What is the long horn sign in hand injury secondary to posterior interosseous nerve (PIN) injury?

The Longhorn Sign in Posterior Interosseous Nerve (PIN) Injury

The longhorn sign refers to the characteristic hand posture seen in posterior interosseous nerve injury, where the patient can extend the wrist but cannot extend the metacarpophalangeal joints of the fingers, resulting in a hand position that resembles longhorns with the index and small fingers partially extended while the middle and ring fingers remain flexed.

Clinical Presentation and Diagnosis

• Posterior interosseous nerve injury typically presents with weak wrist extension with radial drift, inability to extend the fingers, paralysis of thumb extension, and weak thumb abduction 1
• The longhorn sign is a distinctive clinical finding where patients demonstrate partial extension of the index and small fingers with flexion of the middle and ring fingers, creating a pattern resembling longhorns 1
• MRI is the ideal imaging modality for evaluating tendon injuries and helping with surgical planning in cases of PIN injury 2
• Electrophysiological testing (EMG/nerve conduction velocity) is essential for confirming the diagnosis and determining the severity of nerve injury 1

Pathophysiology of the Longhorn Sign

• The posterior interosseous nerve innervates the extensor digitorum communis, extensor digiti minimi, extensor indicis proprius, abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus 1
• In PIN palsy, the extensor digitorum communis to the middle and ring fingers is more severely affected than those to the index and small fingers 1
• The extensor indicis proprius (to index finger) and extensor digiti minimi (to small finger) provide additional extension force to these digits, allowing them to partially extend while the middle and ring fingers remain flexed 1
• This differential innervation pattern creates the characteristic longhorn appearance when the patient attempts to extend all fingers 1

Common Causes of PIN Injury

• Entrapment at the arcade of Frohse (most common cause) 1
• Laceration or trauma to the forearm 1
• Fractures of the radius or elbow 1
• Compression or contusion 1
• Space-occupying lesions such as ganglion cysts or tumors 1

Diagnostic Imaging

• Standard radiographs are typically normal in isolated PIN injuries but should be obtained to rule out fractures or other bony abnormalities 2
• MRI without IV contrast is the preferred advanced imaging modality for evaluating PIN injuries, as it can detect both nerve compression and associated soft tissue abnormalities 2
• MRI of the extensor system has reported sensitivity ranging from 28% to 85% for the detection of extensor hood injuries 2
• CT has limited use for the diagnosis of soft tissue injuries of the hand but may be helpful if bony involvement is suspected 2

Management Considerations

• Initial management often includes splinting to prevent contractures and maximize function while awaiting nerve recovery 3
• Tenodesis extension splinting can allow patients to extend the fingers and thumb via a tenodesis effect at the wrist, enhancing functional use while nerve regeneration occurs 3
• Surgical options include:
  • Neurolysis for nerves in continuity with preserved nerve action potentials 1
  • Direct repair or graft repair for nerves not in continuity 1
  • Nerve transfers, such as supinator motor branches to the posterior interosseous nerve (SUP-PIN), which have shown promising results 4, 5
• In cases of chronic PIN palsy, tendon transfers may be considered to restore finger and thumb extension 3

Prognosis

• With appropriate surgical intervention, most patients can achieve functional recovery with motor strength of Grade 3/5 or better 1
• In a study of 26 patients who underwent surgical treatment, 17 achieved Grade 4/5 strength and 7 obtained Grade 5/5 strength after 4 years of follow-up 1
• Even in chronic cases (>18 months after injury), nerve transfers have demonstrated good outcomes for recovery of finger extension, provided innervation of target muscles is preserved 5

Clinical Pitfalls

• The longhorn sign may be confused with other patterns of finger drop, so careful examination of individual finger extension is crucial 1
• Relying solely on radiographs may lead to missed diagnosis, as PIN injuries primarily affect soft tissues 2
• Delay in diagnosis and treatment can lead to contractures and permanent functional limitations 3
• Patients with higher cervical spinal cord injuries (C5) may have poorer outcomes with nerve transfer procedures 5