Mass Behind the Knee: Causes, Symptoms, Diagnosis, and Treatment

What is a mass behind the knee?

A mass behind the knee, located in the popliteal fossa, can stem from various causes ranging from benign fluid-filled cysts to vascular issues or, less commonly, tumors, and often warrants medical evaluation for accurate diagnosis.

Introduction to the Popliteal Fossa

The area behind the knee, anatomically known as the popliteal fossa, is a diamond-shaped depression that houses vital structures, including major nerves (tibial and common fibular nerves), blood vessels (popliteal artery and vein), and lymph nodes. Given its complex anatomy, any palpable mass in this region demands attention. From an exercise science perspective, understanding the structures within and around this area is crucial for identifying potential sources of discomfort or dysfunction that may manifest as a "mass."

Common Causes of a Mass Behind the Knee

A variety of conditions can lead to the formation of a palpable mass in the popliteal fossa. These range significantly in their severity and required treatment.

• Baker's Cyst (Popliteal Cyst) This is arguably the most common cause of a mass behind the knee. A Baker's cyst is not a true cyst but rather a distension of the gastrocnemius-semimembranosus bursa, which communicates with the knee joint. It typically arises secondary to an underlying knee joint problem, such as:

  • Osteoarthritis or rheumatoid arthritis
  • Meniscus tears
  • Ligament injuries
  • Inflammation of the knee joint (synovitis) Symptoms often include a sensation of tightness or fullness behind the knee, pain (especially with knee extension or flexion), and palpable swelling. If the cyst ruptures, it can cause sudden, sharp pain, swelling, and redness in the calf, mimicking a deep vein thrombosis (DVT).

• Ganglion Cyst While more common around the wrist or ankle, ganglion cysts can also occur in the popliteal fossa. These are benign, fluid-filled sacs that develop from a joint capsule or tendon sheath. They are typically firm, non-tender, and may fluctuate in size. Unlike Baker's cysts, they do not communicate with the knee joint.

• Lipoma A lipoma is a benign tumor composed of fatty tissue. These are generally soft, rubbery, movable, and usually painless masses that grow slowly. They can occur anywhere on the body, including behind the knee.

• Varicose Veins Enlarged, twisted veins can sometimes be felt as a mass, particularly if they are superficial. They may cause aching, heaviness, or discomfort, especially after prolonged standing. While often visible, deeper varicose veins can present as a palpable lump.

• Popliteal Artery Aneurysm This is a serious condition involving a weakened, bulging section of the popliteal artery. It is the most common peripheral artery aneurysm. A key differentiating feature is that an aneurysm will often be pulsatile (you can feel a pulse within it). Symptoms can include pain, swelling, and, critically, signs of limb ischemia (reduced blood flow) if a clot forms within the aneurysm or breaks off. Due to the risk of rupture or clot formation leading to limb loss, prompt medical evaluation is crucial.

• Tendonitis or Bursitis Inflammation of tendons (e.g., hamstrings like the semimembranosus or semitendinosus) or bursae (e.g., the semimembranosus bursa) can cause localized swelling and tenderness that may feel like a mass. This is often associated with overuse or repetitive strain, common in athletes or individuals engaging in activities involving frequent knee flexion and extension.

• Tumors (Rare) Although rare, both benign (e.g., fibroma, schwannoma) and malignant tumors (e.g., sarcoma) can develop in the soft tissues or bone around the popliteal fossa. These are typically less common but must be considered, especially if the mass is rapidly growing, painful, fixed, or associated with other systemic symptoms.

Symptoms and When to Seek Medical Attention

The symptoms accompanying a mass behind the knee can vary widely depending on the underlying cause. Common symptoms include:

• Pain: Localized, radiating, or generalized knee pain.
• Swelling: Visible or palpable enlargement.
• Stiffness: Difficulty bending or straightening the knee.
• Limited Range of Motion: Inability to fully flex or extend the knee.
• Warmth or Redness: Indicating inflammation or infection.
• Numbness or Tingling: Suggesting nerve compression.
• Pulsatile Sensation: A throbbing feeling, characteristic of vascular issues.
• Changes in Skin Color: Discoloration, especially if vascular compromise is present.
• Fever: May indicate infection.

