I.A. Miske, T.E. Parr, H. Seyyedhosseinzadeh, H. Hosseinzadeh
Keywords: metamaterial, orthopedics, implant, femoral, bone resorption
Summary:In the proximal femoral, mechanical failure of an implant is mainly due to prosthesis-bone de-bonding and periprosthetic bone resorption. The mechanical basis for this phenomenon seems to be mechanical stress shielding by the stiff stem, and micromovements between stem and surrounding bone during lower limb motions due to difference in flexural stiffness between stem and cortical bone. To address this problem, we need to manufacture the stems with a metal that has the flexural stiffness the same as a cortical bone while keeping the yield strength, impact strength, and toughness in the ASTM standard range for hip implants. Methods We selected a bending-dominant open cell structure that is more useful when the environmental stress is not as predictable and unidirectional. To measure the stiffness, we used a sub-physiologic force of 1,000 N (mean force on hip joint is 2,500 N and maximum force happening during jogging is 5,000 N in a 100 kg person) and performed a static compression test. To measure the modulus of elasticity, we performed nonlinear simulations on the models with all the same settings. We did the same process for shear, tension, and torque. The fatigue test was performed using normal physiological walking loads (1000 N) for ASTM standard number of cycles (10,000,000 cycles). We also prepared a CAD model of the normal femur to accept standard length, triple tapered proximal fitting cementless stem (Taperloc, Zimmer-Biomet) made out of our Ortho-metamaterial and simulated the assembly under physiologic conditions of single-leg stance and walking to find the stress distribution pattern in the stem and surrounding bone. Results The measured stiffness is between 2.673x106 N/m and 6.514x105 N/m, which is much lower than the stiffness of cortical bone (1.246x106 N/m). Two of the designed models had stiffness comparable to cortical bone. All models showed isotropic properties. The measured MOE of the model is calculated around 1.5 times the index material (Ti Eli) bulk, i.e., 162 GPa to 176 GPa for our models and 113 GPa for Ti Eli bulk. The fatigue test was performed using normal physiological walking loads (1000 N) for ASTM standard number of cycles (10,000,000 cycles) and found no damage in the model as the result of fatigue. The computational studies show that when the stem is made out of our lattice metamaterial with stiffness the same as cortical bone, while the stem bears 2.0107 N/m2, the surrounding proximal cortical bone experiences around 1.0×107 N/m2. Discussion The bending-dominant metamaterial (Ortho-Metamaterial) is a structure when built from TiEli, can have the stiffness exactly identical to cortical bone. At the same time, in spite of its porosity, it has enough strength and durability for the femoral implant based on the ASTM standards. When the implant made out of Ortho-Metamaterial is inserted into the femur, the pattern of load distribution in the proximal femur is the same as a normal femur, distributing the load true both cortical bone and the stem, theoretically, preventing the proximal femur periprosthetic bone resorption.