Mechanical assessment of two hybrid plate designs for pancarpal canine arthrodesis under cyclic loading

Abstract

Pancarpal canine arthrodesis (PCA) sets immobilization of all three carpal joints via dorsal plating to result in bony fusion. Whereas the first version of the plate uses a round hole (RH) for the radiocarpal (RC) screw region, its modification into an oval hole (OH) in a later version improves versatility in surgical application. The aim of this study was to mechanically investigate the fatigue life of the PCA plate types implementing these two features-PCA-RH and PCA-OH. Ten PCA-RH and 20 PCA-OH stainless steel (316LVM) plates were assigned to three study groups (n = 10). All plates were pre-bent at 20° and fixed to a canine forelimb model with simulated radius, RC bone and third metacarpal bone. The OH plates were fixed with an RC screw inserted either most proximal (OH-P) or most distal (OH-D). All specimens were cyclically tested at 8 Hz under 320 N loading until failure. Fatigue life outcome measures were cycles to failure and failure mode. Cycles to failure were higher for RH plate fixation (695,264 ± 344,023) versus both OH-P (447,900 ± 176,208) and OH-D (391,822 ± 165,116) plate configurations, being significantly different between RH and OH-D, p = 0.03. No significant difference was detected between OH-P and OH-D configurations, p = 0.09. Despite potential surgical advantages, the shorter fatigue life of the PCA-OH plate design may mitigate its benefits compared to the plate design with a round radiocarpal screw hole. Moreover, the failure risk of plates with an oval hole is increased regardless from the screw position in this hole. Based on these findings, the PCA plate with the current oval radiocarpal screw hole configuration cannot be recommended for clinical use.

Keywords: fatigue; hybrid plate; locking compression plate; pancarpal canine arthrodesis; radiocarpal hole.

FIGURE 1

LCP Pancarpal Arthrodesis Plate 2.7/3.5 mm with round (up) and oval (down) standard radiocarpal screw hole, 12 holes, length 151 mm.

FIGURE 2

Pre-bending of round-hole PCA plates to 20° joint fusion angle; (A) Photograph showing a plate fixed within the bending press; (B) A consistent bending angle of all plates was obtained.

FIGURE 3

Test setup with a specimen mounted for mechanical testing, adopted from (Zderic et al., 2021).

FIGURE 4

Cycles to failure in the three study groups presented in terms of mean value and SD. Circular points indicate cycles to failure for individual specimens. Star shows significant difference.

FIGURE 5

Photographs visualizing failure modes after fatigue testing; (A) asymmetric plate fracture through the locking part of a distal radial combination hole, posterior view; (B) asymmetric plate fracture through the compression part of a distal radial combination hole, anterior view. Debris in the locking part indicate fractured screw; (C) excessive plate bending at the most distal screw hole following 1st and 2nd metacarpal screw breakage, lateral view; (D) breakage of 3rd and 4th (distal) radial screws at their head-shaft interface, with numbers indicating length scale in centimeters; (E) asymmetric plate fracture through the radiocarpal hole, posterior view.

FIGURE 6

Finite element analysis of the plate pre-bending process demonstrating residual stresses after its completion (Zderic et al., 2021). Residual stresses also exist around the distal radial locking screw hole, as indicated by the red arrow, and are not limited to the bending point.