A Biomechanical Study Comparing Different Configurations of the Ilizarov Mini-fixator and Plate Fixation

Abstract

Objectives: The Ilizarov mini-fixator, an external fixator developed for digits, has a structure different from that of general unilateral external fixators. The purpose of this study was to provide information for clinical external fixator selection by clarifying the biomechanical properties of the Ilizarov mini-fixator.

Methods: Three types of loads were applied to the fracture fixation model using a carbon fiber cylinder as the bone model: axial pressure, 4-point bending, and torsion. Ilizarov mini-fixators with six 1.5 mm pins (6-pin group), four 1.5 mm pins (4-pin group), and four 2.0 mm-threaded-pins (2 mm-pin group) and a group using a 1.5-mm plate (plate group) were set, and the stiffness was compared between these four groups.

Results: Compared to the 6-pin group, which is the basic configuration of the Ilizarov mini-fixator, the 4-pin group had lower stiffness against axial load and bending, and there was no significant difference in stiffness against torsion. The 2 mm-pin group had high stiffness against axial load and bending and low stiffness against torsion. The plate group exhibited high stiffness against axial pressure and low stiffness against bending and torsion.

Conclusions: The Ilizarov mini-fixator is a reasonable option for fixing small phalanges. However, other options should be considered for the fixation of larger bones with higher loads. In particular, a configuration with fewer pins such as the 4-pin model may have insufficient strength during early postoperative exercise, even for fixing the phalanges.

Keywords: Ilizarov; biomechanical study; external fixator; hand surgery.

Figure 1.

Fracture model and Ilizarov mini-fixator configuration.

Figure 2.

Schemas and photographs of the four configurations. (a) Six pins with a 1.5-mm diameter, (b) four pins with a 1.5-mm diameter, (c) four pins with a 2.0-mm diameter, and (d) titanium plate and six screws.

Figure 3.

Loading tests. (a) Axial loading, (b) 4-point bending, and (c) torsion.

Figure 4.

Difference in pin directions. This figure shows the cross-section of a digit. The pins are inserted at an angle of 45° to avoid interference of the pins with the extensor tendon.

Figure 5.

Load–displacement curve under 4-point bending. These are the load–displacement curves of the three different configurations under 4-point bending. The stiffness of the entire bone-fixing device was calculated from the slope of this curve. When the displacement reached 4 mm, the 6-pin configuration model was loaded with 98.2 N, and the 4-pin configuration model was loaded with 34.9 N. The 2 mm-pin configuration model (without rotation) reached the yield point before it was displaced by 4 mm.

Figure 6.

Stiffness in axial loading test. Box plots of the four different configurations under axial loading. In each configuration, three samples are measured five times each. **** indicates statistical significance (p<0.0001). The bottom and top of the box represent the first and third quartiles, the line inside the box is the median, the error bars are the largest and smallest values within a 1.5 interquartile range of the nearer quartile, and any outliers are shown as dots.

Figure 7.

Stiffness in 4-point bending test. Box plots of the five different configurations under 4-point bending. **** indicates statistical significance (p<0.0001). *** indicates statistical significance (p<0.001).

Figure 8.

Stiffness in torsion test. Box plots of the four different configurations under torsion. *** indicates statistical significance (p<0.001). NS indicates no significant difference.