Mechanical Evaluation of Two Hybrid Locking Plate Designs for Canine Pancarpal Arthrodesis

Abstract

Hybrid locking pancarpal arthrodesis plates were designed with either a round (RH) or an oval (OH) radiocarpal hole, the latter allowing varied screw positioning. Due to concerns about potential decreased structural properties of the OH design, our aim was to compare the mechanical behavior of the contrasting plates using combined finite element analysis (FEA) and mechanical testing. Pancarpal arthrodesis plates with RH or OH design were assigned to three fixation techniques (n = 6), prebent at 20°, and fixed to canine forelimb models with simulated radius and radiocarpal and 3rd metacarpal bones. OH plates were instrumented with a radiocarpal screw inserted either most proximal (OH-P) or most distal (OH-D). Specimens were axially loaded to 300 N over 10 ramped cycles at 0.5 Hz. Plate strains were measured with strain gauges placed at areas of highest deformations as predicted by FEA under identical loading conditions. FEA predicted the highest strains (μm/m) adjacent to the radiocarpal hole (2,500 [RH], 2,900 [OH-P/OH-D]) and plate bending point (2,250 [RH], 1,900 [OH-P/OH-D]). Experimentally, peak radiocarpal hole strains were not influenced by the OH screw position (3,329 ± 443 [OH-P], 3,222 ± 467 [OH-D]; P = 0.550) but were significantly higher compared to the RH design (2,123 ± 154; P < 0.001). Peak strains at the bending point were significantly lower for OH-P (1,792 ± 174) and OH-D (1,806 ± 194) versus RH configurations (2,158 ± 114) (P ≤ 0.006). OH plates demonstrated highest peak strains next to the radiocarpal hole and were associated with more heterogenous plate strain distribution. Structural weakening associated with radiocarpal OH plate design could result in decreased fixation strength and increased risk of plate fatigue failure.

Figure 1

Limited contact locking pancarpal arthrodesis plate 2.7/3.5 mm with round (top) and oval (bottom) radiocarpal hole, 12 holes, and length 151 mm. Double arrow indicates the bending point for each plate.

Figure 2

Lateral view of the limited contact locking pancarpal arthrodesis plate 2.7/3.5 mm with round (a) and oval (b) radiocarpal holes in its prebent (top) and original straight (bottom) shapes.

Figure 3

Illustration of the two-step FEA with virtual prebending of the oval hole (OH) plate and simulation of in vivo loading conditions by axial compression; OH plate fixed between two rollers before (a) and after (b) the bending process. Residual stresses derived from the prebending phase were transferred for the second step of the FEA. (c) Boundary conditions for simulation of in vivo loading conditions were based on the experimental set-up. Magnification shows radiocarpal hole and bending point with applied refined mesh (0.8 mm) in this region.

Figure 4

Test set-up with a specimen mounted for mechanical testing. Vertical arrow indicates loading direction. Magnification shows radiocarpal screw placed in the distal aspect of the oval plate hole (OH-D) construct with applied strain gauges at the weakest locations predicted by FEA: the plate bending point, adjacent to occupied/unoccupied radiocarpal hole region.

Figure 5

Principal strains with color-coded strain scale (bottom) in the radiocarpal hole region and at the bending point under 300 N axial compression, shown for round hole (RH) (a), screw insertion at distal margin of oval hole (OH-D) (b), and screw insertion at proximal margin of oval hole (OH-P) (c), indicating for RH a lower strain magnitude next to the radiocarpal hole and a higher strain magnitude at the bending point, compared to both OH-D and OH-P.

Figure 6

Outcome measures for the four parameters of interest derived from experimental testing, namely, peak strain next to RC hole (a) and at bending point (b), as well as construct compliance (c) and maximum angular deformation (d), shown in terms of mean and SD values for each plate and screw configuration separately. ∗ indicates significant differences between fixation techniques.