Bone implants Flashcards
What are materials used in the body called?
Biomaterials
What are the two classes of biomaterials?
Bio-inert: joint replacements, metals and polymers, fixation plates
Bio-active: materials which resorb or enhance bone growth, tissue-engineered scaffolds
What does biocompatible mean?
Does not damage or impair the function of the implanted materials or the tissues of the musculoskeletal system
What is UHMWPE?
Ultra-High Molecular Weight Polyethylene
What is the elastic modulus of UHMWPE?
N/A - non-linear material
What are stainless steels typically used for?
Bone fracture plates
Screws
Hip nails
Joint replacements
What are cobalt chromium alloys used for?
Bone plates
Joint replacements
What is titanium typically used for?
Bone plates
Joint replacements
What is UHMWPE used for?
Bearing material
What is Polymethyl Methacrylate used for?
Fix components to bone
What are the advantages of stainless steels?
- Good balance of strength, ductility and fatigue performance
What are the disadvantages of stainless steels?
-Corrosion/Pitting (address by including Cr and Mo)
What are the benefits of cobalt chromium alloys?
- Range of mechanical properties for varied purposes
- Resistant to corrosion and fatigue failure
- Ductile
What are the disadvantages of cobalt chromium alloys?
- made via casting, leading to unevenly distributed carbides (addresses by forging)
What is the benefit of UHMWPE?
- High impact strength
- Adjustable props via radiation
What are the disadvantages of UHMWPE?
- Excessive crosslinking lowers ductility
- Needs controlled environment for cross linking
What are the benefits and drawbacks of screw fixation?
(+): Simple, Minimally invasive
(-): Challenging to align fracture surfaces
What are the benefits of plates, rods and nails?
(+): can resist bending, plates can be guided for tapping bone
(-): could result in stress shielding
What are the benefits of external fixators?
(+): can be adjusted to place more load on bone
(-): cumbersome for patient, risk of infection
What are two considerations when designing fixation devices?
Mechanical behaviour of the device: crucial if it must carry all load
Mechanics of the bone-fixator structure: crucial to prevent stress shielding
What is the goal of a joint replacement?
Replicate the native condition as much as possible
What are the two classes of joint replacements?
Replacements of the articulate surface (Acetabular cups, femoral and tibial components of TKA): load transferred to underlying trab bone
Replacement of the articular surface and substructure of the bone
What is PMMA Fixation vs Cementless Fixation?
PMMA: at the cement-bone interface, pmma interlocks with the roughness of bone. At bone-implant, pmma weakly adheres to metal and rigid fixation by mechanical interlocks
Cementless: coat prosthesis with porous mat’l to encourage bony ingrowth
What is the typical lifespan of an implant?
10-15 years
Why is fatigue of implants an important consideration in implant design?
Bone defects will repair themselves but damage to the implant will not
Why is stress shielding an issue?
If most of the load is carried by the implant, bone loss will occur in some regions. This will reduce the strength of bone, potentially leading to fracture and more load on the implant increases it’s chance of failure
What is osteolysis?
If enough debris is created between articulating surfaces, WBCs may attack the bone as a response to foreign debris
What are the pros and cons of survival analyses?
(+): identifies specific design which offers best performance
(-): does not identify specific design features
What are the pros and cons of retrieval analyses at revision or autopsy?
(+): best source for evaluating in vivo performance
(-): challenging to coordinate retrieval
What are the pros and cons of long term follow up studies?
(+): acquire info about specific design
(-): expensive, subjective
What are the pros and cons of small-scale in vivo studies?
(+): squire info about specific design
(-): limited sample size, FDA approval
What are the pros and cons of ex vivo cadaveric studies?
(+): acquire info about specific design
(-): expensive, specialized knowledge and equipment
What are the pros and cons of preclinical in vivo animal studies?
(+): acquire info about specific design
(-): may not reflect human condition
What are the pros and cons of in silico subject specific FE modelling?
(+): can evaluate multiple designs
(-): model accuracy is questionable, specialized knowledge and software, time consuming
