Overview

Mallet finger is a disruption of the terminal extensor tendon’s insertion into the distal phalanx at the distal interphalangeal (DIP) joint. This injury results in an inability to actively extend the DIP joint, creating the characteristic “drop” deformity. While common in athletes (e.g., basketball, baseball), it is a critical clinical entity because improper management can lead to permanent deformity and functional impairment.

Pathophysiology and Classification

The terminal extensor tendon inserts on the base of the distal phalanx. A mallet injury occurs when an abrupt force drives the finger into flexion, causing a failure of the extensor mechanism.

1. Tendinous Mallet (Soft Tissue)

A complete or partial rupture of the tendon without a significant bony fragment. This is a pure soft tissue injury.

2. Bony Mallet (Avulsion Fracture)

The tendon avulses a fragment of bone from the dorsal base of the distal phalanx. These are further categorized by:

• Fragment Size: Small fragments may be treated conservatively; larger fragments (typically involving $>1\mathrm{/}3$>1/3 of the articular surface) may predispose the joint to instability.
• Joint Stability: Whether the DIP joint remains congruent or is subluxed/dislocated.

3. Simple vs. Complex Mallet

• Simple Mallet: Isolated injury to the DIP joint.
• Complex Mallet: Associated with other injuries, such as a PIP joint dislocation, volar plate injury, or collateral ligament rupture. These carry a higher risk of stiffness and swan-neck deformity.

Clinical Evaluation and Diagnosis

Physical Examination

• Inspection: Obvious flexion deformity (droop) of the DIP joint.
• Active Motion: Patient is unable to actively extend the DIP joint against gravity.
• Passive Motion: The clinician can passively extend the DIP joint fully, though tenderness will be present over the dorsal aspect.
• PIP Joint Assessment: It is imperative to evaluate the Proximal Interphalangeal (PIP) joint. If the PIP is hyperextended or unstable, it indicates a complex mallet finger.

Imaging

• Radiography (Gold Standard): AP and Lateral views are mandatory.
  • Lateral view is most sensitive for detecting bony avulsion fragments and assessing DIP joint subluxation.
  • Assessment: Look for the size of the fragment and whether the articular surface is disrupted or displaced.
• MRI/Ultrasound: Rarely indicated in acute settings. MRI may be used in chronic cases to evaluate the degree of tendon retraction or joint degeneration.

Management Strategies

Conservative Management (First-Line)

The primary goal is to maintain the DIP joint in neutral or slight hyperextension to allow the tendon/bone to heal.

1. Splinting Protocol

• Indications: Simple tendinous mallets and stable bony mallets without significant subluxation.
• Mechanism: Use of a dynamic extension splint (e.g., Stack splint) or a static splint. The DIP must be held in 0° to 10° of hyperextension.
• Duration: Continuous wear for 6–8 weeks.
• Critical Warning: A single episode of flexion during the healing phase can stretch the newly formed collagen/callus, potentially resetting the healing clock or resulting in a permanent extension deficit.
• PIP Joint Mobility: The PIP joint must remain free to move to prevent stiffness and secondary deformities.

2. Pharmacotherapy

• NSAIDs may be used for acute pain and edema management, though their role in long-term tendon healing is negligible.

Surgical Intervention

Surgery is indicated when conservative management is contraindicated or likely to fail.

Indications for Surgery:

1. Bony Mallet with Subluxation: If the DIP joint is subluxed $\ge 30^{\circ }$≥30∘ or there is significant articular incongruity.
2. Large Articular Fragments: Avulsion involving a significant portion of the distal phalanx base.
3. Complex Mallets: Concurrent PIP instability.
4. Failed Conservative Treatment: Chronic mallet with significant deformity (though results are less predictable in chronic cases).

Surgical Techniques:

• Percutaneous Pinning: K-wire fixation is used to stabilize the bony fragment and maintain joint congruency for 4–6 weeks.
• Open Reduction Internal Fixation (ORIF): Indicated for complex fractures or where percutaneous pinning fails. Small screws or plates may be utilized.
• Tendon Grafting/Tenodesis: Reserved for chronic cases where the tendon has retracted or scarred, though functional outcomes are often inferior to acute repair.

Complications and Long-term Prognosis

Swan-Neck Deformity

The most significant complication of an untreated or poorly treated mallet finger is the development of a Swan-Neck deformity (PIP hyperextension with DIP flexion).

• Mechanism: The loss of tension in the terminal extensor tendon allows the volar plate at the PIP joint to stretch, while the remaining extensor apparatus creates a disproportionate pull that hyperextends the PIP.

Joint Stiffness and Arthritis

Chronic mallet injuries often lead to secondary osteoarthritis of the DIP joint due to prolonged instability or improper alignment during healing.

Pediatric Considerations

In children, an avulsion fracture may involve the epiphyseal plate (growth plate). While these generally heal well, monitoring for growth arrest is necessary if a significant crush injury occurred.

Rehabilitation and Recovery Timeline

• Weeks 0–8: Continuous splinting; active ROM of the PIP joint.
• Week 8+: Gradual weaning from the splint. Patients are taught to perform gentle active extension exercises.
• Expected Outcome: Complete restoration of full active extension is rare. Most patients accept a residual extension deficit of $5^{\circ }\text{–}15^{\circ }$5∘–15∘, which is functionally insignificant.
• Full Recovery: Full strength and resolution of swelling typically occur between 3 and 6 months post-injury.