When the knee joint hurts upon full extension, this symptom is often referred to as a “locked” knee. This describes pain experienced when the leg is fully straightened or pushed to the end of its range of motion. A more serious “true lock” occurs when a physical object prevents the knee from moving at all. Understanding the difference between a mechanical obstruction and pain caused by soft tissue strain is the first step toward finding relief.
Understanding Mechanical Obstruction
A true locked knee occurs when a physical structure within the joint acts like a doorstop, preventing the leg from fully straightening. The most common cause is a torn meniscus. A specific type of injury called a bucket-handle tear is notorious for causing this mechanical blockage. A large fragment of the meniscus rips loose and flips into the joint space, physically jamming the knee.
Another cause of mechanical locking is the presence of loose bodies, which are small fragments of bone or cartilage floating within the joint fluid. These fragments, sometimes called joint mice, can result from trauma, injury, or degenerative conditions like osteoarthritis. When the knee moves, a loose body can momentarily get caught between the femur and tibia bones. This causes a sharp, painful catch that limits the final extension of the joint. Because these obstructions can lead to further damage, they often require professional intervention.
Alignment and Soft Tissue Strain
Pain upon full extension can also arise from issues not involving a physical blockage, often referred to as pseudo-locking. Patellofemoral Pain Syndrome is a frequent culprit where the kneecap (patella) does not track correctly within its groove on the thigh bone. When the knee is extended, this misalignment increases friction and pressure on the cartilage surface beneath the kneecap, causing pain. This pain is often felt under or around the kneecap and is aggravated by activities like squatting or prolonged sitting.
Soft tissue tightness in the surrounding muscle groups also contributes to strain upon locking. Tightness in the hamstrings, quadriceps, or the iliotibial (IT) band can pull the knee joint out of optimal alignment. When the leg attempts full extension, the increased tension from these tight muscles causes compressive forces across the joint surfaces. This chronic strain can lead to inflammation, making the final degrees of straightening the leg painful. Addressing these muscular imbalances is a primary focus of non-surgical treatment.
When to Seek Professional Diagnosis and Treatment
Seeking Immediate Care
Certain symptoms are considered red flags and require immediate medical attention to prevent further damage. If the pain began with a loud popping sound, if you cannot bear weight on the leg, or if the knee appears visibly deformed, seek urgent care. A true lock where the knee is completely stuck and cannot be unlocked also warrants an immediate evaluation. These signs suggest a severe structural injury, such as a large meniscal tear or a fracture.
Diagnosis
For less severe or chronic pain, scheduling an appointment with a healthcare provider is appropriate. Diagnosis typically begins with a physical examination to test the joint’s stability and range of motion. Imaging tests like X-rays check for bone spurs, alignment issues, or loose bodies containing bone fragments. Magnetic Resonance Imaging (MRI) is the preferred tool for visualizing soft tissues like the menisci and ligaments to confirm a tear.
Treatment Options
Treatment starts with conservative management for most issues. Non-surgical options include the RICE protocol (Rest, Ice, Compression, Elevation) to manage initial swelling and pain, along with nonsteroidal anti-inflammatory drugs (NSAIDs). Physical therapy is a cornerstone of recovery, focusing on strengthening the muscles around the hip and knee to correct tracking issues and improve stability. Specific exercises help restore full range of motion.
In cases of confirmed mechanical obstruction, such as a large displaced meniscal tear or a persistently symptomatic loose body, a surgeon may recommend arthroscopic surgery. This minimally invasive procedure involves small incisions to allow the surgeon to view the joint and either repair the torn cartilage or remove the physical obstruction. Surgery is generally considered after conservative treatments have failed to resolve the symptoms.