Thumb Duplication (Pre-Axial Polydactyly) | Symptoms & Causes

At Boston Children’s Hospital, our orthopedic and plastic surgery teams know that you’re concerned about your baby’s hand difference. It might ease your mind to know that we’ve developed innovative surgical treatments for children with variations of thumb duplication. Learning more about this condition may help you feel more confident and informed as we work toward healing your child.

What is thumb duplication?

Thumb duplication is a form of polydactyly (an extra digit in the hand or foot) in which your child’s hand (usually just one side) has more than one thumb.

How is thumb duplication diagnosed?

An extra thumb can often be seen by ultrasound prenatally, and by eye at birth. Your doctor will use x-rays to assess the underlying structure of your baby’s finger and determine a course of treatment.

Causes of thumb duplication (pre-axial polydactyly)

During normal prenatal development (while the baby is in the womb), the hand initially forms in the shape of a paddle, and then eventually — in about the sixth or seventh week of gestation — splits into separate fingers. Thumb duplication and other forms of polydactyly result if there’s an irregularity in this process: An extra digit forms when a single digit splits in two.

The vast majority of the time, this occurs sporadically, meaning that the condition happens without an apparent cause. Some may result from a genetic defect or underlying hereditary syndrome, particularly if your child has a thumb with three bones instead of two. Caucasians, Asians and Native-Americans are more likely to inherit the condition than African-Americans.

When to see a specialist

Your child’s doctor will know by sight that your baby’s hand has an extra thumb. The doctor will refer you to a hand specialist, who will guide you to a more detailed diagnosis and treatment plan.

Questions to ask your doctor

If your child is diagnosed with thumb duplication, you may feel stressed and lose track of the questions that occur to you. Lots of parents find it helpful to jot down questions as they arise — that way, when you talk to your child’s doctors, you can be sure that all of your concerns are addressed.

Some of the questions you may want to ask include:

  • What is happening to my child, and why?
  • What will x-rays reveal?
  • What actions might you take after you review my child’s x-rays?
  • Is surgery necessary? Are there alternative therapies?
  • Will my child be OK after surgery?
  • Will there be restrictions on his activities or capabilities?
  • What will be the long-term effects?
  • What can we do at home?

Frequently Asked Questions

Are there different variations and degrees of severity?

Yes, although almost all forms of thumb duplication require complex surgical correction and hand reconstruction.

  • The extra thumb can occur at the joint of the outermost bone (distal phalanx).
  • The closer to the hand that the extra thumb occurs, the more complex the congenital difference is — such as bent or angular deformities, stiffness, limited range of motion, and underdevelopment.

Are certain ethnic groups predisposed to have a certain type of polydactyly?

Yes, Asians, Caucasians, and Native Americans are more likely to have an extra thumb, while African-American children are more likely to have an extra little (pinkie) finger.

How common is thumb duplication?

Thumb duplication is one of the more common congenital hand conditions, affecting about one out of every 1,000 babies. Usually, only one of a child’s thumbs is affected.

Does thumb duplication cause my baby pain?

No, an extra thumb isn’t usually painful.

For parents

It’s natural for parents whose babies are born with an extra thumb to feel concern. Depending on the extent of your child’s condition, his treatment and recovery may be fairly straightforward, or may require more than one surgery.

Even though you understand the importance of surgery and therapy for your child, you still might experience his treatment and recovery as a stressful time. If you feel frustrated or depressed, speak to your doctor or counselor to get help. Professionals in Boston Children’s Hale Family Center for Families can provide you with important resources and referrals.

Thumb duplication glossary

  • The Hale Family Center for Families at Boston Children’s: dedicated to helping families find the information, services and resources they need to understand their child’s medical condition and take part in their care
  • congenital: present at birth
  • embryonic development: development of the fetus in the womb
  • in utero: in the womb (uterus)
  • occupational and physical therapy: services offered by trained professionals to help restore function or (re)teach basic life skills, like walking or grasping objects
  • orthopedics: the medical specialty concerned with diagnosing, treating, rehabilitating and preventing disorders and injuries to the spine, skeletal system and associated muscles, joints and ligaments
  • orthopedic surgeon, orthopedist: a physician specializing in surgical and non-surgical treatment of the spine, skeletal system and associated muscles, joins and ligaments
  • prenatal (fetal) ultrasound: ultrasound performed at several stages of pregnancy; can detect polydactyly in the fetus
  • polydactyly: a condition in which the hand has one or more extra fingers
  • post-operative (post-op): occurring after surgery
  • prehension: grasp, an important function of the thumb; surgery for thumb duplication aims to ensure good grasping function
  • pre-operative (pre-op): occurring before surgery
  • range of motion (ROM) exercises: physical therapy exercises designed to improve or restore flexion and extension of joints
  • radial (pre-axial) polydactyly: another term for thumb duplication (an extra thumb)
  • reconstructive surgery: surgery performed to repair and/or restore a body part to normal or as near normal as possible
  • sporadic: occurring without an apparent genetic cause
  • thumb duplication: an extra thumb on the hand; polydactyly of the thumb
  • x-rays: a diagnostic test that uses invisible ionizing radiation to produce images of internal tissues, bones, and organs onto film; usually the diagnostic tool of choice for thumb duplication