Posterior (Forearm) Interosseous Membrane

Note: Although "posterior interosseous membrane" can refer to the membrane of the forearm as seen from its posterior aspect, anatomically the forearm has one interosseous membrane spanning the interosseous borders of the radius and ulna. The term "posterior interosseous membrane" is sometimes used loosely to describe this structure, or specifically to contrast it with the oblique cord (anterior cord). The following covers the forearm interosseous membrane comprehensively, including its posterior relations.

Anatomy

• Proximally: begins just inferior to the radial tuberosity (free upper margin)
• Distally: ends at the distal radio-ulnar joint; a small circular aperture exists in its distal third

• Collagen fibers run predominantly obliquely downward and medially - from the radius (proximally) to the ulna (distally), at approximately 20 degrees to the long axis of the forearm
• This obliquity is mechanically significant: it allows load transfer from the radius to the ulna

• The IOM has a marked central thickening called the central band (also called the interosseous ligament)
• It is approximately 3.5 cm wide
• Fibers run obliquely from proximal-radial to distal-ulnar
• The central band contributes ~71% of the longitudinal stiffness of the IOM after radial head resection (Hotchkiss et al., 1989) - Rockwood & Green's Fractures in Adults, 10th ed.

Central band of the interosseous membrane (arrows) - Rockwood & Green's Fractures in Adults

Related Structure: The Oblique Cord

Functions

1. Load / Force Transfer

2. Syndesmotic Joint

3. Muscle Attachment

• Posterior/dorsal side: Extensor pollicis longus, extensor pollicis brevis, abductor pollicis longus, extensor indicis (origin from dorsal surfaces of radius, ulna, and IOM)
• Anterior/volar side: Flexor digitorum profundus, flexor pollicis longus

4. Compartment Separator

Vascular Apertures

• Proximal free margin (superior): The posterior interosseous artery passes over/dorsal to the proximal margin of the IOM from the anterior compartment into the posterior compartment
• Distal aperture: The anterior interosseous artery passes through this aperture distally to contribute to the dorsal carpal arch
• Anterior interosseous artery: Runs along the anterior surface of the IOM throughout its course, supplying the anterior compartment muscles

Clinical Significance

Essex-Lopresti Injury

1. Fracture/dislocation of the radial head (the primary longitudinal stabilizer)
2. Rupture of the IOM (especially the central band)
3. Disruption of the distal radio-ulnar joint (DRUJ)

• Plain X-ray often shows only an isolated radial head fracture (easily missed)
• Look for positive ulnar variance and widened DRUJ
• MRI is the study of choice for IOM integrity
• Ultrasound is increasingly used Management: Urgent orthopedic referral (within 7-10 days). Options include radial head replacement, IOM central band reconstruction, ulnar shortening.

Forearm Compartment Syndrome

Congenital Radioulnar Synostosis

Surgical Importance: Posterior Interosseous Nerve (PIN)

Summary Table

Posterior Interosseous Nerve (PIN)

Origin and Root Value

Course

1. Cubital Fossa / Lateral Epicondyle Region

• The radial nerve enters the forearm between the brachioradialis and brachialis muscles
• It gives off branches here to: brachioradialis, ECRL (and sometimes ECRB) - before dividing
• It then divides into the superficial radial nerve (sensory) and the deep branch (future PIN)

2. Radial Tunnel

• The deep branch dips beneath the origin of ECRB, entering the radial tunnel - a potential space approximately 5 cm long
• Boundaries of the radial tunnel:
  • Posterior: capitulum and radiocapitellar joint
  • Medial: brachialis
  • Anterolateral: brachioradialis, ECRL, ECRB
• It innervates ECRB and supinator here

3. Through the Supinator (Arcade of Frohse)

• The deep branch pierces the supinator muscle through its fibrous proximal edge - the Arcade of Frohse (first described by Fritz Frohse in 1908)
• It courses between the superficial and deep layers of the supinator, winding laterally and posteriorly around the proximal radius
• It emerges from the distal border of supinator as the posterior interosseous nerve

4. Posterior Compartment of Forearm

• The PIN travels between the superficial and deep extensor muscle layers, running on the posterior surface of the interosseous membrane
• It gives off 6 branches (from proximal to distal) to: ECU, EDM, EDC, EPB+EPL, APL, and EIP
• It terminates as articular branches to the dorsal wrist capsule, located on the floor of the 4th extensor compartment

Posterior interosseous nerve course - Rockwood & Green's Fractures in Adults, 10th ed.

Muscles Innervated

Mnemonic for first vs last innervated: First = Brachioradialis; Last = EIP

Sensory Function

Sites of Compression / Entrapment (Radial Tunnel)

Clinical Syndromes

PIN Palsy (Posterior Interosseous Nerve Syndrome)

• Weakness/paralysis of finger and thumb extensors at the MCP joints
• ECU paralysis → wrist deviates radially on attempted extension (because ECRL and ECRB are spared)
• Cannot extend fingers at MCP joints (distinguish from extensor tendon rupture by normal wrist tenodesis test)
• No wrist drop (ECRL and ECRB are intact)
• No sensory loss

1. Complete - all muscles supplied by PIN are paralyzed
2. Incomplete - only one or a few muscles are affected

• EMG/NCS confirms diagnosis
• MRI identifies space-occupying lesions

• Non-operative: activity modification, splinting, NSAIDs (first line)
• Surgical decompression if no recovery by 3 months - good/excellent results in ~85% - Miller's Review of Orthopaedics

Radial Tunnel Syndrome

• Pain only in the lateral proximal forearm - no motor or sensory deficit
• Point of maximal tenderness is 3-4 cm distal to the lateral epicondyle (vs. lateral epicondylitis at the epicondyle itself)
• Provoked by: resisted long finger extension (reproduces pain) and resisted supination
• EMG is typically inconclusive/normal
• Can coexist with lateral epicondylitis in a small % of cases
• Treatment: prolonged non-operative (up to 1 year); surgical decompression gives good-excellent results in only 50-80% - less predictable than for PIN palsy - Miller's Review of Orthopaedics

Surgical Relevance

• During radial head surgery, the forearm is kept in pronation to move the PIN away from the operative field and reduce compression risk - Rockwood & Green's
• Retractors should not be placed around the anterior radial neck (risk of PIN compression)
• During posterolateral approach to the elbow, the interval between EDC and ECRL/B is developed down to the level of the PIN entering the supinator at the arcade of Frohse - Campbell's Operative Orthopaedics

Summary