19 Bartel Chapter 7 Bone Implant Systems

19.0.0.0.1 Introduction

19.0.0.0.1.1 Primary considerations as engineers

Orthopaedic implants serve a structural purpose – must be structurally sound

Biocompatibility is critical

Implants attach to bone – interfaces

• –a common challenge in engineering, now compounded due to a biological environment

Implants alter load paths in the bone

• The change depends on material and geometry

• Load paths effect bone remodeling

Intermediate materials can be involved

• Porous coatings for bone in-growth

• Polymethylmethacrylate cement

• Screws/anchors/fasteners

Joint loads are large relative to body weight

Strength of materials is relevant

http://www.businessworld.in/index.php/Pharma/Device-Malfunction.html\ http://www.uclb.com/newsevents

19.0.1 Implant materials

19.0.1.1 Biomaterials

19.0.1.1.1 Biomaterials

• Biocompatible

  • Materials function must not be damaged by the body environment

  • Material must not damage function of tissue

  • Materials that are biocompatible are often called biomaterials

• Bio-inert

  • Some metals

  • Some polymerics

• Bio-active

  • Porous layers (for in-growth)

  • Roughened surfaces (for in-growth)

  • Resorbable

19.0.1.1.2 Biomaterials

http://www.livescience.com/health/human-images/heart-scaffold-heart-attacks-100810.html\ Heart Scaffold

• Example: A tissue defect may be filled with scaffold seeded with cells (tissue engineering)

• Initial structural integrity and subsequent growth

• Scaffold resorbs and gives way to tissue over time

19.0.1.1.3 Bio-active coatings

image

19.0.1.1.4 Testing for biocompatibility

http://www.ethoxsts.com/toxicology-tests.html\ Ex: Toxicology tests

Long process – in vitro (in a controlled environment) and in vivo (in the living organism) evaluation

Early testing done on small animals (ie the rat lab)

Late stage tests based on clinical experience – constantly evolving and evaluating

This process requires significant oversight on research ethics (HSIRB)

Where bulk materials are fine, particulates can cause problems (wear and/or tissue interactions) – thus: multi-scale problems

Different patients have different tolerances – allergic reactions

Exposure to air prior to insertion can degrade materials (oxidation)

We’ll limit discussion to bio-inert materials and polymers

19.0.1.2 Typical mechanical properties of implant materials

19.0.1.2.1 Common biocompatible materials

• Metals

  • Stainless steels

  • Cobalt-chromium alloys

  • Titanium alloys

• Polymers

  • Ultrahigh molecular weight polyethylene (UHMWPE)

  • Polymethylmethacrylate (PMMA)

19.0.1.2.2 Table of nominal properties

Bartel

19.0.1.2.3 Nominal properties

• Metals are 5-10x as stiff as cortical bone

• Polys are about 10x more compliant than cortical bone

• Cobalt-chrome less ductile but stronger than steel – this has fatigue implications

• Titanium has excellent strength to weight ratio

• Polys can have properties which depend on compression vs tension

• The range of this data can be large (especially for polymers)

  • Sensitivity to manufacturing conditions

  • Use known values if you are a designer

19.0.1.3 Metals

19.0.1.3.1 Stainless steel alloys

http://www.albu-medizintechnik.com/en/products_01.html

• Very common – relatively inexpensive

• Often used for temporary fixation (plates, screws, etc)

  • Good balance of ductility, strength, fatigue

  • Can be plastically deformed in the OR to conform

19.0.1.3.2 Stainless steel alloys

• Manufacturing – cast, forged, extruded

• Corrosion resistant (dependent on alloy – be careful)

  • Coatings of cobalt chrome and nickel oxide provide good in-vivo corrosion resistance

• Also used in joint replacements

19.0.1.3.3 Cobalt-chromium alloys

http://www.bonesmart.org/public_forum/45-years-life-span-for-hip-replacement-t414.html

• Wide range of properties depending on alloy

• Oxidation on outer surfaces allows excellent resistant to corrosion, especially in-vivo crevice corrosion

