Hamate Bone: Anatomy, Articulations, Function, Imaging

Topic

Hamate

The hamate bone is a wedge-shaped carpal bone located on the ulnar (medial) side of the distal carpal row of the wrist. It is distinguished by a prominent hook-like projection on its palmar surface known as the hamulus or hook of hamate, which serves as a key landmark and attachment point for ligaments and tendons in the ulnar wrist region.

It articulates with the capitate, triquetrum, fourth and fifth metacarpal bases, forming part of the carpometacarpal (CMC) and midcarpal joints. The hamate plays a vital role in grip strength, ulnar wrist stability, and force transmission from the hand to the forearm during grasping activities.

Synonyms

Unciform bone
Os hamatum

Location and Structure

Situated on the ulnar side of the distal carpal row, between the capitate (laterally) and triquetrum (proximally).
Shaped like a pyramidal wedge, with a base directed distally toward the metacarpals and an apex oriented proximally.
The hook (hamulus) projects from the palmar surface toward the radial side, curving slightly laterally.
Internal structure consists of cortical shell with trabecular cancellous bone containing fatty marrow.

Surfaces

Proximal surface: Articulates with the triquetral bone.
Distal surface: Divided into two facets for the bases of the fourth and fifth metacarpals.
Medial surface: Non-articular, giving attachment to ligaments.
Lateral surface: Articulates with the capitate.
Palmar surface: Prominent hook forming the ulnar border of the carpal tunnel and the radial wall of Guyon’s canal.
Dorsal surface: Rough for ligamentous attachments.

Articulations

Proximal: Triquetrum
Distal: Fourth and fifth metacarpals
Lateral: Capitate

Relations

Palmar aspect: Covered by flexor tendons and traversed by the ulnar nerve and artery through Guyon’s canal.
Dorsal aspect: Related to the extensor tendons of the ulnar digits.
Medial aspect: In proximity to the pisiform bone and flexor carpi ulnaris tendon.
Lateral aspect: Adjacent to the capitate and deep palmar arch.

Attachments

Ligamentous:
- Flexor retinaculum attaches to the hook of hamate (along with pisiform).
- Pisohamate and pisometacarpal ligaments arise from the hamulus.
Muscular:
- Flexor carpi ulnaris: via pisohamate ligament indirectly.
- Flexor digiti minimi brevis and opponens digiti minimi originate from the hook of hamate.

Function

Structural support: Stabilizes ulnar side of the distal carpal row.
Force transmission: Transfers loads from the ring and little fingers to the wrist.
Muscle anchor: Provides attachment for hypothenar muscles and ligaments forming the carpal tunnel.
Protection: Guards the ulnar nerve and artery as they pass through Guyon’s canal.

Clinical Significance

Hook of hamate fracture: Common in athletes (golf, racquet, baseball) from direct impact of handle on palm. Presents with ulnar-sided wrist pain and weak grip.
Nonunion: May occur due to poor vascularity of the hook fragment.
Ulnar nerve compression: Hook fractures or ganglion cysts can compress the ulnar nerve in Guyon’s canal.
Osteoarthritis: May involve hamate–metacarpal joints, particularly the fourth and fifth CMC articulations.
Imaging role: MRI and CT are critical for diagnosing occult fractures and assessing nerve or tendon involvement.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark rim).
- Marrow: Bright signal from fatty trabecular bone.
- Fractures: Appear as linear low-signal lines with adjacent marrow edema (intermediate to low).
- Hook of hamate: Clearly visualized on oblique coronal or axial planes.
T2-weighted images:
- Cortex: Dark low signal.
- Marrow: Bright signal, slightly less intense than on T1.
- Edema or fracture: Hyperintense signal within the hook or body.
- Cystic or arthritic changes: Bright foci near articulations.
STIR:
- Normal marrow: Intermediate to dark.
- Pathology (fracture, bone bruise, osteitis): Bright hyperintense signal with surrounding soft-tissue edema.
- Sensitive for occult fractures and inflammatory changes.
Proton Density Fat-Saturated (PD FS):
- Normal bone: Intermediate to dark signal.
- Pathologic: Bright hyperintense foci corresponding to bone marrow edema or fracture line.
- Excellent for detecting subtle hook fractures and bone bruising.
T1 Fat-Sat Post-Contrast:
- Normal bone: Mild homogeneous enhancement.
- Fracture or infection: Peripheral or patchy enhancement at the margins of low-signal marrow areas.
- Inflammatory arthropathy: Synovial enhancement around adjacent CMC joints.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation with crisp margins.
Trabecular bone: Homogeneous fine trabecular pattern.
Hook of hamate: Best visualized on axial or coronal oblique planes.
Fracture: Appears as a lucent line or cortical step-off in the hook or body.
Degenerative changes: Subchondral sclerosis, cyst formation, or osteophytes at CMC articulations.