Proximal Interphalangeal Joints of the Hand
The proximal pnterphalangeal (PIP) joints of the fingers are extremely important for gripping, more specifically, what is called the ‘power’ grip. Being a hinge joint, the joint’s articular surface and soft tissue do not permit any lateral movement. Its uniaxial nature allows for a good range of movement but only in one plane, relying on the more proximal metacarpophalangeal joint for a more varied range of motion such as adduction and abduction. The ligaments surrounding this joint are taut during flexion creating an effective gripping mechanism. As a result, the PIP joint has a crucial role to play in the ‘power grip’, which along with the ‘precision grip’ enabled the human race to develop the ability to manufacture and employ tools.
The PIP joint is formed by an articulation of the head of the proximal phalanx and the base of the intermediate phalanx. The head of the proximal phalanx contains a lateral pit into which the collateral ligaments are attached, and a transversely directed apex, leading to an inter-condylar concavity on the articular surface.
The Intermediate phalanx contains two concavities, designed for articulation, separated by a saddle shaped ridge and a tubercle for attachment to the extensor slip of the extensor expansion. There is also a site of attachment for the distal palmar plate, which is sandwiched by small palmar tubercles that provide attachments for parts of the joint capsule and flexor sheath.
There are many soft tissue structures around the PIP joints that protect the joints and restrict unwanted movement. The main components that form this protective envelope are the:
- joint capsule
- volar plate
- collateral ligaments (true and accessory)
- central slip of the extensor tendon hood
The extensor tendon, collateral ligaments and volar plate all increase the surface area of the joint, and their malleable nature helps improve joint congruence. In regards to soft tissue producing movement, flexor digitorum superficialis and flexor digitorum profundus flex the joint, while extensor digitorum, the lumbricals and interossei extend. The PIP joint also includes a synovial membrane.