If you have thumb arthritis at the base of your thumb, you may have noticed something strange. Your thumb may start to bend backwards in a strange way. It may also pull inward toward your palm. This study from 1989 looked closely at what happens to the small muscles in your thumb when arthritis gets worse.
Key Findings at a Glance
- Advanced thumb arthritis causes the thumb to pull inward and the joint to bend backwards, creating a Z shape deformity
- The small muscles at the base of the thumb become weaker and change their pulling direction
- One key muscle, the flexor pollicis brevis, loses about 50 percent of its size
- The muscles lose their activity and can no longer balance each other properly
- This muscle imbalance leads to the unusual bent shape of the thumb
Researchers studied eight hands with arthritis and ten healthy hands. They measured the muscles. They also used special tests to see how the muscles were working. Here are three key things they found:
- The direction in which the thumb muscles pull changes when you have arthritis
- Some muscles get much smaller and weaker
- The muscles stop working together as a team
What Is Rhizarthrosis?
What Happens to Your Thumb?
How Do the Muscles Change?
Why Does the Z Shape Happen?
Frequently Asked Questions
Conclusion
Disclaimer
Scientific Source
What Is Rhizarthrosis?
According to this study, when rhizarthrosis becomes advanced, three main things happen to your thumb. First, the joint at the base slips out of place. This is called subluxation. The bones do not sit together the way they should anymore.
Second, your thumb pulls inward toward your palm. This is called an adduction deformity. You may notice that your thumb looks closer to your other fingers than it used to.
Third, the middle joint of your thumb bends backwards too much. This is called hyperextension. All three changes together make your thumb look bent in a Z shape.
What Happens to Your Thumb?
You may be wondering why this matters. The shape of your thumb affects how well you can use it. When your thumb bends in this Z shape, it becomes harder to pinch and grip things.
The researchers wanted to understand why this Z shape happens. They looked very carefully at the small muscles in the thick part of your thumb near your palm. This area is called the thenar. The thenar muscles are the ones that help your thumb move in many directions.
Research shows that these muscles do not work the same way when you have advanced arthritis. The scientists measured two important things. They measured the direction in which the muscles pull. They also measured how big and strong the muscles are.
How Do the Muscles Change?
Let us look closer at what the study found. The researchers focused on two muscles. One is called the abductor pollicis brevis. This muscle normally pulls your thumb away from your hand. The other muscle is called the flexor pollicis brevis. This muscle bends your thumb.
According to this study, the abductor pollicis brevis muscle has a changed vector in people with arthritis. This means it pulls in the wrong direction. It cannot do its normal job anymore.
Here is where it gets interesting. The researchers also measured something called the physiological cross section. This is a way to measure how strong a muscle can be. A bigger cross section means a stronger muscle.
Research shows that the superficial head of the flexor pollicis brevis muscle becomes much smaller. It loses about fifty percent of its size. That means this muscle becomes only half as strong as it should be.
What Did the Electrical Tests Show?
The scientists also did electrical tests on the muscles. These tests are called electromyography, or EMG for short. EMG tests show how active a muscle is. They show if the muscle is working hard or if it is quiet.
The EMG records showed something important. The short flexor and abductor muscles lose a lot of their activity in patients with thumb arthritis. They do not fire as much as they should. They become weak and quiet.
Why Does the Z Shape Happen?
Now we can put all the pieces together. Your thenar muscles normally work as a team. There are two muscles that pull straight and one muscle that pulls at an angle. Together, they keep your thumb in the right position.
But when you have advanced arthritis, everything changes. The abductor muscle pulls in the wrong direction. The flexor muscle becomes much smaller and weaker. The muscles lose their activity. They cannot talk to each other the way they should.
This creates what the researchers call a functional imbalance. The muscles are no longer balanced. Some pull too hard. Others are too weak to pull at all. The abductor muscle cannot maintain the coordinating system anymore.
When this balance is lost, the Z deformity of the thumb results. Your thumb joint slips out of place. Your thumb pulls inward. The middle joint bends backwards. The Z shape appears.
Why This Research Matters for You
Understanding what happens to your muscles helps explain why your thumb looks and feels the way it does. It also helps doctors think about the best ways to treat thumb arthritis. If the muscles are part of the problem, then treatments that help the muscles may also help your thumb.
Some treatments focus only on the joint itself. But this research reminds us that the muscles around the joint are just as important. When the muscles are weak or pulling in the wrong direction, the whole thumb suffers.
About This Site
This site simplifies medical science for people living with thumb pain and rhizarthrosis. We use only published scientific articles to bring you clear, trustworthy information. Every post is based on real research. We translate complex medical studies into plain language that anyone can understand. We invite you to explore more content on this site to learn about your condition and your treatment options.
Frequently Asked Questions
What is the Z deformity in thumb arthritis?
The Z deformity is a bent shape that your thumb takes when you have advanced arthritis at the base. Your thumb pulls inward toward your palm and the middle joint bends backwards. Together, these changes make your thumb look like the letter Z.
Why do my thumb muscles get weaker with arthritis?
Research shows that the muscles at the base of your thumb lose size and strength when you have advanced arthritis. One important muscle can lose about fifty percent of its normal size. The muscles also lose their activity and stop working together as a balanced team.
Can the muscle changes be reversed?
This study did not look at whether the muscle changes can be reversed. It focused on describing what happens to the muscles. Talk to your doctor about treatments that may help your muscles and your thumb function.
What causes the thumb to slip out of place?
When the cushion in your thumb joint wears away, the bones do not fit together well anymore. The muscles that usually hold your thumb in place become weak and pull in the wrong directions. This combination allows the joint to slip out of its normal position.
Conclusion
This study helps us understand the complex changes that happen to your thumb when you have advanced arthritis. It is not just the joint that is affected. The muscles at the base of your thumb also change in important ways. They become smaller. They pull in different directions. They lose their activity. And they can no longer work together as a team.
All of these muscle changes lead to the Z shaped deformity that many people with thumb arthritis experience. Understanding these changes is the first step toward finding better treatments.
If you are experiencing thumb pain or changes in the shape of your thumb, talk to your doctor. They can help you understand what is happening and what treatment options are available for you.
Disclaimer
This article is for informational purposes only. It is not medical advice. It does not replace the advice of your doctor or healthcare provider. Always consult a qualified healthcare professional about your specific condition and treatment options. Do not make changes to your treatment based on this article alone.
Scientific Source
This article is based on research by Schoneberger, M. and Koebke, J., published in Handchir Mikrochir Plast Chir in 1989. The full study is available at: