Popping and cracking sounds in the course of the movement of human joints are natural—and often disconcerting. These sounds are made in two general circumstances: when the sound is accompanied by pain in the moving joint, and when the sound occurs with the movement alone.
Most joints in the body have common construction features. Two or more bones create the skeletal component, with the bones at their point of contact covered by a slick, friction-reducing material known as articular cartilage. The joint is encased in a capsule that contains synovial fluid, a substance also designed to reduce friction in the joint. Present in the joint is often a bursa, a gel-filled fibrous sac that acts to cushion impacts received at the joint. The bones at the joint are secured to one another by ligaments; the joint usually is powered by muscle and tendon groups secured to the bones.
Joint noises that are not accompanied by pain often occur in the joints of the ankle, back, knee, knuckles, and neck. Such sounds are caused in joints for a variety of reasons. For example, the ankle has three different ankle joints, created by the conjunction of each of the talus (the ankle bone), and the tibia and the fibula (the bones of the lower leg). Through movement, if one of the three corresponding sets of ligaments that secure these ankle joints rubs the joint, the sound may occur. Another example is the back. The bony structure of the back is the spinal column, the arrangement of vertebrae and the ligaments known as facets. Slight misalignments that do not impair either the intravertebral disks or surrounding muscles can cause a cracking sound on movement.
In the knees, movements such as suddenly rising from a seated to an upright position, or stretching the knee from a flexed to an extended position can produce this often prominent sound in the joint. Common causes of knee joint popping or cracking include a slight misalignment of the patella (knee cap), or the movement of the different ligament structures across the joint. When the ligaments move, it is often against the bones of the joint, which creates a slight stretching and then mild snapping back to their accustomed position. The knuckles are the metacarpophalangeal joints, which are often the site of joint noise that results when people deliberately pop or crack their knuckles. The act of pulling on the knuckle using the attached finger creates a gas bubble in the synovial fluid that surrounds the joint. By pulling on the knuckle, the joint is increased in size by approximately 15%. The popping sound occurs twice in this transaction, once as the finger is pulled to create the larger space in the fluid sac surrounding the knuckle, and again as the joint returns to its natural size and position. Repeated deliberate popping of these knuckle joints tends to cause an inflammation in the joint. And finally, the sounds made in the neck occur in a similar fashion to those in the back, by the action of the spinal column; the neck is supported by the cervical vertebrae. A common expression used to describe the sensation prior to the cracking sound or sensation is a "crick" in the neck.
When the joint noise is accompanied by a painful sensation, the noise is usually a symptom of a joint injury or degenerative condition. When the noise from the knee occurs in conjunction with pain in the joint, the cartilage in the knee, known as the meniscus, is commonly torn or otherwise worn. Pain will also occur in the knee if a piece of cartilage has broken away from its position and it is floating within the synovial fluid in the joint. When cartilage pieces float into the space between the bones of the joint, there will be pain. Ankle pain often has a similar cause.
In most joints, pain that accompanies the sound of a pop or a crack is a symptom of arthritis. It is common for arthritic pain to manifest itself without any sound from the joint on movement.
A popping sound is also a common contemporaneous event with a serious rupture of a ligament or a tendon. Such occurrences are usually painful, but not exclusively so. A popping sound is often experienced by athletes who sustain an anterior cruciate ligament (ACL) tear, an Achilles tendon rupture, or a rupture of the hamstring. The pop is caused by the structure being torn completely in two.
A pop in the knee joint may also occur when medical attention is being provided to a damaged knee. In some circumstances, the floating cartilage present in a damaged knee joint may cause the knee to become locked into position. In the course of the gentle maneuvering required to loosen the joint, the knee may generate a pronounced popping sound.