Infected Flexor Sheath (Pyogenic Flexor Tenosynovitis) Info In-depth Evidence

What you're feeling

An infected flexor sheath is an infection in the tunnel that a finger's bending tendon slides through. The whole finger often swells evenly, along its full length, rather than in one sore spot. The finger may sit slightly bent and be hard to straighten. Straightening it, or someone gently straightening it for you, usually hurts the most, especially near the base of the finger where the swelling can show.

The pain sits along the line of the tendon, from the base of the finger down towards the palm, rather than over one joint. It tends to be there all the time and worsens when the finger is pushed straight. Bending the finger a little often eases it, which is why the finger curls up. The swelling and pain can make everyday tasks difficult: gripping a kettle, turning a key, doing up buttons, holding a pen or holding a handrail.

This infection can move quickly, and it is one that should not wait. If your finger or hand becomes hot, red, swollen and painful, especially with a fever, go to an emergency department the same day. You do not need a GP referral first. The same applies if your finger turns pale, cold, white or blue, or if you suddenly lose feeling or movement in it.

What's actually happening

Along the underside of each finger runs a bending tendon, a strong cord that pulls the fingertip towards the palm. That tendon slides inside a closed tunnel called the flexor sheath. The tunnel is lined with a slippery layer that keeps the tendon fed, lubricated and gliding freely, a bit like a cable running through a greased sleeve.

When bacteria get inside that sleeve, usually through a cut, a puncture or a bite, the closed space fills with pus. Because the sleeve is sealed, the pus sits under pressure against the very surface the tendon needs to glide on. The pressure squeezes the tendon's blood supply and starts scarring between the tendon and its lining. That is why the whole finger swells evenly, why it curls up, and why straightening it hurts so much: every part of the tunnel is inflamed at once.

The infection can travel. The thumb's tunnel connects to a fluid space in the palm called the radial bursa, and the little finger's connects to one called the ulnar bursa. In many people these spaces run right up towards the wrist, so infection in one finger can spread towards the palm or even across to the thumb or little finger, forming what is called a horseshoe abscess.

The stakes come down to timing. The tendon depends on gliding, and pus trapped against it does lasting damage the longer it sits. Treated early, most fingers recover well. Left too long, the tendon itself can die, the finger can scar stiff, and in severe cases the finger can be lost. Even with prompt antibiotics and surgery, some stiffness in the finger afterwards is common, because the healing tissue tends to grip rather than glide.

Your surgeon will check how far the infection has spread before deciding how urgently to drain it.

What we can do about it

Dr Kieran Hirpara, an upper-limb surgeon at Mater Private Hospital Rockhampton, treats an infected flexor sheath as an emergency and starts treatment straight away. This infection does not wait, and neither should you. The aim is to clear the infection before it damages the tendon that lets your finger bend.

The first step is antibiotics given through a drip in your arm. Antibiotics work with the drainage, not instead of it. Because the sheath is a sealed tunnel, antibiotics alone often cannot clear the infection on their own. Draining the pus takes the pressure off the tendon and lets the antibiotics reach the whole area.

Most cases need surgery to drain the sheath. We make small incisions, small cuts in the skin, so we can open the tunnel and wash the pus out. Washing the sheath out promptly protects the gliding surface your tendon needs. The exact timing and type of surgery depend on how far the infection has spread and how early you arrive.

After surgery, your finger may need a splint to rest it. Your hand therapy after surgery is with Ruby Doolan at Extend Rehabilitation. Ruby is a hand therapist: she guides your exercises and makes any splint you need. Her focus is getting the tendon gliding again, because scarring between the tendon and its lining is what limits movement later.

Some infections come from unusual germs, such as mycobacteria or fungi, which need specific antibiotic or antifungal treatment over a longer course. If the infection has badly damaged the tendon, rebuilding it is a separate operation that can be done once the infection has fully settled.

The outlook depends heavily on timing. Coming in early is the one thing that most protects your finger, so if your finger is hot, red, swollen and painful, especially with a fever, go to an emergency department the same day.

What to expect

The outlook for an infected flexor sheath depends mostly on how quickly it is treated. Coming in early gives your finger the best chance of bending normally again. Waiting lets the pus sit under pressure against the tendon, and that pressure does damage that cannot be undone later.

With prompt treatment, most fingers settle and start to move again over the following weeks and months. Even so, some stiffness afterwards is common, even in otherwise healthy people who had antibiotics and surgery quickly. The healing tissue tends to grip rather than glide, so the finger may not bend or straighten as freely as it did before. Hand therapy works on this, but it takes time and steady effort.

If the infection is severe or left too long, the outlook is harder. The tunnel around the tendon can be destroyed, and scarring can leave the finger permanently stiff. In some cases the swelling becomes so severe that it squeezes the blood supply to the finger itself, a problem called compartment syndrome. In the most severe infections, the finger can die and need amputation, even when treatment was timely and thorough.

