Orthopaedic Implant Materials
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Direct answer
Orthopaedic implant metals are chosen by balancing stiffness, strength, corrosion resistance and wear. Titanium alloy Ti-6Al-4V has the lowest elastic modulus of the three main metals, at roughly 110 GPa, against about 200 GPa for 316L stainless steel and about 230 GPa for cobalt-chrome. It is also the most corrosion resistant, but its poor wear behaviour keeps it off articulating surfaces.
What you must remember
- Stainless steel (316L): the low-carbon austenitic grade used for trauma plates, screws and temporary fixation, with good strength and adequate corrosion resistance.
- Titanium alloy (Ti-6Al-4V): lowest modulus of the metals, excellent corrosion resistance from a passive oxide layer, but poor wear and notch sensitivity.
- Cobalt-chrome: high strength, good fatigue and wear resistance, and the usual choice for femoral heads and other articulating bearing surfaces.
- Cortical bone modulus: about 18 GPa, so every metal implant is far stiffer than the bone it sits on.
- Stress shielding: load bypasses the bone under a stiff implant, and Wolff's law predicts resorption, seen as cortical thinning beneath a plate or stem.
- Mixed metals: contact between dissimilar metals, such as stainless steel with titanium, risks galvanic corrosion, and crevice corrosion occurs at screw-plate interfaces.
- Ceramics: alumina and zirconia bearings are hard with very low wear, but they are brittle and can fracture.
Where the exam tries to catch you
Most implant-material questions are won by separating corrosion resistance from wear resistance, and by remembering which property drives stress shielding.
- Trap: choosing titanium for a femoral head because it is the most corrosion resistant metal. Correct: titanium wears badly against articulating partners, so cobalt-chrome or ceramic bearings are used on articulating surfaces.
- Trap: believing stress shielding means the implant has failed. Correct: it is bone resorption from load transfer, and a lower-modulus material reduces it.
- Trap: calling stainless steel the stiffest common implant metal. Correct: cobalt-chrome has the highest modulus of the three, so it is stiffer than 316L.
Frequently asked questions
Which implant metal has the lowest elastic modulus?
Titanium alloy Ti-6Al-4V has the lowest elastic modulus of the common orthopaedic implant metals, at roughly 110 GPa. Stainless steel and cobalt-chrome sit near or above 200 GPa. Cortical bone is far lower, at about 18 GPa.
What is stress shielding in orthopaedic implants?
Stress shielding is bone loss beneath a stiff implant that carries load which would otherwise pass through the bone. It follows Wolff's law and appears as cortical thinning around plates or stems. Lower-modulus materials and load-sharing designs reduce stress shielding.
Why is titanium not used for hip bearing surfaces?
Titanium alloy has poor wear resistance and wears badly against its articulating partner, so it is unsuitable for hip bearing surfaces. Cobalt-chrome or ceramic bearings are preferred on articulating surfaces. Titanium remains useful for cementless femoral stems and spinal implants.
Which grade of stainless steel is used for orthopaedic implants?
Austenitic stainless steel grade 316L is the standard orthopaedic grade. Its low carbon content improves corrosion resistance by reducing carbide precipitation at grain boundaries. It is used mainly for trauma plates, screws and temporary fixation rather than long-term articulating implants.