The knuckle joint is the joint in the hand where the finger connects to the hand, or, more formally, where one of the proximal phalanx bones articulates with the corresponding metacarpal bone. Similar joints can also be found on the feet, and people often refer to the joints between individual phalanx bones in the fingers as knuckles as well. The distinctive bulgy appearance of the knuckles is caused by the shape of the bone, with the bones having rounded protuberances which are designed to facilitate articulation so that the bones can move more smoothly.
The knuckles where the fingers join the hand are especially prominent, particularly when people make a fist. In fact, the appearance of the knuckle joint is even referenced in the slang term “knuckle sandwich” for a punch delivered to the mouth. Some people also have large joints between their fingers, depending on their anatomy, age, and type of work that they do.
A swollen knuckle joint is a common problem for many people. The hands see a great deal of hard use, and the joints can become inflamed and swollen in response to temperature extremes, exposure to chemicals, repeated hard work, repetitive motion, and many other factors. The knuckle joint also becomes swollen in people with arthritis, and sometimes the joints are twisted, as well, making it difficult for people to use their hands.
One especially common problem with swollen knuckles is the trapping of jewelry, especially in men. Men who never remove their wedding rings may find that the rings are actually impossible to remove due to swelling in the knuckle, and in some cases rings need to be cut off in emergencies because there is no way to slide the ring over the joint. In non-emergency situations, people can try icing the joint to bring the swelling down, or lubricating the joint with soap to see if this will encourage the ring to slide off.
The knuckle joint is also famous for the distinctive popping or crackling sound made when the joint is flexed. Knuckles can pop naturally with certain kinds of movement, and people can also crack their knuckles deliberately by putting stress on the joint. The crackling happens because stretching the joint lowers the pressure inside, allowing dissolved gases to precipitate out as bubbles, and the bubbles pop as the pressure drops. Since the pressure takes a moment to equalize after the joint is relaxed, it is usually not possible to pop the knuckles right after they have been popped.