A bulge on the side of the knee often indicates inflammation or damage to the joint’s complex network of ligaments, tendons, or fluid-filled sacs. The specific location, texture, and presence of pain provide important clues about the underlying cause. Determining if the bulge is soft and fluid-filled, firm and structural, or a soft tissue growth helps narrow down the potential issue. This article explores common causes of lateral or medial knee bulges, but it is not a substitute for professional medical advice.
Swelling Caused by Inflamed Fluid Sacs
Soft, tender bulges around the knee are often caused by bursitis, which is the inflammation of a bursa. Bursae are small, lubricating fluid-filled sacs that cushion tendons, bones, and skin. When irritated by friction or pressure, these sacs can swell dramatically, and the location of the inflamed bursa determines where the bulge appears.
On the inner (medial) side of the knee, a common cause is Pes Anserine Bursitis. This produces soft, localized swelling a few inches below the joint line. The bursa is situated beneath the tendons of three muscles and often becomes inflamed due to overuse, particularly in runners or individuals with knee osteoarthritis. The bulge is typically tender, with pain that worsens when rising from a chair or climbing stairs.
On the outer (lateral) side, a similar condition is often associated with Iliotibial Band (ITB) friction syndrome. The IT band is a thick band of tissue running from the hip to the shinbone, and its repetitive movement can cause friction at the knee, inflaming a nearby bursa. The resulting bulge can be warm, tender, and associated with a burning pain that intensifies during activities like running or cycling.
Lumps Related to Meniscus and Tendons
When a bulge is firmer and located precisely at the joint line, it may indicate a structural issue within the knee, such as a Meniscal Cyst. These cysts form when a tear in the meniscus allows joint fluid to escape and collect in a localized pocket outside the joint capsule. They are often firm and non-mobile, typically appearing on the lateral side of the knee and are linked to an underlying meniscal tear.
The appearance of the cyst may change with movement, sometimes becoming more prominent as the knee straightens. Since the cyst is a secondary effect of a meniscal tear, the lump is often accompanied by mechanical symptoms like clicking, locking, or a sensation of the knee giving way.
Acute injuries to the collateral ligaments can also cause a localized bulge, which is usually a sign of swelling and bruising around the injured ligament tissue. A severe Medial Collateral Ligament (MCL) tear on the inner side can result in swelling and bruising directly over the ligament’s path. Similarly, an injury to the Lateral Collateral Ligament (LCL) on the outer side causes localized swelling and tenderness. These bulges are typically accompanied by immediate, significant pain and a feeling of instability following a specific traumatic event.
Soft Tissue Growths and Other Possibilities
Beyond fluid sacs and joint structures, some bulges are slow-growing masses originating from the surrounding soft tissue layers. Lipomas are the most common benign soft tissue tumors, consisting of a collection of mature fat cells. They present as soft, rubbery lumps just beneath the skin that are usually painless and easily movable with slight finger pressure.
Ganglion Cysts are another possibility, arising from a tendon sheath or joint capsule, and they contain a thick, jelly-like fluid. These can feel small and firm, and while typically non-tender, they may cause discomfort if they press on a nearby nerve or blood vessel. These masses generally develop slowly over time.
While rare, any persistent or growing lump requires professional evaluation to rule out more serious pathological masses, such as sarcomas. These malignant growths are often deep, fixed to surrounding tissues, and may grow rapidly, differentiating them from the slow-growing, mobile nature of a benign lipoma. The characteristics of the lump—its depth, mobility, and growth rate—are important factors in diagnosis.
Recognizing When to Seek Medical Attention
While many causes of a knee bulge are manageable with conservative care, certain signs indicate the need for prompt medical consultation. Urgent evaluation is required for:
- Any bulge accompanied by fever, chills, or significant redness and warmth, suggesting a possible infection.
- A sudden inability to bear weight on the affected leg or a visible deformity, which may indicate a fracture or severe ligament tear.
A bulge that grows rapidly over a few weeks or feels hard and fixed rather than soft and movable warrants timely medical review to investigate the possibility of a solid tumor. Mechanical symptoms, such as the knee locking, catching, or giving way, also require a professional assessment to check for meniscal tears or joint instability. For any persistent bulge that does not improve with rest and ice over several days, consulting a physician or physical therapist is necessary for an accurate diagnosis and treatment plan.