Knee Cartilage Cysts: Types, Causes, Symptoms, Diagnosis, and Treatment

What is a cyst on the knee cartilage?

A cyst on the knee cartilage is typically a benign, fluid-filled sac that forms in association with the menisci (cartilage pads) or joint capsule of the knee, often as a secondary result of underlying cartilage damage or tears.

Understanding Cysts in the Knee Joint

A cyst is generally defined as a closed sac-like structure, abnormal in the body, that contains fluid, air, or semi-solid material. In the context of the knee, these cysts are most commonly filled with synovial fluid, the lubricating fluid found within joints. While the term "on the knee cartilage" might imply a growth directly on the surface of the articular cartilage (the smooth covering on the ends of bones), these cysts are more often associated with tears or degeneration of the meniscal cartilages or are located adjacent to the joint capsule, sometimes in response to intra-articular pathology.

Anatomy of Knee Cartilage

To understand knee cysts, it's crucial to first grasp the basic anatomy of knee cartilage:

• Articular Cartilage (Hyaline Cartilage): This is the smooth, glistening white tissue that covers the ends of the femur (thigh bone), tibia (shin bone), and the back of the patella (kneecap). Its primary role is to reduce friction and absorb shock, allowing for smooth, pain-free movement of the joint. Cysts directly on this cartilage are rare; rather, subchondral cysts can form beneath it within the bone.
• Meniscal Cartilage (Fibrocartilage): The knee contains two C-shaped menisci (medial and lateral) located between the femur and tibia. These tough, rubbery cartilages act as shock absorbers, stabilize the joint, and help distribute weight evenly across the articular surfaces. Cysts are most commonly associated with these menisci.

Types of Cysts Associated with Knee Cartilage

Several types of cysts can occur in or around the knee, often linked to cartilage health:

• Meniscal Cysts: These are the most common type of cyst directly associated with knee cartilage. They typically form adjacent to a meniscal tear, especially a horizontal or complex tear. Synovial fluid leaks through the tear and collects in a sac outside the meniscus but within the joint capsule, forming a palpable lump. They are more common on the lateral (outer) side of the knee.
• Baker's Cyst (Popliteal Cyst): While not on cartilage, a Baker's cyst is a common fluid-filled sac that forms at the back of the knee (popliteal fossa). It's usually a symptom of an underlying knee problem, such as osteoarthritis, rheumatoid arthritis, or a meniscal tear, which causes excess synovial fluid production. The fluid builds up and pushes through the joint capsule, forming a bulge.
• Ganglion Cysts: These are benign, non-cancerous lumps that most commonly develop along the tendons or joints, including the knee. While not directly "on" cartilage, they can be found near the joint line or associated with joint capsules. Their exact cause is unknown but may involve trauma or repetitive stress.
• Subchondral Cysts: These cysts form within the bone directly beneath the articular cartilage. They are often seen in advanced osteoarthritis, where the cartilage has worn away, leading to bone-on-bone friction and subsequent bone remodeling and cyst formation.

Causes and Risk Factors

Cysts associated with knee cartilage are often a secondary condition, indicative of an underlying issue within the joint:

• Meniscal Tears: The primary cause of meniscal cysts. A tear in the meniscus can create a one-way valve effect, allowing synovial fluid to extrude and become trapped outside the meniscus.
• Degenerative Joint Disease (Osteoarthritis): The wear and tear of articular cartilage in osteoarthritis can lead to increased synovial fluid production (contributing to Baker's cysts) or the formation of subchondral cysts as the bone reacts to altered loading.
• Trauma or Injury: Acute injuries to the knee, such as a direct blow or twisting injury, can cause meniscal tears or other damage that predisposes to cyst formation.
• Inflammation: Chronic inflammation within the knee joint from conditions like rheumatoid arthritis can lead to excess fluid and cyst development.
• Repetitive Stress: Overuse or repetitive loading on the knee can contribute to the degeneration of cartilage over time, increasing the risk of tears and subsequent cysts.

Symptoms

The symptoms of a knee cartilage cyst can vary depending on its size, location, and the underlying cause:

• Pain: Often localized to the area of the cyst, which may worsen with activity, especially movements that load the affected cartilage.
• Swelling or Lump: A palpable, often firm, lump or bulge may be noticeable, particularly with meniscal cysts on the joint line or Baker's cysts at the back of the knee.
• Stiffness: Difficulty bending or straightening the knee.
• Limited Range of Motion: The cyst can mechanically impede full knee extension or flexion.
• Clicking, Popping, or Catching: These sensations may occur if the cyst or underlying meniscal tear interferes with joint mechanics.
• Weakness or Instability: Less common, but significant pain or mechanical interference can lead to perceived knee weakness or a feeling of "giving way."

