A 48-year-old woman with seronegative spondyloarthropathy (psoriatic arthritis) involving her hands shows a 'pencil-in-cup' deformity on X-ray. This deformity indicates:

• A Marginal erosions causing juxta-articular osteoporosis at the MCP joints
• B Soft tissue swelling obscuring the joint space bilaterally
• C New bone formation (periostitis) bridging the IP joints
• D Severe erosive arthritis mutilans where the central tapering of the proximal phalanx (pencil) articulates with a widened distal erosion (cup) ✓

Explanation

The 'pencil-in-cup' deformity is pathognomonic of erosive psoriatic arthritis mutilans. Aggressive osteolysis causes the distal aspect of the proximal phalanx to taper to a point ('pencil') which fits into a cup-shaped resorption cavity of the distal bone. This contrasts with rheumatoid arthritis which causes marginal erosions without this degree of central bone resorption. Arthritis mutilans is the most severe and destructive form of psoriatic arthritis.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.