Which muscle is most likely affected by nerve injury in an extension-type supracondylar (above the condyle of the humerus) fracture with neurological deficit?

Anterior Interosseous Nerve Injury is Most Likely in Extension-Type Supracondylar Humerus Fractures

The anterior interosseous nerve (a branch of the median nerve) is most likely affected in a 4-year-old boy with an extension-type supracondylar humerus fracture presenting with neurological deficit. 1

Nerve Injury Patterns in Supracondylar Humerus Fractures

• Extension-type supracondylar humerus fractures are associated with nerve injuries in approximately 11% of cases 2
• The median nerve (including its anterior interosseous branch) is the most commonly injured nerve in extension-type fractures, accounting for 62% of all nerve injuries 2
• In a prospective study of nerve injuries in supracondylar fractures, median nerve injuries (including anterior interosseous nerve) occurred at a rate of 12%, compared to radial nerve (8%) and ulnar nerve (3%) 3

Mechanism of Injury

• In extension-type fractures, the distal fragment is typically displaced posteriorly, which can cause the median nerve to be stretched over the proximal fragment 1
• The anterior interosseous nerve is particularly vulnerable due to its anatomical position anterior to the fracture site 1
• High-resolution ultrasound studies have consistently shown post-traumatic median nerve neuroma at the fracture site in affected patients 1

Clinical Presentation

• Patients with median nerve injury in supracondylar fractures typically present with a clinical syndrome showing predominant but not exclusive affection of anterior interosseous nerve-innervated muscles 1
• This presents as weakness in the flexor pollicis longus and flexor digitorum profundus to the index finger, resulting in inability to make an "OK" sign 1
• Electrodiagnostic studies in these cases show axonal motor and sensory median nerve neuropathy 1

Prognosis and Recovery

• The majority (92%) of nerve injuries associated with pediatric extension-type supracondylar humerus fractures recover completely without acute nerve decompression 2
• Median nerve injuries tend to recover more quickly than other nerve injuries, with 70% of isolated median nerve injuries recovering within 3 months, compared to only 42% of radial nerve injuries 2
• Complete functional recovery can be expected with conservative management, though motor recovery may require up to 11 months 1

Management Considerations

• The American Academy of Orthopaedic Surgeons (AAOS) guidelines do not provide specific recommendations for indications for electrodiagnostic studies or nerve exploration in patients with nerve injuries associated with pediatric supracondylar fractures 4
• Recent evidence suggests that prioritizing urgent surgery in patients with nerve injury did not improve ultimate neurological recovery, though earlier surgical intervention was associated with earlier partial recovery 5
• Closed reduction with pin fixation is the preferred treatment for most displaced supracondylar fractures of the humerus 4

Important Clinical Caveat

• A reliable neurological examination can be difficult to obtain in a young child in pain, so careful assessment by an experienced clinician is essential to avoid misdiagnosing pre-existing nerve injuries as iatrogenic injuries 3
• In contrast to extension-type fractures, flexion-type supracondylar fractures (which are much rarer at 2-3% of cases) are more commonly associated with ulnar nerve injuries 6