A lump on the side of the knee can be concerning, but these growths are often benign and related to the knee’s complex structure. The knee joint is an intricate system composed of bone, cartilage, ligaments, tendons, and small fluid-filled sacs called bursae. The location of a lump, whether medial (inner) or lateral (outer), helps medical professionals narrow down its potential origin. Observing the lump’s characteristics is important because it can occasionally signal an issue requiring attention. Understanding the differences between non-traumatic, slow-growing masses and those arising from injury or infection is the first step in determining the need for professional care.
Common Non-Traumatic Causes
Many lumps that develop slowly on the side of the knee are benign soft-tissue growths unrelated to a recent injury. Lipomas are among the most frequent findings, representing non-cancerous tumors made entirely of mature fat cells. These growths are typically soft, rubbery, and highly mobile beneath the skin, often feeling doughy. Lipomas generally cause no pain unless they grow large enough to press on nearby nerves or are located in an area subject to constant friction.
Another common type of lump is a ganglion cyst, a fluid-filled sac arising from a joint capsule or tendon sheath. Unlike lipomas, ganglion cysts contain thick, gelatinous synovial fluid, making them feel firmer or rubbery upon palpation. While a classic Baker’s cyst is located at the back of the knee, related cysts can appear on the sides, often near a joint or tendon. Their mobility is usually more restricted than a lipoma’s due to their attachment to deeper structures.
Benign fibrous growths, such as fibromas, may also present as hard or firm lumps on the side of the knee. These masses originate from connective tissue and usually grow very slowly. A lump that is not painful and has remained unchanged for a long time often points toward one of these non-traumatic conditions.
Lumps Related to Injury and Inflammation
Lumps that appear quickly, are associated with pain, or feel warm often involve inflammation or trauma. Bursitis is a frequent cause of inflammatory swelling, resulting from the irritation of a bursa, a small sac that cushions tendons and bone. On the medial side, the pes anserine bursa is a common site for inflammation, causing pain and tenderness two to three inches below the joint line. This area may feel puffy or swollen as the bursa produces excess fluid.
Trauma, such as a direct blow or severe bruise, can result in a hematoma, a localized collection of blood outside the blood vessels. A hematoma typically presents as a firm, tender lump that may be discolored, appearing dark red or purple beneath the skin. This type of lump is correlated with a recent injury and may subside as the body reabsorbs the pooled blood.
The presence of an infection, such as an abscess or cellulitis, is indicated by a lump accompanied by specific, urgent signs. An abscess is a tender collection of pus, often signaling a bacterial infection; the surrounding skin will usually be red and distinctly warm. Infectious lumps require immediate medical attention, especially if accompanied by systemic symptoms like a fever or chills. The combination of localized heat, rapid swelling, and pain indicates an active, acute process.
Assessing the Lump and Knowing When to Seek Care
Evaluating a lump on the side of the knee involves checking several physical characteristics, including its size, texture, and mobility. A lump that is soft, easily movable under the skin, and has not changed in size over months or years is less likely to represent a serious issue. Conversely, a mass that feels hard and fixed to the underlying tissue, such as bone or deep muscle, warrants a prompt evaluation. Color changes, such as unexplained redness, or an area that feels significantly warmer than the surrounding skin should also be noted.
Warning Signs (Red Flags)
Specific warning signs indicate the need for immediate medical consultation. Seek urgent care if you notice any of the following:
- The lump is growing rapidly.
- You experience severe, unrelenting pain that disrupts sleep.
- You are unable to bear weight on the affected leg.
- Signs of systemic illness, including a persistent fever or chills, are present.
Diagnostic Tools
When consulting a medical professional, they will perform a physical examination to assess the lump’s characteristics and its relationship to underlying joint structures. The physician may utilize imaging techniques to gain a better understanding of the lump’s composition.
An ultrasound can distinguish between a solid mass and a fluid-filled sac, while an X-ray can identify any bony involvement or growths. These diagnostic steps help confirm the cause, ensure that benign conditions are monitored, and determine the appropriate management plan.