Pencil-in-cup deformity is a distinctive pattern of bone destruction visible on X-ray, where one end of a bone is whittled down to a tapered point while the opposing bone surface widens into a cup-shaped cavity. The result looks remarkably like a sharpened pencil sitting inside a small cup. It is most strongly associated with psoriatic arthritis, particularly its most destructive form, but it also appears in a handful of other conditions that cause aggressive joint erosion. The finding usually signals that significant, irreversible bone damage has already occurred.
What the X-Ray Actually Shows
On a standard hand or foot radiograph, pencil-in-cup deformity appears at a joint where the head of one bone has been eroded into a pointed, conical shape while the base of the adjacent bone has been scooped out or expanded. In psoriatic arthritis, this most often involves the small joints of the fingers and toes. A case report of systemic sclerosis documented a textbook example: the head of the proximal phalanx was whittled down while the base of the middle phalanx broadened, producing a characteristic pencil-in-cup appearance at the fifth finger’s proximal interphalangeal joint.1Rheumatology. Systemic sclerosis with pencil‐in‐cup deformity The deformity can affect a single joint or show up at multiple joints simultaneously, depending on how widespread the underlying disease process is.
The name is purely descriptive and refers only to what the bones look like on imaging. It does not, by itself, tell you the cause. But the pattern is so distinctive that when a radiologist spots it, the differential diagnosis narrows considerably. It immediately raises the question of psoriatic arthritis or one of a small number of other erosive conditions.
Why Psoriatic Arthritis Is the Primary Culprit
Psoriatic arthritis is the condition most closely linked to pencil-in-cup deformity, and the most severe variant of psoriatic arthritis, called arthritis mutilans, is where the finding is most dramatically expressed. Arthritis mutilans involves extreme osteolysis, a process in which bone is actively broken down and resorbed by the body’s own cells. Digits can shorten so much that the overlying skin folds and telescopes, a phenomenon sometimes called “opera glass” fingers because the fingers can be pulled in and out like the tubes of a collapsible telescope.
A systematic review examining the radiographic features of psoriatic arthritis mutilans found that pencil-in-cup change was present in about 16% of cases studied, while broader bone resorption appeared in roughly 41% and ankylosis (joint fusion) in about 21%.2The Journal of Rheumatology. Psoriatic Arthritis Mutilans: Clinical and Radiographic Criteria. A Systematic Review That 16% figure might seem low for a finding so closely associated with the condition, but the explanation is partly definitional: researchers use varying criteria for what counts as arthritis mutilans, and pencil-in-cup deformity represents a specific stage in a spectrum of destruction. Some joints progress past the pencil-in-cup stage into complete dissolution or fusion before an X-ray is taken.
Arthritis mutilans itself is rare. A Nordic population study estimated its prevalence at roughly 3.7 per million adults.3PubMed. Psoriatic arthritis mutilans (PAM) in the Nordic countries: demographics and disease status. The Nordic PAM study But because the consequences are so severe and the joint damage irreversible, identifying patients headed toward this outcome has become a priority in rheumatology research. An international initiative through GRAPPA (the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis) has focused on finding clinical predictors and biomarkers that could flag at-risk patients early, before joints reach the pencil-in-cup stage and beyond.
How Bone Gets Destroyed From the Inside Out
The pencil-in-cup shape does not happen because bones wear down mechanically. It results from a biological attack driven by the immune system. In psoriatic arthritis, the synovium, the tissue lining the joint capsule, becomes inflamed and thickened. This inflamed tissue, sometimes called pannus, invades the adjacent bone. The key players in this destruction are osteoclasts, specialized cells whose entire function is to dissolve bone.
Osteoclasts are not inherently harmful. They are part of normal bone maintenance, constantly breaking down old bone so that new bone can be laid down. But in psoriatic arthritis, the system tips dramatically out of balance. Inflammatory signaling molecules, especially TNF-alpha and a protein called RANKL, flood the joint environment and drive the production of far more osteoclasts than normal. Research on psoriatic arthritis joints has shown that osteoclasts congregate in resorption lacunae, which are the pits they carve into bone, right at the border where inflamed tissue meets bone. RANKL was found to be intensely expressed across the synovial lining layer, while its natural counterweight, a protein called osteoprotegerin (OPG), was confined to blood vessel walls away from the sites of active erosion.4JCI Insight. Mechanisms of TNF-α– and RANKL-mediated osteoclastogenesis and bone resorption in psoriatic arthritis In other words, the brakes on bone destruction are physically separated from the accelerator.
