Effects of Plate-Bone Contact on Bridge Plate Fracture Fixation Biomechanics: Computational, Experimental, and Analytical Modeling

Abstract

Bridge plating is commonly used for internal fixation of comminuted fractures. The inner working length between screws has been established as a key parameter controlling postoperative biomechanical stability. However, plate-bone contact may affect these biomechanics in complex ways, and the offset between the plate and bone is variable across surgeries. The objective of this study was to examine the effects of construct and loading parameters on interfragmentary motion and maximum plate stress of bridge plating constructs. Finite element models were developed with variations in inner working length, plate-bone offset, fracture gap size, and loading type and magnitudes. Experiments with synthetic bones were conducted in parallel to support model credibility. Analytical models were also developed based on beam bending and torsion of the plate, assuming rigidity outside the inner working length. Finite element and experimental results of axial and torsional loading scenarios without plate-bone contact confirmed linear relationships between inner working length and interfragmentary motion. Analytical predictions of interfragmentary motion showed very good agreement with the finite element simulations in these scenarios. Conversely, in cases with plate-bone contact, a shorter effective working length was formed, and results were dependent on additional variables such as fracture gap. The study shows how the mechanics of bridge plating can be understood and predicted based on beam theory up to the point of plate-bone contact, and how interfragmentary motions and maximum plate stresses are influenced by the interaction of surgical variables in the presence of plate-bone contact.

Keywords: beam bending model; biomechanics; bridge plating; bridge span; finite element analysis; fracture fixation; fracture plate; interfragmentary motion; plate clearance; plate working length.

Figure 1

(A) An example of the meshed model showing the middle section of the construct with increased mesh density present at the center region of the plate. Variable input parameters for FE model including (B) variations in inner working length; (C) variations in the initial plate‐bone offset; (D) variations in the simulated fracture gap size; and (E) the three loading types and their respective boundary conditions with axial offsets.

Figure 2

Experimental setup schematics for (A) axial compressive testing with spherical joints at the proximal and distal fixtures and (B) torsional testing with a planar bearing fixture at the proximal end and a rigid fixture at the distal end. (C) A construct with four motion capture clusters on the synthetic bone. Red circles indicate the virtual resolved points from the motion capture data from which axial and shear IFMs were calculated.

Figure 3

Schematics for pure bending loading of the (A) commercial plate model (Section 2.1), (B) validation FE model (Section 2.2), and (C) analytical plate model (Section 2.3.), which included plate bending within the inner working length, and rigid body rotation outside the inner working length. Geometric parameters involved in the analytical model including (D) the inner working length L _IW and outer length L _O (E) e the eccentric distance from the axis of load application to the centroid of the plate, p _c the distance between plate centroid and the near cortex, d the bone diameter, w the width of the plate, and t the thickness of the plate, (F) the angle of the plate at the inner screw due to bending θ _IW with red circles representing the points used for calculating IFM, and (G) the vertical deflection of the plate δ _P .

Figure 4

Maximum plate stress and IFM_Axial for (A) 150 N and (B) 250 N of applied axial compressive load, for constructs having different combinations of inner working length, plate‐bone offset, and fracture gap. Red circles indicate cases where plate‐bone contact was observed.

Figure 5

Representative FE results including construct deformations and von Mises stresses of plates and screws. Minimum principal strain in bones is also displayed, including surrounding plate‐bone contact regions. (A), (B), and (D) are showing cross‐sectional views during axial compression, whereas (B) shows torsion. (A and B) The longest, middle, and shortest inner working lengths with a 1‐mm plate‐bone offset are shown after being subjected to either (A) 250 N of axial compression or (B) 4 Nm of torsion. (C) Results are shown for 156 mm inner working length with three different plate‐bone offsets at 250 N of axial compression. (D) Results are shown for 84 mm inner working length with a 1‐mm plate‐bone offset at three axial compressive loads.

Figure 6

Maximum plate stress and IFM_Shear for 8 Nm of torsional load for 25 mm fracture gap constructs having different combinations of inner working length and plate‐bone offset.

Figure 7

Comparisons between IFM from experimental (n = 3) results and validation FE models for the three (long, middle, short) working lengths for (A) 150 N of axial compression, and (B) 4 Nm of torsional loading. Error bars indicate ± 1 standard deviation.

Figure 8

Interfragmentary motion comparisons between parametric FE models and analytical model results for 4 Nm of pure bending, 150 N of axial compression (IFM_Axial), and 4 Nm of pure torsion (IFM_Shear) when varying the inner working length. FEA models all had a 1 mm plate‐bone offset and 10 mm fracture gap. *Contact occurred in the finite element model.