3 20 pts A patient is being treated for a fracture from a hu

3. (20 pts) A patient is being treated for a fracture from a hunting accident. Lead shot remains in the wound. The surgeon decided a bone plate should be used. What is your recommendation: use of a Co-Cr or Ti6Al4V plate. Disregard alloy and passivation layer effects. Please support your recommendation mathematically.

Solution

Ans:

Corrosion is one of the major issues resulting in the failure of biomedical implant devices. The nature of the passive oxide films formed, and the mechanical properties of the materials form some of the essential criteria for selection of alternative or development of new materials. In clinical terms, the biggest improvements could be made by better material selection, design, and quality control to reduce, or possibly eliminate corrosion in implant devices. Co-Cr or Ti6Al4V plates are have several applications like orbit reconstruction, dental implants, orthopedic fracture plates, heart valves, spinal rods, joint replacement prostheses and cranial plates, orbit reconstruction, maxillofacial reconstruction, dental implants, dental wires, orthopedic fracture plates, joint replacement prostheses, stents, ablation catheters. Due to their corrosion resistance the titanium and its alloys are presently employed to address implant cases. Titanium alloys such as Ti-6Al-4V, Ti-5Al-2.5Fe, and Ti-6Al-7Nb provide ideal strength and corrosion resistance characteristics. The main advantage of titanium and its alloys is the non-reactivity of the passive film that is formed; the main disadvantages are its susceptibility to fretting as well as oxygen diffusion during fabrication, causing embrittlement. The mechanical properties of the alloys compared to human bone are presented in the table given below.

In cases where there are galvanic couples arising from the combination of dissimilar metals, such as 316L stainless steel and the Co-Cr-M alloy or Ti-6Al-4V alloy, the stainless steel will be attacked and these combinations should be avoided. Galvanic effects can also occur by using metal alloys that have undergone slightly different metal processing (cast vs. wrought Co-Cr-Mo). The placement of polymeric inserts between metal-metal interfaces will eliminate galvanic corrosion and would considerably reduce fretting corrosion, however alloy selection is critical since a crevice situation would certainly be the result and a more aggressive corrosion issue could develop.

Mechanical properties of implant alloys and human bone.

Material

Tensile Strength (MN/m)²

Yield Strength (MN/m)²

Vickers Hardness (H)

Young’s Modulus (GN/m)²

Fatigue Limit (GN/m)²

Wrought Co-Cr Alloy

1540

1050

450

541

0.49

Co-Cr Alloy

690

490

300

241

0.30

Ti-6Al-4V

1000

970

---

121

---

Human Bone

137.3

-

26.3

30

-

Since Co-Cr or Ti6Al4V plates properties are similar or more comparable with the human bone, they can be effectively utilized in implantation cases. These orthopedic bone plate now excels over other types of internal fixators in bone fracture fixation