The appearance of a bump on a knuckle often causes immediate concern, but these growths represent a wide range of underlying conditions, many of which are benign. The term “knuckle” refers to the joints of the finger: the metacarpophalangeal (MCP) joints connecting the fingers to the hand, and the interphalangeal (IP) joints (PIP and DIP) within the fingers. Since the hand contains complex structures of bone, cartilage, and connective tissue, a bump can arise from any of these components. Understanding the location and consistency of the growth is the first step in identifying its origin.
Cysts and Fluid-Filled Growths
Bumps that feel soft, rubbery, or spongy beneath the skin often originate from a fluid-filled sac associated with the nearby joint or tendon. The most common non-bony mass is the ganglion cyst, which frequently forms on the back of the hand or wrist but can also appear near the knuckle joints. These growths are typically filled with a thick, jelly-like fluid that has leaked from a joint capsule or tendon sheath. They can fluctuate in size but are almost always non-cancerous.
A specific form of ganglion cyst is the digital mucoid cyst, which appears at the distal interphalangeal (DIP) joint closest to the fingertip. These cysts are often smooth and translucent, sometimes appearing near the nail bed. Their proximity to the nail can cause a groove or deformity in the fingernail plate due to pressure on the nail matrix. Mucoid cysts are frequently linked to underlying degenerative changes, such as early-stage osteoarthritis in the DIP joint. If a cyst breaks open, it creates an open pathway for bacteria, leading to a risk of joint infection that requires medical attention.
Bumps Related to Joint Deterioration
Some growths are hard and unyielding, signifying a solid change in the underlying bone structure, often related to long-term wear-and-tear conditions. Osteoarthritis (OA), where joint cartilage breaks down over time, is a frequent cause of these firm, bony enlargements. As cartilage erodes, the body attempts to repair the joint by growing new bone tissue, resulting in osteophytes, or bone spurs, that manifest as nodes.
These bony nodes are named based on their location on the finger joints. Heberden’s nodes form on the distal interphalangeal (DIP) joints, nearest the fingertip. Bouchard’s nodes occur on the proximal interphalangeal (PIP) joints, the middle knuckles. Both types of nodes are signs of advanced OA in the hand and can lead to stiffness and limited motion.
A different type of firm lump is the rheumatoid nodule, associated with Rheumatoid Arthritis (RA), an inflammatory autoimmune disease. Unlike the bony spurs of OA, rheumatoid nodules are inflammatory soft-tissue growths that form under the skin, often over the metacarpophalangeal (MCP) joints. These nodules feel firm but may be slightly less rigid than OA nodes, suggesting a need for specific diagnosis to manage systemic inflammation.
Acute Injury and Other Deposits
A sudden lump on a knuckle is often the result of an acute injury, such as a sprain or a small fracture. A direct blow or an awkward twist can cause rapid swelling and inflammation of the soft tissues or joint capsule. These traumatic bumps are typically accompanied by immediate pain and often bruising, but the swelling usually resolves as the injury heals. If a fracture has occurred, the bump will be hard and painful, requiring imaging for diagnosis.
Other causes of solid deposits include gouty tophi, which are accumulations of uric acid crystals deposited in the tissues around the joints. Gout is a form of inflammatory arthritis caused by high levels of uric acid in the blood, leading to sudden, intensely painful flares. Tophi appear as hard, white or yellowish lumps under the skin, signifying chronic gout that has not been adequately controlled. If left unaddressed, they can cause joint damage and deformity.
When to Seek Medical Attention
While many knuckle bumps are harmless, a medical evaluation is advised if the growth is accompanied by certain warning signs. Any lump that begins to grow rapidly or changes significantly in shape requires prompt professional assessment. Swelling, warmth, or redness around the bump, especially when combined with a fever, could indicate an infection that needs immediate treatment.
Severe or unremitting pain, numbness, or tingling in the finger suggests the growth may be pressing on a nerve or causing internal joint damage. If the lump limits your ability to move the finger or hand normally, a doctor can determine if the underlying cause requires intervention. A healthcare provider, such as a primary care physician, orthopedist, or rheumatologist, can use diagnostic tools like X-rays or aspiration to provide a definitive diagnosis and recommend the appropriate course of action.