Biomechanical Analysis of Supracondylar Humerus Fracture Pinning for Slightly Malreduced Fractures

Background: The most common position of malreduced type III supracondylar humerus (SCH) fractures is internal rotation and medial collapse of the distal fragment. The purpose of this study was to determine the effect of SCH fracture rotational deformity on stability with various pin configurations. Specifically, is the biomechanical stability lost when an SCH fracture is pinned in slight malreduction ( compared with anatomical pinning) improved by adding a third pin? Methods: Sixty-four synthetic humeri were sectioned in the midolecranon fossa to simulate an SCH fracture. Specimens were randomized to an anatomically reduced group or a group with 20 degrees of distal fragment internal rotation (n = 32 per group). Each was randomized to one of 4 pin configurations: 2 laterally divergent pins, 2 crossed pins, 3 laterally divergent pins, or 2 lateral with 1 medial pin ( n = 8 per group). All fractures were stabilized with 1.6 mm (0.062 in) Kirschner wires. Models were tested in extension, varus, and valgus for 10 cycles between 5 N and 50 N. Internal and external rotations were tested between +/- 1 Nm. Data for fragment stiffness ( newtons per millimeter or newton millimeters per degree) were analyzed with a 2-way analysis of variance (p < 0.05). Results: Internally rotated fractures were significantly less stable than the anatomically reduced group for external rotation, internal rotation, and varus loading regardless of pin configuration. Within the malreduced group, 3-pin configurations were more stable than 2-pin configurations in internal rotation, varus, and extension loading. Two lateral divergent pins were similar to 2 crossed pins, except in extension, where 2 lateral pins had greater stiffness. Conclusions: Construct stiffness for malreduced specimens after pinning was less than those pinned with an anatomical reduction when loaded in varus, internal rotation, and external rotation. For simulated fractures with residual internal rotation, the addition of a third Kirschner wire compared with an anatomically reduced 2-crossed-pin configuration resulted in increased stiffness of the model for all loading directions. Clinical Relevance: Consider a 3-pin pattern, either 3 laterally divergent pins or 2 lateral pins and 1 medial pin, for SCH fractures when a less than complete anatomical reduction is obtained.