Give an example of a metal commonly used in joint repairs.
Stainless steel: Iron +~20%Chromium + 10%Nickel + carbon
Cobalt Chromium alloys - high resistance to wear and corrosion
Titanium: good corrosion resistance, light. Expensive
Give properties of ceramics that make it good for joint replacements
Good wear properties, hard,
Why choose a polymer over a metal in joint replacements?
Lower friction coefficient. Polymers include PMMA, PTFE. UHMWPE
Give an advantage and a disadvantage of using metals+polymer, metal+metal and ceramic+ceramic
Metals+polymer: Predictable lifespan, lots of people use it, cost effective. Polymer debris, aseptic loosening.
Metal+metal: Longer lifespan, larger femoral head decreases chance of dislocating. Metallosis, cancer risk from metal ions
Ceramic+Ceramic: Low friction and debris. Inert. 10% squeak! Expensive
Give an advantage and a disadvantage of using metal+metal
- Longer lifespan
- Larger femoral head
- Decreases chance of dislocating.
- cancer risk from metal ions
What materials are used in modern hip replacements?
Acetabulum shell and metal head
Ceramic head on plastic shell ( for young active patients but squeak, probs awks during sex)
What is hip resurfacing?
Metal cap is placed on top of femoral head.
Less bone is removed and revision is easier
More common in younger patients
Explain the anatomic approach to TKA
Just the articular surfaces replaced or resurfaced
Most soft tissue constraints preserved
Issues: complex geometries are difficult to manufacture
Surgery considered too difficult for most surgeons
Most surgeons considered cruciate ligament resection necessary to correct deformity.
Explain the functional approach to total knee arthroscopy
Mechanics of the knee simplified by resection of the condyles and the cruciate ligaments
No attempts to be anatomical
Explain the design challenge of total knee arthroscopy (trade off)
Congruent surfaces vs incongruent surfaces
- Congruent surfaces: They give you low contact stresses, minimal wear and less resorption of wear.
- BUT they also produce high constraint forces and lead to more loosening.
- Incongruent surfaces: Gives you unrestricted movements, low constraint forces and less loosening.
- BUT you get maximal wear and more resorption of bone
How do you solve the design challenge of TKA?
Create more conformity between components
Still allow varus valgus rotations and some axial rotation
Still the approach taken for many total knee systems
What is the design for mobile bearings? How do they compare to fixed bearing designs?
Large congruent surfaces: minimal wear, less resorption of bone
Mobile: Unrestricted rotational movements, low constraint forces, decreased loosening
But are more likley to dislocate and may cost more than fixed breaking implants
What are the three types of knee replacements? Draw them?
Total, unicompartmental, kneecap replacement. See notes for diagrams
Which one of these is a good candidate for partial knee replacement?
Exhibit A because lateral compartment has normal joint space. Medial compartment has severe arthiritis. Exhibit B has severe arthiritis in both medial and lateral parts
What are the advantages and disadvantages of an uncemented knee joint vs an cemented knee joint?
- It’s for young people and you don’t have to worry about cement breaking down
- Better long term bond
- DisadvantagesHealthy bones needed
- Extended recovery period and limited activities for 3 months
Advantages and disadvantages of cemented vs uncemented
- Suitable for osteoporotic patients
- Cement dries quickly
- Antibiotic can be added to cement
- Breakdown of cement leads to loosening
- Cement debris causes inflammation.It can enter bloodstream and lungs in rare cases
What are types of shoulder replacement? (SHould change this to being able to recognise photos)
Total, hemi-athroplasty, resurfacing hemiarthoplasty
What characteristics does a successful implant need to have?
- Long-lasting (wear resistant)
- Obviously MUST restore function and relieve pain
What ways can a joint replacement fail?
- Most frequent complication is a long-term loosening aka asceptic loosening which is:
- Gradual process where mechanical integrity of implant-bone interface is lost
- Fibrous tissue is formed between surfaces, leading to pain and restricted function
Given enough service time, every prosthesis will loosen
What Factors are there affecting implant performance?
(design concepts, maaterials production etc)
- Surgeon factorsIndication for operation, experience/skill
- Younger surgeons are more experienced?
- Patient factors
- Weight and dimensions, age activity level, musculoskeletal condition
What is the swedish hip registry
SOme phat database
How can new designs be tested prior to patient use?
- Experimental: Cadaveric/bone substitute specimens
- Computer models: FEA
- Animals models
What are the main complications with implants
Wear (WHPE, alumina), squeaky hips, stress transfer
What complications does wear of implants bring? How does this process work?
- Wear particles can (rarely) lead to allergic reactions
- Leads to osteolysis = bone resoprtion
- Particle debris generated by wear leads to pseudosynovial membrane at interface between implant and bone.
- Leads to infiltration of fibrocytes and macrophages
- Cytokines released by these cells induce bone resorption by activating osteoclasts
How can you create resistance to wear?
Additives or irradiation
What are the three mechanisms of wear in acetabular cups
- Adhesive wear
- Surface asperities adhere to metal surface
- Either polymer film formed or polymer particles are released and trapped in the joint
- Abrasive wear
- Due to polymer particles, cement particles and/or asperities on metal surface
- Fatigue wear
- Due to creep, folds and cracks that cause small polymer particles to break off.
What do stress patterns depend on in a bone-prosthesis structure
- Depends on the:
- Bond between the interfaces
- Relative magnitudes of their elastic moduli
What is axial rigidity and what is its formula?
Resistance to axial force. Ratio of the axial rigidity of the stem to that of the combined axial rigidities of the stem and bone
What is bending rigidity? Give its formula
- Resistance to bending.
- Ratio of the bending rigidity of the stem to that of the combined bending rigidities of the stem and bone.