Bodily Oddities

Linburg–Comstock variation

Also called: Linburg–Comstock syndrome · Linburg–Comstock anomaly · Linked thumb and index finger tendons

An extra tendon link in the forearm that ties the thumb to the index finger. When the thumb bends into the palm, the index fingertip bends with it. Roughly 1 in 5 people have it, but counts vary widely.

Muscle Variation Structural Try it Vestigial

How common

About 1 in 5 people. Studies range from 5% to 60%. Hand tests often give higher and less reliable numbers than dissection.

Do you have this?

How it works

Two long forearm muscles matter here. The flexor pollicis longus, the long thumb flexor, bends the thumb. The flexor digitorum profundus, the deep finger flexor, bends the fingertips1.

In some people, a slip of tendon joins the thumb flexor to the part of the deep flexor that moves the index finger12. Sometimes it reaches the middle finger too1. Then bending the thumb also curls the index fingertip4.

A 2018 review asks for the word “syndrome” only when the link causes symptoms. Otherwise, it is a variation3.

The link may be a leftover from evolution. Other primates have one flexor muscle for all the fingers. In humans, the thumb flexor is separate1. A 2017 study found the link no more often in secretaries who type six or more hours a day. Its authors think people are born with it6.

How common it is

The number depends on the method. The 1979 study found it in 31% of 194 patients by hand test, and in 25% of 43 bodies by dissection2. A 2019 hand test in Serbia found 60%5. A 2021 study found 5% to 32%, depending on how the thumb was bent. Its two examiners agreed weakly or not at all. Dissection in that study found 23%4. A 2018 meta-analysis of 14 studies found it more often in women. By ancestry, rates went from 8.8% in African groups to 34.5% in Hispanic groups3. Overall estimates are around 21%1.

Try it

  1. Hold one hand up, palm toward you, fingers straight.
  2. Slowly bend your thumb across the palm toward the base of your little finger.
  3. Watch your index fingertip. If it bends too, you probably have the link.
  4. Hold the index finger straight with your other hand and try again.
  5. If the thumb cannot bend fully and the forearm or wrist cramps or aches, that is also a sign1.
  6. Do not force it. Stop if it hurts.

This test is not reliable. Trained examiners often disagree4.

Is it a problem?

Usually no. Most people with the link have no symptoms1. Some get forearm or wrist pain with heavy or repeated hand use1. Musicians are the group most often described6. In one study, between 21% and 25% of people with the link reported forearm pain6. See a doctor if bending your thumb causes forearm or wrist pain that does not go away.

History and culture

Doctors knew about the link as early as the 1800s1. Linburg and Comstock described it in detail in 1979. In four of their patients, it probably caused long-lasting inflammation around the tendons. Surgeons removed the link in all four2.

Sources and further material

  1. Wikipedia Linburg–Comstock variation
  2. PubMed Anomalous tendon slips from the flexor pollicis longus to the flexor digitorum profundus Linburg and Comstock, Journal of Hand Surgery (American Volume), 1979 The first detailed report. Hand exams of 194 patients and dissection of 43 cadavers.
  3. PubMed Linburg-Comstock variation and syndrome. A meta-analysis Yammine and Erić, Surgical and Radiologic Anatomy, 2018 14 studies, 4,132 forearms or hands.
  4. PubMed An investigation into the reliability and methodology of the Linburg-Comstock anomaly clinical test Leyva, Laslovich, Roberts and Kane, Journal of Hand Therapy, 2021 Hand tests on 134 limbs and dissection of 106 limbs.
  5. PubMed Prevalence of the Linburg-Comstock variation through clinical evaluation Erić, Yammine, Gosh, Feigl and Marić, Surgical and Radiologic Anatomy, 2019 Hand tests on 215 people in Serbia.
  6. NCBI Linburg-Comstock: Is Overuse an Etiological Factor? Bulut, Tahta, Ozturk, Zengin, Ozcan and Sener, Plastic Surgery, 2017 313 medical secretaries compared with 323 people without hand-heavy jobs.

Published .

Missing something your body does? Tell me about it.

Muscle Variation

Palmaris longus

A thin tendon that stands out in the wrist when you press your thumb and little finger together. About 1 in 7 people do not have it at all, and they lose nothing.

Missing in about 15% of people
Muscle Variation

Plantaris muscle

A small calf muscle with a very long, thin tendon that medical students mistake for a nerve. Between 7% and 20% of people do not have it.

Missing in 7–20% of people
Bones Variation

Hitchhiker's thumb

A thumb whose top joint bends back past 50 degrees, once taught as a simple recessive gene. Thumb angle is really a smooth range, and the genetics are not that simple.

24–36% of people
Bones Variation

Joint hypermobility

About 1 in 5 people have joints that bend further than usual, measured with a nine-point test called the Beighton score. For most people it causes no trouble.

10–25% of people