A few less common things can also happen. Sometimes the infection does not fully clear and needs a second washout. Rarely, an unusual germ can appear in the finger weeks after treatment and need further care.

The one thing you can control is timing. If your finger or hand becomes hot, red, swollen and painful, especially with a fever, go to an emergency department the same day. You do not need a GP referral first. The same applies if your finger turns pale, cold, white or blue, or if you suddenly lose feeling or movement in it.

When to see someone

This infection is an emergency, and it moves fast. Go to an emergency department the same day if your finger or hand becomes hot, red, swollen and painful, especially with a fever. The same applies if your finger turns pale, cold, white or blue, or if you suddenly lose feeling or movement in it. You do not need a GP referral first. If you cannot reach the clinic, for example out of hours or on a weekend, go to your nearest emergency department.

The warning signs follow the shape of the infection: the whole finger swells evenly, it sits bent, it hurts along the line of the tendon, and straightening it is very painful. You do not need all of these signs before getting checked. A finger that shows only some of them can still have this infection.

See your GP or ask for a specialist review if symptoms are not settling, are getting worse over weeks, or stop you using the hand.

In more depth

Advanced reading: the deeper science (optional)

This section goes further than you need for your own treatment decisions. An infected flexor sheath is worth the extra reading because it is the one hand infection that is a genuine emergency, and because two of the things that most determine how it ends were decided before you reached hospital — one of them years before.

The diagnosis rests on four signs from the 1930s that have never been properly tested

Allen Kanavel described four signs of an infected flexor sheath: a finger swollen along its whole length rather than in one spot, held slightly bent, tender all the way along the tendon sheath rather than over one joint, and, the most useful of the four, severe pain when someone gently straightens the finger [1].

Nearly a century later, those four signs are still how the diagnosis is made, and their sensitivity, specificity and interobserver reliability have never been properly established [1]. That is a remarkable gap for a diagnosis whose delay costs fingers.

The practical consequence is the part worth carrying: not all four signs appear in every case, particularly in children, and the absence of one or more does not exclude the diagnosis [1]. A finger that fails only one of Kanavel's tests is not a finger that has been cleared.

How it is drained changes how well the finger moves afterwards

Once the diagnosis is made the sheath has to be decompressed, and there are two broad ways to do it: open the sheath surgically, or pass a fine catheter into it and irrigate through a much smaller wound.

A systematic review covering 763 patients found that catheter irrigation produced better range of motion than open washout, and that using antibiotics as part of treatment, rather than relying on drainage alone, also improved movement [2].

Note that this is the opposite of the position on a felon, where a properly drained pulp abscess needs no antibiotic at all. The difference is the anatomy: a felon is a closed pocket you can empty completely, while a sheath is a long tube lined with the very surface the tendon must glide against. You cannot debride it clean without damaging what you are trying to save, so the antibiotic does work the surgery cannot.

The risk of losing the finger is mostly a property of the patient

This is the least comfortable finding and the most useful one. In the pooled series, amputation rates were driven overwhelmingly by background health rather than by surgical technique: 39% in patients with diabetes, 64% with renal failure, and 71% with peripheral vascular disease, all statistically significant [2].

Those numbers describe a different disease from the one a healthy person with a splinter injury has. They are why an infected sheath in someone with diabetes or poor circulation is treated with more urgency and a lower threshold for repeat washout, and why the honest answer to "will I keep the finger" depends more on the answer to "what else is going on with your health" than on anything that happens in theatre.

Why the delay matters more than almost anything else

The sheath is a closed space with a poor blood supply and a tendon inside it that depends on gliding. Pus under pressure inside that space does two things at once: it strangles the tendon's blood supply, and it seeds the adhesions that later limit movement. Both are time-dependent, which is why early treatment consistently improves outcomes [2] and why the standard advice for this specific infection is an emergency department rather than a GP appointment [3].

Stiffness afterwards is common even when everything is done correctly and promptly. How much stiffness is set largely by how long the sheath was under pressure, which is the one variable a patient can influence, by coming in early.

Infected Flexor Sheath (Pyogenic Flexor Tenosynovitis) Info In-depth Evidence

What you're feeling

What's actually happening

What we can do about it

What to expect

When to see someone

In more depth

The diagnosis rests on four signs from the 1930s that have never been properly tested

How it is drained changes how well the finger moves afterwards

The risk of losing the finger is mostly a property of the patient

Why the delay matters more than almost anything else

Overview

Anatomy & Pathophysiology

Flexor Tendon Anatomy and Sheath Structure

Pathophysiology of Pyogenic Flexor Tenosynovitis

Classification

Investigations

Treatment

Operative

Outcomes and Prognosis

Complications

Recovery

Key Evidence

References