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Movement at radioulnar joint

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Movement at the Radioulnar Joint

Overview

The radioulnar joints are two pivot (trochoid) joints that act together as a single functional unit to produce pronation and supination of the forearm. The ulna remains stationary while the radius rotates around the ulna.

Types of Radioulnar Joints

Feature	Proximal RUJ	Distal RUJ
Location	Below elbow	Above wrist
Articulation	Head of radius in radial notch of ulna	Head of ulna in sigmoid notch of radius
Joint type	Pivot (trochoid)	Pivot (trochoid)
Stabilizer	Annular ligament	TFCC (Triangular Fibrocartilage Complex)

Movement: PRONATION & SUPINATION

Axis of Movement

Runs obliquely from the centre of the radial head (proximal RUJ) to the fovea of the ulnar head / styloid process of ulna (distal RUJ).

Range

Supination: 0 - 90°
Pronation: 0 - 90°
Total rotation: ~180° (with shoulder adding ~270°)

Easy Drawable Diagram

SUPINATION                        PRONATION
(Palm faces UP)                   (Palm faces DOWN)

PROXIMAL VIEW:                    PROXIMAL VIEW:
  [Radius] [Ulna]                   [Radius crosses
   parallel                          over Ulna]
      ↑                                  ↑
  Annular lig.                      Annular lig.
  holds radius                      holds radius
  in radial notch                   in radial notch


ANTERIOR VIEW (Right forearm):

SUPINATION          PRONATION
  R    U             U
  |    |            / \
  |    |           R   |
  |    |           |   |
  R = Radius, U = Ulna
  (parallel)      (radius crosses over ulna)


AXIS LINE:
  Radial head (above)
       |
       |  (oblique axis)
       |
  Ulnar styloid (below)


DISTAL RUJ CROSS SECTION:

SUPINATION:     NEUTRAL:       PRONATION:
  [R][U]         [R][U]         [U][R]
  side by side   in contact    radius crossed

Muscles Producing Movement

SUPINATION (stronger movement)

Muscle	Nerve	Note
Biceps brachii	Musculocutaneous (C5, C6)	Most powerful supinator; best when elbow flexed at 90°
Supinator	Posterior interosseous nerve (C6)	Works at all positions
Brachioradialis	Radial nerve	Returns from mid-position

Mnemonic: "SUPinator + BICeps = SUPination" - The Biceps also SUPinates

PRONATION (weaker than supination)

Muscle	Nerve	Note
Pronator teres	Median nerve (C6, C7)	Main pronator
Pronator quadratus	Anterior interosseous nerve (C8)	Works alone in slow pronation
Flexor carpi radialis	Median nerve	Weak accessory
Brachioradialis	Radial nerve	Returns from full supination

Mnemonic: "Pronator Teres + Pronator Quadratus = Pronation"

Connecting Structure: Interosseous Membrane

A fibrous sheet between the shafts of radius and ulna
Transmits force from radius to ulna (e.g., falls on outstretched hand)
Fibers run obliquely: downward and medially from radius to ulna
Functionally links proximal and distal RUJ - movement of one joint is always accompanied by movement of the other
Central band is thickest = main load transfer

Easy Diagram: Interosseous Membrane

RADIUS (lateral)          ULNA (medial)
    |    \\\\\\\\\\\\\\\\\\ |
    |     \\\\\\\\\\\\\\\\\ |
    |      \\\\\\\\\\\\\\\\ |  ← Oblique fibers
    |       \\\\\\\\\\\\\\\ |     (radius → ulna,
    |        \\\\\\\\\\\\\\ |      downward + medially)
    |                       |
  [Radial                [Ulnar
   tuberosity]            attachment]

Biomechanics at the Distal RUJ

In neutral position: styloid process of ulna is aligned with sigmoid notch of radius
In pronation: ulnar head rotates - styloid process moves to the palmar side; radius migrates proximally (increases ulnar variance)
In supination: styloid process moves to the dorsal side
The two distal articular surfaces are in closest contact (congruent) only in neutral/semipronated position

Diagram: Radial head cross-section (Proximal RUJ)

(Based on Thieme Atlas, p. 302)

Cross section of proximal radioulnar joint showing axis shift from supination to pronation

Note: The radial head is slightly oval - the pronation/supination axis shifts ~2 mm radially during pronation. Articular cartilage is thicker on the pronation side (adapts to higher pressure in pronated position).

Diagram: Distal RUJ - Rotation of radius around ulna

Distal radioulnar joint showing supination, neutral, and pronation positions

Clinical Points (High Yield)

1. Pulled Elbow (Nursemaid's Elbow)

Common in children < 5 years
Mechanism: sudden pull on the hand when the forearm is pronated
Radial head slips out of the annular ligament
Child holds arm in slight flexion and pronation, refuses to supinate
Treatment: hyperpronation or supination + flexion manoeuvre - feel/hear a click

2. Monteggia Fracture-Dislocation

Fracture of the proximal ulna + dislocation of the radial head from the proximal RUJ
Disrupts the radioulnar joint
Must always X-ray the elbow when there is a forearm fracture
Rule: A line through the radial neck/head should always pass through the capitulum in all views

3. Distal Radioulnar Joint (DRUJ) Disruption

Seen with distal radius fractures (Galeazzi fracture)
Signs: pain at DRUJ, prominence of ulnar head (piano key sign), restricted and painful pronation/supination
Stable DRUJ is essential for painless forearm rotation

4. TFCC (Triangular Fibrocartilage Complex) Injury

Stabilizes the DRUJ
Injury causes ulnar-sided wrist pain worse on pronation/supination and grip
Diagnosed by MRI; treatment ranges from physiotherapy to arthroscopic repair

5. Radioulnar Synostosis

Bony or fibrous union between radius and ulna
Forearm is fixed in pronation - loss of supination
Can be congenital or post-traumatic (after severe forearm fractures, especially if both bones fractured at same level)

6. Superior Radioulnar Joint in Rheumatoid Arthritis

Synovitis leads to destruction of the annular ligament
Causes instability of the radial head and loss of rotation

Summary Table

Feature	Detail
Type of movement	Pronation and Supination
Joint type	Pivot / Trochoid
Axis	Radial head to ulnar styloid/fovea
Range	~90° pronation, ~90° supination
Strongest supinator	Biceps brachii
Strongest pronator	Pronator teres
Proximal stabiliser	Annular ligament
Distal stabiliser	TFCC + dorsal/palmar RU ligaments
Link between joints	Interosseous membrane
Moving bone	Radius (rotates around the ulna)
Fixed bone	Ulna (relatively stationary)

Sources: General Anatomy and Musculoskeletal System, Thieme Atlas of Anatomy (p. 270-302); Imaging Anatomy Text and Atlas Vol. 3, Biomechanics of the Arm and Forearm; Rockwood and Green's Fractures in Adults, 10th ed. 2025

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