Some people have thumbs that are hypermobile. This means they bend backward beyond the typical range. The medical term for hitchhiker’s thumb is distal hyperextensibility. It is also known as Z-shaped deformity of the thumb.
Hitchhiker’s thumb is not painful and does not make it harder to use your hands.
Unlike your fingers, which have three bones (phalanges), your thumb has only two bones. The first bone starts at the tip of your thumb and extends to the knuckle. The second bone starts at the knuckle and extends to the base of your thumb.
There is only one joint in your thumb. Some people with hitchhiker’s thumb can bend this thumb joint backward as far as 90 degrees.
Causes of Hitchhiker’s Thumb
There haven’t been many studies carried out on hitchhiker’s thumb. But researchers say that it’s an inherited condition.
A study published in 1953 was one of the first to analyze hitchhiker’s thumb. A total of 450 families were studied.
A protractor was used to measure the angle of people’s thumbs. The researchers gave the name “hitchhiker’s thumb” to the thumbs that could bend at an angle equal to or greater than 50 degrees. They noted that many people had one hitchhiker’s thumb and one straight thumb.
The conclusion from this 1953 study was that hitchhiker’s thumb is a recessive trait.
In 2011, researchers found that in a random sample of 310 people in Nigeria, 32.3% had hitchhiker’s thumb. Of those with hitchhiker’s thumb, 15.5% were male and 16.8% female.
In a 2007 study of populations of different regions in India, hitchhiker’s thumb was found to be rarer in those from northern, western, and eastern India. Nearly 30% of participants from southern India had hitchhiker’s thumb.
Hitchhiker’s Thumb and Genetics
Our genes contain information that specifies traits or characteristics that are passed down from your biological parents. Their genes influence traits such as your height, eye color, blood type, and disease risk. You receive one version of a gene, called an allele, from each of your parents.
If the alleles are different, the recessive one is masked by the dominant allele, which means only the dominant trait shows. The trait from a recessive gene won’t show unless you have two copies, one from each parent.
This means that if both your parents have hitchhiker’s thumb, you should have it too. But if one of your parents has the dominant allele for straight thumb, and the other has the recessive allele, you won’t have hitchhiker’s thumb.
But there has been some debate about the science behind hitchhiker’s thumb. An expert has argued that thumbs don’t fall into only two neat categories of hitchhiker’s thumb and non-hitchhiker’s thumb. Instead, they said that the angles of people’s thumbs vary, with no clear division between hitchhiker and non-hitchhiker thumbs.
Conditions Associated with Hitchhiker’s Thumb
These are some of the health conditions that have hitchhiker’s thumb as a symptom:
Diastrophic dysplasia. This is a rare genetic disorder that’s also known as diastrophic dwarfism. People with this condition are usually short, with very short arms and legs. They may also have bones that develop abnormally, resulting in club feet, spinal deformities, cleft palate, and hitchhiker’s thumb.
Diastrophic dysplasia is inherited in an autosomal recessive pattern. This means that both parents have the gene, but they usually don’t have any symptoms.
A related rare disorder, atelosteogenesis type II, has similar symptoms to diastrophic dysplasia, including hitchhiker’s thumb. But this is a far more severe disorder with life-threatening complications shortly after birth.
Hypermobility spectrum disorders. An estimated 30% of adults have hypermobile joints, meaning that they are what many of us call “double-jointed.” This means that your joints, such as your hips, shoulders, hands, and ankles, can move beyond the standard range of motion. For some people, like athletes and dancers, this can be an advantage. But joint hypermobility is also associated with hypermobility spectrum disorders and hypermobile Ehlers-Danlos syndrome (EDS).
Hypermobility spectrum disorders include disorders related to your connective tissues. These disorders usually have symptoms like chronic pain, fatigue, and joint instability.
One of these hypermobility spectrum disorders is hypermobile Ehlers-Danlos syndrome. This is an inherited disorder that affects collagen, the protein that holds your body together. Among the symptoms are:
Rheumatoid arthritis. Problems in the joints of your hands and feet are common symptoms of rheumatoid arthritis. Rheumatoid arthritis is an autoimmune and chronic inflammatory disorder in which your immune system mistakenly attacks your body’s tissues.
When the lining of your joints is attacked, a layer of abnormal tissue forms. This can destroy the cartilage and bone within the joint. Your tendons and ligaments holding the joint together weaken. The joint gradually loses its shape.
Hitchhiker’s thumb is one of the possible joint deformities. Some other joint deformities include swan-neck deformity, in which the base and top joint of your finger bend and the middle joint straightens, and boutonniere deformity, where your middle finger joint won’t straighten while the joints at the end bend back.
These days, it may be possible to prevent joint deformities with medication.