What Is Flexor Synovitis?

Flexor tenosynovitis (FT), also called flexor tendon synovitis or flexor synovitis, refers to the inflammation of the synovial sheath and tendons of the finger joints of the hands. It is usually caused by infection, but acute and chronic inflammatory pathophysiologic states, such as arthritis and diabetes, may also cause this condition. The diagnosis of infectious or septic FT is dependent on the four Kanavel signs, which include finger positioned in slight flexion, swelling, tenderness along the sheath of the flexor tendon, and pain elicited when the affected digit is passively extended. This inflammation can speedily destroy the functional capacity of a person’s fingers, so it is considered one of the orthopedic emergencies. Proper treatment should be initiated immediately.

Penetrating trauma is usually the main cause of infectious flexor synovitis. Other causes include bite wounds and hematogenous or bloodborne dissemination of bacteria from other infected sites of the body. Trauma disrupts the normal anatomy and physiology of the hand, allowing native skin flora, such as Staphylococcus and Streptococcus, to invade the tissues under the skin. Most cases of infectious flexor synovitis are due to Staphylococcus aureus, but there are many others.

Non-infectious flexor synovitis may occur secondary to amyloidosis, crystalline deposition, sarcoidosis, psoriatic arthritis, rheumatoid arthritis, diabetes mellitus, and systemic lupus erythematosus. Studies show that when flexor tenosynovitis is diagnosed with magnetic resonance imaging (MRI), it strongly predicts early rheumatoid arthritis. People who have diabetes are also at higher risk of having multiple digits affected by flexor synovitis.

A tendon sheath has both an inner layer, called the visceral layer, and an outer layer, called the parietal layer. The visceral layer is closely approximated to the flexor tendon, and the space between these layers, called the synovial space, contains synovial fluid. Based on this anatomical organization, an infection of the tendon sheath is called a closed-space infection. When pus accumulates in the flexor tendon sheath, pressure increases and leads to decreased blood flow to the area, or ischemia. With the occurrence of tendon ischemia, the risk of necrosis and rupture is high and may lead to loss of flexor function.

Non-infectious flexor synovitis occurs when there is a proliferation of certain substances, such as amyloid or crystals, within the joint space. These substances impinge on the nearby tendons, leading to swelling and pain. With repetitive microtrauma or overuse syndrome, the tendon tissues are not able to adapt to the chronic injury, leading to inflammation, proliferation, and maturation.

A person with infectious flexor tenosynovitis often complains of symptoms of pain, redness, and fever, and physical findings include the four Kanavel signs. The Kanavel signs might not be present if the patient is in an early stage of the disease, recently took antibiotics, is chronically infected, or has an immunocompromised state. A person with non-infectious flexor synovitis often has swelling of the knuckles as the initial symptom and complains of restricted motion and pain.

Treatment of this condition involves medical therapy, including intravenous antibiotics, elevation of the affected area, and rehabilitation with range of motion (ROM) exercises. People who are immunocompromised, have diabetes, or have flagrant pus in their tendons have to undergo surgical management. Surgical management involves an incision over the affected area, drainage of the pus, irrigation, splinting, and elevation, as well as concurrent antibiotic therapy.