Pencil-in-cup sign is sometimes called pencil-in-cup change or deformity. It’s an abnormal change in one of the small hinge-like joints of your fingers or toes. It happens when new bone begins to grow in places it shouldn’t, or old bone cells aren’t replaced. Sometimes both of these things can happen in different places.
Pencil-in-cup can happen with a number of conditions, but most cases happen if you have arthritis mutilans. This is the rarest, most severe form of psoriatic arthritis (PsA).
How Does Pencil-in-Cup Happen?
If you have PsA, some of the messaging networks that regulate your immune system no longer work normally. This causes your immune system to mistakenly fire off messages that send cells to certain areas that attack the tissues of your body, including your bones.
The activity of these bone-gobbling cells is what causes pencil-in-cup sign. The bone on one side of the joint is eaten away until it’s scooped out so it looks like a cup. The bone on the other side of the joint gets whittled down, too, so it takes on a shape like the sharpened end of a pencil. If they continue to grind away, more and more of the bone will disappear. When this happens, the “pencil tip” – and the finger – gets shorter and shorter.
If enough bone is lost, the skin and muscle of the finger can become floppy. When this happens, you can push the finger in on itself and pull it back out, like a telescope.
Pencil- in-cup is more common in some joints than others. It often happens in the joint closest to the nail. But it can also occur in the middle joint or the knuckle joint.
Who Gets Pencil-in-Cup?
It’s not clear how many people develop pencil-in-cup sign. But according to studies, arthritis mutilans itself is relatively rare. Scientists aren’t sure, but they think it only happens in about 5% of people with PsA.
So which people with PsA end up getting arthritis mutilans? Your chances are greatest if you haven’t gotten treatment for your PsA, or you have and the disease hasn’t responded. If you have PsA in your hands or feet, it will get worse if you don’t treat it. It will progress the fastest within the first year.
Pencil-in-cup may not cause symptoms in itself, unless it’s so advanced the fingers have become noticeably shorter. But prolonged, uncontrolled inflammation resulting from the abnormal immune activity can cause pain.
Inflammation in the joint lining can, in turn, make the entire digit swell up until it looks like a sausage (dactylitis). This kind of swelling is a common symptom of PsA, and it happens more often in the feet than in the hands. Dactylitis is also a sign of aggressive disease.
Erosion of the joint can also allow the bones in the joint to dislocate, or move out of their normal positions, which can also cause pain. Over time, the finger can permanently change shape and bend in an unnatural direction.
Besides bone loss, arthritis mutilans can cause stiffness and less range of motion in joints. Abnormal bone growth can happen too. This can eventually fuse bones in a joint together so you can no longer move it at all.
Your doctor can only diagnose pencil-in-cup when an imaging test – usually an X-ray – shows the distinctive pattern of bone loss. The joint damage that you can see on the X-ray is one of the signs doctors look for to diagnose arthritis mutilans.
Unfortunately, once these changes have happened, you can’t reverse them. That’s one of the reasons that doctors are now trying to detect PsA as early as possible, before it has a chance to cause this kind of damage.
No studies to date have looked at whether early, aggressive treatment of PsA can prevent pencil-in-cup sign. But there’s evidence that this approach can often slow or stop the progress of the disease overall.
If you’ve been newly diagnosed with pencil-in-cup caused by PsA, your doctor will probably prescribe one of several drugs. These medications can treat the ongoing inflammation that damages bone. You and your doctor can talk about which approach is best for you.
The first treatments your doctor might try for PsA include over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs) to ease pain and swelling. Other medications include steroids and DMARDs (disease-modifying antirheumatic drugs).
These medications may not work as well when PsA is at this advanced stage, so your doctor may recommend a type of drug called a biologic. These medications can cost several thousands of dollars per month, so you’ll need to be sure if you don’t have insurance, or your plan doesn’t cover them.
Your doctor may also refer you to a hand therapist. These health professionals use a variety of strategies to ease symptoms and help you keep as much normal function as possible. For example, a hand therapist might prescribe devices that ease strain on the joint and exercises for flexibility and mobility.
If damage to the joint is extremely severe, your medical team may suggest surgery.
It’s not possible to reverse the damage that results in pencil-in-cup sign. But early treatment to get inflammation under control will give you the best chance of keeping your mobility.