The Proximal Interphalangeal joint (PIP joint) is the central hinge in the fingers, connecting the first and second bones of each digit. This joint is fundamental to the dexterity and strength of the human hand, enabling movements necessary for gripping and pinching. Its structure makes it highly functional, but also prone to injury, pain, and stiffness.
Pinpointing the Location and Structure
The PIP joint is the knuckle closest to the palm. It is the articulation point between the head of the proximal phalanx (the first finger bone) and the base of the middle phalanx (the second finger bone). This bony arrangement forms a synovial hinge joint, allowing motion primarily in a single plane.
The joint’s stability relies on a complex network of soft tissues. Strong collateral ligaments on the sides prevent excessive side-to-side (lateral) movement. On the palm side, the volar plate, a thick fibrocartilaginous structure, prevents hyperextension by stopping the finger from bending backward too far.
The entire structure is enclosed by a joint capsule lined with a synovial membrane, which produces fluid to lubricate the joint surfaces. The design of the phalanges promotes stability during bending and straightening motion. The PIP joint is an inherently stable structure, provided the surrounding ligaments and the volar plate remain intact.
The Role of the PIP Joint in Hand Movement
The PIP joint functions almost exclusively as a hinge, primarily facilitating flexion (bending) and extension (straightening). A healthy joint can typically flex up to 100 to 110 degrees. This movement is powered by the flexor digitorum superficialis muscle, which attaches to the middle phalanx.
This range of motion is responsible for approximately 85% of the total finger movement required for a functional grip. Flexibility in the PIP joint is necessary for the “power grip” used when holding heavy objects. The joint’s limited movement to a single plane helps focus force and precision, essential for fine motor tasks like writing or manipulating small items.
Common Issues and Injuries
The PIP joint is one of the most commonly injured joints in the hand due to its frequent use and exposed location. Acute injuries often result from direct impact or sudden, forceful movements, particularly in sports. A common mechanism is a “jammed finger,” where an axial load or hyperextension causes a sprain or tear of the volar plate.
If the force is severe enough, the collateral ligaments can be stretched or completely ruptured, leading to joint instability. In serious acute cases, the joint can dislocate, forcing the middle phalanx out of alignment with the proximal phalanx. Dislocations frequently involve a tear of the volar plate and collateral ligaments, sometimes resulting in an avulsion fracture.
The joint is also a frequent target for chronic conditions that cause pain and stiffness. Osteoarthritis, a degenerative “wear-and-tear” condition, often affects the PIP joint, causing protective cartilage to break down. This causes pain, swelling, and loss of motion. Rheumatoid arthritis, an autoimmune disease, frequently involves the PIP joints early on, causing inflammation and eventual joint destruction. Stiffness is a concern with all PIP joint issues, as the joint tends to scar and lose motion rapidly.