Ray Vanderby, VANDERBY@surgery.wisc.
edu 03/01/17
BME 515
PROSTHETIC JOINTS
Orthopedics: That branch of surgery which is concerned with the preservation
and restoration of function of the skeleton, its articulations and
associated structures.
Orthopedic biomaterials: Materials for joints and structures
Orthopedic issues:
Rapid return to function
Low morbidity
Naturally occurring orthopedic biomaterials (tissues):
Bone
- Cancellous
- Cortical
Cartilage
-Articular
Ligament
Tendon
Meniscus
Intervertebral disc
- Nucleus pulposus
- Anulus fibrosus
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Some orthopedic tissues have little or no healing potential making
tissue replacement necessary:
Cartilage (artificial joints)
Some parts of the meniscus (removal)
Intercapsular ligaments (ACL, PCL) (ligament grafts)
All orthopedic tissues have pathologies that reduce healing
Osteoporosis
Smoking
Immuno-compromise
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
Most tissues will regenerate if damage is in very young (making
tissue engineering an important area of research)
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
Traditional artificial orthopedic biomaterials for joint prostheses:
1. Metals
- 316L stainless steel
- Co-Cr-Mo alloy (cast)
- Co-Cr-W-Ni alloy (wrought)
- Titanium
- Ti-6Al-4V Alloy
- Issues
- Note, Ni and Cr are toxic ions in first 3 metals
- The more biocompatible alloys are more expensive
- Most compatible is not the hardest
2. Ceramics Aluminum oxide
- Issues
- Hard and biocompatible but brittle
- Osteophilic
3. Polymers
- Bone cement (PMMA)
- Issues
- Exothermic
- Toxic effects of monomer
- Shrinkage with polymerization
- Grout, not adhesive
- UHMWPE (polyethylene)
- Issues
- Lipid absorption
- Wear particles
- Low coefficient of friction and creep resistant
www.healthandage.com
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
Requirements for replacement materials:
1. Must re-establish mechanical function with healing or replacement
- Structural and mechanical criteria
- Strength
- Stiffness
- Fatigue life
- Wear
- Wear debris
2. Must not adversely affect biological environment
- Biocompatibility criteria
- No infammatory response
- Inflammation causes lymphocyte invasion, vascular
occlusion, tissue necrosis, fibrous tissue formation,
prosthesis / implant loosening
- Not carcinogenic
- Not toxic
- Not mutagenic
- No immunogenic response
- Must not overload or underload surrounding tissues
- Overloaded tissues will die
- Underloaded tissues will resorb or atrophy
- Unloaded tissues will not heal as rapidly and may have
compromised tissue organization and microstructure
- Must be appropriate size and not disturb surrounding tissues and
blood supply, particularly those necessary for tissue healing
- Must not allow too much or too little micromotion (relative motion at
bone-bone or bone-implant interface)
3. Must not be adversely affected by biological environment
- Non-corrosive (metals)
- Stress
- Galvanic
- Crevice
- Lipid absorption (polymers) causing property degradation
4. Manufacturing and use
- Ductility (plates must be contoured during use)
- Machinability
5. Cost
6. Requisite size (determined by mechanical properties) versus available
space
7. Sterilizable
8. Size of material (i.e. wear debris) biocompatibility doesnt fix things
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
Mechanical Properties of Orthopedic Biomaterials
Typical properties of materials associated with total joint replacement. Variations in failure stress and fatigue
stress for metals arise from how the material was formed (i.e. cast, cold worked, host forged, etc.), the
surface condition, surface treatment and other factors.
Material Modulus Poisson's Failure Stress Fatigue
(GPa) Ratio (MPa) Stress
(MPa at 106)
Stainless steel (AISI 316) 193 0.30 480-1,300 240-700
Titanium alloy (Ti-6Al-4V) 110 0.32 800-1,500 350-600
Cobalt-chrome alloy (Co-Cr-Mo) 220 0.30 800-1,000 310-950
PMMA 2-3 0.35 25-40 (tension) 14
90-100 (compression)
Cortical Bone 10-20 0.39 51-133 (tension) 60-100
133-195 (compression)
Cancellous Bone (porous structure) 0.5-1.5 0.32 3-10 (compression)
PMMA-bone interface 7-10 (tension) 0.06
2-4 (shear)
PMMA-metal interface 5-10 (tension) 2.0
5-8 (shear)
UHMWPE 1
Fibrous tissue 0.001
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
OBJECTIVES
This BME 515 lecture on orthopedic prosthetic joints is intended to
introduce you to the topic and to the engineering issues. At the end
of the lecture you should be able to answer the following questions on
total joint replacement:
What problems require joint replacement?
Arthritic pain
Cartilage damage
Tumors
Fractures
What joints can be replaced with prosthetics?
Hips
Knees
Shoulders
Ankles
Wrists
Elbows
Fingers
What are overall goals of joint replacement for the patient?
Reduce pain
Restore function
Last throughout life
What are overall design requirements for prosthetic joints?
Geometric requirements for fit and stability
Geometric requirements for function
Ease and reliability of implantation
Minimize bone loss
Minimize trauma to surrounding tissue
Minimize changes in bone loading
Durability
Cost
What are key design criteria for artificial biomaterials?
Biocompatibility
Wear properties
Fatigue strength
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
What artificial biomaterials are most commonly used?
Titanium alloys
Co-Cr-Mo alloys
Stainless steel
Ceramics
UHMWPE
PMMA
What are alternatives for implant fixation and when is each used?
Bone cement (PMMA)
Porous coatings - used when bone has good healing potential
How successful are prosthetic joints?
Relieve pain
Restore function (although it is never normal)
Most (>90%) last for >10 years
What are the main problems and limitations?
Wear debris
Infection
Bone loss
Finite life (wear out faster in larger and/or active patient)
- Revisions are less successful and do not last as long
- Young patients would need many over normal life span
How much do they cost?
Approximately $20K for everything - primary
Approximately $4 to 5K for implant - primary - modular, off-the-shelf size
- Why so much? Liability and FDA costs
For custom procedure - 5X
What are some technologies that did not work and why?
Robotic implantation - cost and little improvement
CAD/CAM designs - cost and little improvement
Surface replacements - implant stability
Fiber reinforced PMMA - wear debris
Composite components to replace metal - durability
What future improvements can come from biomedical engineering?
Tissue engineering to regenerate or heal pathological tissues
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Ray Vanderby, VANDERBY@surgery.wisc.edu 03/01/17
Improvements in biomaterials (osteoconductive and/or osteoinductive,
wear, load-sharing stiffness)
Improvements in surgical instrumentation
Improvements in geometry
