Biomechanical efficacy of AP, PA lag screws and posterior plating for fixation of posterior malleolar fractures: a three dimensional finite element study

Abstract

Background: Clinically there are different fixation methods used for fixation of the posterior malleolar fractures (PMF), but the best treatment modality is still not clear. Few studies have concentrated on this issue, least of all using a biomechanical comparison. The purpose of this study was to carry out a computational comparative biomechanics of three different commonly used fixation constructs for the fixation of PMF by finite element analysis (FEA).

Methods: Computed tomography (CT) images were used to reconstruct three dimensional (3D) model of the tibia. Computer aided design (CAD) software was used to design 3D models of PMF. Finally, 3D models of PMF fixed with two antero-posterior (AP) lag screws, two postero-anterior (PA) lag screws and posterior plate were simulated through computational processing. Simulated loads of 500 N, 1000 N and 1500 N were applied to the PMF and proximal ends of the models were fixed in all degrees of freedom. Output results representing the model von Mises stress, relative fracture micro-motion and vertical displacement of the fracture fragment were analyzed.

Results: The mean vertical displacement value in the posterior plate group (0.52 mm) was lower than AP (0.68 mm) and PA (0.69 mm) lag groups. Statistically significant low amount of the relative micro-motion (P < 0.05) was observed in the posterior plate group.

Conclusions: It was concluded that the posterior plate is biomechanically the most stable fixation method for fixation of PMF.

Keywords: Biomechanical; Finite element analysis; Fixation; Posterior malleolar fracture; Three dimensional.

Fig. 1

3D ankle model (a) Initial 3D modeling, (b) Meshing of the processed model, (c) Finite element model showing cortical and cancellous portions of the bone

Fig. 2

Model of posterior malleolar fracture showing three different fixation strategies. a Two AP lag screws, b Two PA lag screws, c Posterior plate

Fig. 3

Finite element model showing boundary conditions and load direction. a AP lag group, b PA lag group, c Plate group

Fig. 4

Von Mises Stress (VMS) pattrens in three models with loads of three different magnitudes

Fig. 5

Peak von Mises Stress (VMS) stress distribution in models using three different fixation strategies. a AP, b PA, c Posterior plate models

Fig. 6

Tibial plafond showing the model and implant displacements (mm). Star represents the displacemnt of fracture fragment only. Arrow represnts the movement of medial malleolus. a AP lag, b PA lag, c Posterior plate models

Fig. 7

Representation of displacement patterns in X,Y and Z axes in the higest load group (1500 N)

Fig. 8

Relative micro-motion (RM) of the fracture in (a) AP, (b) PA and (c) posterior plate models

Fig. 9

Graphical representation of Relative micro-motion (RM) of the fracture in three different fixation models