When your knee “pops out of place,” your kneecap (patella) has slipped out of the groove it normally sits in at the front of your knee joint. This is called a patellar dislocation, and it’s one of the most common knee injuries, especially in young, active people. The kneecap almost always shifts toward the outside of the leg, stretching or tearing the soft tissues that normally hold it centered. Sometimes it slides back on its own within seconds. Other times it stays visibly displaced until a medical professional pushes it back.
Dislocation vs. Subluxation
There’s an important distinction between the two things people describe as a knee “popping out.” In a full patellar dislocation, the kneecap gets pushed completely out of its groove. You can often see or feel it sitting off to the side of your knee, and the joint locks up. In a subluxation, the kneecap only slides partway out before slipping back. Subluxations are less dramatic but still painful, and they signal that the knee is unstable. People with chronic patellar instability often experience repeated subluxations, where the kneecap shifts and catches with certain movements but never fully displaces.
What It Feels and Looks Like
A patellar dislocation is hard to miss. Common symptoms include an audible pop at the moment it happens, intense knee pain, and visible displacement of the kneecap. Your knee will swell quickly as fluid rushes into the joint, and bruising typically follows within hours.
Functionally, the knee becomes unreliable. It will either lock in place, unable to straighten or bend, or it will catch and pop when you try to move it. The joint buckles when you try to stand on it, and walking is usually impossible until the kneecap is back in position. Even after it’s relocated, the knee feels loose and unstable for days or weeks.
What Gets Damaged Inside
The pop itself is just the beginning. When the kneecap jumps out of its groove and snaps back, it damages the soft tissues around it. The structure most commonly injured is a ligament on the inner side of the knee that acts like a leash, restraining the kneecap from sliding outward. When the kneecap dislocates toward the outside of the leg, this ligament gets stretched or torn. That torn restraint is the main reason dislocations tend to happen again.
The cartilage lining the back of the kneecap and the groove it sits in can also be damaged. Small pieces of cartilage or bone sometimes break off during the dislocation, creating loose fragments inside the joint. These fragments can cause catching, locking, and further cartilage wear if they aren’t addressed.
True Knee Dislocation: A Different Injury Entirely
It’s worth knowing that a “dislocated knee” can refer to something far more serious than a kneecap slipping out. A tibiofemoral dislocation happens when the two main bones of the leg, the thighbone and shinbone, separate at the knee joint. This is a surgical emergency caused by severe trauma like a car accident or a major fall. Up to 40 percent of patients with this type of dislocation sustain damage to the major artery behind the knee, which can threaten the entire leg if not repaired quickly. If you’ve been in a high-force accident and your knee looks deformed, you can’t feel your foot, or the lower leg turns pale or cold, that requires immediate emergency care.
What Happens at the Hospital
If the kneecap hasn’t slid back on its own, a doctor will manually guide it back into the groove, a procedure called reduction. It’s quick but uncomfortable. After that, imaging helps assess the damage. X-rays are taken first to check for fractures and confirm the kneecap is properly seated. An MRI often follows to get a detailed look at the torn ligament, cartilage damage, and any loose fragments floating in the joint. These findings determine whether you’ll need surgery or can heal with bracing and rehab alone.
Recovery Without Surgery
Most first-time patellar dislocations are treated conservatively. Your knee will be placed in a brace, splint, or cast for several weeks to let the torn tissues begin healing. Physical therapy follows, focusing on strengthening the muscles around the knee, particularly the inner quadriceps, which help keep the kneecap tracking properly.
Most people can return to normal activities within 6 to 8 weeks. Returning to sports takes longer and requires meeting specific benchmarks: you should be able to run, jump, and cut at angles on the injured leg without pain or limping, and the strength in your injured leg should match your uninjured side. Rushing back before meeting those markers raises the risk of re-injury significantly.
When Surgery Becomes the Better Option
Surgery is typically considered when loose cartilage or bone fragments are floating in the joint, when the ligament restraining the kneecap is completely torn, or when dislocations keep recurring. The most common procedure reconstructs the torn inner ligament to restore the kneecap’s stability. Interestingly, a large Cochrane review found that the evidence comparing surgery to non-surgical treatment after a first dislocation is still uncertain. No large, high-quality trial has definitively shown that early surgery produces better long-term outcomes than rehabilitation alone. This means the decision often depends on your individual anatomy, activity level, and how many risk factors you have for it happening again.
Chances It Happens Again
Recurrence is the central concern after a first patellar dislocation. Overall, the re-dislocation rate is estimated at 15 to 44 percent, with roughly one in three patients experiencing another episode. But that number varies enormously based on individual risk factors like age (younger patients dislocate more often), the shape of the groove the kneecap sits in, and how lax the surrounding ligaments are.
When no anatomical risk factors are present, the chance of another dislocation drops to about 8 to 14 percent. With two risk factors, it jumps to 29 to 60 percent. With three, the re-dislocation rate reaches 70 to 78 percent. This is why imaging and a thorough evaluation after the first event matter so much. Knowing your specific risk profile helps determine whether aggressive rehabilitation is enough or whether surgery to stabilize the kneecap makes more sense upfront.
Long-Term Effects on the Joint
Even a single dislocation leaves its mark. Cartilage damage to the kneecap and its groove is present in the majority of patients after a dislocation, and that damage gets progressively worse with each additional episode. Injured cartilage can partially recover, but early degradation tends to progress over time. Within 25 years of the initial injury, roughly half of patients develop arthritis in the patellofemoral joint, the area where the kneecap meets the thighbone. Patients who experience multiple dislocations face an even higher risk. This is one of the strongest arguments for taking rehabilitation seriously after the first event and addressing instability before it leads to repeated episodes and cumulative cartilage loss.