Unilateral external fixator and its biomechanical effects in treating different types of femoral fracture: A finite element study with experimental validated model

Abstract

Previous works had successfully demonstrated the clinical effectiveness of unilateral external fixator in treating various types of fracture, ranging from the simple type, such as oblique and transverse fractures, to complex fractures. However, literature that investigated its biomechanical analyses to further justify its efficacy is limited. Therefore, this paper aimed to analyse the stability of unilateral external fixator for treating different types of fracture, including the simple oblique, AO32C3 comminuted, and 20 mm gap transverse fracture. These fractures were reconstructed at the distal diaphysis of the femoral bone and computationally analysed through the finite element method under the stance phase condition. Findings showed a decrease in the fixation stiffness in large gap fracture (645.2 Nmm-1 for oblique and comminuted, while 23.4 Nmm-1 for the gap fracture), which resulted in higher displacement, IFM and stress distribution at the pin bone interface. These unfavourable conditions could consequently increase the risk of delayed union, pin loosening and infection, as well as implant failure. Nevertheless, the stress observed on the fracture surfaces was relatively low and in controlled amount, indicating that bone unity is still allowable in all models. Briefly, the unilateral fixation may provide desirable results in smaller fracture gap, but its usage in larger gap fracture might be alarming. These findings could serve as a guide and insight for surgeons and researchers, especially on the biomechanical stability of fixation in different fracture types and how will it affect bone unity.

Keywords: External fixator; Femur; Finite element analysis; Fracture healing; Pin tract infection.

Fig. 1

3D model of a (a) healthy bone, (b) Unilateral external fixator, and fitted bone-fixator construct for (c) oblique fracture, (d) 20 mm gap transverse fracture, and (e) complex AO32C3 comminuted fracture, along with the boundary conditions applied.

Fig. 2

Validation test where the (a) femoral bone was compressed under axial compression, mimicking the (b) 3D FE femoral bone model, and the (c) graphical presentation of the results describing the correlation of the two tests.

Fig. 3

Stiffness calculated for the three different fracture type.

Fig. 4

Displacement and IFM for (a) oblique, (b) comminuted, and (c) gap fracture, as well as the deformation for (d) oblique, (e) comminuted, and (f) gap fracture.

Fig. 5

Stress contour and histogram of VMS distribution specifically on the fracture gap between the three types of fracture for (a) Oblique, (b) Comminuted, and (c) 20 mm gap transverse.

Fig. 6

Stress contour between the three types of fracture.