Sagittal plane alignment affects the strength of pin fixation in supracondylar humerus fractures

Closed reduction with percutaneous pin fixation is commonly used to treat pediatric supracondylar humerus fractures. Various pin configurations of varying biomechanical strength have been described. However, to our knowledge, no biomechanical study has focused on pin alignment in the sagittal plane. Our goal was to compare the stability of fixation using 3 different pin constructs: 3 lateral pins diverging in the coronal plane but parallel in the sagittal plane (3LDP), 3 lateral pins diverging in the coronal and sagittal planes (3LDD), and 2 crossed pins (1 medial and 1 lateral). Transverse fractures were made through the olecranon fossa of 48 synthetic humeri, which were then reduced and pinned in the 3LDP, 3LDD, and crossed-pin configurations (16 specimens per group) using 1.6-mm Kirschner wires. The sagittal plane pin spread was significantly greater in the 3LDD group than in the 3LDP group, whereas we found no difference in the coronal plane. Sagittal extension testing was performed from 0° to 20° at 1°/s for 10 cycles using a mechanical torque stand. The torque required to extend the distal fragment 20° from neutral was compared between groups using one-way analysis of variance with multiple comparison post-hoc analysis. P values ≤.05 were considered significant. The 3LDD configuration was more stable than the 3LDP and crossed-pin configurations. The mean torque required to displace the pinned fractures was 5.7 Nm in the 3LDD group versus 4.1 Nm in the 3LDP group and 3.7 Nm in the crossed-pin group (both, P < .01). We found no difference in stability between the 3LDP and crossed-pin groups (P = .45). In a synthetic biomechanical model of supracondylar humerus fracture, sagittal alignment influenced pin construct stability, and greater pin spread in the sagittal plane increased construct stability when using 3 lateral pins. The lateral pin configurations were superior in stability to the crossed-pin configuration. Level of Evidence: Level V.