Circular External Fixation Frames with Divergent Half Pins: A Pilot Biomechanical Study

The use of hexapod circular external fixators has simplified the ability to correct complex limb deformities without cumbersome frame reconfigurations. These frames are applied primarily using half pin mountings and may be difficult to utilize given the constraints of traditional half pin constructs. We compared the biomechanical performance of simplified divergent half pin frames to mountings currently being utilized for application of hexapod frames. Three 6-mm half pins per limb segment were placed into sawbones at 60 degree divergent angles in both the sagittal and coronal planes in a 2-cm diaphyseal fracture gap model. Pin mountings were attached to a standardized four-ring construct. This was compared to similar four-ring frames with two differing pin/wire configurations: (1) two tensioned wires per ring placed at 90 degree angles, a total eight wires; and (2) two 5-mm half pins per ring placed at 90 degree angles, a total eight half pins. The divergent 6-mm half pin frames demonstrated similar performance compared the standardized tensioned wire and 5-mm half pin frames in terms of axial micromotion and angular deflection. Based on the mechanical performance of these divergent half pin frames we believe they can be used clinically without detrimental consequences.