Biomechanical considerations in the design of patient-specific fixation plates for the distal radius

Abstract

Use of patient-specific fixation plates is promising in corrective osteotomy of the distal radius. So far, custom plates were mostly shaped to closely fit onto the bone surface and ensure accurate positioning of bone segments, however, without considering the biomechanical needs for bone healing. In this study, we investigated how custom plates can be optimized to stimulate callus formation under daily loading conditions. We calculated implant stress distributions, axial screw forces, and interfragmentary strains via finite element analysis (FEA) and compared these parameters for a corrective distal radius osteotomy model fixated by standard and custom plates. We then evaluated these parameters in a modified custom plate design with alternative screw configuration, plate size, and thickness on 5 radii models. Compared to initial design, in the modified custom plate, the maximum stress was reduced, especially under torsional load (- 31%). Under bending load, implants with 1.9-mm thickness induced an average strain (median = 2.14%, IQR = 0.2) in the recommended range (2-10%) to promote callus formation. Optimizing the plate shape, width, and thickness in order to keep the fixation stable while guaranteeing sufficient strain to enhance callus formation can be considered as a design criteria for future, less invasive, custom distal radius plates. Graphical abstract ᅟ.

Keywords: Corrective osteotomy; Finite element analysis; Implant design; Patient-specific implant; Volar distal radius plate.

Graphical abstract

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Fig. 1

Steps for preoperative planning and design of a custom implant. a 3D reconstruction of the affected bone. b Distal and proximal parts of the affected bone model registered onto the mirrored image of the healthy contralateral radius. c Virtual box used to select the target bone surface for the implant design. d Selected bone surface extruded into a 3D plate, cut and then repositioned via the transformation matrix (M_T). e Repositioned parts of extruded plate footprint with interpolated connection

Fig. 2

Definition of daily loading conditions. a Forty percent of the axial load was exerted onto the lunate and 60% onto the scaphoid fossa. b Bending moment about the axis defining wrist flexion-extension motion. c Torsion

Fig. 3

Definition of corresponding distal and proximal point pairs (D_i, P_i) at the osteotomy border on the distal and the proximal bone segments used to calculate the average strain in the gap

Fig. 4

Implant types: a standard anatomical plate (2.4-mm LCP Volar Column Distal Radius Plate, Synthes, West Chester, Pennsylvania, USA). b Initial custom implant configuration. Five screw holes were positioned distally (D1, D2, D3, D4, D5) and four proximally (P1, P2, P3, P4). c Custom implant with the new screw configuration. d Custom implant with the new screw configuration and a reduced plate size

Fig. 5

a Color maps showing Von Mises stress distribution under axial compression (50 N), bending moment (1 Nm), and torsion (1 Nm). Stresses are expressed as a percentage of the yield stress of the compounding materials. Location of the highest stress, as indicated by each applicable color scale, is also red circled. Regions with stress below the minimum value along the scale are colored white. a Standard plate. b Initial custom plate. High stress concentrations were observed at the level of the proximal screw closer to the gap. c Custom plate with modified screw configuration

Fig. 6

Maximum Von Mises stress value, in percent of yield stress (YS) for implant-screw assembly under axial compression (50 N), bending moment (1 Nm), and torsion (1 Nm) in the standard, initial, and modified custom plates

Fig. 7

Boxplots representing a the distribution of maximum Von Mises stress and b distribution of the gap interfragmentary strain ε_IF in five patient cases as a function of the modified custom plate thickness (2.4, 1.9, 1.5, and 1.0 mm) under axial compression (50 N), bending moment (1 Nm), and torsion (1 Nm). Based on Perren’s theory, each strain range induces a different biological response