Orthomech L6-joint replacement

Stainless steel: Iron +~20%Chromium + 10%Nickel + carbon

Cobalt Chromium alloys - high resistance to wear and corrosion

Titanium: good corrosion resistance, light. Expensive

Good wear properties, hard,

Lower friction coefficient. Polymers include PMMA, PTFE. UHMWPE

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

• Advantages:
• Longer lifespan
• Larger femoral head
• Decreases chance of dislocating.

• Disadvantage:
• Metallosis
• cancer risk from metal ions

Acetabulum shell and metal head

Ceramic head on plastic shell ( for young active patients but squeak, probs awks during sex)

Metal cap is placed on top of femoral head.

Less bone is removed and revision is easier

More common in younger patients

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.

Mechanics of the knee simplified by resection of the condyles and the cruciate ligaments

No attempts to be anatomical

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

Compromise

Create more conformity between components

Still allow varus valgus rotations and some axial rotation

Still the approach taken for many total knee systems

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

Total, unicompartmental, kneecap replacement. See notes for diagrams

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

• Advantages
• It’s for young people and you don’t have to worry about cement breaking down
• Better long term bond

• Disadvantages
• Healthy bones needed

• Extended recovery period and limited activities for 3 months

• Suitable for osteoporotic patients
• Cement dries quickly
• Antibiotic can be added to cement

• Disadvantages
• Breakdown of cement leads to loosening
• Cement debris causes inflammation.It can enter bloodstream and lungs in rare cases

Total, hemi-athroplasty, resurfacing hemiarthoplasty

• Biomcompatibility
• Load-bearing
• Long-lasting (wear resistant)
• Reliable
• Revise-able
• Obviously MUST restore function and relieve pain

• 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

Implant factors (design concepts, maaterials production etc)

• Surgeon factors
• Indication for operation, experience/skill
• Younger surgeons are more experienced?

• Patient factors
• Weight and dimensions, age activity level, musculoskeletal condition

SOme phat database

• Experimental: Cadaveric/bone substitute specimens
• Computer models: FEA
• Animals models

Wear (WHPE, alumina), squeaky hips, stress transfer

• 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

Additives or irradiation

• 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.

• Depends on the:
• Bond between the interfaces
• Relative magnitudes of their elastic moduli

Resistance to axial force. Ratio of the axial rigidity of the stem to that of the combined axial rigidities of the stem and bone

• Resistance to bending.
• Ratio of the bending rigidity of the stem to that of the combined bending rigidities of the stem and bone.