Flexor Tendon Sheath Infection (Pyogenic Flexor Tenosynovitis)

A flexor tendon sheath infection (pyogenic or suppurative flexor tenosynovitis) is a closed-space bacterial infection of the synovial sheath that surrounds the flexor tendons of a digit, usually following penetrating trauma over a flexion crease. It is a surgical emergency: pus under pressure within the sheath rapidly destroys the tendon's gliding surface and precarious blood supply, so standard treatment is admission, empirical intravenous antibiotics and, in the majority of patients, urgent surgical washout of the sheath. In the UK, management is defined by the British Society for Surgery of the Hand (BSSH) trauma standard on flexor sheath infection, supported by the general framework of the July 2025 BOAST on musculoskeletal soft tissue infections.

Why is a flexor sheath infection a surgical emergency?

The flexor sheath is a sealed, double-walled synovial tube with a limited vascular supply; the tendons within it depend partly on synovial diffusion for nutrition. Bacterial proliferation in this closed space raises intrasheath pressure (recorded up to around 30 mmHg), which impairs tendon perfusion and gliding and can progress to tendon necrosis, rupture and digital ischaemia within days.

The anatomy also explains proximal spread. The little finger sheath communicates with the ulnar bursa, and the thumb sheath with the radial bursa; the two bursae communicate through the space of Parona in roughly 80% of people. Infection of the thumb or little finger can therefore track proximally and across the wrist as a horseshoe abscess, and the space of Parona itself can become infected, threatening the median nerve.

Pathogens: Staphylococcus aureus is the dominant organism (around three-quarters of cases), with MRSA reported in up to about a third of cases in some series. Beta-haemolytic streptococci, S. epidermidis and Pseudomonas also occur. Think of Eikenella corrodens after human bites, Pasteurella multocida after animal bites, and polymicrobial or Gram-negative infection in immunocompromised patients and people with diabetes.

How does it present? Kanavel's cardinal signs

Diagnosis is clinical, based on the four signs described by Kanavel in 1912:

• Fusiform (symmetrical) swelling of the whole digit — the most common sign, present in 97% of patients in Pang and colleagues' series
• Semi-flexed posture of the digit at rest
• Pain on passive extension of the digit — generally the earliest sign
• Tenderness along the course of the flexor sheath — typically the last sign to appear

Taken together the four signs are highly sensitive (reported at 91–97%), but not every patient shows all four, particularly early or partially treated presentations. Take blood cultures and inflammatory markers, and obtain a plain radiograph to exclude a retained foreign body; ultrasound may demonstrate sheath fluid but must never delay definitive treatment. Good practice is to actively consider the differentials — septic arthritis, felon, dorsal cellulitis, gout and herpetic whitlow — because a whitlow, in particular, should not be incised.

What is the immediate management?

The BSSH flexor sheath infection standard sets out first aid and referral:

• Elevate the limb and splint in a position of safe immobilisation (POSI)
• Start empirical IV antibiotics according to local protocols, covering Staphylococcus and Streptococcus species, "plus gram negative rods and anaerobes in immunocompromised patients" — after cultures where possible
• Refer as category orange (BSSH hand trauma triage): the patient "may need to go to theatre within 24 hours and so should be referred to a department where they will be treated under the direct supervision of a Consultant Hand Surgeon"

The July 2025 BOAST on musculoskeletal soft tissue infections adds the system-level requirements: locally agreed multi-specialty pathways naming the responsible specialty, immediate initiation of the sepsis six in any patient with evidence of sepsis, and formal operating theatre drainage for deep or complex collections. If there is any suspicion of necrotising infection, that BOAST is explicit that surgery "should not be delayed by medical and intensive care management, imaging, or inter-hospital transfer".

When are antibiotics alone enough, and when is washout needed?

