Orthopedics

Question 1

Applications of orthopedic implants

Answer 1

• fracture fixation enhancement, joint replacement (arthroplasty), and dynamic stabilization (like for the spine)

Question 2

Design criteria

Answer 2

• orthopedic materials must withstand cyclic loads
• metal alloys generally have best combination of material properties
• ceramics used widely as well, have a higher fabrication cost

Question 3

Material properties needed for orthopedics

Answer 3

• high strength, corrosion resistance, ductility, fracture toughness, hardness, formability, and biocompatibility

Question 4

Most common orthopedic biomaterials:

Answer 4

• Metals: Ti alloys, Co-Cr-Mo alloy, stainless steel
• Polymers: PMMA, ultra-high-molecular-weight-polyethylene (UHMWPE)
• Ceramics: alumina, zirconia,

Question 5

Bone Structure

Answer 5

• compact cortical bone (dense bone along shaft of long bones) has principle inorganic material, hydroxyapatite (OHAp) and a protein component like collagen
• four levels of organization

Question 6

Levels of Bone Structure

Answer 6

• Level 1 (Molecular): collagen triple helical structure and OHAp crystallography, HCC cell
• Level 2 (Ultrastructural): OHAp crystals are found both inter and intrafibrillary
• Level 3 (Microstructural): fibrillary composites of collagen and OHAp form fibers/bundles, pack into lamellar units
• Level 4 (Macroscopic): Bone is anisotropic and inhomogeneous, displays viscoelasticity (not too brittle)
• have not yet mimicked these properties with synthetic material!

Question 7

Total Hip Arthroplasty (THA) History

Answer 7

• study hip anatomy… need for hip replacements stems from wearing away of cartilage in the socket joint
• first prosthetic was made of a carved ivory femoral head
• 1923, the ‘mold’ approach was first broached using a glass cup that fit between the femoral head and acetabular cup… created fibrous capsule
• cobalt-chromium alloy Vitallium, which shows corrosion resistance, biocompatibility, and performance
• used screws to affix the mold
• tried a short stem (high shear stress, didn’t work)

Question 8

More recent iterations

Answer 8

• long-stemmed prostheses with fenestrations to allow for bone growth, studding for fixation
• PMMA bone cement is used to reduce loosening, UHMWPE acetabular cup is low friction (most popular today)

Question 9

Total Disc Arthroplasty (TDA)

Answer 9

• more contemporary, alternative to spinal fusion in treating disc degeneration
• first tried using Co-Cr spheres (not good results)
• now utilize metal-polymer, polymer-polymer, and metal-metal interfaces or elastic core technology

Question 10

Current TDA designs

Answer 10

• SB Charite the III artificial disc: cobalt chrome endplates, calcium phosphate coating, UHMWPE core
• Maveric Disc: Co-alloy endplates
• NUBAC: PEEK, replaces only the nucleus
• Freedom Lumbar Disc: viscoelastic polymer
• Theken eDic: elastic polyurethane, provides load-based measurements

Question 11

THA Design Considerations

Answer 11

• femoral stem is typically titanium, CoCr, or stainless-steel alloys
• PMMA bone cement/press fitting
• femoral head is CoCr alloy or ceramic
• UWHMWPE or ceramic cup
• titanium or CoCr cup liner cemented/screwed into place
  -90% success rate at 7 years

Question 12

Titanium vs. Cobalt vs. Stainless Steel

Answer 12

• titanium alloys are more inert, less wear resistance
• Stainless-steel is most ductile, but mechanically weaker
• Co-Cr-Mo is strongest, hardest, and most fatigue resistant

Question 13

Alternate Design options

Answer 13

• alumina and zirconia: decreasing fracture rates, low wear
• Zirconium and tantalum alloys have high strength, stability, resistance to wear/corrosion due to thick surface oxide
• costly, often used for hypersensitivity or metal allergies
• MUCH harder than cobalt/titanium