If you look at your fingers, they each have three sections called phalanges (pleural of phalanx). The thumb has two phalanges. Each phalanx (proximal, middle, and distal) is separated by a joint with the one closest to the nail called the DIP or distal interphalangeal joint. The PIP or proximal interphalangeal joints form the middle knuckle joints of the fingers. The thumb has only one IP or interphalangeal joint.
On each side of an IP joint are collateral ligaments, which provide joint stability from excessive side-to-side motion. These ligaments can tear, partially or fully, when enough force is applied to the joint. The collateral ligament can tear at one of two locations, (1) at its attachment to the proximal phalanx or (2) at its attachment to the volar plate and middle phalanx (see Figure 1). The PIP joint is the one most commonly involved in collateral ligament injuries. The collateral ligament tears are the cause of finger sprains and usually happen when a person forcefully jams their finger. These type of injuries are collectively known as the "jammed finger" injuries.
Collateral ligament injuries present as pain, swelling or deformity at the affected joint, and inability to fully move the joint, If the joint is stable and there is no avulsion fracture where the ligament has pulled off a piece of bone from the joint surface of the middle phalanx, the injury can be treated with buddy taping of the affected finger.