The proximal interphalangeal (PIP) joint is located in the middle of each finger, allowing for bending and straightening. This joint is crucial for the hand’s mobility and dexterity, enabling both strong grips and fine manipulations.
Anatomy and Position
The proximal interphalangeal joint refers to the articulation between the proximal and middle phalanx in each finger, excluding the thumb which has only one interphalangeal joint. This hinge joint is formed by the head of the proximal phalanx connecting with the base of the middle phalanx. The head of the proximal phalanx features two curved condylar processes, which create a shallow groove for articulation.
Surrounding and supporting this joint are several structures that ensure stability and facilitate movement. A fibrous joint capsule encloses the PIP joint, lined internally by a synovial membrane that produces fluid for lubrication. On either side of the joint, proper and accessory collateral ligaments originate from the proximal phalanx and insert onto the middle phalanx and volar plate, preventing excessive side-to-side movement. The volar plate, a thick fibrocartilaginous structure on the palm side, prevents the joint from hyperextending.
Role in Hand Movement
The PIP joint is a hinge joint, allowing movement in a single plane: flexion (bending) and extension (straightening). The flexor digitorum superficialis and flexor digitorum profundus muscles are the main contributors to PIP joint flexion, while the extensor digitorum, lumbricals, and interossei muscles facilitate extension.
The PIP joint can flex between 100-110 degrees, which is important for tasks requiring a strong grip and precise control. This range of motion allows the fingers to curl and adapt to the shapes of objects, which is essential for activities like gripping tools, pinching small items, and writing. The PIP joint accounts for approximately 85% of the motion needed for a functional grip. While stable in flexion and extension, the joint offers limited tolerance to angular, axial, and rotational stress.
Common Conditions Affecting the PIP Joint
The proximal interphalangeal joint is susceptible to various issues, from acute injuries to chronic degenerative conditions. Given its position and extensive use, it is one of the most frequently injured joints in the fingers. Injuries often occur due to direct trauma or forces that push the joint beyond its normal range of motion.
Sprains and dislocations are common acute injuries to the PIP joint, frequently resulting from hyperextension or a laterally directed force, such as during sports activities. A sprain involves damage to the ligaments, categorized by severity, while a dislocation means the bones of the joint are forced out of alignment. Dorsal dislocations, where the middle phalanx shifts backward, are particularly common and often involve injury to the volar plate.
Chronic conditions like osteoarthritis and rheumatoid arthritis can also significantly affect the PIP joint, leading to pain, stiffness, and deformity. Osteoarthritis involves the breakdown of cartilage within the joint, causing bones to rub together, while rheumatoid arthritis is an autoimmune condition that causes inflammation of the joint lining. Other conditions include Boutonnière deformity, which results from damage to the extensor tendon mechanism, preventing the PIP joint from straightening.