A small cut over the knuckle after a punch is not a minor wound. A tooth can enter the MCP joint capsule, inoculate the joint with human oral bacteria, and lacerate the extensor tendon — all at once, through a wound that looks trivial. This is a dual injury: septic arthritis and tendon laceration. It requires OR irrigation and tendon evaluation the same day.
Antibiotics alone will not sterilize an established septic joint. Every hour of delay allows bacteria to destroy articular cartilage. The damage is permanent.
The Core Problem
The fight bite is the most commonly mismanaged hand injury in emergency medicine. The wound looks insignificant — a small laceration or puncture over the knuckle. The patient often minimizes the injury or denies the mechanism entirely. The treating provider closes the wound, prescribes antibiotics, and sends the patient home. Days later, the patient returns with a destroyed MCP joint.
What makes a fight bite uniquely dangerous is that it is not one injury — it is two. When a tooth strikes the closed fist at the MCP joint, it can simultaneously penetrate the joint capsule, inoculating the joint space with human oral bacteria, and lacerate the extensor tendon that runs over the knuckle at that level. Both injuries require surgical assessment and treatment. Neither can be adequately addressed by antibiotics and wound closure alone.
The extensor tendon injury is particularly easy to miss because of the wound position shift. When the fist was closed at the moment of impact, the extensor tendon was positioned directly over the MCP joint — directly in the path of the tooth. When the patient opens the hand and the laceration is examined, the skin wound has moved proximally relative to the tendon, which has shifted back under it. The skin laceration no longer sits over the tendon laceration beneath it. Only surgical exploration under appropriate anesthesia can assess both injuries properly.
Fight bites are one of the most reliably undertreated injuries in emergency medicine. The same three factors appear repeatedly in delayed presentations — and understanding them is the difference between a saved joint and a destroyed one.
The Mechanism
The MCP joint sits directly beneath the skin and extensor tendon at the knuckle. The joint capsule is thin. The tooth of an adult human can penetrate the skin, the extensor tendon, and the joint capsule in a single impact — leaving a wound that may be only a few millimeters in length but has reached the joint space.
Human teeth carry one of the most virulent bacterial profiles of any bite wound. Human oral flora include Eikenella corrodens, multiple Streptococcal species, Staphylococcal species, and various anaerobes — organisms that are highly adapted to cause invasive soft tissue and joint infections. Eikenella is particularly notable because it is resistant to first-generation cephalosporins and clindamycin — the antibiotics most commonly prescribed for hand infections at urgent care — meaning empiric treatment with these agents will fail.
Once bacteria are deposited inside the MCP joint, they begin multiplying in a nutrient-rich, enclosed space with limited immune access. The inflammatory response releases enzymes that degrade articular cartilage. The process begins within hours. Cartilage, once destroyed, does not regenerate. A septic joint that is not surgically irrigated promptly will sustain permanent damage — loss of smooth articular surface, chronic pain, stiffness, and eventual degenerative joint disease.
Left: with the fist closed, the skin wound, extensor tendon laceration, and joint capsule penetration all line up along the same vertical track. Right: when the fist opens, the skin slides proximally — the surface laceration no longer sits over the joint or tendon injury beneath. The wound appears superficial. The damage is hidden.
This anatomy is why every wound over the MCP joint must be explored surgically — surface examination in the open hand position consistently underestimates the depth of injury.
Microbiology
Human bite wounds carry a polymicrobial inoculum that is specifically adapted to cause invasive infections. The critical organism for antibiotic selection is Eikenella corrodens — resistant to the antibiotics most commonly prescribed for hand wounds at urgent care.
Surgical Treatment
When joint penetration is confirmed or strongly suspected, the treatment is OR irrigation and debridement the same day — not antibiotics and follow-up. Dr. Graham addresses both injuries simultaneously: the joint is irrigated under pressure to remove the bacterial inoculum, and the extensor tendon is assessed and repaired if lacerated, all in the same operative setting.
The wound is explored in the position of injury — with the MCP joint in flexion — to properly assess the depth of penetration relative to the joint capsule and tendon. The capsule is opened, the joint irrigated with liters of saline under pressure, and any devitalized tissue debrided. The tendon is examined through its full excursion to identify partial or complete lacerations that may not be visible in the open hand position.
Wounds are left open or only loosely closed — primary tight closure of a contaminated fight bite wound risks trapping residual bacteria and promoting deep abscess formation. A second-look procedure may be planned at 48 hours if the infection is severe or contamination was significant.
IV antibiotics with appropriate polymicrobial oral flora coverage — including Eikenella — are started immediately and continued post-operatively. Transition to oral antibiotics is made as clinical improvement is confirmed. Culture results from the OR guide antibiotic duration and selection.
Polymicrobial oral flora coverage started on presentation — before surgery. Ampicillin-sulbactam or equivalent provides Eikenella, Streptococcal, Staphylococcal, and anaerobic coverage. Do not wait for culture results to start antibiotics.
Wrist or more proximal regional block. Outpatient procedure at Baptist Beaches Hospital or Horizon Surgery Center — most cases do not require general anesthesia.
MCP joint placed in flexion to replicate the position at impact. Wound is extended and explored. The skin wound in the open-hand position systematically underestimates the depth of injury — exploring in flexion reveals the true track to the joint.
Joint capsule opened. MCP joint irrigated with copious saline under pressure — removing the bacterial inoculum and any necrotic debris. Cultures obtained from joint fluid and any purulent material. Thorough debridement of devitalized tissue.
Extensor tendon examined through full excursion. Partial lacerations addressed with repair. Complete lacerations repaired primarily if tissue quality allows. Tendon reconstruction deferred until infection is fully cleared if tissue is too contaminated for reliable repair.
Wound left open or very loosely approximated — never primarily closed. Dressing applied. Return to OR at 48 hours for second-look if significant contamination or infection. IV antibiotics continued; transitioned to oral when clinically appropriate.
Bacteria in the MCP joint begin destroying articular cartilage from the moment of inoculation. The longer surgery is delayed, the more irreversible damage accumulates. A joint irrigated within hours looks clean. A joint irrigated after 48 hours looks destroyed.
Bacteria established in joint space. Cartilage intact. Irrigation at this stage — excellent joint preservation. Best outcomes.
Inflammatory response releasing proteases. Cartilage degradation beginning. Irrigation still effective — urgency high.
Significant cartilage loss. Joint preservation possible but outcome less predictable. Chronic joint disease likely even with treatment.
Severe cartilage destruction. Joint fusion or chronic septic arthritis. In worst cases — amputation. Irreversible damage.
"I cut my knuckle and the ER gave me antibiotics and sent me home. Two days later my hand was so infected I could barely move my fingers. Dr. Graham operated that day and told me if I had waited another day I might have lost the use of that joint. He was direct, skilled, and took it seriously when no one else did."
Common Questions
Questions from patients — and from the ER providers who call about these cases.