Mechanics of dynamic compression plate application in fracture fixation

Abstract

Background: Dynamic compression plating is a fundamental type of bone fracture fixation used to generate interfragmentary compression. The goal of this study was to investigate the mechanics of the surgical application of these plates, specifically how plate prebend, screw location, fracture gap, and applied torque influence the resulting compressive pressures.

Methods: Synthetic bones with transverse fractures were fixed with locking compression plates. One side of the fracture was fixed with locking screws. On the other side of the fracture, a nonlocking screw was inserted eccentrically to induce interfragmentary compression. A pressure mapping sensor within the fracture gap was used to record the resulting pressure distribution. Plate prebends of 0 mm, 1.5 mm, and 3 mm were tested. Three locations of the eccentric screw, four levels of screw torque, and two initial fracture gap conditions also were tested.

Findings: With increasing plate prebend, fracture compression pressures shifted significantly toward the far cortex; however, compression force decreased (P < 0.05). The 1.5 mm prebend plate resulted in the greatest contact area. Increasing screw torque generally resulted in greater fracture compression force. The introduction of a 1 mm fracture gap at the far cortex prior to dynamic compression resulted in little or no fracture compression.

Interpretation: The model showed that increasing plate prebend results in an increasing shift of fracture compression pressures toward the far cortex; however, this is accompanied by decreases in compressive force. Initial fracture gaps at the far cortex can result in little or no compression.

Keywords: Dynamic compression plate; Eccentric screw; Fracture compression; Plate prebend.

Figure 1.

Experimental setup and variables. (A) plate prebends; (B) compression screw locations, determined relative to the fracture plane (red line), and locking screw locations (labeled ‘L’) (C) sawbone and plate construct mounted to the fixtures on both ends (the red fixture allowed rotation, and the blue fixture allowed rotation plus free translation in the direction of the bone axis); (D) fracture gap of 1 mm (0 mm not shown); (E) close up of construct showing locations of locking screws, and compression screw—in this example in location 2; (F) example of resulting pressure distribution measured by pressure sensor located in the fracture.

Figure 2.

Representative samples, having the three different plate prebends, after compression screw tightening. On the right, corresponding pressure results measured in the fracture site are shown, superimposed on an annulus representing the bone end surface. Note the typical results showing contact pressures concentrated at the near cortex (top of annulus) for the 0 mm prebend, near the far cortex for the 3 mm prebend, and a more even distribution for the 1.5 mm prebend. Differences in final angulation of the bones can also be seen.

Figure 3.

Fracture compression results derived from pressure sensor measurements, including total force, contact area, and the center of force (pressure) location. Results are shown for the middle screw location 2 only (results for the other two locations were similar) and initial fracture gap = 0 mm (at far cortex). Error bars represent ± one standard error of the mean.

Figure 4.

Comparison of total fracture compression force for the 0 mm and 1.0 mm initial fracture gap conditions. In the 1.0 mm group, two samples resulted in no compression force.

Figure 5.

Effects of initial conditions on dynamic compression. (A) For a prebend of 0 mm (and 0 mm initial far cortex fracture gap), the near cortex compresses with large force but cannot be further closed. As the eccentric screw head translates further to the right, it causes the screw to tilt and leads to opening of the far cortex (red circle). (B) For a prebend of 1.5 mm, assuming the bone fragments are brought flush with the plate before compression, a gap of approximately 1 mm at the near cortex is present initially. As the screw head translates relative to the plate 1 mm, that 1 mm near cortex gap is precisely closed and balanced pressures across the fracture are obtained. (C) For a prebend of 3.0 mm, a gap of approximately 1.5 – 2 mm at the near cortex is formed in our model. The screw head translates 1 mm, which is not enough to close the near cortex gap, but induces far cortex pressures. (D) An initial gap at the far cortex 1 mm or greater may not close during dynamic compression.