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What Is Thumb Hypoplasia and Aplasia?

Medically Reviewed by Dan Brennan, MD on July 26, 2022

Thumb hypoplasia and aplasia are two versions of a congenital defect. Congenital problems like these occur when your baby is still in the womb. They’re also known as birth defects. 

In the case of thumb hypoplasia, your baby is born with a thumb that’s smaller and weaker than normal. Thumb aplasia occurs when a thumb doesn’t form at all. 

What Is Hypoplasia?

Hypoplasia is caused by a problem with the way that your baby’s hand develops. Under normal circumstances, your baby’s hand starts out like a fleshy mitten without any distinguishing characteristics. Then, the individual fingers and thumbs take shape. This involves a complex interweaving of a variety of tissues including: 

  • BonesThese provide structural support throughout your baby’s hands and body.
  • Muscles. These tissues give your baby’s hand strength. 
  • LigamentsThese stretchy tissues connect bones. They provide a lot of support, particularly at the joints. 
  • TendonsThis type of tissue connects your bones to your muscles, adding further support to body parts like hands. 

In cases of hypoplasia, some or many parts of this normal developmental process are disturbed. Thumb hypoplasia can include mild and severe developmental abnormalities. Different thumb hypoplasia types include hands with: 

  • Small thumbs. The structures within your child’s thumb are shaped normally, but the thumb is smaller and weaker than it should be. 
  • Abnormal musculature. The muscles in your child’s hand don’t grow or form connections where they should within your child’s thumb. The bones tend to be smaller than normal, and the middle thumb joint is unstable. It can wiggle when your child tries to apply pressure with this digit. There will likely be too little skin between the thumb and forefinger. This tight webbing restricts the thumb’s movement. 
  • Absent musculature. This abnormality creates significant muscle problems. Your child’s thumb will be weak, and the central joint will be unstable. There will be limited movement due to webbing that’s too tight.
  • Abnormal bones. In this case, bones are present, but they’re not correctly formed or sized. There are similar mobility and strength issues to those found in thumbs with musculature problems. 
  • Absent bones. In this version of hypoplasia, the thumb doesn’t have any bone-based support at all. There can still be bones within the fleshy thumb region, but they’ll be free-floating. They won’t be anchored to the rest of the skeleton in your child’s hand. Only skin and soft tissue will connect your child’s thumb to the rest of their body.  

Hypoplasia can occur in either one or both hands. Even if one of your child’s hands looks normal, your doctor should still thoroughly examine it to check for signs of mild dysplasia. 

What Is Aplasia?

Aplasia is a specific, severe type of hypoplasia. Thumb aplasia is the term that’s used when your child’s thumb is completely absent. 

How Is Thumb Hypoplasia Related to Radial Dysplasia?

Thumb hypoplasia is a type of radial dysplasia. Radial dysplasia is sometimes referred to as radial club hand. This condition has been studied since 1733. It involves a wide range of developmental abnormalities centered around your child’s hand. 

Other versions of radial dysplasia can include:

  • Hands that are perfectly formed, but small
  • Musculature issues throughout the hand
  • Bone formation issues throughout the hand 

All types of radial dysplasia, like thumb hypoplasia, are more common in males than females. Males are more likely to be born with one of these issues at a 3:2 ratio. 

More data is needed to understand the exact rate of radial dysplasia, but one Finnish study estimated that some form of radial dysplasia occurs in one out of every 5,000 births. 

What Causes Thumb Hypoplasia and Aplasia?

The exact cause of thumb hypoplasia and aplasia remains unknown. The condition often occurs at the same time, though, as other disorders that are present at birth. In such cases, these congenital defects are likely related to the other conditions. 

There’s no evidence that this condition is caused by any actions that the mother takes when pregnant. For example, it’s not linked to drug abuse or other harmful activities. It’s also not currently linked to any vitamin deficiencies or diseases that the mother could have. 

Is thumb hypoplasia hereditary? There does seem to be a genetic component to many types of radial dysplasia. One gene that scientists are currently investigating is a so-called sonic hedgehog gene. A mutation, or change, in the code of this gene, may be partially to blame for some of the developmental issues seen in radial dysplasia. 

This gene may cause a decrease in fibroblast growth factors, which are biological signals that aid in development. Fibroblast growth factors help cells multiply, survive, and transform into different types of tissues. 

We need a lot more research to understand the causes of all forms of radial dysplasia, though, including thumb dysplasia.  

What Is the Best Thumb Hypoplasia Treatment?

The best treatment for your child’s thumb hypoplasia depends on the severity of the condition and other parental considerations. Possible treatment options include: 

  • Strengthening techniques. Some hyperplastic thumbs can function almost normally. They just need to be properly adjusted. Your child can work with physical and occupational therapists to adapt to their modified thumb and strengthen the appendage. This can include a variety of techniques, like the use of external braces to stretch out your child’s hand. 
  • Home exercises. These are ways to strengthen your child’s thumb that you can take advantage of without any special equipment. A physical or occupational therapist can suggest the best ones for your child’s unique situation. These exercises work for many forms of thumb hyperplasia. They’re also necessary during the weeks before surgery for best results.  
  • Pollicization. This is the most common form of surgery in cases of severe hypoplasia and aplasia. This procedure modifies your child’s index (pointer) finger so it can function as a thumb. The hyperplastic thumb may need to be removed first. This surgery normally takes place when your child is between the ages of six and 18 months old. 
  • Other surgeries. Surgeries for milder forms of thumb hypoplasia can range from procedures that rearrange the ligaments and tendons within the thumb to skin grafts that extend the webbing between the thumb and index finger. 

Ultimately, the decision about how to treat thumb hypoplasia lies with the child’s parents. Children can learn to adapt to life without a finger. They can learn how to use their index and longest fingers to pinch and grab. The main downsides include decreased pinching strength and difficulties holding large objects. 

No matter what choice you make, you can help encourage and support your child while they learn to navigate the world with this condition. 

Show Sources

SOURCES: 

Boston Children’s Hospital: “Thumb Hypoplasia and Aplasia.” 

Nemours Kids Health: “Thumb Hyposplasia.” 

Olson, N., Hosseinzadeh, S. StatPearls, “Radial Dysplasia,” StatPearls Publishing, 2022.

Stanford Children’s Health: “Thumb Hypoplasia and Aplasia (Small or Missing Thumb).” 

Tissue Engineering: “Fibroblast Growth Factors: Biology, Function, and Application for Tissue Regeneration.” 

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