Unit 1 - Orthopaedic Implant Mechanics & Materials

Question 1

List the criteria for a successful orthopaedic implant?

Answer 1

1. Be tolerated by the human body
2. Relieve pain and enable sufficient mobility
3. Function without failure
4. Implanted successfully by a surgeon of average
   ability
5. Acceptable cost

Question 2

What are the main structural requirements for an implant?

Answer 2

Strength and stability

Question 3

What structural factors must be considered in the design of all orthopaedic implants?

Answer 3

1. Strength
2. Stiffness
3. Lubrication
4. Wear
5. Fatigue

Question 4

What kinetic factor must be considered in the design of all orthopaedic implants?

Answer 4

Motion

Question 5

What are the important requirements of an orthopaedic implant that are essential for biocompatibility?

Answer 5

1. Biological Integration

2. Functional Integration

Question 6

Most orthopaedic implants are attached to bone and form what?

Answer 6

A composite structure with bone

Question 7

What are the 2 types of bone?

Answer 7

Compact (cortical) and cancellous bone

Question 8

The stiffness property of a material is defined by what?

Answer 8

Its Young’s modulus

Question 9

Young’s modulus is a ratio of what?

Answer 9

Stress to stain

Question 10

What does isotropic mean?

Answer 10

Mechanical properties are the same no matter what direction they are loaded

Question 11

What does anisotropic mean?

Answer 11

Young’s modulus depends on the direction in which it is being loaded

Question 12

What is stress shielding?

Answer 12

Reduction in the load that would normally be taken by a bone. This occurs because some of the load is taken by an implant. Bone resorbs when understressed and this can lead to loosening of the implant.

Question 13

Structural stiffness is determined by what?

Answer 13

1. Material stiffness

2. Geometrical stiffness

Question 14

What is the formula for shear modulus?

Answer 14

G = shear stress/shear strain

Question 15

What is the formula for stiffness?

Answer 15

S=Force/stress or S=E x A/L

Question 16

What are the geometrical factors that affect the axial stiffness of a structural component?

Answer 16

1. Cross sectional area

2. Length

Question 17

What is the formula for axial rigidity?

Answer 17

R=EA

Question 18

What is the formula for bending rigidity?

Answer 18

R=EI

Question 19

What is the formula for torsional rigidity?

Answer 19

R=GJ

Question 20

What are the 2 main methods of biological fixation?

Answer 20

1. Porous beads

2. Ceramic coatings

Question 21

What are the 3 important features required of an orthopaedic implant material ?

Answer 21

1. High degree of biocompatibility
2. Suitable structural properties
3. Ease of manufacture

Question 22

Name 3 types of corrosion?

Answer 22

1. Galvanic corrosion
2. Fretting corrosion
3. Crevice corrosion

Question 23

Name 2 methods for improving corrosion resistance?

Answer 23

1. Nitric acid immersion

2. Titanium nitride coating

Question 24

Name 7 adverse effects implanted materials can have on the body?

Answer 24

1. Growth of a fibrous layer between the bone and the implant
2. Local infection
3. Body sensitisation to metals
4. Tissue inflammation in regions of implant corrosion
5. Tissue necrosis in the region of bone cement
6. Immunological reaction to wear particles
7. Tumours occurring near sites of long term implants

Question 25

Which metal implant materials are thought to have the best corrosion resistance?

Answer 25

Titanium and its alloys