Ulna Bone: Anatomy, Structure, Function, and Imaging

Topic

Ulna

The ulna is one of the two long bones of the forearm, located on the medial (ulnar) side, parallel to the radius. It plays a critical role in forming the elbow joint (proximally) and the distal radioulnar joint (distally), acting as a stabilizing structure of the forearm. It provides attachment for numerous muscles and ligaments responsible for forearm motion, grip strength, and elbow stability.

The ulna is larger proximally, tapering distally, and forms the olecranon and coronoid processes, which together create the trochlear notch that articulates with the humerus. It serves as the mechanical axis of the forearm, transmitting forces between the hand and the upper arm.

Synonyms

Medial forearm bone
Ulnar shaft
Bone of the cubital side of the forearm

Location and Structure

The ulna lies along the medial aspect of the forearm, extending from the elbow to the wrist.
It articulates proximally with the humerus and radius, and distally with the radius and triangular fibrocartilage complex (TFCC).
The ulna consists of:
- Upper end: Olecranon, coronoid process, trochlear notch, and radial notch
- Shaft: Long, prismatic, with anterior, posterior, and interosseous borders
- Lower end: Head and styloid process

Parts of the Ulna

1. Upper End:

Olecranon process: Large curved projection forming the bony prominence of the elbow; insertion site for triceps brachii.
Coronoid process: Anterior projection fitting into the coronoid fossa of the humerus during flexion; gives attachment to brachialis.
Trochlear notch: Formed by olecranon and coronoid processes; articulates with the trochlea of the humerus.
Radial notch: On the lateral side; articulates with the head of the radius, forming the proximal radioulnar joint.

2. Shaft:

Long and triangular in cross-section.
Anterior border: Gives attachment to flexor digitorum profundus.
Posterior border: Subcutaneous distally; provides attachment to deep fascia and extensor carpi ulnaris.
Interosseous border: Sharp lateral margin for interosseous membrane attachment.

3. Lower End:

Head: Rounded, articulates with ulnar notch of the radius and the articular disc of the wrist.
Styloid process: Medial projection providing attachment for the ulnar collateral ligament of the wrist.

Relations

Proximally: Articulates with humerus (trochlea) and radius (head).
Distally: Articulates with radius and articular disc (no direct carpal articulation).
Anteriorly: Flexor muscles of forearm (flexor digitorum profundus, pronator quadratus).
Posteriorly: Extensor muscles (extensor carpi ulnaris, anconeus).
Laterally: Interosseous membrane separates it from the radius.
Medially: Subcutaneous border along distal two-thirds palpable beneath skin.

Ossification

Primary center appears in the shaft during the 8th week of fetal life.
Secondary centers:
- Olecranon: appears around age 10 and fuses by 15–16 years.
- Lower end: appears around age 6 and fuses by 18–20 years.

Function

Structural support: Provides mechanical axis of the forearm.
Joint articulation: Forms part of the elbow and distal radioulnar joints.
Muscle attachment: Serves as a major anchor for forearm flexors and extensors.
Stabilization: Maintains forearm alignment and assists in pronation-supination with the radius.
Force transmission: Transfers load between wrist and humerus during gripping and pushing.

Clinical Significance

Fractures: Common sites include olecranon (direct trauma), coronoid process (elbow dislocation), and shaft (nightstick fracture).
Monteggia fracture-dislocation: Proximal ulnar fracture with anterior dislocation of the radial head.
Stress fractures: Seen in athletes and manual laborers.
Osteomyelitis: May occur secondary to open injuries.
Elbow instability: Results from disruption of coronoid and olecranon processes.
Surgical relevance: Important in ORIF (open reduction internal fixation), prosthesis alignment, and donor bone graft harvesting.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright signal due to fatty marrow composition in adults.
- Periosteum: Thin, low-signal line outlining cortex.
- Muscles: Intermediate signal; perimuscular fat is bright.
- Fracture or lesion: Linear low-signal disruption of cortex with adjacent intermediate to bright marrow signal.
T2-weighted images:
- Cortex: Dark, sharply defined.
- Marrow: Bright, slightly less than T1 but clearly higher than muscle.
- Periosteal reaction or edema: Hyperintense signal outlining bone surface.
- Pathology:
  - Fractures: Linear low-signal lines with bright adjacent marrow edema.
  - Osteomyelitis: Mixed bright marrow and cortical disruption.
  - Tumors or cysts: Focal bright intramedullary lesions with possible cortical thinning.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Pathologic marrow: Bright hyperintense signal in edema, contusion, or infection.
- Highly sensitive for early bone stress and inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal cortex: Dark, sharply defined.
- Normal marrow: Intermediate-to-dark.
- Pathology: Bright hyperintense regions representing marrow edema, periostitis, or soft-tissue infiltration.
- Ideal for detecting subtle fractures and bone contusions.
T1 Fat-Sat Post-Contrast:
- Normal bone: Homogeneous mild enhancement.
- Osteomyelitis: Patchy marrow enhancement with cortical irregularity.
- Tumors: Heterogeneous or nodular enhancement.
- Fracture repair: Linear peripheral enhancement around healing callus.

CT Appearance

Non-Contrast CT:

Cortex: High-attenuation dense outline.
Medullary cavity: Low-attenuation trabecular pattern.
Anatomic landmarks: Olecranon, coronoid process, and styloid clearly visualized.
Pathology:
- Detects fractures, dislocations, cortical irregularities, osteophytes, or sclerosis.
- Useful for preoperative planning, especially in olecranon fractures or prosthesis alignment.

Post-Contrast CT (standard):

Bone: Enhances uniformly.
Soft tissues: Show enhancement in inflammatory or neoplastic processes.
Periosteal reaction or abscess: Appear as enhancing rims around low-attenuation marrow or cortical lesions.

MRI images

CT image

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