Paratenon

Paratenon, also referred to as the peritenon in some texts, is a layer of loose connective tissue that surrounds a tendon when the tendon is not enclosed by a synovial sheath. It lies between the tendon proper and the surrounding tissues, blending with the epitenon and adjacent fascia. The paratenon contains small blood vessels and nerves and serves to permit smooth gliding of the tendon within its surrounding environment.

In tendons without a synovial sheath, the paratenon forms the primary covering and contributes to the vascular supply of the tendon through its network of vessels. It facilitates movement by reducing friction between the tendon and adjacent structures during contraction and relaxation.

Paratenonitis is inflammation of the paratenon, typically arising from overuse or repetitive microtrauma. It most commonly affects superficial tendons such as the Achilles, and presents with diffuse tenderness and swelling along the tendon. Chronic inflammation can coexist with or contribute to tendinopathy, potentially affecting tendon function.

Diagnosis is usually clinical, supported by imaging when needed. Ultrasound or MRI may show thickening of the paratenon and fluid around the tendon. Treatment generally includes activity modification, rest, and nonsteroidal anti-inflammatory drugs, followed by physical therapy that emphasizes gradual loading and eccentric strengthening. In some cases, injections are considered near the paratenon but away from the tendon substance to minimize injury risk.