Diagnosis

Accurate diagnosis is crucial for effective treatment and involves a combination of physical examination and imaging:

• Physical Examination: A healthcare professional will assess the knee for swelling, tenderness, range of motion, and stability. They may palpate for the presence of a mass and perform specific tests to evaluate meniscal integrity.
• Imaging Studies:
  • X-rays: Primarily used to rule out bony abnormalities, fractures, or significant joint space narrowing (indicating osteoarthritis). They do not directly visualize soft tissue cysts or cartilage.
  • Magnetic Resonance Imaging (MRI): The gold standard for diagnosing knee cysts and associated soft tissue pathology. An MRI provides detailed images of cartilage, menisci, ligaments, and fluid collections, clearly showing the size, location, and nature of the cyst and any underlying tears or degeneration.
  • Ultrasound: Can be used to confirm the fluid-filled nature of a palpable mass and distinguish it from a solid tumor. It's particularly useful for dynamic assessment of Baker's cysts.

Treatment Options

Treatment for a knee cartilage cyst depends on its type, size, symptoms, and the underlying cause.

• Conservative Management:
  • Rest, Ice, Compression, Elevation (RICE): To reduce pain and swelling, especially for acute symptoms.
  • Non-steroidal Anti-inflammatory Drugs (NSAIDs): Over-the-counter or prescription medications to manage pain and inflammation.
  • Physical Therapy: A structured rehabilitation program focusing on strengthening the muscles around the knee (quadriceps, hamstrings, glutes), improving flexibility, balance, and proprioception. This can help manage symptoms and improve joint mechanics.
  • Activity Modification: Avoiding activities that exacerbate pain or stress the knee.
  • Aspiration: Draining the fluid from the cyst with a needle. While it provides temporary relief, cysts often recur if the underlying cause (e.g., meniscal tear) is not addressed.
  • Corticosteroid Injections: May be injected into the joint or sometimes directly into the cyst to reduce inflammation and pain, but also often provide temporary relief.
• Surgical Intervention:
  • Arthroscopic Cyst Excision: For symptomatic meniscal cysts, arthroscopic surgery is often performed to remove the cyst and, critically, address the underlying meniscal tear. This involves repairing or trimming the torn meniscus to prevent recurrence of the cyst.
  • Treatment of Underlying Pathology: For Baker's cysts, surgical intervention is rarely aimed at the cyst itself unless it's very large and symptomatic. Instead, the focus is on treating the primary problem in the knee (e.g., arthroscopic debridement for severe osteoarthritis, meniscal repair).

Rehabilitation and Management

Following conservative or surgical treatment, rehabilitation plays a crucial role in recovery and preventing recurrence:

• Progressive Strengthening: Focus on strengthening the muscles surrounding the knee, particularly the quadriceps, hamstrings, and gluteal muscles, to improve joint stability and support.
• Range of Motion and Flexibility: Restore full, pain-free movement through targeted stretches and mobilization exercises.
• Proprioception and Balance Training: Exercises that challenge balance and body awareness help to improve neuromuscular control around the knee, reducing the risk of re-injury.
• Gradual Return to Activity: A carefully phased return to sports or high-impact activities, ensuring proper biomechanics and avoiding excessive stress on the healing joint.
• Weight Management: Maintaining a healthy body weight reduces stress on the knee joint, which can be particularly beneficial for those with osteoarthritis.
• Proper Footwear and Biomechanics: Addressing gait abnormalities or improper movement patterns can help prevent future issues.

When to Seek Medical Attention

It's important to consult a healthcare professional if you experience:

• Persistent knee pain or swelling that doesn't improve with rest.
• A noticeable lump or bulge around the knee.
• Difficulty bearing weight on the affected leg.
• A feeling of the knee "locking," "giving way," or persistent instability.
• Sudden, severe pain or swelling following an injury.

Conclusion

A cyst on the knee cartilage is typically a benign, fluid-filled sac that forms as a response to an underlying issue within the knee joint, most commonly a meniscal tear or degenerative changes. While often symptomatic, accurate diagnosis through physical examination and advanced imaging like MRI is key to identifying the specific type of cyst and its root cause. Treatment strategies range from conservative management aimed at symptom relief and rehabilitation to surgical intervention that addresses both the cyst and the predisposing joint pathology. Understanding the nature of these cysts is vital for effective management and maintaining optimal knee health.