Interphalangeal joints of the hand

Location: Within the fingers and thumb, between neighbouring phalanges: each finger has a proximal (PIP) and a distal (DIP) interphalangeal joint; the thumb has a single interphalangeal joint.

Bones taking part

• Proximal phalanx of the thumb (head)
• Distal phalanx of the thumb (base)
• Proximal phalanx of the index finger
• Middle phalanx of the index finger
• Distal phalanx of the index finger
• Proximal phalanx of the middle finger
• Middle phalanx of the middle finger
• Distal phalanx of the middle finger
• Proximal phalanx of the ring finger
• Middle phalanx of the ring finger
• Distal phalanx of the ring finger
• Proximal phalanx of the little finger
• Middle phalanx of the little finger
• Distal phalanx of the little finger

Articulating surfaces

• PIP joint: the pulley-shaped (bicondylar) head of the proximal phalanx with the base of the middle phalanx
• DIP joint: the head of the middle phalanx with the base of the distal phalanx
• Thumb IP joint: the head of the proximal phalanx of the thumb with the base of its distal phalanx
• Each phalangeal head has two small condyles with a groove between them, and each base has two shallow facets separated by a ridge, so the surfaces interlock and allow movement in one plane only.

Structural

Synovial joints (hinge type).

Functional

Diarthroses (freely movable).

Synovial subtype

Hinge (ginglymus).

Axes

Uniaxial: flexion and extension about a transverse axis.

Capsule & cavity

Each joint has its own fibrous capsule, strengthened on each side by collateral ligaments and in front by a palmar ligament (palmar plate). The back of the capsule is covered by the extensor expansion.

• Curl and straighten the fingers for gripping, pinching and fine manipulation.
• Allow the fingertips to be placed precisely, for example when writing or picking up small objects.

Stability vs mobility: Stable in the side-to-side direction because the interlocking surfaces and collateral ligaments allow only one plane of movement, and the palmar plates prevent hyperextension. The price is that the joints are uniaxial: flexion and extension only.

• Mallet finger: the extensor tendon is torn from (or avulses a fragment of) the base of the distal phalanx, so the DIP joint droops in flexion and cannot be actively straightened.
• Boutonnière deformity: damage to the central slip of the extensor expansion over the PIP joint causes PIP flexion with DIP hyperextension.
• Swan-neck deformity: PIP hyperextension with DIP flexion, seen for example in rheumatoid arthritis.
• In osteoarthritis, bony swellings form at the DIP joints (Heberden's nodes) and PIP joints (Bouchard's nodes).
• Hyperextension injuries ('jammed finger') can tear the palmar plate of the PIP joint.

• Classify them as synovial hinge joints, uniaxial: flexion and extension only.
• Count them correctly: two per finger (PIP and DIP), one in the thumb.
• Know which muscle flexes which joint: superficialis the PIP, profundus the DIP (and PIP), flexor pollicis longus the thumb IP.
• Distinguish them from the MCP joints, which are condylar and allow abduction and adduction.
• Recognise mallet, boutonnière and swan-neck deformities by which joint is flexed or extended.

• Saying the interphalangeal joints allow abduction and adduction; that happens at the MCP joints.
• Giving the thumb a PIP and a DIP joint; it has one IP joint.
• Saying flexor digitorum superficialis flexes the DIP joint; it inserts on the middle phalanx and does not reach it.
• Calling them condylar joints because the phalangeal heads have two condyles; functionally and structurally they are hinges.