Damage in a Distal Radius Fracture Model Treated With Locked Volar Plating After Simulated Postoperative Loading

Abstract

Purpose: "Damage" is an engineering term defining a period between a state of material perfection and the onset of crack initiation. Clinically, it is a loss of fixation due to microstructural breakdown, indirectly measured as a reduction of stiffness of the bone-implant construct, normalized by the cross-sectional area and length of the bone. The purpose of this study was to characterize damage in a cadaver model of extra-articular distal radius fracture with dorsal comminution treated using 2-column volar distal radius plates.

Methods: Ten matched distal radii were randomly divided into 2 groups: group I specimens were treated with a volar distal radius plate with an independent, 2-tiered scaffold design; group II specimens (contralateral limbs) were treated with a volar plate with a single-head design for enhanced ulnar buttressing. Specimens were cyclically loaded to simulate a 6-month postoperative load-bearing period. We report damage after a defined protocol of cyclical loading and load to failure simulating a fall on an outstretched hand.

Results: Group II specimens experienced more damage under cyclic loading conditions than group I specimens. Group I specimens were stiffer than group II specimens under load-to-failure conditions. Ultimate force at failure in group I and group II specimens was not different. Specimens failed by plate bending (group I, n = 6/10; group II, n = 2/10) and fracture of the lunate facet (group I, n = 4/10; group II, n = 8/10).

Conclusions: Group I specimens had less screw cutout at the lunate facet than group II specimens under cyclic loading as indicated by lower damage measures and fewer facet fractures during load-to-failure testing. The overall strength of the construct is not affected by plate design.

Clinical relevance: Microstructural damage or a loss of fixation due to an overly rigid volar plate design may cause malunion or nonunion of fracture fragments and lead to bone-implant instability.

Keywords: Distal radius fracture; distal radius plating; volar locking plate; wrist biomechanics; wrist fracture.

Figure 1

A. Anteroposterior view of the hand and forearm showing the distal radius bone segmentation (purple) using the MIMICS software. B. Axial view of the wrist showing a 2D slice of the distal radius bone (purple) at the location of simulated fracture. MIMICS measurement tools were used to calculate the cross-sectional area of the bone at this location, for use in subsequent damage calculations.

Figure 1

A. Anteroposterior view of the hand and forearm showing the distal radius bone segmentation (purple) using the MIMICS software. B. Axial view of the wrist showing a 2D slice of the distal radius bone (purple) at the location of simulated fracture. MIMICS measurement tools were used to calculate the cross-sectional area of the bone at this location, for use in subsequent damage calculations.

Figure 2

Experimental testing fixture showing radial shaft mounted to load frame actuator. The scaphoid/lunate load applicator is mounted to an angle vise to adjust contact angle to achieve 60/40 load distribution across the distal articular surface. A mini C-arm is positioned to obtain intra-cyclic x-ray images for failure analysis. A digital camera is positioned to capture failure during ramped loading.

Figure 3

A: Representative force-displacement hysteresis curves of a single specimen from Group I and contralateral limb in Group II. A reduction in stiffness of each specimen is noted by the increase in displacement under constant loading. Note that the Group II specimen has a greater increase in displacement. B: Representative damage plots of the same two specimens over the 5000 cycle test period. Much of the damage in each specimen accumulated within the first 1000 cycles with little damage progression after that period.

Figure 3

A: Representative force-displacement hysteresis curves of a single specimen from Group I and contralateral limb in Group II. A reduction in stiffness of each specimen is noted by the increase in displacement under constant loading. Note that the Group II specimen has a greater increase in displacement. B: Representative damage plots of the same two specimens over the 5000 cycle test period. Much of the damage in each specimen accumulated within the first 1000 cycles with little damage progression after that period.

Figure 4

Representative images of the failed specimens showing A. distal fragment collapse leading to plate bending (Group I, n=6/10; Group II, n=2/9), and B. fracture of the lunate facet (Group I, n=4/10; Group II, n=8/9).

Figure 4

Representative images of the failed specimens showing A. distal fragment collapse leading to plate bending (Group I, n=6/10; Group II, n=2/9), and B. fracture of the lunate facet (Group I, n=4/10; Group II, n=8/9).