S66.009 Unspecified injury of long flexor muscle, fascia and tendon of unspecified thumb at wrist and hand level

ICD-10-CM Code S66.009: Unspecified injury of long flexor muscle, fascia and tendon of unspecified thumb at wrist and hand level

ICD-10-CM code S66.009 designates an unspecified injury of the long flexor muscle, fascia, and tendon of an unspecified thumb at the wrist and hand level. The thumb's primary flexor tendon is the flexor pollicis longus, which attaches to the distal phalanx to facilitate bending. Flexor tendon injuries are frequently caused by trauma and diagnosed through clinical physical examinations. 3 5 6

Plain-language overview

Flexor tendons are cord-like structures extending from forearm muscles to the hand that facilitate thumb and finger movement. The thumb relies on one major flexor tendon, the flexor pollicis longus, while the four fingers each have two flexor tendons. 5

What this code represents

ICD-10-CM code S66.009 is designated for an unspecified injury of the long flexor muscle, fascia, and tendon of an unspecified thumb at the wrist and hand level. The semantic type for this concept is classified as an injury. 3

Clinical context

The flexor pollicis longus is attached to the volar aspect of the distal phalanx of the thumb. The flexor tendon system of the hand comprises tendons, synovial sheaths, pulleys, and muscles that guide movement and increase mechanical advantage. 6

Flexor tendon injury symptoms include pain, swelling, and bruising in the affected area, along with difficulty or inability to bend the affected finger or thumb. Patients may also experience a sensation of catching or popping when attempting to flex the finger. 5

Documentation considerations

Diagnosing a flexor tendon injury begins with a thorough medical history and physical examination by an experienced hand surgeon. Physical exam findings often suggest the presence or absence of injury by correlating the laceration, overlying skin injury, and mechanism. 1 5

The most common cause of flexor tendon lesions is trauma, such as a laceration, bite, or crush injury. Lesions of the flexor tendon mechanism can occur at any level, but they are most frequent at the metacarpophalangeal joint and the proximal interphalangeal joint. 6

What the sources add

While S3 defines the exact ICD-10-CM code S66.009 as an unspecified thumb flexor injury, S6 provides clinical context that flexor tendon lesions are most commonly caused by trauma. S5 further supplements this by detailing that the thumb has one major flexor tendon. 3 5 6

S1 establishes that primary repair is the optimal treatment for flexor tendon injuries, though this technique is not always feasible in severe hand trauma. S6 adds that the current standard of treatment for flexor tendon repair is a 6-strand core suture and venting of the pulley. 1 6

Sources

1. Bookshelf — ncbi.nlm.nih.gov; accessed 2026-07-25.
2. Bookshelf — ncbi.nlm.nih.gov; accessed 2026-07-25.
3. EVS Explore — evsexplore.semantics.cancer.gov; accessed 2026-07-25.
4. EVS Explore — evsexplore.semantics.cancer.gov; accessed 2026-07-25.
5. Flexor Tendon Injury | San Francisco — The Buncke Clinic — thebunckeclinic.org; accessed 2026-07-25.
6. Flexor tendon mechanism — surgeryreference.aofoundation.org; accessed 2026-07-25.