Locked Knee: Causes, Diagnosis, and Treatment

What is the most common cause of a locked knee?

The most common cause of a true locked knee, characterized by an inability to fully straighten the leg, is a meniscal tear, where a torn piece of cartilage obstructs the knee joint's normal movement.

Understanding the "Locked Knee" Phenomenon

The sensation of a "locked knee" can be alarming and debilitating. It refers to a sudden inability to fully extend or flex the knee joint, as if something is physically blocking its movement. This mechanical blockage differs from a knee that feels stiff or painful but can still be moved through its full range of motion. A true knee lock implies a structural impediment within the joint.

The Most Common Culprit: Meniscal Tears

The most frequent cause of a true mechanical locked knee is a meniscal tear. The menisci are two C-shaped pieces of tough, rubbery cartilage (medial and lateral menisci) located between your thigh bone (femur) and shin bone (tibia). They act as shock absorbers, stabilize the knee joint, and help distribute weight evenly across the joint surfaces.

Meniscal tears commonly occur due to:

• Acute trauma: Often from a twisting injury to the knee, especially when the foot is planted and the body rotates, or from a deep squat. This is common in sports like football, basketball, and skiing.
• Degeneration: In older individuals, the menisci can weaken and fray over time, making them more susceptible to tears from relatively minor movements or even normal daily activities.

While various types of meniscal tears exist (e.g., radial, longitudinal, flap), the type most notorious for causing true locking is a bucket-handle tear. In this specific tear, a large, often U-shaped fragment of the meniscus detaches but remains connected at both ends, resembling the handle of a bucket.

How Meniscal Tears Lead to Locking

When a bucket-handle tear, or any sufficiently large and mobile meniscal fragment, occurs, the torn piece of cartilage can displace and become wedged between the femur and tibia. This physical interposition directly obstructs the smooth gliding and rolling motion necessary for the knee to fully straighten (extend) or bend (flex). The knee literally "locks" in a partially bent position, and no amount of effort or manipulation can fully straighten it without the fragment moving out of the way. This can sometimes be accompanied by an audible pop or click as the fragment shifts.

Other Potential Causes of Knee Locking

While meniscal tears are the most common cause of true mechanical locking, other conditions can also lead to this symptom:

• Loose Bodies (Joint Mice): These are fragments of bone or cartilage that have broken off within the knee joint due to trauma, degenerative conditions like osteoarthritis, or osteochondritis dissecans. These free-floating fragments can occasionally get caught between the joint surfaces, causing sudden, intermittent locking.
• Anterior Cruciate Ligament (ACL) Tears: While primarily causing instability, a large fragment of a torn ACL can sometimes become impinged within the joint, leading to mechanical blockage. However, true locking is less common than with meniscal tears, and ACL tears more often cause a sensation of "giving way."
• Patellar Dislocation or Subluxation: If the kneecap (patella) slips out of its groove (dislocation) or partially slips (subluxation), it can cause the knee to feel locked until the patella relocates back into its proper position. This is often associated with a feeling of the knee "buckling."
• Plica Syndrome: The plica are normal folds in the synovial membrane (the lining of the joint capsule). If a plica becomes inflamed, thickened, or irritated (often due to repetitive motion or trauma), it can get caught between the bones, leading to a catching sensation or, less commonly, true locking.
• Advanced Osteoarthritis: In severe cases of knee osteoarthritis, the extensive loss of cartilage, combined with the formation of bone spurs (osteophytes), can lead to mechanical impingement and a locked sensation, particularly if fragments of cartilage or bone break off.

Differentiating True Locking from Pseudolocking

It is crucial to distinguish between true mechanical locking and pseudolocking or "giving way."

• True Mechanical Locking: Involves a physical obstruction that prevents full range of motion. It is typically sudden, reproducible, and often requires specific manipulation or rest for the knee to unlock.
• Pseudolocking (or "Giving Way"): This is a more common sensation where the knee feels like it's catching, buckling, or giving out due to pain, muscle spasm, or reflex inhibition. The knee can usually be straightened with effort, though it may be painful. Conditions like patellofemoral pain syndrome, IT band syndrome, or general knee instability can cause this sensation.

Diagnosis and When to Seek Medical Attention

If you experience a true locked knee, it is imperative to seek prompt medical attention. A healthcare professional, such as an orthopedic surgeon or sports medicine physician, will conduct a thorough evaluation, which typically includes:

• Detailed History: Asking about the mechanism of injury, onset of symptoms, and any accompanying sounds or sensations.
• Physical Examination: Assessing your knee's range of motion, stability, tenderness, and performing specific tests (e.g., McMurray's test for meniscal tears) to pinpoint the source of the problem.
• Imaging Studies:
  • X-rays: Primarily used to rule out bone fractures, dislocations, or advanced arthritis.
  • Magnetic Resonance Imaging (MRI): The gold standard for visualizing soft tissue structures like menisci, ligaments, and cartilage. An MRI can definitively diagnose meniscal tears and identify other potential causes of locking.
• Arthroscopy: In some cases, a minimally invasive surgical procedure called arthroscopy may be performed. This allows the surgeon to directly visualize the inside of the joint, confirm the diagnosis, and often treat the problem simultaneously.

Treatment Approaches

Treatment for a locked knee depends entirely on the underlying cause:

• Meniscal Tears:
  • Conservative Management: For small, stable tears, especially in areas with good blood supply, rest, ice, compression, elevation (RICE), non-steroidal anti-inflammatory drugs (NSAIDs), and physical therapy may be sufficient.
  • Surgical Intervention: For tears causing persistent locking, significant pain, or those in areas with poor healing potential (e.g., bucket-handle tears), arthroscopic surgery is often recommended. This may involve meniscal repair (stitching the torn pieces back together) or partial meniscectomy (removing only the torn, problematic fragment).
• Loose Bodies: Arthroscopic surgery is typically performed to remove the loose fragments.
• Patellar Instability: Treatment can range from physical therapy to strengthen surrounding muscles to surgical procedures to stabilize the kneecap.
• Plica Syndrome: Initial treatment usually involves rest, ice, NSAIDs, and physical therapy. In persistent cases, corticosteroid injections or arthroscopic plica excision may be considered.

Prevention and Management

While not all causes of knee locking are preventable, certain strategies can reduce your risk:

• Proper Warm-up and Cool-down: Prepare your muscles and joints for activity and aid recovery.
• Strength Training: Focus on strengthening the muscles around the knee, especially quadriceps, hamstrings, and glutes, to improve joint stability.
• Proprioception Training: Exercises that challenge balance and coordination can enhance your body's awareness of joint position, reducing the risk of awkward movements.
• Gradual Progression: Increase the intensity and duration of your workouts gradually to allow your body to adapt.
• Proper Technique: Learn and use correct form for exercises and sports-specific movements to minimize undue stress on the knee.
• Listen to Your Body: Do not ignore persistent knee pain, clicking, or catching sensations. Early intervention can prevent more severe issues.

Conclusion

A true locked knee is a significant symptom that warrants prompt medical evaluation. While other conditions can cause a similar sensation, a meniscal tear, particularly a bucket-handle tear, stands out as the most common mechanical culprit. Understanding the anatomy, biomechanics, and various potential causes of knee locking is crucial for accurate diagnosis and effective management, ensuring a return to optimal knee function and overall well-being.