Felon

 Updated 3/2017-- photos and all links removed as many are no longer active and it was easier than checking each one.

Felons and paronychia make up one third of all hand infections. The thumb and index finger are most often involved. Wooden splinters or minor cuts are common predisposing causes, yet over half of patients will give no history of injury. 

Felons are closed-space infections of the fingertip pulp (padding beneath the finger print area). Fingertip pulp is divided into numerous small compartments by vertical septa that stabilize the pad. Infection occurring within these compartments can lead to abscess formation, edema, and rapid development of increased pressure in a closed space. This increased pressure may compromise blood flow and lead to necrosis of the skin and pulp. It is most often caused by staph aureus. Felons that are untreated, are incorrectly treated, or have a prolonged course may lead to osteomyelitis. X-rays evaluation of the bone should be done in severe cases.

Treatment is incision and drainage. This will remove any pus present, but will also reduce the pressure within the closed compartment and restore blood flow.

1. The finger can be numbed with local anesthesia (digital block).
2. Make short skin incision with a number 11 blade over the area of maximum tenderness. Incise only the skin with scalpel. Do not cross the DIP joint crease (can create a contraction)
3. Evacuate any pus (and culture) using a blunt instrument, like a small hemostat. This will decrease the chance of injury to the digital nerve or the tendon sheath (can lead to acute tenosynovitis). Do not divide vertical fascial strands (septa) as this makes the fingertip pulp unstable.
4. Pack gauze loosely into the wound to prevent skin closure. Apply a loose dressing, splint finger, and elevate hand above the heart.
5. Followup in 2-3 days.

Incision Tips:
A longitudinal incision in the midline is effective without serious iatrogenic complications (nerve injury, tendon sheath injury).
Lateral or transverse incisions frequently cause ischemia and anesthesia by injuring one or both neurovascular bundles. If a lateral incision is used, it is best to incise the non-contact aspect of the finger (ulnar side of fingers, radial side of the thumb).
Fish-mouth incision can lead to an unstable painful fingertip.

Empirical antibiotic coverage for S aureus and streptococcal organisms should be provided. Given the rapid emergence of community-acquired methicillin-resistant S aureus, treatment with a drug more likely to be effective against this agent should be considered. Coverage for E corrodens may be indicated for immunosuppressed patients. Dicloxacillin, erthromycin, Keflex, nafcillin, and Bactrim DS are good ones to start the patient on while waiting for the culture. The recommended length of treatment varies from five to 14 days and depends on the clinical response and severity of infection.

Complications:
Osteomyelitis involving the distal phalanx.
The most serious complication is acute tenosynovitis. It may result from natural, contiguous spread of infection. It most often iatrogenic from inadvertent nicking of flexor tendon sheath with scalpel.
Other complications include skin necrosis, deformity of the fingertip, septic arthritis, and instability of the finger pad.


References

Infections of the Hand: A Guide to the Surgical Treatment of Acute and Chronic Suppurative ... By Allen Buckner Kanavel (Google eBook)

Wheeless' Textbook of Orthopedics Online
Common Acute Hand Infections--AAFP
Felon by Glen Vaughn, MD--eMedicine article