It is imperative to seek medical attention if you experience any of the following:

• Sudden onset of a mass with severe pain.
• Rapid growth or change in the size or nature of the mass.
• A mass that is pulsatile or feels like it has a heartbeat.
• Numbness, tingling, or weakness in the lower leg or foot.
• Changes in skin color or temperature of the lower leg.
• Fever, chills, or unexplained weight loss.
• A mass that is fixed, hard, or tender to the touch.
• Any mass that interferes with daily activities or causes persistent discomfort.

Diagnosis and Treatment Overview

Accurate diagnosis of a mass behind the knee requires a thorough medical evaluation.

• Physical Examination: A healthcare professional will assess the size, consistency, mobility, tenderness, and pulsation of the mass, along with a comprehensive knee examination.
• Imaging Studies:
  • Ultrasound: Often the first-line imaging, excellent for differentiating fluid-filled cysts from solid masses and evaluating vascular structures (e.g., popliteal artery aneurysm, DVT).
  • MRI (Magnetic Resonance Imaging): Provides detailed images of soft tissues, useful for characterizing cysts, tumors, and underlying joint pathology.
  • X-ray: Primarily used to rule out bony abnormalities or arthritis, though it won't visualize soft tissue masses directly.
  • CT Scan: May be used for more detailed cross-sectional imaging, especially if bony involvement or complex vascular issues are suspected.
• Blood Tests: May be ordered if infection or inflammatory conditions are suspected.
• Aspiration: In some cases, fluid may be drawn from a cyst for analysis.
• Biopsy: For solid masses suspected to be tumors, a biopsy may be necessary for definitive diagnosis.

Treatment is entirely dependent on the underlying cause. It can range from conservative measures (rest, ice, compression, elevation - RICE, physical therapy, anti-inflammatory medications) for inflammatory conditions or small, asymptomatic benign masses, to aspiration, injection, or surgical intervention for larger cysts, aneurysms, or tumors.

Prevention and Management Strategies (Fitness Perspective)

While you cannot prevent all causes of a mass behind the knee, maintaining optimal knee health can reduce the risk of conditions like Baker's cysts, tendonitis, and bursitis, which often stem from overuse or underlying joint issues.

• Maintain a Healthy Weight: Excess body weight places increased stress on the knee joints, contributing to wear and tear that can lead to arthritis and subsequent Baker's cysts.
• Regular, Low-Impact Exercise: Engage in activities that strengthen the muscles supporting the knee (quadriceps, hamstrings, glutes, calves) without excessive impact. Examples include swimming, cycling, elliptical training, and brisk walking.
• Proper Warm-up and Cool-down: Always begin workouts with a dynamic warm-up to prepare muscles and joints, and conclude with a static cool-down to improve flexibility and aid recovery.
• Stretching and Flexibility: Regularly stretch the hamstrings, quadriceps, and calf muscles to maintain good range of motion and reduce muscle tightness that can contribute to tendonitis or bursitis.
• Strength Training: Incorporate exercises that promote balanced strength in the muscles around the knee. Focus on controlled movements and proper form to avoid placing undue stress on the joint.
• Listen to Your Body: Avoid pushing through pain. If an activity causes discomfort, modify it or take a rest day. Overtraining can exacerbate existing conditions or lead to new ones.
• Appropriate Footwear: Wear supportive shoes that provide adequate cushioning and stability, especially during physical activity, to help absorb shock and maintain proper biomechanics.

Conclusion

A mass behind the knee is a symptom, not a diagnosis, with a broad spectrum of potential causes. While many are benign, some can indicate serious underlying conditions requiring urgent medical attention. As an active individual, recognizing the potential sources of such a mass and understanding the importance of prompt, professional evaluation is paramount for accurate diagnosis and effective management, ensuring the long-term health and function of your knee joint.