The knee is a complex, weight-bearing hinge joint where the thigh bone (femur) meets the shin bone (tibia), stabilized by strong ligaments and muscles. When people describe their knee “popping out,” they are usually referring to a sudden, painful event that causes the joint to feel unstable or visually deformed. Understanding the specific structures involved helps inform the correct response to this common concern.
What Happens When the Knee “Pops Out”?
The feeling of a knee “pop out” typically describes one of two distinct injuries, which differ significantly in frequency and severity. The most common event is a patellar subluxation or dislocation, which involves the kneecap (patella). Patellar dislocation occurs when the kneecap slides completely out of the shallow groove at the end of the femur, known as the trochlear groove, most often moving to the outside of the leg. A subluxation is a partial or temporary slippage, where the patella only briefly moves out of the groove before spontaneously returning to its normal position.
A far more serious and rare event is a tibiofemoral dislocation, often called a true knee dislocation. This injury involves the complete separation of the thigh bone from the shin bone at the main knee joint, usually resulting from high-energy trauma, such as a motor vehicle accident or significant sports impact. Because major ligaments, including the anterior and posterior cruciate ligaments, are often torn, a true knee dislocation is a medical emergency with a high risk of nerve or arterial damage to the lower leg. Patellar dislocations, while painful, are much more frequent and predominantly affect young athletes.
Why Some People Are More Susceptible
A person’s underlying anatomy and history can increase their risk of experiencing patellar instability. One factor is the shape of the trochlear groove, the V-shaped channel on the femur where the kneecap tracks. If this groove is too shallow or flat, a condition called trochlear dysplasia, the patella has less bony guidance and is more likely to slide out of place. Another anatomical risk is patella alta, where the kneecap sits higher than normal on the thigh bone, reducing the stabilizing contact with the groove during initial knee bending.
The alignment of the leg bones also plays a role, with an increased angle between the thigh and shin bones, often seen in individuals with “knock knees,” placing greater lateral force on the patella. Weakness or imbalance in the muscles surrounding the knee, particularly the vastus medialis oblique (VMO)—the inner part of the quadriceps—can contribute to poor patellar tracking. Furthermore, a previous patellar dislocation stretches the medial patellofemoral ligament (MPFL), the primary soft tissue restraint, which significantly increases the chance of recurrence.
Immediate Care After a Knee Injury
If a knee injury occurs and the joint feels unstable or appears deformed, stop all activity and avoid attempting to move or straighten the limb. Never try to manually force a dislocated joint back into place, as this can cause severe damage to nerves, blood vessels, or cartilage. Immediate first aid should focus on the RICE protocol: Rest, Ice, Compression, and Elevation.
Rest involves immobilizing the injured area and avoiding any weight-bearing activity, often requiring crutches. Applying a cold pack wrapped in a thin towel to the knee for 15 to 20 minutes every few hours helps reduce pain and swelling. Compression with an elastic bandage minimizes swelling, but it must be wrapped snugly without causing numbness or tingling. Elevating the knee above the level of the heart helps gravity drain excess fluid from the injury site.
Any suspected dislocation, even one that appears to have spontaneously reduced, requires prompt medical evaluation to assess for associated ligament, cartilage, or bone damage.
Preventing Future Instability
For individuals who have experienced knee instability, the long-term strategy for preventing recurrence centers on rehabilitation and muscle strengthening. Physical therapy is often recommended to restore proper mechanics and build dynamic support around the joint. A primary focus is strengthening the quadriceps, especially the VMO, which helps ensure the kneecap tracks correctly within its groove.
Strengthening the hip abductor and gluteal muscles is also important, as these muscles control the alignment of the leg during movement and reduce stress on the knee joint. Exercises like single-leg squats, step-ups, and lateral band walks help improve muscle balance and coordination. In cases of recurrent instability or significant anatomical abnormalities, surgical procedures may be considered to repair damaged ligaments or correct underlying bone alignment issues.