Construction of FEA model
A 3D model of the composite femurs (4th generation, Sawbones Worldwide, WA, USA) was constructed by computed tomography (CT) imaging analysis (Mimics 16, Materialise, Software & Services for Biomedical Engineering, Leuven, Belgium) of the data obtained from X-ray CT (Eclos-4S, Hitachi, Otawara, Tochigi, Japan) . The periprosthetic femoral fracture model was assembled in a 3D-computer aided design software (UG NX 5, SIEMENS, Plano, TX, USA), and the stem position was determined based on radiographs and CT data of an experimental THA model. A transverse fracture was created 10 mm below the tip of the Exeter femoral stem (Stryker, Kalamazoo, MI, USA) and the construct was fixed using two different fixation methods (Fig 1). The single-plating method was fixed laterally using a 9-hole LCP-DF locking plate (Depuy Synthes, West Chester, PA, USA) with four proximal uni-cortical locking screws and three distal bi-cortical locking screws. The double-plating method was fixed similarly to the single-plating method with an additional anterior 7-hole metaphyseal locking plate (Depuy Synthes) with two proximal uni-cortical locking screws and three distal bi-cortical locking screws. To add fixation to the proximal fragments, two cerclage cables (Depuy Synthes) were used with a tension of 400 N.
All sections were assigned isotropic material properties with an elastic modulus of 16.3 GPa for cortical bone , 0.15 GPa for cancellous bone , 2.8 GPa for polymethylmethacrylate (PMMA) cement , 195 GPa for Orthinox stainless steel , and 110 GPa for Titanium . A Poisson’s ratio of 0.3 was used for all materials .
A finite element pre-processor was generated using HyperMesh 13 (Altair Engineering, Troy, MI, USA). Tetrahedral primary elements were used, whereas the numbers for elements and for nodes were 1,023,382 and 224,630 in the single-plate fixation method, and 1,047,309 and 231,601 in the double-plate fixation method, respectively. To set up the boundary conditions, the cortical and trabecular bones were fixed by glue, with a coefficient of friction of 0.1, 0.1, 0.3, 0.1 and 0.1 used at the bone-stem, bone-plate, bone-screw interface, bone-cable, and cable-fastener interface, respectively [28,29]. The distal end of the femoral model was fixed with cement. These constructs were positioned at 20 degrees adduction in the frontal plane and aligned vertically in the sagittal plane. This position was to simulate the anatomical one-legged stance. Thereafter, the constructs were tested under an axial load of 1,500 N (Fig 2) as previously described [30,31], and the results were then analysed using a nonlinear FEA software (MSC Marc 2017, MSC Software, Newport Beach, CA, USA).
Testing and analysis
Biomechanical testing was conducted using synthetic composite femurs (Sawbones Worldwide). Composite bones were placed in a bench-mounted vice grip, and then neck osteotomy, trochanteric reaming, and rasping were performed. Polymethyl methacrylate (PMMA) cement (Simplex P, Stryker) was pressurized into composite bone, and an Exeter hip prosthesis (Stryker) was manually inserted. Stem alignment was checked using the X-ray (data not shown). To add fixation to the proximal fragments, two cerclage cables (Depuy Synthes) were used with a tension of 400 N (Fig 3A). The strain gage (KFG-2N-120-C1, Kyowa, Chofu, Japan) was attached on the surface of the LCP-DF, parallel to axis of the plate, at the defect level (Fig 3B). The distal end of the composite bone was placed in an 80-mm-wide threaded steel pipe and fixed with two steel bolts for anti-rotation. The constructs were further fixed by pouring the cement into the steel pipe, and the fracture fixation models were made with the mechanical test equipment (AGS-H, Shimadzu, Japan). To achieve maximum vertical load directly on the head of the prosthesis, the mounting platform was placed to facilitate biaxial translation of the specimen (Fig 3C). For the axial loading test, a sequentially vertical loading test was performed on the prosthetic head at a velocity of 5 mm/min up to 1500 N. The test was repeated thrice for each construct. The maximum displacements and strains were calculated based on the load-displacement and load-strain curves generated by the static compression tests.
Data analysis and statistics
Statistical analysis was performed using Student’s t-test to compare the differences between two independent groups, and the results were considered significant when P < 0.05. Data are presented as means ± SEM.