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Nov. 9, 2004
By Robert H. Shmerling, M.D.
Beth Israel Deaconess Medical Center
Remember that kid in grade school who could bend her thumb backwards until
it touched her wrist? Or the one who could wrap his legs behind his head?
Did they tell you they were double-jointed? Did you ever wonder what that
means?
It might sound as though they have two joints in places others have only
one, or that they have twice the normal amount of motion, but these are
myths. As members of the same species, the vast majority of humans have the
same number of bones and joints. While it's true that some people have more
flexibility in their joints than others, the expression "double-jointed"
implies something that isn't true.
"Double-Jointed" Means Hypermobile
People with more than normal flexibility of their joints are considered to
have hypermobility - which simply means their joints (and surrounding
structures, including ligaments and tendons) are able to bend farther than
"normal," or, more accurately, farther than average. In most cases, the
reason for the extra mobility is not known. The most important questions are
whether a person's hypermobility is a health liability in some way, or
whether it's simply a curiosity. Although the latter is more common than the
former, there are many examples of both.
Keep in mind that flexibility can change over time; in particular, it tends
to decline with age. Through stretching and other exercises, one can
maintain flexibility well into advanced age, but on average, people over age
75 will have less flexibility in their joints than people in their 20s. The
specific causes are not clear, although tiny injuries in tendons and
ligaments (with limited ability of these tissues to repair damage
accumulated over time), diminished muscle mass and tone, and biochemical
changes in aging connective tissues all may play a role. It is not clear,
however, that loss of flexibility is inevitable in all people. In fact,
there are notable examples of people of advanced age maintaining remarkable
flexibility.
Conditions Associated With Hypermobility
Most people with hypermobility are otherwise normal and healthy. It's also
possible that they have an as-yet-undiscovered gene mutation, especially
when others in the family have hypermobility as well. They may be able to
bend forward and touch their palms on the floor, extend their elbows well
past straight, or bend their fingers back toward the back of the hand
farther than would be comfortable for others. Although club foot or
dislocation of the shoulder, hip, or kneecap occasionally complicate
hypermobility, most people who have these dislocations are not hypermobile.
While hypermobility is usually harmless, there are notable exceptions, such
as Marfan's syndrome and Ehlers-Danlos syndrome, which are associated with
hypermobility as well as other potentially serious problems.
Marfan's syndrome is an inherited condition in which a gene mutation leads
to abnormal connective tissue such as tendons, ligaments, blood vessels
bones and cartilage. Because structural support in these tissues is reduced,
flexibility may be increased, but other problems, including reduced vision
(because ligaments holding the lens and other elastic tissues around the eye
are abnormal), chest-wall deformity, leaky heart valves and rupture of large
blood vessels may develop. Affected people tend to be tall, with long,
slender fingers because of abnormal bone development.
There are at least 6 types of Ehlers-Danlos syndrome, each with different
manifestations, since different gene mutations affect different tissues.
Most, but not all, cause hypermobile joints. Other problems include easy
bruising, rupture of arteries, lax skin, scoliosis (curvature of the spine)
and eye problems.
For people with one of the more serious inherited conditions, screening for
complications and genetic testing are recommended. For some, doctors may
recommend that affected people avoid strenuous exercise or contact sports,
have regular eye examinations, and undergo tests to monitor the heart and
large blood vessels. For the rest, however, there is usually no need for
specific treatment or concern.
Does Hypermobility Matter?
Hypermobility is common; in one study, 1 percent of athletes were
hypermobile; another study found that 30 percent of healthy medical students
were unusually flexible. When it is not associated with more serious,
systemic (body-wide) abnormalities in connective tissue (such as Marfan's
syndrome), hypermobility may cause no symptoms at all or cause annoying (but
not life-threatening) joint pain, muscle pain, fibromyalgia, osteoarthritis
and back pain. These may be treated in a similar way to people who are not
hypermobile, with exercises and/or braces, pain medicines, anti-inflammatory
medications and muscle relaxants.
The Bottom Line
If you have a family history of highly flexible joints or other features
suggestive of an inherited connective-tissue disease, learn more about
Marfan's syndrome and Ehlers Danlos syndrome at the links below. There are
important ways to prevent complications in these conditions. Recognize,
though that most people with hypermobility have nothing so serious.
The notion of being "double-jointed" suggests more than the normal allotment
of joints or that the joints have twice the normal motion, but neither is
the case. It is true that different species have evolved to have different
numbers of bones and joints with different capacities for movement, having
adapted over centuries to thrive in their environments. But humans have a
set number of bones and joints and only rarely is there significant
variation.
Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess
Medical Center and associate professor at Harvard Medical School. He has
been a practicing rheumatologist for over 20 years at Beth Israel Deaconess
Medical Center. He is an active teacher in the Internal Medicine Residency
Program, serving as the Robinson Firm Chief. He is also a teacher in the
Rheumatology Fellowship Program.
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