The two materials I selected for the femoral stem are TNTZ and CFR-PEEK

Properties

Elastic Modulus Ultimate Tensile Strength Fracture Toughness Wear Corrosion Resistance Biocompatibility and Other Properties
Ti-29Nb-13Ta-4.6Zr(TNTZ) 46 GPa for metastable beta-type TNTZ.
The elastic modulus can be as low as 35 GPa through mechanical processing. UTS: 967-1192 MPa
Yield Strength: 920 MPa Fracture toughness for TNTZ is not found, but it is expected to have a similar value as Ti-6Al-4V or other titanium alloys (84-107 MPa.m1/2) The wear weight losses for TNTZ under different temperatures are all smaller than Ti-6Al alloys in both low and high loading conditions. By means of surface processing such as nitriding, the wear resistance is further improved;. TNTZ his demonstrated high corrosion resistance in standard physiological solutions. Furthermore, the bonding energy between Ti and neighbouring atoms in the TNTZ alloy is higher than that of the metallic bonds in pure Ti metal. The niobium (Nb) element will form a stable oxide when placed in oxidizing environment. Biocompatibility: Low Young’s modulus prevents bone resorption (good mechanical biocompatibility). Greater direct contact between newly formed bone tissue and TNTZ surface (excellent biological biocompatibility).

In addition, TNTZ does not induce a significant cytotoxin effect. | | Carbon Fibre Reinforced Polyetheretherketone (CFR-PEEK) | 18 GPa | 138 MPa | CFR-PEEK can withstand one million loading cycles of 2240 N of 5 Hz without any visual signs of fracture. | Wear rate is higher than polyethylene. Has lower wear debris weight than titanium alloys. | No evidence of corrosion of CFR-PEEK was identified under a simulated in vivo environment. Also resistant to various acidic solutions under room temperature. | Biocompatibility: Immune responses to PEEK is rare. Studies on the systemic and intracutaneous toxicity and sensitisation tests all showed on adverse effects. Although the PEEK material itself is inert in biological context and has limited fixation with the bone, its bioactivity can be improved by coating with titanium of hydroxyapatite, or by creating porous PEEK networks for bone ingrowth. |

<aside> 🧐 Conclusions based on material properties:

  1. Although being a member of the titanium alloy “family”, TNTZ was recently developed and its application in the biomedical field is already promising. In addition, TNTZ exhibits the great mechanical properties for an ideal material for the femoral stem (high ultimate tensile strength and high wear resistance), while also obtains distinctive differences than other titanium alloys, such as low elastic modulus, high corrosion resistance, and even great biocompatibility.
  2. CFR-PEEK is a material commonly used in orthopedic implants, especially for spinal cages or knee replacement products. However, I believe that CFR-PEEK can also be used to create durable and biocompatible femoral stem with several modifications and strengthening techniques. The mechanical properties of CFR-PEEK are highly customizable by changing the orientation, concentration, and composition of the carbon fibre. The only potential issue with this material is that it has a relatively low ultimate tensile strength, which may not be able to withstand the load exerted by our patient’s body.

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Processing

Coatings Drug Delivery Options Corrosion/Wear Resistance Manufacturing and Strengthening Techniques
TNTZ The are several coating options that can be applied to improve the performance of TNTZ femoral stem. First, the powder of CaP invert gas can be applied to the surface of TNTZ and heated. The fatigue endurance limit of the coated titanium alloy is found to be 15% than that of uncoated TNTZ.
Another coating that can be applied to TNTZ is silicon carbide, which decreases wear and provides a bone-like microstructure that can enhance osseointegration. Antibiotic delivery on titanium stem can lower the probability of revision surgery as 40.4% of revisions occur due to post-operative infections.
Ultrasonic nanocrystal surface modification can be appl8ied to TNTZ alloy to improve wear resistance and bifunctionality (more than 7 times higher than the untreated one).
Surface hardening of TNTZ by gas nitriding can also be used to improve corrosion resistance. Through thermomechanical processing, the mechanical properties of TNTZ can be controlled similar to that of conventional titanium alloys.
While maintaining the low elastic modulus, the tensile strength of TNTZ can be improved by work hardening, grain refinement strengthening, precipitation strengthening, and dispersion strengthening.
The elastic modulus can be further reduced through the introduction of porosity.
CFR-PEEK Coatings and modifications are needed since the hydrophobic surface of CFR-PEEK will prevent bone apposition. However, materials known to encourage bone tissue growth such as hydroxyapatite or titanium can be used as coatings. Diffusion is the major drug release system that can be applied on CFR-PEEK. The drug can be directly incorporated into the carrier through a matrix system.
Another drug release system is polymer chain degradation. The graphene oxide coating can reduce wear rate of CFR-PEEK by 83%. 1. The 3D printed CFR-PEEK material allows the development of customized patient-specific implants to fit precisely into the defect space. 3D-printing techniques provide faster and more efficient productions.
  1. Manufactures can increase the material’s strength, stiffness, and impact properties with proper selection of fibre type and concentration.
  2. Manufacturing processes include autoclave molding, compression molding, filament winding, and pultrusion to create simple or complex part geometries with specific fibre orientations for the desired design. |

*Cover image obtained from the Unsplash database: https://unsplash.com/photos/FdRHUdfNrtg?utm_source=63921&utm_medium=referral