Relative stability of conventional and locked plating fixation in a model of the osteoporotic femoral diaphysis

Abstract

Background: This study investigated the stiffness and strength of bridge plating with uni-cortical and bi-cortical locking plate constructs relative to a conventional, non-locked construct in the osteoporotic femoral diaphysis.

Methods: Four bridge plating configurations were applied to a validated model of the osteoporotic femoral diaphysis. A non-locked conventional configuration served as baseline. Locked configurations included bi-cortical locked plating, uni-cortical locked plating and mix-mode locked plating, which combined uni and bi-cortical locking screws. For all configurations, an 11-hole plate was applied with 4.5-mm screws placed in the 1st, 3rd, and 5th plate hole. Five specimens of each configuration were dynamically loaded until failure in torsion, axial compression, and bending to determine construct stiffness, strength and failure modes.

Findings: In torsion and bending, locked plating constructs provided a significantly lower stiffness and strength than the conventional construct. The uni-cortical locked construct was 69% weaker (P<0.001) in torsion than the conventional construct, but its torsional strength improved 73% (P<0.001) by adding one bi-cortical locked screw. In axial compression, construct stiffness varied by less than 10% between the four groups. However, the bi-cortical and mixed-mode locked constructs provided a significant increase in strength of 12% (P=0.001) and 11% (P=0.002), respectively, compared to the conventional construct.

Interpretations: Locked plating in the osteoporotic diaphysis can improve fixation strength under axial loading, but may reduce fixation strength in bending and torsion compared to conventional plating. Adding one bi-cortical locked screw to an otherwise uni-cortical construct is recommended to improve torsional strength.

Figure 1

Bridge plating constructs, representing four diaphyseal fixation configurations: conventional non-locked plating (CP), bi-cortical locked plating (BLP), uni-cortical locked plating (ULP), and mixed-mode (uni- and bi-cortical) locked plating (MLP). Construct failure was isolated to the osteoporotic diaphyseal side of each construct.

Figure 2

Construct testing under three distinct loading conditions: four-point bending in gap-closing direction; torsions; and axial compression. Interfragmentary motion in response to progressive dynamic loading was recorded by motion sensors (S).

Figure 3

Construct stiffnesses in torsion, axial compression and four-point-bending. Asterisks (*) denote significant differences between groups.

Figure 4

Construct failure loads in torsion, axial compression, and four-point-bending. Asterisks (*) denote significant differences between groups.

Figure 5

Failure modes of constructs: a) in torsion, failures included spiral fracture at the plate end (CP) and screw breakage between the elevated plate and the bone; b) in axial compression, all constructs failed by screw migration, resulting in elongation of the near cortex screw holes; c) in bending, all constructs failed by transverse fracture through the screw hole at the plate end.