Pisiform Bone: Anatomy, Relations, Function, Clinical Significance, Imaging

Topic

Pisiform

The pisiform is a small, pea-shaped sesamoid bone located on the anteromedial aspect of the proximal carpal row, situated within the tendon of the flexor carpi ulnaris (FCU). It articulates only with the triquetral bone, forming the pisotriquetral joint. Despite being the smallest carpal bone, it serves as a key biomechanical lever for the FCU tendon and provides attachment sites for important ligaments and muscles that contribute to wrist stability and ulnar-sided grip strength.

The pisiform is one of the last carpal bones to ossify, typically appearing around ages 9–12 years in females and slightly later in males.

Synonyms

Pisiform carpal bone
Os pisiforme
Pea bone of the wrist

Location and Structure

Position: Palmar and medial to the triquetral bone in the proximal carpal row.
Shape: Pea-shaped with a rounded anterior surface and a flattened dorsal articular surface.
Articulation: Single articulation with the triquetral bone forming the pisotriquetral joint.
Surface anatomy:
- Anterior surface: Subcutaneous and palpable at the heel of the hand.
- Posterior surface: Articulates with the triquetral bone, covered by cartilage.
- Lateral surface: Gives attachment to ligaments connecting to the hamate and fifth metacarpal.
- Medial surface: Serves as attachment for the flexor retinaculum and hypothenar muscles.

Ossification

Type: Sesamoid ossification within the tendon of the flexor carpi ulnaris.
Ossification center appearance: Around 9–12 years of age.
Fusion: Fully ossified and fused in adulthood.

Relations

Anteriorly: Flexor carpi ulnaris tendon and palmar fascia.
Posteriorly: Triquetral bone forming the pisotriquetral joint.
Laterally: Pisohamate and pisometacarpal ligaments extending to hamate and fifth metacarpal.
Medially: Ulnar nerve and artery (in Guyon’s canal) passing adjacent to the pisiform.

Attachments

Tendon: Embedded within the tendon of flexor carpi ulnaris (FCU).
Ligamentous attachments:
- Pisohamate ligament – connects pisiform to hook of hamate.
- Pisometacarpal ligament – extends to the base of the fifth metacarpal.
- Both ligaments act as distal continuations of the FCU tendon.
Muscular attachments:
- Abductor digiti minimi originates partly from the pisiform.

Function

Sesamoid function: Increases the mechanical leverage of the flexor carpi ulnaris tendon.
Force transmission: Enhances flexion and ulnar deviation power of the wrist.
Joint stability: Contributes to ulnar carpal stabilization through pisohamate and pisometacarpal ligaments.
Protection: Shields the ulnar artery and nerve within Guyon’s canal.

Clinical Significance

Pisiform fracture: Usually results from direct trauma or fall on an outstretched hand; may cause pain and tenderness over ulnar wrist.
Pisotriquetral arthritis: Common in repetitive wrist use or degenerative joint disease.
Ulnar nerve entrapment: Inflammation or fracture of pisiform can compress the ulnar nerve within Guyon’s canal.
Accessory ossicle (os pisiforme bipartitum): Rare developmental variant.
Surgical relevance: Pisiform excision may relieve chronic ulnar wrist pain or nerve compression.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark) outlining the bone.
- Marrow: Bright, reflecting fatty content in normal adults.
- Cartilage (pisotriquetral joint): Thin intermediate signal layer between pisiform and triquetrum.
- Pathology: Fracture line as linear low-signal band; marrow edema as intermediate-to-bright change.
T2-weighted images:
- Cortex: Low signal.
- Marrow: Bright, slightly less than on T1 but clearly hyperintense compared to muscle.
- Articular cartilage: Intermediate-to-bright signal, smooth in normal state.
- Joint fluid: Bright hyperintense outlining pisotriquetral space.
- Pathology: Fracture, contusion, or arthritis show focal hyperintensity and cortical irregularity.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Abnormal marrow (edema, fracture, infection): Bright hyperintense regions.
- Excellent for detecting occult fractures and inflammatory changes.
Proton Density Fat-Saturated (PD FS):
- Normal bone: Intermediate-to-dark with uniform appearance.
- Pathologic changes: Bright hyperintensity in bone marrow edema, synovitis, or pericapsular inflammation.
- Pisotriquetral arthritis: Focal cartilage thinning and pericapsular signal increase.
T1 Fat-Sat Post-Contrast:
- Normal pisiform: Mild homogeneous enhancement of marrow.
- Inflammatory or arthritic changes: Focal or diffuse enhancement around pisotriquetral joint.
- Fracture or osteomyelitis: Patchy marrow enhancement with cortical irregularity.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply defined contour.
Marrow cavity: Homogeneous low attenuation core representing cancellous bone.
Articulation: Pisotriquetral joint clearly seen with smooth cortical surfaces.
Pathology: Fractures, cortical irregularities, or sclerosis visible with excellent spatial detail.
Ideal for evaluating bony fragments, degenerative changes, and nonunion fractures.