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Flexor retinaculum of wrist

The flexor retinaculum of the wrist, also known as the transverse carpal ligament, is a strong fibrous band forming the roof of the carpal tunnel on the palmar aspect of the wrist. It serves as a restraining structure, holding the flexor tendons close to the wrist during movement and protecting the underlying neurovascular structures, most notably the median nerve.

It converts the concave carpal arch into a tunnel through which tendons of the flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus pass, along with the median nerve. The integrity and tightness of the retinaculum are crucial for efficient finger flexion and wrist stability.

Synonyms

  • Transverse carpal ligament

  • Anterior annular ligament of the wrist

Location and Structure

  • The flexor retinaculum is located on the anterior aspect of the wrist, forming a strong fibrous bridge over the carpal bones.

  • It is approximately 2–3 cm wide and 1–1.5 mm thick in most adults, composed of dense collagen fibers.

  • Converts the carpal groove into the carpal tunnel, protecting the underlying tendons and median nerve.

Attachments

  • Medially: Pisiform bone and hook of the hamate

  • Laterally: Tubercle of the scaphoid and crest of the trapezium

  • Superficial relations: Palmaris longus tendon and palmar cutaneous branch of median nerve

  • Deep relations: Median nerve and flexor tendons within the carpal tunnel

Relations

  • Superiorly (superficial): Palmar fascia, palmaris longus tendon, and palmar cutaneous branches of median and ulnar nerves

  • Inferiorly (deep): Median nerve, flexor pollicis longus tendon, and tendons of flexor digitorum superficialis and profundus

  • Medially: Ulnar artery and nerve (outside the carpal tunnel)

  • Laterally: Thenar muscles (abductor pollicis brevis, opponens pollicis)

Function

  • Mechanical support: Maintains position of flexor tendons, preventing bowstringing during wrist flexion

  • Protective role: Safeguards tendons and the median nerve within the carpal tunnel

  • Force transmission: Provides leverage for flexor tendons across the wrist

  • Tunnel formation: Forms the roof of the carpal tunnel, maintaining its structure

  • Clinical relevance: Contributes to carpal tunnel syndrome when thickened or fibrosed

Clinical Significance

  • Carpal tunnel syndrome: Compression of the median nerve beneath the flexor retinaculum, causing paresthesia and weakness

  • Thickening or fibrosis: May result from repetitive wrist motion or inflammatory disorders (e.g., rheumatoid arthritis)

  • Surgical importance: The retinaculum is divided during carpal tunnel release surgery to decompress the median nerve

  • Trauma or iatrogenic injury: Can lead to altered tendon gliding and wrist instability

  • Imaging role: MRI and CT are critical for evaluating retinacular thickness, carpal tunnel contents, and postoperative changes

MRI Appearance

  • T1-weighted images:

    • Flexor retinaculum: low signal (dark linear band) overlying the carpal tunnel

    • Surrounding fat: bright signal, accentuating the ligament’s contour

    • Normal thickness: 1–1.5 mm; thickening (>2 mm) suggests pathology

    • Median nerve: intermediate signal; flattening or compression indicates entrapment

  • T2-weighted images:

    • Retinaculum: low signal intensity, darker than adjacent tendons and muscle

    • Thickening or fibrosis: persistent low signal with irregular contour

    • Inflammation or edema: may cause mild intermediate hyperintensity

    • Median nerve compression: nerve shows focal swelling with hyperintense signal proximally

  • STIR:

    • Normal retinaculum: dark (low signal)

    • Pathologic changes: bright hyperintensity if inflamed or edematous

    • Adjacent fluid in the carpal tunnel appears bright, enhancing nerve-tendon contrast

  • Proton Density Fat-Saturated (PD FS):

    • Normal: dark homogeneous band over carpal tunnel

    • Pathology: thickened retinaculum with mild periligamentous hyperintensity in synovitis or post-surgical fibrosis

    • Excellent for visualizing median nerve swelling and tendon sheath fluid

  • T1 Fat-Sat Post-Contrast:

    • Normal ligament: minimal or no enhancement

    • Synovitis or postoperative scar: enhancing surrounding soft tissues

    • Perineural enhancement: indicates inflammation or entrapment neuropathy

CT Appearance

Non-Contrast CT:

  • Retinaculum: seen as a thin soft-tissue density band spanning across carpal bones

  • Carpal tunnel boundaries: clearly visualized, especially scaphoid and hamate attachments

  • Thickening: visible as soft-tissue prominence overlying the carpal groove

  • Useful for identifying calcifications, ossification, or postoperative changes

Post-Contrast CT (standard):

  • Retinaculum itself enhances minimally

  • Surrounding soft-tissue enhancement may indicate inflammation, fibrosis, or scar formation

  • Contrast highlights median nerve swelling and peritendinous vascularity in carpal tunnel syndrome

CT Arthrography Appearance

  • Technique: Contrast injected into the radiocarpal or midcarpal joint outlines adjacent structures, including the deep surface of the flexor retinaculum

  • Normal appearance: Smooth, sharply defined low-density band overlying the contrast-filled carpal tunnel

  • Pathologic findings:

    • Thickening or irregularity of the retinaculum contour

    • Contrast leakage into adjacent tissue planes in trauma or surgical defects

    • Better delineation of carpal tunnel contents, synovial recesses, and retinacular adhesions

  • Clinical utility: Excellent for evaluating postoperative fibrosis, carpal instability, and tenosynovitis extending beneath the retinaculum

MRI image

flexor retinaculum of wrist axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

flexor retinaculum of wrist MRI sagittal image