The process is amplified by a network of inflammatory cytokines. IL-17, a molecule increasingly recognized as central to psoriatic disease, ramps up RANK expression on osteoclast precursor cells, making them more responsive to the RANKL signal and more likely to mature into active bone-eating cells. Other cytokines such as IL-1 and IL-6 further increase RANKL production from mesenchymal stem cells in the joint.5PubMed Central. Immune response and cytokine pathways in psoriatic arthritis: A systematic review The result is a self-reinforcing cycle: inflammation drives osteoclast production, osteoclasts destroy bone, and the debris and ongoing inflammation perpetuate the process.
The Skin Connection
One of the more surprising findings in recent psoriatic arthritis research is that the skin itself contributes to bone destruction, even in joints far from the skin lesions. Studies have shown that inflamed psoriatic skin releases a cocktail of factors, including RANKL, that can drive monocytes (a type of white blood cell) to differentiate into active osteoclasts. When researchers blocked RANKL with osteoprotegerin in these experiments, osteoclast formation decreased but was not eliminated, suggesting that the skin produces additional bone-destroying signals through pathways that do not depend on RANKL at all.6PubMed. Psoriatic cutaneous inflammation promotes human monocyte differentiation into active osteoclasts, facilitating bone damage
This finding has practical implications. It helps explain why patients with more severe skin psoriasis sometimes develop worse joint damage, and it suggests that controlling skin disease is not merely a cosmetic goal but may help protect bones and joints. Serum levels of RANKL, along with osteoclast numbers and activity, were found to be influenced by the severity of the cutaneous disease in patients with psoriasis, regardless of whether they had been formally diagnosed with joint involvement.7PubMed. Psoriatic cutaneous inflammation promotes human monocyte differentiation into active osteoclasts, facilitating bone damage The skin is not just a bystander in the story of joint destruction; it is an active participant.
Other Conditions That Produce the Same Finding
Although psoriatic arthritis dominates the differential, pencil-in-cup deformity is not exclusive to it. Several other conditions can produce the same X-ray appearance, and distinguishing among them usually requires combining the imaging with clinical history and laboratory findings.
- Systemic sclerosis: This autoimmune condition, which causes skin thickening and blood vessel damage, can lead to severe bone resorption in the fingertips (acro-osteolysis) and, in rare cases, pencil-in-cup deformity in the finger joints. The case described earlier involved erosive and destructive changes across multiple joint groups, with soft tissue atrophy and flexion deformities alongside the pencil-in-cup finding.8Rheumatology. Systemic sclerosis with pencil‐in‐cup deformity
- Multicentric reticulohistiocytosis: A rare systemic disease in which abnormal immune cells infiltrate the skin and joints, sometimes causing dramatic bone destruction. A case report documented bilateral hand involvement with lytic defects, pathological fractures, and the classic pencil-in-a-cup appearance suggesting arthritis mutilans.9Clinics in Orthopedic Surgery. Bilateral Total Hip Arthroplasty in a Rare Case of Multicentric Reticulohistiocytosis
- Rheumatoid arthritis: While the typical erosion pattern in rheumatoid arthritis differs from psoriatic arthritis, advanced erosive rheumatoid disease can occasionally produce pencil-in-cup changes, particularly in the fingers.
- Gout: Chronic tophaceous gout, where urate crystal deposits erode bone over years, can mimic some features of pencil-in-cup deformity in severe cases, though the overall joint pattern typically looks different.
In practice, the location of the deformity, the distribution across joints, and the presence or absence of features like skin psoriasis, rheumatoid factor, or tophi (gout nodules) usually point clinicians toward the correct diagnosis. Pencil-in-cup deformity in the distal interphalangeal joints of the fingers, for instance, is much more suggestive of psoriatic arthritis than of rheumatoid arthritis, which tends to spare those joints.
Why Early Detection Matters So Much
By the time pencil-in-cup deformity is visible on an X-ray, the bone damage is substantial and permanent. No current treatment can rebuild bone that has been resorbed in this way. This is what makes the finding both clinically important and sobering: it represents a point of no return for the affected joint. The functional consequences can be severe. A study of patients with psoriatic arthritis mutilans in Nordic countries found that about one in five reported impaired functional capacity with little or no ability to perform self-care or everyday tasks.10PubMed. Disease activity in and quality of life of patients with psoriatic arthritis mutilans: the Nordic PAM Study Shortened, unstable digits make it difficult to grip objects, button clothing, or perform fine motor tasks.
The emphasis in rheumatology has therefore shifted toward prevention, specifically identifying patients with psoriatic arthritis who are at risk for progressing to arthritis mutilans and starting aggressive treatment before pencil-in-cup changes develop. This is easier said than done, because the disease trajectory varies enormously from person to person, and reliable predictive biomarkers remain elusive. But the principle is straightforward: once bone is gone, you cannot get it back, so the window for intervention is before the radiographic damage becomes established.
