for each pinky finger, elbow, knee, shoulder, etc., » Vigorchik says. Basically, it's an easy way for doctors to diagnose hypermobility without genetic testing for a related disorder. You have to score at least a 5 to have generalized hypermobility, says Hand, but from there, the looseness of your joints is on a spectrum and can range from moderate to severe.
Ehler-Danlos National Foundation / Via ednf.org