BackgroundThis study aimed to present a safe zone for distal pin insertion for external fixation using magnetic resonance imaging (MRI) images.MethodsAll patients who took at least one upper arm MRI from June 2003 to July 2021 were searched via a clinical data warehouse. For measuring the humerus length, proximal and distal landmarks were set as the highest protruding point of the humeral head and lowermost margin of ossified bone of the lateral condyle, respectively. For children or adolescents with incomplete ossification, the uppermost and lowermost ossified margin of the ossification centers were set as proximal and distal landmarks respectively. The anterior exit point (AEP) was defined as the location of the radial nerve exiting the lateral intermuscular septum to the anterior humerus and distance between the distal margin of the humerus and AEP was measured. The proportions between the AEP and full humeral length were calculated.ResultsA total of 132 patients were enrolled for final analysis. The mean humerus length was 29.4 cm (range 12.9–34.6 cm). The mean distance between the ossified lateral condyle and AEP was 6.6 cm (range 3.0–10.6 cm). The mean ratio of the anterior exit point and humeral length was 22.5% (range 15.1–30.8%). The minimum ratio was 15.1%.ConclusionA percutaneous distal pin insertion for humeral lengthening with an external fixator may be safely done within 15% length of the distal humerus. If pin insertion is required more proximal than distal 15% of the humeral shaft, an open procedure or preoperative radiographic assessment is advised to prevent iatrogenic radial nerve injury.