Biomechanical Analysis of a New Carbon Fiber/Flax/Epoxy Bone Fracture Plate Shows Less Stress Shielding Compared to a Standard Clinical Metal Plate

Abstract

Femur fracture at the tip of a total hip replacement (THR), commonly known as Vancouver B1 fracture, is mainly treated using rigid metallic bone plates which may result in "stress shielding" leading to bone resorption and implant loosening. To minimize stress shielding, a new carbon fiber (CF)/Flax/Epoxy composite plate has been developed and biomechanically compared to a standard clinical metal plate. For fatigue tests, experiments were done using six artificial femurs cyclically loaded through the femoral head in axial compression for four stages: Stage 1 (intact), stage 2 (after THR insertion), stage 3 (after plate fixation of a simulated Vancouver B1 femoral midshaft fracture gap), and stage 4 (after fracture gap healing). For fracture fixation, one group was fitted with the new CF/Flax/Epoxy plate (n = 3), whereas another group was repaired with a standard clinical metal plate (Zimmer, Warsaw, IN) (n = 3). In addition to axial stiffness measurements, infrared thermography technique was used to capture the femur and plate surface stresses during the testing. Moreover, finite element analysis (FEA) was performed to evaluate the composite plate's axial stiffness and surface stress field. Experimental results showed that the CF/Flax/Epoxy plated femur had comparable axial stiffness (fractured = 645 ± 67 N/mm; healed = 1731 ± 109 N/mm) to the metal-plated femur (fractured = 658 ± 69 N/mm; healed = 1751 ± 39 N/mm) (p = 1.00). However, the bone beneath the CF/Flax/Epoxy plate was the only area that had a significantly higher average surface stress (fractured = 2.10 ± 0.66 MPa; healed = 1.89 ± 0.39 MPa) compared to bone beneath the metal plate (fractured = 1.18 ± 0.93 MPa; healed = 0.71 ± 0.24 MPa) (p < 0.05). FEA bone surface stresses yielded peak of 13 MPa at distal epiphysis (stage 1), 16 MPa at distal epiphysis (stage 2), 85 MPa for composite and 129 MPa for metal-plated femurs at the vicinity of nearest screw just proximal to fracture (stage 3), 21 MPa for composite and 24 MPa for metal-plated femurs at the vicinity of screw farthest away distally from fracture (stage 4). These results confirm that the new CF/Flax/Epoxy material could be a potential candidate for bone fracture plate applications as it can simultaneously provide similar mechanical stiffness and lower stress shielding (i.e., higher bone stress) compared to a standard clinical metal bone plate.

Osteogenesis and cytotoxicity of a new Carbon Fiber/Flax/Epoxy composite material for bone fracture plate applications

“…Not only did the current CF/ Flax/Epoxy has promising biological properties, but the present authors previously showed it has lower axial stiffness (for reducing "stress shielding") and adequate bending stiffness (for proper fracture site immobilization) [11]. Moreover, in a prior biomechanical study, the current authors have used this plate to fix a Vancouver B1 femoral fracture under clinical-type static and dynamic loading conditions with comparable results to a medical grade 316L stainless steel fixation plate [50].…”

“…stiffness) for almost 90% of its fatigue life, which is almost never the case for homogeneous metallic materials [26]. More importantly, as a consequence of its biomechanical properties, this composite can potentially minimize the common major problem of "stress-shielding", which is typically present when using metallic materials to fix bone fractures [27]. Also, this composite provides adequate immobilization that avoids any gross motion at the fracture site leading to proper healing of the fractured bone [27].…”

“…More importantly, as a consequence of its biomechanical properties, this composite can potentially minimize the common major problem of "stress-shielding", which is typically present when using metallic materials to fix bone fractures [27]. Also, this composite provides adequate immobilization that avoids any gross motion at the fracture site leading to proper healing of the fractured bone [27]. However, no prior studies exist which have assessed this unique composite material's cytotoxicity, its effect on osteogenetic differentiation necessary for bone fracture healing, and its wettability characteristics.…”

Failure of Fiber-Reinforced Composites

“…The second type of heat source is friction inside the material or between the support and the test specimen. While the generation of heat due to the latter is a general problem especially during fatigue testing of composites [199][200][201], the friction inside the material is key to assist in interpretation of material failure. During periodic cyclic loading experiments, existing crack surfaces can rub against each other and therefore generate a noticeable amount of heat.…”

Carbon Fiber Biocompatibility for Implants

“…The modulus for a metal bone plate is between 110 GPa and 220 GPa compared to human long cortical bone of around 17–20 GPa [ 66 ]. Cellular bone formation and bone loss are balanced so that when higher loads are applied osteogenic bone formation occurs to counteract the extra force [ 65 – 67 ]. Consequently, with metal plates, the bone fracture is “shielded” or under-stressed and prevented from healing normally even for tissues ingrown into the fracture site by resorbing into weaker bone according to Wolff's law [ 65 – 67 ].…”