Anatomy and Biomechanics of the Finger

Within this extensor mechanism, the cruciate ligaments play a vital role. These ligaments are shaped like a cross and serve to stabilize the joint during movement. They help manage the distribution of forces as the finger transitions between flexion and extension, ensuring that the extensor apparatus remains centered over the joint axis.

Flexor Tendons: FDS and FDP

The movement of the fingers is driven by two primary tendons that travel through the carpal tunnel and into the digits: the Flexor Digitorum Superficialis (FDS) and the Flexor Digitorum Profundus (FDP).

• Flexor Digitorum Superficialis (FDS): This tendon is responsible for flexing the Proximal Interphalangeal (PIP) joint. It has a split tendon design that allows the FDP tendon to pass through it.
• Flexor Digitorum Profundus (FDP): This is the deeper tendon, which is the only structure capable of flexing the Distal Interphalangeal (DIP) joint.

The coordination between these two tendons allows for the complex, multi-joint flexion required for grasping objects of varying shapes and sizes.

Clinical Deformities: Boutonniere and Swan-Neck

When the balance between the extensor hood and the flexor tendons is disrupted, specific clinical deformities occur. Two of the most common are the Boutonniere deformity and the Swan-neck deformity.

Boutonniere Deformity

A Boutonniere deformity occurs when the central slip of the extensor mechanism is damaged. This results in an inability to extend the PIP joint, causing it to remain flexed, while the DIP joint may become hyperextended. This is essentially a failure of the extensor mechanism at the middle joint.

Swan-neck Deformity

In contrast, the Swan-neck deformity is characterized by hyperextension of the PIP joint and flexion of the DIP joint. This often results from an imbalance where the volar plate (the stabilizer on the palm side of the joint) is weakened or the extensor mechanism is pulling too strongly on the distal phalanx.

Exam Tip: To distinguish these two, look at the PIP joint. In Boutonniere, the PIP is flexed (bent inward). In Swan-neck, the PIP is hyperextended (bent backward).

Trigger Finger

Another common pathology is "Trigger Finger," medically known as stenosing tenosynovitis. This condition occurs when there is an inflammation or a nodule on the flexor tendon (FDS or FDP) or within the tendon sheath.

As the finger attempts to move from a flexed to an extended position, the swollen tendon or nodule gets caught on the A1 pulley (the first pulley at the base of the finger). This causes the finger to "catch" or "lock" in a bent position, often releasing with a painful snap or pop when the person forces the finger straight.