Ch. 7 - Bone-Implant Systems

Question 1

Give 3 examples of treatment for orthopaedic disorders that involve the use of implants attached to bone.

Answer 1

1. Devices for fracture fixation
2. Joint replacement for arthritic joints
3. Devices to promote fusion of bony segments

Question 2

What does load distribution btw bone and device depend on?

Answer 2

The relative amount of load carried on bone and device respectively depends on the ratio of elastic properties and respective geometries.

Question 3

What is specific strength?

Answer 3

Ultimate tensile stress / density of material

A high specific strength material would be both strong and lightweight

Question 4

List the 4 major design principles on which the design of joint replacement prosthesis is based.

Answer 4

1. Restoration of joint kinematics
2. Restoration of joint mechanics
3. Mechanical stability
4. Wear and friction of the artificial joint

Question 5

Describe the main components and materials of a knee prosthesis.

Answer 5

• Femoral component: cobalt chromium alloy
• Tibial component: cobalt chromium alloy or titanium alloy
• Articular surface (on tibial side): polyethylene

Question 6

What is a biomaterial?

Answer 6

Both engineered and natural materials used in the body are called biomaterials.

Question 7

What is the difference between bioinert and bioactive materials?

Answer 7

Bioinert materials (metals and polymers in joint replacements) do not initiate a response or interact when introduced to a biological tissue.
Bioactive materials (coatings in joint replacement) elicit a specific biological response at the interface between the tissue and the material.

Question 8

Describe difference between in vitro and in vivo experimental testing for biocompatibility.

Answer 8

In vitro - cultivating cells in petri dishes in contact with the material and assess cell survival rate as a function of exposure time.
In vivo - placement of small implants inside the body of animals and assess biological response once animal has been sacrificed after a fixed amount of time

Question 9

List the 5 most used materials for ortho applications.

Answer 9

1. Stainless steel
2. Cobal chromium alloys
3. Titanium alloys
4. Ultrahigh molecular weight polyethylene (UHMWPE)
5. Polymethyl methacrylate (PMMA)

Question 10

What observations can you make regarding the mechanical properties of the materials on the table?

Answer 10

1. Stainless steel and cobalt chromium both have high elastic modulus
2. Cortical bone has a modulus with an order of magnitude lower
3. Polymer components have moduli even an order of magnitude lower
4. Cobalt chromium and titanium are usually stronger but also more brittle than stainless steel
5. The stronger a material is made during the fabrication process by different hardening strategies, the more brittle it becomes. It can withstand higher stresses without yielding but once yielding sets in, it will break catastrophically at lower inelastic deformation.

Question 11

Describe the process of cold working and its consequences (good & bad).

Answer 11

Cold working is a primary strengthening mechanism. It involves subjecting the metal to permanent deformation at room temperature. This leads to a change in its crystalline structure, decrease in grain size and a pileup of dislocations. This is turn leads to increase in strength. However, mechanical anisotropy can be induced by this process which is often undesirable, and ductility is reduced.

Question 12

What is stainless steel and what is it used for?

Answer 12

Stainless steels are a family of iron-based alloys that contain a minimum of approx. 11% chromium. The presence of the chromium prevents the iron from corroding due to a formation of a thin chromium rich oxide film at the surface.
Stainless steels are primarily used for temporary implants (e.g. fixation plates, screws)

Question 13

List 3 different types of corrosion.

Answer 13

1. Pitting
2. Intergranular corrosion
3. Crevice corrosion

Question 14

Explain the pitting corrosion mechanism.

Answer 14

Pitting corrosion is a localised form of corrosion by which cavities are produced in the materials, e.g. due to localised attack of chemical at flaws in the oxide film. Pitting is more dangerous than uniform corrosion because it is more difficult to detect as corrosion products often cover the pits.

Question 15

Explain the mechanism of intergranular corrosion.

Answer 15

Intergranular corrosion is a form of corrosion where crystal grains are more susceptible to corrosion than the bulk material. This can happen in otherwise corrosion resistant alloys when the concentration of the corrosion inhibiting elements in the grain boundaries is very low or depleted.

Question 16

Explain the mechanism of crevice corrosion.

