Biomechanical analysis of four augmented fixations of plate osteosynthesis for comminuted mid‑shaft clavicle fracture: A finite element approach

Abstract

INTRODUCTION: The clavicle is one of the most commonly fractured bones in the human body, accounting for 2.6-5 % of adult fractures and 44 % of injuries to the shoulder girdle. Nearly 80% of clavicle fractures occur in the mid-shaft, and most are displaced comminuted fractures. For these patients, nonunion rates with conservative treatment can be as high as 15% that makes them candidates for surgical intervention [1]. Comminuted clavicle fractures with multiple fragments or cortical splits continue to pose a challenge to stable fixation that allows early postoperative rehabilitation. Plate fixation is often recommended for simple clavicle fractures, however, its fixation strength is insufficient for comminuted clavicle fractures. To achieve better stability, augmented supports like cerclage wirings and interfragmentary screws are used with the plate fixation. The cerclage wiring can wrap the clavicle fracture below or above the plate surface. For the interfragmentary lag screw, it can be placed either separately or passed through the plate. Clinical studies suggested that both cerclage wires and interfragmentary screws can produce satisfactory surgical outcomes. However, there were cases of implant failure and complications, and the optimal augment fixation design remains controversial in the biomechanical context. Finite element (FE) analysis is a powerful biomechanical tool, which allows the control of condition parameters, such as loading forces, fracture type, and implants, otherwise difficult to be assessed by in vivo or cadaveric experiments [2]. Regarding application of FE analysis on clavicle fracture, it has been used to study the fixation stability, bone adaptation and optimize implant design. This study endeavored to investigate the biomechanical stability of augmentations to plate fixation. The purpose of this study was to analyze the biomechanical stability of clavicle plate fixation under four kinds of augmentations: (1) cerclage wirings below the plate; (2) cerclage wirings above the plate; (3) single interfragmentary screw; and (4) double interfragmentary screws. Meanwhile, their effect on fracture union will be investigated. Bone and implant stress, model displacement, and fracture micro-motion would be evaluated. We hypothesized that double interfragmentary screws fixation would provide better stability than the other augmentations. METHODS: Based on CT images of a Chinese male, a finite element model of the clavicle was constructed. The type 2B1 comminuted fracture model was created according to the Robinson classification [3] with a butterfly segment at the mid-shaft of the clavicle. The clavicle fracture fixation was simulated by an 8-hole, 3.5mm locking compressive plate (LCP), which was placed on the superior surface of the clavicle (Figure 1). The internal and lateral parts of the clavicle fracture was fixed by 4 screws and 3 screws respectively. For the butterfly segment, four types of fixations were used to hold it with the main fracture: (1) single 3.5mm interfragmentary screw (SIS), (2) double interfragmentary screws (DIS), (3) double inner cerclage wirings (DICW), and (4) double outer cerclage wirings (DOCW). Two types of loads, i.e. compressive and bending forces of 100 N were applied at the 15-mm distal end of the clavicle to simulate the condition of arm abduction. The sternal end of clavicle was fixed in all degrees of freedom. The biomechanical performances including stress distribution, model displacements, and fracture micro-motions were evaluated and compared. RESULTS: Under both loading conditions, the stresses concentrated on the cerclage wirings were relatively higher than those in the interfragmentary screws. For the LCP, the DOCW construct led to the highest stress, followed by SIS, DICW, and DIS. For the clavicle fracture, the DICW construct led to the highest stresses. The bone stresses under DOCW and SIS fixation were relatively lower, but some segments still sustained high stresses. Only with DIS fixation, the bone stresses were within the normal range of cortical bone (Figure 2). The displacement analysis showed that the DIS fixation led to the smallest displacements and fracture micro-motions, followed by SIS and DICW. The displacements and fracture micro-motions of DOCW fixation was the largest. DISCUSSION: The FE results showed that the augment fixations greatly changed the clavicle fracture fixation stability, and could implicate fracture union. For the cerclage wirings, if they wrapped the clavicle, high stresses appeared on the bones, as well as the implants, and exceeded the yield strength of the cortical bone. This may lead to complications and re-fractures. However, if the cerclage wires were placed around the LCP, the plate would act as a support, and the stresses beard by the bones would decrease greatly. Among the two cerclage wiring fixations, the study showed that the biomechanical stability of DICW was better than that of DOCW, which maybe related with the larger room between the plate and clavicle of DOCW. The interfragmentary screw fixations showed better biomechanical results than those of cerclage wirings. Meanwhile, the study showed that DIS was more favorable to SIS in terms of lower stress magnitudes and better stability. This finding is compatible with our hypothesis. The displacement analysis showed that the augmented fixations present similar stresses level and acromial displacements under compression. However, in the bending condition, the results varied greatly. This indicated that the augment fixation could not effectively resist the bending forces, and the suspension break of affected limb after operation was necessary. In addition, the weight-bearing rehabilitation of affected limb, or any practice which can apply bending forces to clavicle, should also be avoided. (Figure Presented).