The appearance of a lump on the inside of the foot can be concerning, but it is a relatively common issue. The foot’s intricate anatomy, composed of bones, joints, muscles, tendons, and ligaments, supports the body’s weight and facilitates movement. Any alteration in these components, from soft tissue inflammation to bony changes, can manifest as a palpable mass on the medial (inner) side of the foot or ankle. While many lumps are benign and easily treatable, identifying the origin is necessary for accurate management. A physical examination by a specialist is the first step in differentiating between potential causes, which range from simple fluid-filled sacs to structural deformities.
Common Soft Tissue Masses
Lumps arising from soft tissues (excluding tendons and ligaments) are frequently encountered and often the least concerning. A ganglion cyst is a common example, appearing as a non-cancerous, fluid-filled sac that typically develops near a joint or a tendon sheath. These cysts contain a thick, jelly-like fluid and their texture can range from firm to spongy. Ganglion cysts may fluctuate in size, sometimes becoming smaller when rested or larger with activity, and typically cause pain only if they compress a nearby nerve or rub against footwear.
A lipoma is another soft tissue mass, defined as a benign tumor composed of fatty tissue. Lipomas are soft to the touch and easily movable beneath the skin, usually remaining painless unless they grow large enough to press on adjacent structures. Conversely, plantar fibromas are fibrous knots embedded within the plantar fascia, the thick band of tissue spanning the arch of the foot. These masses are firm and fixed to the fascia, often feeling like a hard lump directly in the arch. Discomfort can arise when pressure from standing or walking bears down upon the fixed nodule.
Structural and Bony Causes
Some lumps on the inside of the foot are rigid protrusions resulting from changes in the underlying skeleton, rather than soft tissue growths. The most recognized is a bunion, known medically as a hallux valgus deformity. This forms a bony prominence where the big toe meets the foot, caused by a misalignment where the first metatarsal bone drifts inward. The resulting lump is fixed, rigid, and often becomes inflamed, leading to chronic pain and difficulty wearing shoes.
A specific bony prominence on the medial midfoot is associated with Accessory Navicular Syndrome. This involves an extra bone or piece of cartilage, called the accessory navicular, located on the inner side of the foot just above the arch. This extra bone is incorporated within the posterior tibial tendon and creates a visible, hard lump that becomes painful and swollen, particularly after physical activity or due to shoe irritation. Osteophytes, or bone spurs, can also develop in the medial ankle or midfoot joints due to chronic stress or arthritis. These bony projections are rigid and fixed, forming in response to cartilage deterioration, and they can cause a palpable bump that limits joint motion and causes pain.
Swelling Related to Tendons and Ligaments
Lumps or swelling on the inside of the foot can signify inflammation or damage to the tissues that stabilize the foot’s arch. Posterior Tibial Tendon Dysfunction (PTTD) is a significant cause of medial foot swelling, as this tendon is a primary support structure for the arch. When this tendon becomes inflamed or strained, the area along its course, which wraps around the inner ankle bone and attaches to the midfoot, can become visibly swollen.
The swelling associated with PTTD or general tenosynovitis (tendon sheath inflammation) is often elongated, following the path of the tendon, and is typically tender. Unlike a discrete cyst or bone spur, this type of swelling is accompanied by warmth, redness, and pain that increases with activities like walking or standing. Over time, chronic PTTD can impair the tendon’s ability to support the arch, leading to adult-acquired flatfoot.
Next Steps for Evaluation and Treatment
While most foot lumps are benign, any newly discovered or changing mass warrants professional evaluation for accurate diagnosis and management. Certain “red flag” symptoms should prompt an immediate consultation with a podiatrist or orthopedic specialist.
Red Flag Symptoms
- Rapid growth
- Increasing pain
- Numbness or tingling
- Signs of infection like fever
The diagnostic process typically begins with a thorough physical examination, where the specialist assesses the lump’s location, texture, mobility, and tenderness. Imaging studies are often employed to determine the mass’s composition. X-rays identify bony causes like bunions or osteophytes, while ultrasound or MRI can visualize soft tissue lesions such as cysts, lipomas, or tendon damage. Treatment varies widely depending on the underlying cause, ranging from conservative measures like custom orthotics, footwear modifications, and anti-inflammatory medication. Localized treatments include corticosteroid injections. For masses that are persistently painful, large, or that do not respond to non-surgical care, surgical removal or correction may be recommended to restore comfort and function.