Answer 16

Crevice corrosion occurs in confined spaces to which the axis of the access of the surrounding fluid is limited, called crevices. E.g. gaps, cracks

Question 17

How are cobalt-chromium alloys made and what are they used for?

Answer 17

Cobalt-chromium is produced by a fusion of pure cobalt and chromium metals at high temperatures under inert atmosphere conditions or in a vacuum.
They are used for fracture fixation or joint replacement (e.g. knee, hip).

Question 18

Name 2 types of cobal-chromium alloys used for medical applications.

Answer 18

1. Cobalt chromium molubdenum

2. Cobalt nickel chromium molybdenum

Question 19

What is a special feature of titanium-aluminium-vanadium that makes it especially well suited for medical applications?

Answer 19

In addition to titanium-aluminium-vanadium featuring excellent corrosion resistance due to theEmation of a layer of titanium oxide at the surface, it also provides an excellent surface to bone tissue which can grow right up to the implant surface without substantial fibrous layers (“osteointegration”).

Question 20

What is the main disadvantage of titanium-aluminium-vanadium?

Answer 20

It is a notch sensitive material which can lead to problems with porous coatings.

Question 21

What is the effect of cross-linking in the production process of UHMWPE?

Answer 21

It increases abrasive and adhesive wear resistance needed in hip replacement, but decreases toughness and fatigue resistance.

Question 22

State the 3 step process to produce UHMWPE.

Answer 22

1. Irradiation to promote cross-linking
2. Thermal processing to remove residual stresses and free radicals
3. Sterilization

Question 23

What 5 functions must bone cement fulfill?

Answer 23

1. Fixation of artificial joints
2. Anchoring of implant to bone
3. Load transfer from prosthesis to bone
4. Optimal stress strain distribution btw implant and bone
5. Release antibiotics (some cases)

Question 24

Outline how PMMA may be used to cement a femoral component.

Answer 24

1. Medually canal is machined to fit basic implant shape plus part of cement mantle
2. Plug is inserted at the bottom to restrict flow
3. Cement is then inserted into cavity by means of cement guns
4. Implant is inserted into the cement
5. Polymerisation occurs within minutes

Question 25

How is temperature affected by polymerisation and why is it (not) an issue?

Answer 25

Polymerisation is exothermic, so temperature of the cement increases. This can potentially damage the bone as collagen starts degenerating at 50-60C. However, the substantial amount of heat is dissipated into the metallic implant, which has a higher contractility than bone and a larger thermal mass. Therefore the damage to the bone is only minimal and necrosis due to overheating is clinically not usually observed.

Question 26

Why is “bone cement” a misnomer for PMMA?

Answer 26

The term does not describe PMMA in the context of implant fixation very well. PMMA does not bond to the implant or bone chemically. It fills up the open space btw the implant and the bone, thereby creating a physical fixation through an interlocking mechanism.

Question 27

What are the mechanical properties of PMMA?

Answer 27

PMMA is brittle, weak in tension and strong in compression, with a low endurance limit.

Question 28

List the 4 phases of PMMA handling and delivery.

Answer 28

1. Mixing
2. Waiting
3. Application
4. Settling

Question 29

Explain how porosity affects the material properties of PMMA.

Answer 29

Especially for fatigue properties, it is not necessarily true that decrease in porosity of bone cements leads to improved properties of the bone cement composite. While pores act as stress concentrators, and therefore can aid crack initiation, they can also lead to crack arrest when a crack hits a pore of sufficient size, due to the sudden increase in crack radius. Furthermore, the mechanical behaviour of the bone-cement composite is also strongly influence by gaps at the bone cement interface, which influence the bone transfer and can lead to interfacial delamination and act as crack initiation sites. Therefore, reducing the internal porosity of the PMMA might not increase the apparent properties of the bone polymer composite if failure is initiated mainly at the bone cement interface.

Question 30

List the design criteria for a fracture fixation device.

Answer 30

1. Sufficiently stiff and strong
2. Compatible with biological system / Biocompatible
3. Internal devices must be non-toxic and resistant to corrosion
4. Evaluate mechanics of the device alone AND of the bone + fixator composite structure
5. Knowledge of actual loads in vivo