Finger Extensor Tendon Injuries: Mallet Finger and Boutonnière Deformity | Guide | Artur Soczka, MD, PhD

Finger deformity after extensor tendon injury

Extensor tendon injuries in the fingers can lead to characteristic deformities: mallet finger, boutonnière deformity, and a dropped thumb due to extensor pollicis longus (EPL) rupture.

Each deformity results from damage to a different part of the extensor mechanism and requires accurate diagnosis and appropriate treatment.

Why extensor tendons are at risk

Extensor tendons run just beneath the skin surface, making them vulnerable even to minor trauma — a small cut or a direct blow can be enough.

In the fingers, they form a complex extensor apparatus made up of multiple interconnected slips. Damage to any single component disrupts the entire balance of finger extension — even a small injury can produce a significant deformity.

For this reason, careful clinical examination of each finger joint is essential for correct diagnosis.

Mallet finger

Mallet finger typically results from a direct blow to the fingertip — common in ball sports or a forceful bending injury. It damages the terminal slip of the extensor apparatus at the distal interphalangeal (DIP) joint.

The hallmark sign is a drooping fingertip with inability to actively extend the DIP joint.

Treatment of mallet finger

In most cases, continuous splinting of the DIP joint in full extension for 6–8 weeks is sufficient. Consistency is critical — even a brief moment of flexion during this period can disrupt healing.

The terminal slip is approximately 1 mm thick — an extremely delicate structure. Surgery is considered for large bony fragments, joint instability, or failure of conservative management.

Boutonnière deformity

Boutonnière deformity follows damage to the central slip of the extensor mechanism at the proximal interphalangeal (PIP) joint. It causes characteristic PIP flexion combined with DIP hyperextension.

Left untreated, the deformity can become fixed.

Treatment of boutonnière deformity

In the early phase, splinting and orthoses — sometimes dynamic — are used. Early surgical repair involves reattaching the damaged central slip to the middle phalanx using a small bone anchor.

Established deformities may require reconstructive surgery using specialised hand surgery techniques.

Extensor pollicis longus (EPL) rupture

The EPL tendon enables extension of the thumb at the interphalangeal joint — for example, lifting the thumb off a flat surface. Rupture can occur after a distal radius fracture, direct laceration, or attritional wear from bony prominences.

The key symptom is inability to actively extend the thumb at the IP joint.

Treatment of EPL rupture

Complete rupture requires surgery. If diagnosed promptly before significant tendon retraction, end-to-end repair is possible.

When the proximal stump has retracted, direct repair is no longer feasible. The most common solution is tendon transfer from the extensor indicis proprius (the index finger's second extensor tendon) to restore thumb extension.

The most common mistake

The most common mistake is minimising the injury and delaying specialist consultation. During this time the deformity can become fixed, and the tendon may retract to a point where direct repair is no longer possible.

Accurate examination of the extensor apparatus and each finger joint allows the type of injury to be identified and the correct treatment to be selected.

When to seek urgent care

Seek prompt evaluation after finger injury if:

• the finger does not extend actively,
• the fingertip droops,
• visible deformity is present.

Early intervention maximises the chance of full recovery.

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