• OK ductility and fatigue

• Excellent biocompatibility in bulk

19.0.1.3.4 Cobalt-chromium alloys

• Two common alloys

  • Cobalt-chromium-molybdenum (CoCrMo)

    • Made by casting – small grain sizes for good fatigue strength

    • Properties can be improved by HIP (hot isostatic pressing)

  • Cobalt-nickel-chromium-molybdenum (CoNiCrMo)

    • Made by forging

    • Post-processing (cold working, annealing) makes very small grain sizes – thus strong and very fatigue resistant

    • This comes at the cost of reduced ductility

19.0.1.3.5 Titanium

http://www.emeraldinsight.com/journals.htm?articleid=1600954&show=html

• Alloys developed in the Aerospace industry

  • Titanium-aluminum-vanadium (Ti-6Al-4V) is common

• Modulus is closer to bone than other metals – more load sharing and less stress shielding

  • Possibility for decreased bone resorption

19.0.1.3.6 Titanium

• Titanium oxide layer forms:

  • is corrosion resistant beyond other implant metallics

  • can be excellent interface for bone growth
    (without fibrous tissues being formed)

• High strength to weight ratio

  • not critical as in aerospace but favorable

• Fatigue less favorable due to notch sensitivity
  – ie lower fatigue strengths

• Lower wear resistance (softer)
  – particles can scratch the surface more easily

19.0.1.4 Ultrahigh Molecular Weight Polyethylene

19.0.1.4.1 Ultrahigh Molecular Weight Polyethylene

http://www.hss.edu/conditions_arthritis-knee-total-knee-replacement.asp

• Good biocompatibility

• Good impact strength

• Good toughness and wear resistance

• Can be machined or net shape molded

19.0.1.4.2 Ultrahigh Molecular Weight Polyethylene

http://www.tsuhp.com/uslesion06.htm

• Sterilized using ethylene oxide or gamma radiation which does effect properties

  • Radiation produces free radicals which encourage cross-linking – the use of it is a relatively recent improvement in implant materials

  • Cross-linking improves wear resistance, decreases fatigue resistance and fracture toughness

  • Oxidation during radiation decreases molecular weight and degrades properties

• Notes: subsequent heat re-sterilization will negatively effect properties

• Commonly used in shoulders (glenoid component)

• Poly is non-linear viscoelastic-viscoplastic – be careful in interpreting the reported moduli

19.0.1.4.3 Polymethylmethacrylate (PMMA)

http://www.polymerprocessing.com/polymers/PMMA.html

• Often called “Bone cement”

• Two parts

  • liquid monomer

  • pre-polymerized powder

• Polymerization initiated in the OR and completed in-situ

  • Time is a factor – waiting in OR until “ready”

• Process is exothermic (watch for burns – historically not major problem)

• Not truly “cement” (very weak chemical bond to the bone). More a volume filling grout. (Fills voids – mechanical bond.)

Margaret A McGee et al

• Cement particles have negative effects

  • Ex: foreign body inflammatory response – resorption and aseptic loosening

  • Extra cement must be carefully removed

• Material is brittle, weak in tension, has low endurance limit

• Loosening of the cement (or implant) is a common failure mode

• Shrinkage can be a problem
  (6-7% during polymerization) – porosity

• OK in compression

• Can be made to leach antibiotics and/or show on x-ray

19.0.1.4.4 Bone cement

image

19.0.1.4.5 Bone cement

http://www.healio.com http://www.healio.com\ http://www.healio.com\ Blue contrast bone cement on tibial (2A), femoral (2B), and patellar (2C) components

19.0.2 Fracture fixation devices

19.0.2.0.1 The healing environment

• Fracture fixation devices promote an appropriate the healing environment

  • Proper biology required including stability

    • Stability levels range from rigid to some movement

    • Healing depends on the level (callous formation vs direct healing)

• Fracture fixation devices include plates, screws, wires, intramedullary rods, and external fixators

