When bacteria enter the flexor tendon sheath, pus builds under pressure inside a closed space with no room to expand. The tendon's blood supply is compressed. Within hours to days, the tendon begins to die. Flexor tenosynovitis requires same-day surgical evaluation — every hour of delay increases the risk of permanent tendon damage and loss of finger motion.
The puncture wound may appear trivial or already healed by the time FTS develops. Do not let the small size of the wound reassure you.
Understanding the Infection
The flexor tendons of each finger run through a closed synovial sheath — a sealed tube of tissue that produces lubricating fluid allowing the tendons to glide smoothly during finger flexion. This sheath extends from the base of the finger into the palm, and in the thumb and small finger it connects directly to the radial and ulnar bursae in the wrist.
Under normal conditions this closed space is sterile. When bacteria enter — through a puncture wound, a bite, a laceration, or hematogenous spread — they find an ideal environment: warm, nutrient-rich, and sealed from the body's normal immune defenses. Infection spreads rapidly through the sheath in both directions. Pus accumulates under pressure with nowhere to go.
The flexor tendons receive their blood supply through vincula — small vascular connections along the sheath wall. As pressure inside the sheath rises, these blood vessels are compressed. The tendon's blood supply is interrupted. Without blood flow, the tendon begins to necrose. A necrotic tendon cannot be repaired — the finger loses active flexion permanently.
In Dr. Graham's experience, the patients who present late — having been reassured at urgent care that their swollen finger was "just a sprain" — are the ones who end up with the most devastating outcomes. Flexor tenosynovitis is one of the few hand conditions where hours genuinely matter.
Pus accumulates inside the sealed flexor tendon sheath (red), building pressure in all directions. The vincula — tiny vessels that supply the tendon through the sheath wall — are compressed. The tendon loses its blood supply and begins to necrose from the inside out.
In the thumb and small finger, the sheath communicates with the radial and ulnar bursae — meaning infection can spread into the wrist and palm without any additional puncture. This is the "horseshoe abscess" pattern — a catastrophic spread that requires urgent bilateral decompression.
Pus under pressure inside the flexor tendon sheath begins destroying the tendon's blood supply from the moment infection is established. The longer surgery is delayed, the more tendon necrosis occurs — and necrotic tendon cannot be repaired. Functional outcome correlates directly with time to surgical drainage.
Bacterial inoculation, sheath inflammation begins. Kanavel's signs developing. Tendon viable. Best window for closed irrigation — fastest recovery, best motion outcomes.
Pus accumulating under pressure. Vincular compression increases. Closed irrigation still often effective. Earlier surgery in this window = better outcome.
Tendon blood supply severely compromised. Necrosis beginning at areas of maximum compression. Open drainage required. Outcome less predictable — stiffness, tendon rupture risk.
Tendon necrosis established. Sheath wall involvement. Possible spread to bursa or adjacent spaces. Even with excellent surgical care, permanent loss of motion is likely.
Treatment Framework
The decision between a brief IV antibiotic trial and immediate surgery depends on the clarity of the diagnosis and the degree of clinical involvement at the time of presentation.
In carefully selected early or equivocal cases — where the clinical picture is not yet fully established and the patient is presenting very early in the course — Dr. Graham may admit for IV antibiotics and observe for 12 to 24 hours. If there is meaningful clinical improvement, surgical drainage may be avoided.
This is not watchful waiting. The patient is admitted, monitored closely, and re-examined frequently. Any failure to improve — any progression of Kanavel's signs — triggers immediate surgery. The threshold to proceed to the OR is low and the window is short.
When Kanavel's signs are clearly established, when there is obvious fluctuance or purulence, when the patient has been symptomatic for more than 24 to 48 hours, or when there is any sign of sheath necrosis or proximal spread — the answer is immediate surgical drainage. There is no role for an antibiotic trial in these cases.
Surgery clears the infected contents from the sheath under direct visualization, decompresses the pressure, restores tendon blood supply, and allows the infection to be cultured for targeted antibiotic therapy. No amount of IV antibiotics can achieve this in an established suppurative FTS.
Surgical Technique
The choice between closed tendon sheath irrigation and open drainage depends on what is found at surgery — specifically, the stage of the infection and whether tendon or sheath necrosis is present. Dr. Graham performs both techniques, selecting based on the clinical situation.
The goal in both cases is the same: eliminate the purulent contents, decompress the sheath, restore the environment for tendon survival, and obtain cultures to guide post-operative antibiotics. Patients are kept on IV antibiotics post-operatively and transitioned to oral antibiotics as clinical improvement is confirmed.
Post-operative hand therapy begins as soon as the infection is controlled — early motion is essential to prevent adhesion formation inside the sheath, which is a major source of long-term stiffness after FTS even after a technically successful drainage.
Microbiology
The responsible organism informs antibiotic selection — empiric broad coverage is started at presentation and narrowed based on intraoperative cultures.
"My finger swelled up overnight after a small cut and I almost didn't go in. Dr. Graham took one look and said I needed surgery that day. He wasn't wrong — it was infected all the way through. He saved the function of my finger. I am so grateful I didn't wait another day."
Common Questions
Questions from patients — and from the ER providers who call about these cases.