Treatment Aimed at Stopping Progression
Because the pencil-in-cup finding reflects irreversible structural damage, treatment is directed at the underlying inflammatory process rather than at the deformity itself. The goal is to halt further destruction and manage pain and disability. Conventional disease-modifying drugs like methotrexate, leflunomide, and cyclosporine have been used for decades, but their ability to prevent structural damage in psoriatic arthritis has been modest at best.
Biologic therapies have changed the landscape considerably. TNF inhibitors were the first to demonstrate clear slowing of joint damage in psoriatic arthritis. More recently, drugs targeting IL-17 (such as secukinumab and ixekizumab) and IL-23 (such as guselkumab) have shown strong efficacy against both skin and joint symptoms. A systematic review of emerging biological therapies noted that bimekizumab, which blocks both IL-17A and IL-17F, demonstrated enhanced efficacy compared to traditional biologics.11PubMed Central. Emerging biological therapies for psoriatic arthritis: A systematic review These drugs work precisely because they interrupt the inflammatory cytokine signals that drive osteoclastogenesis, the process at the heart of pencil-in-cup destruction.
For joints that have already reached the pencil-in-cup stage or beyond, surgical options such as joint fusion (arthrodesis) or joint replacement may be considered. These procedures aim to restore stability or relieve pain rather than restore the original anatomy. In the hands and fingers, fusion of a severely damaged joint can provide a more functional position even if range of motion is sacrificed. For larger joints affected in rare conditions like multicentric reticulohistiocytosis, total joint replacement may be necessary.12Clinics in Orthopedic Surgery. Bilateral Total Hip Arthroplasty in a Rare Case of Multicentric Reticulohistiocytosis
Pencil-in-Cup Versus Other Erosion Patterns
Radiologists and rheumatologists pay close attention to erosion patterns because they help distinguish between diseases that can look similar in the clinic. Pencil-in-cup is one of several characteristic patterns, and understanding how it differs from others helps clarify what makes it distinctive.
In rheumatoid arthritis, erosions typically appear as marginal defects, small bites taken out of bone at the edges of the joint where the inflamed synovium attaches. These tend to affect joints symmetrically and often spare the distal interphalangeal joints. Psoriatic arthritis, by contrast, can affect any finger joint, often asymmetrically, and has a tendency to involve the distal joints. The erosive process in psoriatic arthritis can be more aggressive and more central to the joint surface, leading to the wholesale remodeling seen in pencil-in-cup deformity rather than the more focal marginal erosions of rheumatoid disease.
Another important distinction is that psoriatic arthritis can feature new bone formation alongside bone destruction, something not typically seen in rheumatoid arthritis. A patient with psoriatic arthritis might show pencil-in-cup deformity at one joint and fluffy periosteal bone growth at another, reflecting the paradox of simultaneous destruction and formation that makes psoriatic arthritis a uniquely complex bone disease. This dual pathology, where osteoclasts are overactive in some locations while osteoblasts lay down abnormal new bone elsewhere, remains an active and somewhat puzzling area of research.
Living With Advanced Joint Damage
For people who already have pencil-in-cup deformity, the day-to-day reality involves managing joints that are structurally compromised. Occupational therapy plays a significant role, with hand therapists providing splints, adaptive devices, and strategies for protecting remaining joint function. Custom-made ring splints can stabilize floppy or telescoping fingers enough to improve grip strength. Assistive tools for opening jars, turning keys, and fastening buttons can preserve independence.
The psychological burden is real and often underappreciated. Visible hand deformities affect social interactions, self-image, and the ability to work. The Nordic study on arthritis mutilans patients found that a substantial fraction of individuals had significant limitations in self-care and daily activities, and disease activity scores reflected ongoing pain and inflammation even in patients receiving treatment.13PubMed. Disease activity in and quality of life of patients with psoriatic arthritis mutilans: the Nordic PAM Study Comprehensive care for these patients extends well beyond prescribing medication; it involves addressing the full spectrum of physical, functional, and emotional consequences of living with severe joint destruction.
Imaging Beyond Plain X-Rays
While pencil-in-cup deformity is a finding on conventional radiographs, newer imaging techniques can detect earlier stages of the erosive process, before bone destruction becomes visible on plain film. Ultrasound can reveal synovial thickening, increased blood flow to inflamed joints, and small erosions that are not yet apparent on X-ray. MRI goes further, showing bone marrow edema (a sign of active inflammation within the bone) and early erosive changes in exquisite detail.
These modalities matter because they offer a chance to catch the destructive process earlier. If a patient with psoriatic arthritis shows bone marrow edema on MRI at a finger joint, that joint may be on a trajectory toward the kind of damage that eventually produces pencil-in-cup deformity. Identifying these warning signs can prompt a rheumatologist to escalate treatment before irreversible structural damage sets in. The pencil-in-cup finding on a plain radiograph, while diagnostically useful, essentially documents damage that has already happened. The clinical push is toward catching the process upstream.