Per the BSSH standard, the majority of patients require urgent surgical washout of the flexor tendon sheath. A non-operative trial is the exception, not the rule:

• It may be appropriate only in patients "presenting very early with mild symptoms and signs" — in the literature this generally means within about 48 hours of onset with incomplete Kanavel signs
• The BSSH describes this as "a trial of 24 hours of IV antibiotics" and notes that "senior surgical review may be required to make this difficult clinical decision"
• The patient must be monitored closely and reviewed within 24 hours, with surgery planned "if any deterioration occurs" or if there is no clear improvement

Consent should follow shared decision-making: discuss non-surgical and surgical options, the rehabilitation pathway and likely outcomes, explicitly including the risk of tendon rupture or amputation with delayed presentation or severe infection, in the context of the patient's occupation and hand function needs.

How is the flexor sheath washed out? Closed sheath irrigation

The BSSH standard requires surgery to be "urgent — as early as possible within safe working hours and within a maximum of 24 hours of decision to operate", with staffing and theatre requirements as for the flexor tendon injury standard. The workhorse technique is closed (limited-incision) sheath irrigation:

1. Two limited incisions: proximally over the A1 pulley and distally at the level of the distal interphalangeal joint, planned so they "can be extended and connected if further debridement or access is required"
2. Send sheath fluid and tissue for Gram stain and culture before antibiotics are escalated
3. Pass a fine catheter (typically 16–18G or a paediatric feeding tube) into the sheath and irrigate with high volumes of normal saline, proximal to distal, "until the outflow is clear"
4. "Care must be taken to ensure there is outflow of fluid from the digit so that the tissue pressure is not raised above safe limits"
5. Leave the wounds open or loosely tacked; purulent infections require a planned repeat washout at 48 hours

Open washout through an extensile Bruner or midlateral approach is reserved for late presentations with frank purulence, tissue necrosis or a horseshoe abscess. The evidence base (summarised by Giladi and colleagues, 2015) shows no clear superiority of any single irrigation protocol, and closed catheter irrigation appears as effective as open irrigation when the sheath is not necrotic. The Michon staging is a useful way to frame the operative decision:

Postoperatively, rationalise antibiotics against cultures with microbiology advice; a 7–14 day oral course after adequate washout is usually sufficient. The BSSH mandates early hand therapy: supervised early active motion can begin the day after surgery, with therapy offered at weekly intervals for at least six weeks.

What are the outcomes and complications?

With timely treatment the BSSH expectation is that "the patient should be able to regain a functional range of active motion", but 10–25% of patients are left short of full active range. Adhesions, tendon rupture, pulley destruction, proximal spread and digital amputation are the principal complications. Pang and colleagues (JBJS Am, 2007; 75 patients) identified age over 43, diabetes/peripheral vascular disease/renal failure, subcutaneous purulence, digital ischaemia and polymicrobial infection as predictors of poor outcome, and stratified prognosis:

Units should run rolling audits of microbiological confirmation of diagnosis, time to surgery, tendon rupture rate and total active motion of the digit, as listed in the BSSH standard.

Key points

• Flexor sheath infection is a closed-space surgical emergency; delay costs tendon, motion and occasionally the digit.
• Diagnosis is clinical using Kanavel's four signs — fusiform swelling (commonest), flexed posture, pain on passive extension (earliest) and sheath tenderness (latest).
• Immediate care per the BSSH standard: elevation, POSI splint, empirical IV antibiotics covering staphylococci and streptococci, and orange-category referral to a consultant hand surgeon.
• A 24-hour IV antibiotic trial is reasonable only for very early, mild presentations, decided at senior level with mandatory review within 24 hours.
• Most patients need washout within 24 hours of the decision to operate: closed sheath irrigation via A1 pulley and DIPJ incisions with high-volume saline until clear, ensuring free outflow; purulent sheaths get a planned re-look at 48 hours.
• Subcutaneous purulence and digital ischaemia (Pang group III) predict a 59% amputation rate; early hand therapy from day one protects final motion.