Circular external fixation frames with divergent half pins: a pilot biomechanical study

Abstract

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.

Fig. 1A–B

(A) A divergent half pin frame using three 6-mm half pins above and below the nonunion is shown. Note each pin is divergent in both the coronal and sagittal planes allowing for simplified frame application. (B) This frame avoids the potential for pin/strut impingement as a minimum of fixation points are mounted to the frame and thus allows maximum excursion of the adjustment struts.

Fig. 2A–B

(A) This standard four-ring Ilizarov frame with two wires mounted on each ring with two rings above and below a fracture is similar to the construct tested. (B) The 90° orientation of two wires on each ring is shown with the bone located in the center of the ring. This wire positioning limits frame translation in both AP and mediolateral directions and is the most stable wire orientation.

Fig. 3A–B

(A) A 90° 5-mm half pin frame is mounted using two pins per ring placed perpendicular to each other for a total of eight half pins. Frames were preconstucted prior to mounting and subsequent osteotomy. (B) An axial view demonstrates the 90º pin orientation to each other. These pins do not diverge in the sagittal plane.

Fig. 4A–B

(A) A 60° divergent frame is shown with three 6-mm pins inserted per limb segment. The pins diverge in both the coronal and sagittal plane. (B) Axial view demonstrates a 60º divergence for each pin achieving at least 100º coronal spread for all pins in each limb segment.

Fig. 5

A divergent frame with simulated 2-cm fracture gap (black rectangle seen at midshaft) was mounted in an MTS machine with biplanar video imaging to determine the strain of each construct.