What muscle is usually affected in an Anterior Interosseous Nerve (AIN) injury in an extension-type supracondylar fracture of the humerus?

Muscle Affected in AIN Injury in Extension Type Supracondylar Humerus Fracture

In extension-type supracondylar humerus fractures, the flexor pollicis longus (FPL) is the primary muscle affected in Anterior Interosseous Nerve (AIN) injury, followed by the flexor digitorum profundus (FDP) of the index finger. 1, 2

Anatomical Basis for AIN Injury

• The AIN arises approximately 45 mm from the humeral intercondylar line and is located in the dorsal part of the median nerve before emerging 2
• AIN injuries occur through two primary mechanisms in extension-type supracondylar fractures:
  • Direct contusion of the posterior aspect of the median nerve (where AIN fascicles are located) by the proximal bone fragment 2
  • Stretching of the transitional portion of the AIN (Zone 1), which has less ability to withstand stretching compared to the median nerve and its other branches 2

Clinical Presentation of AIN Injury

• AIN injury presents with weakness or paralysis of the flexor pollicis longus and flexor digitorum profundus of the index finger 1, 3
• This results in impaired flexion of the terminal joint of the thumb and index finger 1
• Unlike complete median nerve injuries, AIN injuries do not cause sensory loss as the AIN is a pure motor branch 1, 3

Diagnosis and Management

• Nerve injuries occur in 6.6-31% of pediatric supracondylar fractures, with approximately one-third being AIN palsies 2
• High-resolution ultrasound can help exclude persistent nerve entrapment and visualize post-traumatic median nerve neuroma at the fracture site 3
• Electrodiagnostic studies typically show axonal motor neuropathy in the affected muscles 3
• For displaced supracondylar fractures with associated nerve injuries:
  • Closed reduction with pin fixation is the preferred treatment for most displaced supracondylar fractures 4, 5
  • Open reduction may be considered in certain cases, particularly with vascular compromise 4, 6

Prognosis

• Most AIN injuries associated with supracondylar fractures have excellent prognosis with conservative management 3
• Complete functional recovery can take up to 11 months and recovery time differs between involved muscles 3
• In patients with at least 3-month neurological follow-up, approximately 97% have complete resolution of nerve palsy 6

Clinical Pearls and Pitfalls

• It's important to distinguish between isolated AIN injury and more extensive median nerve injury, as they may present similarly but have different implications 3
• Detailed neurological follow-up examinations and sonographic exclusion of persistent nerve compression can guide conservative therapy 3
• Delayed time from injury to surgery is associated with higher risk of compartment syndrome in patients with pulseless supracondylar fractures and nerve injuries 6