Posterior Interosseous Nerve Supply
The posterior interosseous nerve (PIN) is the terminal motor branch of the radial nerve that innervates the extensor carpi ulnaris and all extensors of the thumb and fingers, while also providing deep sensory innervation to the wrist ligaments and joints. 1
Motor Innervation
The PIN supplies the following muscle groups:
- Extensor carpi ulnaris - provides wrist extension and ulnar deviation 1
- Thumb extensors - including extensor pollicis longus and brevis 1
- Finger extensors - all digital extensors allowing finger extension 1
- Most extensor muscles of the forearm - the majority of the posterior compartment musculature 2
Sensory Innervation
- Deep sensory fibers to the wrist ligaments and joints, allowing measurement of sensory conduction velocity along this nerve 2
- These deep sensory branches are distinct from cutaneous sensation and provide proprioceptive input from wrist structures 2
Anatomical Course and Clinical Relevance
The PIN follows a specific anatomical pathway that has important surgical implications:
- Origin: The radial nerve divides into the PIN and superficial radial nerve approximately 1.3 cm above the elbow joint line 3
- Radial tunnel passage: The PIN passes through the radial tunnel anterior to the radiohumeral joint, then courses laterally and posteriorly beneath the arcade of Frohse (the proximal edge of the superficial supinator muscle) 3
- Relationship to radius: In the coronal plane, the PIN crosses the radial neck/shaft at a mean of 33.4 mm below the radial head surface in pronation and 16.9 mm in supination 4
- Radial tuberosity landmark: The PIN crosses 4.9 mm distal to the radial tuberosity in pronation and 9.6 mm proximal in supination, making the tuberosity a reliable intraoperative landmark 4
Clinical Presentation of PIN Injury
- Finger drop - inability to extend the fingers at the metacarpophalangeal joints 1
- Partial wrist drop - weakness of wrist extension, though less complete than proximal radial nerve injury due to preserved extensor carpi radialis longus and brevis function 1
- No sensory loss over dorsum of hand - this distinguishes PIN neuropathy from more proximal radial nerve lesions, as the superficial radial nerve (providing dorsal hand sensation) branches proximal to the PIN 1