OTA 1997 Posters - Tibia Fractures

Fracture Site Motion with Ilizarov and Hybrid External Fixation

Cyna Khalily, MD, Michael J. Voor, PhD, David Seligson, MD

Louisville, Kentucky, USA

Purpose: To compare the differences in fracture site motion between a circular external fixator that utilizes two rings on one side of a fracture and two rings on the other side and a hybrid external fixator that utilizes two rings on one side of the fracture and unilateral half pins on the other side. Our hypothesis was that the overall fracture site motion, particularly off-axis motion, in an unstable fracture pattern would be significantly greater when a hybrid system was used.

Methods: A wooden dowel was used to simulate a long bone with a midshaft fracture. An Interfragment Motion Device (IMD) developed in our lab measured relative displacement of the fracture ends. Three external fixator configurations were studied: an Ilizarov external fixator with two rings above and two rings below the fracture connected by four 6.0 mm threaded rods, a hybrid configuration with rings above and half-pins below the fracture, and a hybrid configuration with a "strut" augmentation. Axial, angular, and shear displacement were measured under axial (200N), 4-point bending (10Nm), and torsional (5Nm) loading situations. Under torsional load, rotational displacement was also measured. Four-point bending was performed at 45° increments in order to determine the effect of frame orientation. Statistical analysis was performed with the Students t-test for samples with unequal variances (n=5 for each configuration and loading mode), p values less-than-or-equal to 0.01 were considered significant.

Results: Fracture Site Axial Displacement: Axial fracture site displacement under axial load resulted in compression of the fracture ends, in the bending and torsional modes, axial displacement was in the opposite direction, i.e. the fracture ends moved farther apart. The Ilizarov fixator demonstrated significantly less axial fracture site motion in all three loading modes than either hybrid configuration. The strut improved hybrid performance in the 4-point bending mode but not in the axial or torsional loading modes.

Fracture Site Shear Displacement: The Ilizarov fixator demonstrated significantly less fracture site shear displacement in all three loading modes. The strut improved the performance of the hybrid in the four-point bending and axial loading modes but not in the torsional loading mode.

Fracture Site Angular Displacement: The Ilizarov fixator demonstrated significantly less fracture site angular displacement in all three loading modes except in four-point bending at the 0 and 180 degree frame orientations. The strut improved the performance of the hybrid in the four-point bending and axial loading modes but not in the torsional loading mode.

Fracture Site Rotational Displacement: In the torsional loading mode, the Ilizarov fixator allowed significantly less pure rotation than the hybrid or strut augmented hybrid configurations. The differences between the motion of the Ilizarov and the strut augmented hybrid and between the hybrid and the strut augmented hybrid were statistically significant.

Discussion: Essential characteristics of fracture fixation include stability with adequate apposition and alignment of the bone ends. While rigid fixation with as little off-axis motion as possible is ideal, many studies have demonstrated the positive benefits of axial loading or dynamization. Because unilateral external fixators attempt to control fracture site motion from an asymmetric position, off-axis motion is coupled with axial compression. Axial loading leads to bending in the frame which causes angular and shear displacement of the fracture site. In order to address these limitations, unilateral frames are either too rigid or allow excessive axis motion. Any system that is overly rigid may lead to stress shielding or inadequate stimulation of bone healing, possibly resulting in non-union or malunion and disuse atrophy. The multiplanar/coaxial crossed wire configuration of a ring fixator as described by Ilizarov is well suited to address these shortcomings because it limits shear and bending at the fracture site but still allows axial load sharing at the fracture site. The hybrid external fixator configuration was developed in an attempt to combine the ability of a circular fixator to control a complex fracture with the soft tissue access allowed by a unilateral half-pin frame.

The Ilizarov configuration provided more stability than the hybrid or strut augmented hybrid configurations in this study. In pure axial loading, the fracture site angular displacement allowed by the hybrid or strut augmented configurations averaged 22 times and 12.5 times that of the Ilizarov configuration respectively. The Ilizarov configuration also had significantly less off-axis motion in bending and torsion. The load-deformation curve under pure axial loading illustrates the difference in mechanical behavior between circular fixators and hybrid fixators. The Ilizarov and hybrid frames have similar initial stiffness but the stiffness of the Ilizarov frame increases with increasing deformation while the stiffness of the hybrid decreases. These observations support the hypothesis that this hybrid configuration behaves more like a unilateral half-pin frame than a circular frame. Strut augmentation only minimally improved mechanical performance of the hybrid configuration in this study.

Conclusion: The most mechanically effective external fixation system must discourage off-axis motion while still allowing some degree of dynamic axial load bearing. Our results indicate that in a completely unstable fracture with poor bone apposition, a Ilizarov tensioned-wire system exhibits superior biomechanical behavior than a unilateral hybrid frame.