Injuries to the Hand and Digits

1. THE 2-MINUTE KINEMATICS

Understanding the precise biomechanics of hand injuries is the first step in identifying occult damage and anticipating life-altering functional deficits.

• High-Pressure Injection Injuries: Industrial equipment (e.g., paint or grease guns) operating at 2,000 to 10,000 psi can inject fluids through a seemingly benign pinpoint wound. The initial dissipation of kinetic energy forces the fluid deeply along fascial planes, triggering an intense chemical inflammation and subsequent tissue ischemia.
• The Clenched Fist ("Fight Bite"): When a clenched metacarpophalangeal (MCP) joint strikes a human tooth, the tooth penetrates the extensor tendon and joint capsule. Upon extending the fingers post-injury, the severed tissues glide proximally, trapping highly virulent, polymicrobial flora deep within the joint space.
• Mallet vs. Jersey Finger: A mallet finger results when the distal interphalangeal (DIP) joint is forcibly flexed while actively extending, tearing the extensor mechanism and preventing active distal phalanx extension. Conversely, a "Jersey finger" occurs when a actively flexed DIP is forcibly extended, resulting in a flexor digitorum profundus (FDP) avulsion.

2. THE BEDSIDE ACTION PLAN

• Immediate Stabilization: Prioritize the removal of all rings and constrictive jewelry from the injured extremity before edema sets in.
• Pain Control and Sedation: Procedural sedation or regional nerve blocks (e.g., median, ulnar, or radial blocks at the wrist) are superior to local infiltration, providing excellent analgesia while allowing the patient to participate in active tendon testing without pain-induced guarding,.
• High-Pressure Injection Protocol: Administer broad-spectrum IV antibiotics, update tetanus, elevate the limb, and secure immediate hand surgery consultation. Never perform a digital nerve block for these injuries, as the added volume can critically increase compartmental pressure and accelerate ischemia,.
• Fight Bite Protocol: Never close these wounds primarily in the emergency department. Treat aggressively with broad-spectrum IV antibiotics covering Staphylococcus, Streptococcus viridans, and Eikenella corrodens (e.g., Ampicillin/Sulbactam 3 g IV), and admit for operative exploration and washout.
• Splinting Applications: The optimal "position of function" (intrinsic plus position) requires a volar splint with the wrist in 20 to 30 degrees of extension, MCP joints at 70 to 90 degrees of flexion, and the PIP/DIP joints in full extension.

3. THE DIAGNOSTIC GRID

• The Flexor Tenosynovitis Screen: Kanavel's four classic signs of infectious flexor tenosynovitis are: 1) fusiform swelling of the entire digit, 2) exquisite tenderness along the flexor tendon sheath, 3) the digit held in resting flexion, and 4) severe, disproportionate pain elicited by passive extension of the digit,,.
• Targeted Tendon Testing: Assess the hand's resting posture; an injured flexor tendon will cause the digit to lose its naturally flexed "cascade sign". When evaluating lacerations, replicate the hand's position at the time of injury, as tendons severed while the fingers were flexed will retract proximally and hide from view when the hand is extended.
• Mandatory Neurologic Checks: Test the anterior interosseous nerve (median) using the "OK sign" against resistance and assessing 2-point discrimination on the volar 2nd digit,,. Test the ulnar nerve by resisting finger spread and assessing the volar 5th digit. Test the radial nerve by resisting a "thumbs up" sign and checking the dorsal web space.

4. THE VISUAL BOARD

• Radiographic Rules: Obtain at least three views of the hand or digits (AP, lateral, and oblique) entirely free of overlying splint material.
• Occult High-Pressure Signs: Plain radiographs are vital in injection injuries to track the wide dissemination of radiopaque materials (like lead-based paints or grease) and to identify subcutaneous emphysema tracking deeply along fascial planes,.
• Mallet & Scaphoid Clues: Look for a small bony avulsion fragment on the dorsal base of the distal phalanx indicating a mallet finger. Recognize that scaphoid fractures are notoriously occult on initial imaging; exquisite tenderness over the anatomic snuffbox demands a thumb spica splint and orthopedic follow-up regardless of normal x-rays.

5. THE CLASSIFICATION MATRIX

• Tendon Zones: The hand and wrist are strictly mapped to predict surgical complexity. Flexor injuries are divided into Zones I through V. Extensor injuries are divided into Zones I through VIII. Open injuries over Zone V (the MCP joint) must be treated as human bites until proven otherwise.
• Metacarpal Fracture Angulation Limits: Acceptable conservative management limits for metacarpal neck fractures depend heavily on the digit: 10 to 15 degrees for MC 2 and 3; 30 to 40 degrees for MC 4; and up to 50 to 60 degrees for MC 5 (Boxer's fracture). Fractures exceeding these limits, or demonstrating malrotation, require surgical fixation.

6. THE DANGER ZONE

• The Benign Injection Trap: The absolute deadliest cognitive error is discharging a high-pressure injection injury due to the deceptively small, benign-appearing pinpoint entrance wound,. Delays in operative debridement beyond 6 hours dramatically escalate the risk of compartment syndrome and push amputation rates up to 30%,,.
• Seymour Fracture Mimics: Do not mistake a Seymour fracture (a juxtaepiphyseal Salter-Harris I or II injury with proximal nail fold displacement) for a simple mallet finger. This is an open fracture requiring immediate hand surgery consultation, systemic antibiotics, and operative exploration.
• The Extensor Illusion: A complete extensor tendon laceration on the dorsum of the hand may still present with weak active extension of the digit due to the compensatory transfer of forces through the intact juncturae tendinum.
• Fight Bite Denial: Patients frequently lie about punching someone in the mouth. Regardless of the reported history, any laceration over the MCP joints of the dominant hand must be treated as a highly contaminated human bite,,.

7. MCQ MASTERCLASS

• Buzzword: "Disproportionate pain on passive extension" = Infectious flexor tenosynovitis (Kanavel's sign),.
• Buzzword: "Loss of the normal resting flexion cascade" = Flexor tendon laceration.
• Buzzword: "Paint gun nozzle slipped" or "10,000 psi injection" = Surgical emergency requiring immediate OR washout; never inject a digital block,.
• Exam Distractor: An MCQ will present a deep, dirty puncture wound and offer "Swab the wound for aerobic and anaerobic cultures in the ED" as an option. This is a classic distractor—ED wound swabs are useless, grow only skin flora, and delay definitive antibiotic therapy.
• Buzzword: "Red dot" or "bleeding from a chipped tooth" = Ellis III dental fracture (though frequently tested alongside fight bites).

8. THE BOARDROOM SCRIPT

"I am consulting you for a [Patient Age]-year-old [Right/Left]-hand dominant [Occupation] who sustained a [Mechanism of Injury, e.g., high-pressure paint injection / clenched fist strike] to their [Specific Digit Name—never use numbers, e.g., index finger] at [Tendon Zone]. On my exam, they have [Presence/Absence of Kanavel's signs], a [Loss/Presence] of the normal resting cascade, and the wound is [Clean/Grossly contaminated]. Radiographs show [Fracture Pattern, Displacement, Angulation, or Subcutaneous air]. Distal perfusion is intact with capillary refill under 2 seconds, and 2-point discrimination is intact in the [Median/Ulnar/Radial] distributions. I have avoided digital blocks, administered IV [Antibiotic Name], and placed the hand in an intrinsic plus splint. Post-splinting neurovascular status remains unchanged. They require your emergent evaluation.",,,,,.