Load transfer considerations

Question 1

What are the 4 important design factors that affect interface stresses between the tibial component and the underlying bone?

Answer 1

Thickness of the HDP component
Whether or not the HDP has a metal backing plate
Whether the tibial component has a stem
The stiffness of the HDP material

Question 2

How does HDP thickness affect stress?

Answer 2

The thinner the HDP component the greater it is stressed

Question 3

What is the recommended HDP thickness for HDP components without a metal backing tray?

Answer 3

8mm

Question 4

Why does the HDP component tend to be thinner in HDP tibial components with a metal backing tray?

Answer 4

To limit the total thickness for surgical reasons

Question 5

Which side does the natural tibial plateau take most of the load?

Answer 5

Medial (at least 60%)

Question 6

Which type of bone takes the load proximally and what is it transferred to?

Answer 6

Taken proximally by cancellous bone only and is transferred to the cortical bone more distally

Question 7

What is the purpose of a metal backing plate?

Answer 7

To distribute high contact stresses under the condyles in order to provide an even loading on the bone beneath it

Question 8

What is the downside of a metal backing plate?

Answer 8

If the knee is loaded unevenly stress conc on the underlying medial bone is greater if a metal tray is present than if the componant is only HDP because the metal is so much stiffer

Also be a tensile stress between the plate and the bone laterally

Question 9

Why are central pegs used in most tibial components?

Answer 9

Reduces incidence of loosening

Can resist lateral rotating force

Question 10

How does the stiffness of the HDP affect contact stress?

Answer 10

Higher the Young’s modulus of the HDP the greater the contact stress