19.0.2.0.2 Screws

image

• Provide compression and a pin joint

• Type of thread depends on type of bone

• Threads may span the screw or be a the tip (depending on desired compression)

• Can be pre-drill/tapped or self tapping

19.0.2.0.3 Plates, IM Rods, Nails

image

• Plates, IM Rods, Nails are used (in part) to address bending loads in the bone

• Typical use puts cortical fragments in apposition

19.0.2.0.4 IM rod

image

19.0.2.0.5 Dynamic hip screw

image

19.0.2.0.6 External fixator

image

19.0.2.0.7 Analysis and design of fracture fixation devices

• We must provide strength and stability for fracture support

• Goals range from full immediate weight bearing to no-load fixation

• Ideally, devices may be tuned or adjusted after implant

  • ie decrease the stiffness as healing occurs

  • This is more practical with external fixators but may also be possible with internal fixation

• Design considerations must include the peri-operative period and after full healing has occurred (and growth!?!)

• Can be challenging to determine appropriate design loads

19.0.3 Joint replacement (Guest Lecture)

19.0.4 Joint replacement (Dr. Gustafson)

19.0.4.0.1 Joint replacement

image

19.0.4.0.2 Joint replacement

• Typically, joint replacement is due to arthritis

  • Osteoarthritis

    • Typical joint wear or due to trauma

    • Damage usually limited to the articular surfaces – ie good bone stock available

    • Soft tissues are usually intact

  • Rheumatoid arthritis

    • Systemic disease typically effecting multiple joints

    • Compromised bone tissue and the soft tissue environment (tendons and ligaments may loose their function)

    • Additional restraint may need to be built into the implant

19.0.4.0.3 Joint replacement

• Joint replacement might include total joint or hemiarthroplasty (half joint)

• May want large changes in implants (ie change implant as a adolescent grows)

19.0.4.0.4 Total knee replacement

19.0.4.0.5 Total knee replacement

• Two classes.. surface replacement or substructure replacement

• Tibial components typically a poly insert in a metal tray

  • May be completely of polyethylene

• Patellar component included... might just be a resurfacing

• Goals

  • Normal range of motion plus stabilization if needed – this constrains the shape of the surfaces

  • Over stabilization has negative effects on gait and other joints – the kinematics and loads are coupled

  • Carry loads (obviously)

19.0.4.0.6 Total hip

• Accetabular cup is metal and poly

19.0.4.0.7 Failure and damage

• Load transfer effects

  • Bone adaption to the implant is extremely important – can lead to resorption

  • Unlike bone, implant material never heals... fatigue a concern!

  • Common failure mechanisms:

    • bone fracture

    • implant loosening

    • yield overload

    • fatigue fracture

19.0.4.0.8 Failure and damage

• Articulating surface effects

  • Debris is released as materials wear

  • Particulates collect in the soft tissues – can cause infection and loosening

  • Wear can be abrasive of from pitting and delamination related to corrosion

    • Limit surface loads or improve materials

19.0.4.0.9 Modularity of components

• It is often desirable to design modular systems

  • Surgical flexibility

  • Sizing for variation in anatomy

  • Simpler replacement of worn components

  • “Composite” mechanisms allow multi-function (ie bony in-growth into metals, poly cup a good bearing surface)

19.0.5 Implant systems

19.0.5.0.1 Design of bone-implant systems

A surgical jig

• We cannot forget the implantation system
  must be designed as well

  • Example: computer navigation of knee implants

• Surgeons need jigs, fixtures, instrumentation, etc which allow precise placement

• Current interest in surgical planning software solutions – aid in decision making

19.0.5.0.2 Evaluation of implant performance

• During design phases, experiments and computer models are the best we have...

  • but in-vivo studies are a must to determine the risk/reward.

• Clinical studies determine if viable... construct carefully

  • The FDA has strong influence on this process

• Peer reviewed journals and society conferences provide the primary source of information on performance of existing designs

• Post-failure analysis is a must

  • Companies tend to do this themselves...
    thus the data isn’t widely available

19.0.5.0.3 Questions

19.0.5.0.4

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