The proximal interphalangeal joint, or PIP joint, is the middle knuckle of the fingers and toes, serving as a hinge that enables movement in the digits. This joint is arguably the most important in the hand for function, as its mobility allows for the fine manipulation and powerful grip needed for everyday tasks. Because the PIP joint is frequently subjected to considerable forces and constant use, it is highly susceptible to injury and degenerative conditions.
Anatomical Location and Structure
The PIP joint forms the articulation between the head of the proximal phalanx (the first bone) and the base of the middle phalanx (the second bone). This structure classifies it as a bicondylar synovial hinge joint, functioning primarily to allow movement in a single plane, much like a door hinge. Stability is maintained by a fibrous capsule and a complex arrangement of ligaments that prevent unwanted motion.
Lateral stability is provided by the collateral ligaments, which resist forces that would cause the finger to bend sideways. On the palmar side, the volar plate, a thick fibrocartilaginous structure, reinforces the joint capsule. The volar plate is a crucial stabilizer that specifically prevents the joint from hyperextending. This combination of bony fit and soft tissue support makes the PIP joint vulnerable to angular or rotational stress.
Essential Function and Movement
The primary function of the PIP joint is to permit flexion (bending) and extension (straightening) of the finger, which is fundamental to almost all hand movements. The arc of motion is substantial, ranging from full extension (0 degrees) to approximately 100 to 110 degrees of flexion. This large range accounts for a significant portion of the total movement required for functional grip, estimated to be around 85% of the grasping capabilities of the fingers.
Movement at this joint allows the fingers to curl inward, facilitating both the “power grip” used to hold objects firmly and the “precision grip” needed for fine motor skills. The ability to flex the PIP joint independently from the other joints in the finger grants the hand its remarkable dexterity.
Common Injuries and Conditions
The PIP joint is susceptible to a range of acute injuries and chronic conditions, often due to its high mobility. Acute trauma frequently results from hyperextension or axial loading, leading to sprains or dislocations. A common acute injury involves the volar plate tearing or being avulsed, which can pull a small bone fragment from the middle phalanx and cause joint instability.
Chronic conditions frequently affecting the PIP joint include two distinct forms of arthritis. Osteoarthritis involves the progressive degeneration of cartilage, leading to pain, stiffness, and the formation of bony enlargements called osteophytes. These nodules, known as Bouchard’s nodes, restrict movement and cause a noticeable bump.
Rheumatoid arthritis is an inflammatory autoimmune condition that damages the synovial lining, leading to the destruction of cartilage and bone. This can result in severe deformities, such as the Boutonnière deformity (permanently bent joint) and the Swan-neck deformity (hyperextended joint). Both acute injuries and chronic diseases ultimately result in joint stiffness and a loss of functional range of motion.