Load transfer mechanisms in the stem

Question 1

How is load transferred in an intramedullary stem?

Answer 1

Stem transfers some of its load to the bone proximally, the rest distally with a central region of load sharing

Question 2

What is the advantage of a low stiffness stem?

Answer 2

Reduces stress shielding

Question 3

What is the disadvantage of a low stiffness stem?

Answer 3

High shear stresses at the proximal bone-stem interface or in the cement or at the bone-cement-stem interface

Question 4

Why is cement not included in load sharing calculations?

Answer 4

Because its rigidity is low can be assumed it takes a very small proportion of the load

Question 5

How is load transferred from the stem to the bone?

Answer 5

As a shear force

Question 6

What is an isoelastic stem?

Answer 6

Stem with the same stiffness as bone

Question 7

Why are isoelastic stems not viable?

Answer 7

Better in terms of stress shielding but shear stresses or the interfaces arising from high proximal load transfer can be high enough to cause failure

Question 8

What are the two ways of potentially varying the rigidity of a stem?

Answer 8

Using a material which has varying stiffness along its length
Making the stem hollow

Question 9

What are the magnitude of stresses in the cement dependent on?

Answer 9

Thickness of cement

Stiffness of cement

Question 10

What are the problems if the cement layer is too thin?

Answer 10

Very high cement stresses

Bone resorption at proximal end of the femur due to cement debris)

Question 11

What are the problems if the cement layer is too thick?

Answer 11

High cement stresses

Question 12

Why is a more flexible cement preferred?

Answer 12

Stiffer cement results in higher proximal and distal cement and interface stresses

Question 13

Why does Harris argue that a proximal collar is good?

Answer 13

Allows compressive load transfer from the stem to the bone - reduces stress sheilding - lowers the stresses in the cement in the proximal medial region

Question 14

What is the calcar?

Answer 14

Thick cortical part of femoral neck

Question 15

How should a proximal collar be fitted?

Answer 15

Cut the calcar very accurately to ensure that the collar rests on a substantial part of the bone

Question 16

Why does Ling argue that proximal collars are bad?

Answer 16

Collar-calcar contact area acts as a pivot about which the stem rotates - distal end prone to high stress conc and may fail particularly if cement mantle is deficient
Also debris due to fretting of the collar against the cement or bone

Question 17

How is the stem-cement bond strengthened in practice?

Answer 17

Coating the stem with PMMA

Question 18

What does Ling argue about the cement-metal interface?

Answer 18

Should be free to slide - as it interface is weak so there is a risk of bone reaction to wear particles

Question 19

What does Harris argue about bonding stems?

Answer 19

A collar should be used to prevent unnecessary sinkage - also argues that the bone-cement interface is usually well interlocked so there is no need to the stem-cement interface to have to slip to protect it

Question 20

How does a press fit reduce stress shielding?

Answer 20

By promoting hoop stresses in the bone

Question 21

Why are modern cementless stems coated with hydroxyapatite?

Answer 21

Help bone ingrowth

Potentially eliminates metal debris from bone-metal abrasion

Question 22

What is the downside of fully HAp coated stems?

Answer 22

Promote stress shielding of the bone

Question 23

What issue doe sthe lack of distal contact in cementless stems cause?

Answer 23

Thigh pain

Question 24

How is thigh pain treated in cementless stems?

Answer 24

Custom made plastic sleeves (used to provide good distal contact)

Question 25

What problems are encountered with bone ingrowth with HAp coating?

Answer 25

Bone ingrowth does not occur (thought to be due to movement at interface)
Fibrous tissue grows (preventing future prospects of bone ingrowth)
After a few years bone-HAp bond breaks down (unsure of cause but could be bone resorption due to presences of HDP frgments)

Question 26

What is the difficulty in stem shape in cementless implants?

Answer 26

Needs to be good fit as gap cannot be filled
Hard for commercial replacements to cover wide range of femur shapes (dimensions of femoral canal do not vary with the size of the femur)

Question 27

How does the femoral canal geometry vary with age?

Answer 27

Cortical bone thins and gradually becomes wider (particularly in women after they are 50-60 y/o)

Question 28

How does head offset distance affect the bending moment?

Answer 28

Reducing the offset distance from the head to the neutral axis of the stem

Question 29

How can the offset distance be reduced (to reduce bending moment)?

Answer 29

Reduce length of neck of stem

Increase angle between the long axis and the axis of the neck

Question 30

What are the downsides of reducing offset distance?

Answer 30

Increases joint reaction force (greater wear and acetabular bone-implant stresses)

Question 31

Why is there much less of a need to reduce bending moments in the stem nowadays?

Answer 31

Stem materials are much stronger - fracture of the stem less of a problem

Question 32

Why is there a current trend towards using more anatomically accurate stems (which increases the offset distance)?

Answer 32

Reduces the magnitude of the joint force and therefore reduces joint wear