MRI Anatomy Flashcards · MSK

Thumb Anatomy

Learn to identify every labeled structure on a Thumb MRI, plane by plane.

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Thumb anatomy, structure by structure

Thumb MRI is a focused, high-resolution study, and reading it starts with knowing the normal anatomy cold. This reference walks through every structure in our thumb flashcard deck across the planes you will scan (sagittal and coronal), with a plain-language definition, how each structure looks on MRI, and the pathology you will actually run into at the scanner.

Bones

The skeletal ray of the thumb runs proximal to distal: metacarpal, then proximal phalanx, then distal phalanx. The sagittal plane lines them up end to end; coronal compares the sides of each bone.

Metacarpal

The first (thumb) metacarpal, the long tubular bone of the hand that forms the base of the thumb ray between the wrist and the proximal phalanx.

On MRI: The most proximal long bone of the thumb on sagittal and coronal images, capped by bright marrow fat on T1 with a thin dark cortical rim. Its base articulates with the trapezium at the wrist and its head with the proximal phalanx.

Common pathology: Bennett and Rolando fracture-dislocations at the metacarpal base, stress reaction, and metacarpal osteomyelitis or marrow signal change are the common findings.

Tip: On sagittal, follow the thumb ray from the wrist outward: the first long bone you reach before the knuckle is the metacarpal.

Metacarpal Head

The rounded distal end of the thumb metacarpal that forms the proximal half of the metacarpophalangeal (knuckle) joint.

On MRI: A smooth, rounded articular surface at the far end of the metacarpal on coronal images, covered by intermediate-signal cartilage and underlain by bright marrow fat on T1.

Common pathology: Osteochondral injury, subchondral cysts and edema, and marrow change at the proximal attachment of the thumb collateral ligaments; note that ulnar collateral ligament avulsions most often occur at the distal (proximal phalanx) attachment rather than at the metacarpal head.

Tip: It is the ball of the MCP joint: the curved cap of the metacarpal sitting just proximal to the knuckle line.

Proximal Phalanx

The first phalanx of the thumb, the bone between the metacarpophalangeal joint and the interphalangeal joint.

On MRI: The middle long bone of the thumb ray on sagittal and coronal, sitting distal to the metacarpal head and proximal to the distal phalanx, with bright marrow fat on T1.

Common pathology: Phalangeal fractures, collateral ligament avulsions at its base, and marrow signal change from infection or trauma.

Distal Phalanx

The terminal bone of the thumb, beyond the interphalangeal joint, that supports the nail and fingertip pad.

On MRI: The short, tapering bone at the very tip of the thumb on sagittal and coronal images; its broadened distal tuft anchors the soft tissues of the fingertip.

Common pathology: Tuft fractures, mallet-type avulsions of the extensor insertion, and distal phalanx osteomyelitis from a felon or nail-bed infection.

Tip: It is the last bone in the ray: the small bone past the IP joint that backs the nail.

Joints

Three joints stack along the thumb from the wrist to the tip. Sagittal images open the joint spaces cleanly so you can judge alignment and cartilage.

Carpometacarpal (CMC) Joint of Thumb

The saddle-shaped joint between the trapezium of the wrist and the base of the thumb metacarpal that gives the thumb its wide range of motion.

On MRI: The most proximal thumb joint, seen where the metacarpal base meets the trapezium; the articular cartilage is intermediate signal and the joint space follows fluid signal on T2.

Common pathology: First CMC osteoarthritis (basal joint arthritis) is extremely common: look for joint-space narrowing, subchondral cysts, osteophytes, and subluxation.

Metacarpophalangeal (MCP) Joint

The knuckle joint of the thumb, between the metacarpal head and the base of the proximal phalanx, stabilized by the collateral ligaments and volar plate.

On MRI: The joint between the rounded metacarpal head and the proximal phalanx base; best opened on sagittal, while coronal is the key plane for the collateral ligaments on either side.

Common pathology: Gamekeeper and skier thumb (ulnar collateral ligament injury) centers here, along with volar plate injury and inflammatory or osteoarthritis.

Interphalangeal (IP) Joint

The single hinge joint of the thumb, between the proximal and distal phalanges, that flexes and extends the fingertip.

On MRI: The distal thumb joint on sagittal, between the proximal and distal phalanges; the hinge surfaces carry intermediate-signal cartilage with fluid-signal joint space on T2.

Common pathology: IP joint osteoarthritis, septic arthritis, and articular involvement from mallet-type or intra-articular phalangeal fractures.

Ligaments and tendons

The soft-tissue stabilizers and mover of the thumb. Coronal is the workhorse plane for the collateral ligaments; tendons are best traced where they run along the bone.

Collateral Ligament

A band on the side of the thumb metacarpophalangeal joint connecting the metacarpal head to the proximal phalanx; the ulnar collateral ligament is the one torn in gamekeeper or skier thumb.

On MRI: A low-signal (dark) band running alongside the MCP joint, best evaluated on coronal images; compare the ulnar and radial sides for thickening, discontinuity, or retraction.

Common pathology: Ulnar collateral ligament tear (gamekeeper or skier thumb); a Stener lesion, where the torn end displaces above the adductor aponeurosis, is the key finding that changes management to surgery.

Tip: Read it on coronal across the MCP joint and always check the ulnar side: a balled-up, retracted ligament stump above the aponeurosis is a Stener lesion.

Tendon of Extensor Pollicis Longus

The tendon that runs along the back of the thumb to extend the tip, attaching to the base of the distal phalanx.

On MRI: A thin, low-signal (dark) tendon coursing along the dorsal surface of the thumb on sagittal images, traceable from the metacarpal toward its insertion on the distal phalanx.

Common pathology: Extensor pollicis longus rupture (classically after a distal radius fracture or in rheumatoid arthritis) and tenosynovitis with bright fluid surrounding the tendon on T2.

Tip: Trace the dark line riding along the back of the thumb on sagittal: that dorsal tendon is the extensor pollicis longus heading for the distal phalanx.

Frequently asked questions

What structures are seen on a thumb MRI?

A focused thumb MRI shows the bones of the thumb ray (the metacarpal, metacarpal head, proximal phalanx, and distal phalanx), the three joints (carpometacarpal, metacarpophalangeal, and interphalangeal), and the soft-tissue stabilizers including the collateral ligaments of the MCP joint and the extensor pollicis longus tendon. This page labels each one with its MRI appearance and the pathology techs commonly see.

Which planes are used for thumb MRI?

Thumb MRI is read mainly on sagittal and coronal images. Sagittal lines up the bones of the ray and opens the joint spaces front to back, while coronal is the workhorse plane for the collateral ligaments at the metacarpophalangeal joint. Most protocols also include an axial series, but this deck focuses on sagittal and coronal.

What is the most common reason for a thumb MRI?

Suspected ulnar collateral ligament injury of the thumb MCP joint, known as gamekeeper or skier thumb, is a leading indication. The study is ordered to confirm the tear and, critically, to look for a Stener lesion, where the torn ligament end displaces above the adductor aponeurosis and requires surgical repair.

How do I identify the collateral ligament on a thumb MRI?

Look at the coronal images across the metacarpophalangeal joint. The collateral ligament is a low-signal (dark) band running alongside the joint from the metacarpal head to the proximal phalanx base. Compare the ulnar and radial sides; a thickened, discontinuous, or retracted ulnar band is the abnormal finding.

Do I need an account to use these thumb MRI flashcards?

No. The interactive flashcards and this full labeled reference are open to use, with no account required to start. Creating an account lets you save your progress across devices and track which packs you have mastered.

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