Everyone experiences joint popping occasionally, and stair climbing stretches the tendons in your knees in a way that may produce a such a sound. If the phenomenon is infrequent, it is usually harmless. If the popping is frequent, or accompanied by pain, however, you may have an injury or a degenerative condition and you should seek medical assistance. Treatment is essential to prevent further damage.
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Your thighbone, or femur, meets your shinbone, or tibia, to form the knee joint. The kneecap, also called the patella, is a shield-shaped bone that rides over this joint to protect it. Your knees bear the majority of your body weight and these joints sometimes encounter a lot of stress--for example, when you run on a stiff surface or twist while playing sports. Two tough pieces of cartilage, called the menisci, are attached to either side of the knee and act as a shock absorber system to cushion the bones and to stabilize the joint. Knee popping can be related to damage or degeneration of this cartilage.
Your joints contain synovial fluid--a lubricant that protects bones from the damage they would incur if they rubbed directly against each other. This fluid contains carbon dioxide, oxygen and nitrogen. When you stretch your knee in a certain way--for example, by climbing stairs--these gases escape abruptly from the joint and cause a popping or cracking sound. When you climb stairs, you also alter the position of the tendons surrounding the knee, and the popping you experience may be the movement of the tendons as they return to their normal position. In addition, soft tissue around the knee sometimes moves over the edge of the knee joint and you may feel a pop as it moves back.
Knee popping is one of the symptoms of a tear in the meniscal cartilage. Among younger people, this most often occurs during sports, but as you age and your joint cartilage wears thin, a routine twisting movement--such as turning quickly while standing--can cause a meniscal tear. Usually, you can still walk with such a tear, but you likely will feel pain and the joint will stiffen within a few days. Swelling also is likely. If you leave such an injury untreated, a portion of the meniscus may come between the bones of the joint, which may cause your knee to pop. If you experience knee pain, or if you experienced an event that may have caused such a tear, consult your doctor. Treatment may include rest, immobilization and/or surgery.
If you have arthritis, the surface of the bones in your knee will become increasingly rough over time. The combination of uneven bone surface and decreasing thickness of the cartilage cushion may cause frequent popping or cracking in your knees. If you experience pain from this popping, your doctor may be able to offer some relief by treating the arthritis symptoms. Losing weight will decrease the stress on your knees and gentle exercise to build flexibility will help protect you from injury.