Knee Cysts: Types, Symptoms, Diagnosis, and Treatment

What are the different types of knee cysts?

Knee cysts are fluid-filled sacs that can develop around the knee joint, often as a result of underlying knee conditions or injury, with the most common type being a Baker's cyst.

Understanding Knee Cysts

A knee cyst is essentially a sac filled with synovial fluid, the lubricating fluid found within joints. These cysts are not true tumors but rather benign swellings that can form due to increased pressure or fluid accumulation within the joint, often in response to inflammation, injury, or degenerative conditions like arthritis. While some cysts may be asymptomatic, others can cause pain, swelling, and restrict movement, depending on their size and location.

Common Types of Knee Cysts

While various fluid collections can occur around the knee, a few specific types are most commonly identified:

Baker's Cyst (Popliteal Cyst)

Description: This is the most prevalent type of knee cyst, characterized by a fluid-filled lump that forms at the back of the knee, in the popliteal fossa. It occurs when excess synovial fluid from the knee joint is pushed into a bursa (a small, fluid-filled sac that reduces friction between tissues) located behind the knee, causing it to swell. Causes: Baker's cysts are almost always secondary to an underlying knee problem that causes increased synovial fluid production. Common culprits include:

• Osteoarthritis or rheumatoid arthritis
• Meniscal tears
• Ligament injuries
• Inflammation of the knee joint (synovitis) Symptoms: May include a palpable lump, tightness, pain, and stiffness at the back of the knee, especially during knee flexion or extension. In rare cases, a Baker's cyst can rupture, leading to sharp pain, swelling, and bruising in the calf.

Ganglion Cyst

Description: Ganglion cysts are non-cancerous lumps that typically form along tendons or joints, including the knee. Unlike Baker's cysts, which are usually posterior, ganglion cysts around the knee can appear on the anterior (front) or lateral (side) aspects. They are filled with a thick, jelly-like fluid. Causes: The exact cause is often unknown, but they are thought to arise from joint capsule or tendon sheath degeneration, or as a result of trauma that causes tissue irritation. Symptoms: Often asymptomatic, but can cause pain, tingling, or weakness if they press on a nerve. They are usually firm, movable, and vary in size.

Meniscal Cyst

Description: A meniscal cyst is a fluid collection that develops directly adjacent to a torn meniscus within the knee joint. The synovial fluid escapes through the tear in the meniscus and collects in the surrounding soft tissues, forming a cyst. Causes: These cysts are directly associated with meniscal tears, particularly horizontal or complex tears that allow fluid to egress. Symptoms: Typically presents as a localized, firm swelling or lump along the joint line (often on the outer or inner side of the knee), accompanied by pain that may worsen with activity. The pain and swelling often correlate with the underlying meniscal pathology.

Cystic Lesions of the Cruciate Ligaments

Description: These are relatively rare intra-articular cysts that form within or adjacent to the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) inside the knee joint. They are typically small and often discovered incidentally during MRI scans. Causes: The exact etiology is unclear, but they are thought to arise from synovial proliferation or mucoid degeneration of the ligament. Symptoms: Many are asymptomatic. However, if they grow large enough, they can cause symptoms such as knee pain, clicking, or limited range of motion, particularly if they impinge on other structures.

Synovial Cysts (General)

Description: This is a broader term for any fluid-filled sac lined with synovium that communicates with a joint space. A Baker's cyst is a specific type of synovial cyst, but other synovial cysts can form in less common locations around the knee joint. Causes: Similar to Baker's cysts, they are often secondary to underlying joint pathology leading to increased synovial fluid and pressure. Symptoms: Depend on size and location, ranging from asymptomatic to causing localized pain, swelling, or mechanical symptoms.

Symptoms of Knee Cysts

While specific symptoms can vary by type and size, general indicators of a knee cyst may include:

• Visible or palpable lump: Often felt as a soft, fluid-filled lump around the knee.
• Pain: Localized pain, especially with activity or prolonged standing.
• Stiffness: Difficulty bending or straightening the knee.
• Swelling: General swelling around the knee joint.
• Limited range of motion: Due to pain, stiffness, or the physical presence of the cyst.
• Catching or locking sensation: Less common, but possible if the cyst interferes with joint mechanics.

Diagnosis of Knee Cysts

Diagnosis typically involves a combination of:

• Physical Examination: A healthcare professional will examine the knee, palpate the cyst, and assess the range of motion and stability.
• Imaging Studies:
  • Ultrasound: Often the first-line imaging for cysts, as it can confirm the fluid-filled nature of the lump and differentiate it from solid masses.
  • Magnetic Resonance Imaging (MRI): Provides detailed images of the knee's soft tissues, allowing for precise identification of the cyst type, size, location, and crucially, any underlying joint pathology (e.g., meniscal tears, arthritis) that may be causing the cyst.
  • X-rays: While X-rays don't show soft tissue cysts, they can help identify underlying bone conditions like arthritis.

Treatment Approaches for Knee Cysts

Treatment for knee cysts primarily focuses on addressing the underlying cause rather than just the cyst itself.

• Conservative Management:
  • Rest, Ice, Compression, Elevation (RICE): To reduce pain and swelling.
  • Non-steroidal Anti-inflammatory Drugs (NSAIDs): To manage pain and inflammation.
  • Physical Therapy: To strengthen surrounding muscles, improve knee mechanics, and reduce stress on the joint.
  • Aspiration: Draining the fluid from the cyst with a needle, often followed by a corticosteroid injection to reduce inflammation. This provides temporary relief, but the cyst may recur if the underlying cause isn't addressed.
• Addressing the Underlying Cause: Treating conditions like arthritis or repairing meniscal tears can often resolve the cyst.
• Surgical Intervention: Rarely, surgical removal of the cyst may be considered if it is very large, causing significant symptoms, or if conservative treatments fail. This is more common for meniscal cysts where the underlying meniscal tear is also repaired.

When to Seek Medical Attention

It's important to consult a healthcare professional if you notice a new lump around your knee, experience persistent pain, swelling, or limited mobility. While most knee cysts are benign, a proper diagnosis is essential to rule out other conditions and to identify any underlying problems that may require treatment.

Conclusion

Knee cysts are common, often benign, fluid-filled sacs that can develop around the knee joint. While Baker's cysts are the most frequently encountered, other types like ganglion and meniscal cysts can also occur, each with distinct characteristics and associations. Understanding the different types is crucial, but more importantly, recognizing that these cysts are frequently a symptom of an underlying knee issue – such as arthritis or a meniscal tear – underscores the importance of a thorough medical evaluation for accurate diagnosis and effective management.