BIOMATERIALS ENT 311/4 Lecture 11 Orthopaedic Implant: Internal Fixation Prepared by: Nur Farahiyah Binti Mohammad Date: 15 th September 2008 Email : [email protected]

BIOMATERIALSENT 311/4

Lecture 11 Orthopaedic Implant: Internal

FixationPrepared by: Nur Farahiyah Binti Mohammad

Date: 15th September 2008

Email : [email protected]

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Teaching Plan

COURSE CONTENT

Define various types of internal fixation. Identify failure modes of internal fixation.Describe and compare three types of fixation methodsDescribe and discuss types of joint replacements.Describe and recommend biomaterial use to make components of joint replacement.

DELIVERYMODE

LectureSupplement reading

LEVEL OF COMPLEXITY

KnowledgeRepetitionAnalysis Evaluation

COURSE OUTCOMECOVERED

Ability to select biomaterials that can be used for different medical applications and explain the criteria that will lead to a successful implants

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Purpose: To stabilize fractured bone until natural healing processes have restored sufficient strength so that implant can be removed.

Material requirement: Biocompatible Sufficient strength Corrosion resistance

1.0 Internal Fixation

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Material used: Stainless steel Co-Cr alloys Titanium alloys Biodegradable polymer

To treat minimally loaded fractures Eliminate the need for second surgery

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Types:1. Wires2. Pins3. Screw4. Plates5. Intramedullary nails

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1. WIRES Used to reattach large fragments of bone Useful especially for spiral breaks and

reattaching greater trochanter of hip Wires suffer from twisting and knotting

The deformed region of the wire are more prone to corrosion

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2. PINS Used to hold fragments of bone together

temporarily or permanently and to guide large screw insertion.

Have different tip design Trocar tip – most efficient in cutting

- often used for cortical bone Diamond tip

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3. SCREWS Two types of bone screws:

1. Cortical (Compact) bone screw-small treads

2. Cancellous screws –large tread to get more thread-to bone contact

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It can be used alone or along with other devices.

The general principle is that bone heals better if the fracture fragments are aligned and pressed closely together.

The idea is to stabilize the fracture and keep the bone in anatomic alignment.

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Principle application of bone screw: As interfragmentary fixation devices to lag

or fasten bone fragment together. To attach a metallic plate to bone

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4. PLATES Plates come in several types, and are

named for their function.

In general, there are: Dynamic Compression plates Neutralization plates Buttress (support) plates.

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The dynamic compression plate is one of the most common types of plates, and can be recognized by its special oval screw holes. These holes have a special beveled floor

to them with an inclined surface. If desired, this inclined surface can be

used to pull the ends of the bone together as the screws are tightened.

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Compression plates are used for fractures that are stable in compression. They may be used in combination with lag

screws, and they may provide dynamic compression when used on the tension side of bone.

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Neutralization plates are designed to protect fracture surfaces from normal bending, rotation and axial loading forces. They are often used in combination with

lag screws.

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Resorbable bone plate

As the strength of the fracture site increase due to normal healing processes, the resorption of the implant begins to take place.

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Buttress plates are used to support bone that is unstable in compression or axial loading. These plates are often used in the distal

radius and tibial plateau to hold impacted and depressed fragments in position once they have been elevated.

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Buttress plate bridging a humeral neck fracture -

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5. INTRAMEDULLARY NAILS

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Used as internal struts to stabilize long bone fracture.

Inserted into medullary cavity Should have some spring to provide

elastic force and prevent rotation. IM nails are better than plates at

resisting multi-directional bending However, torsional resistance is less

than plate

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When designing IM nails: A high polar moment of inertia is desirable

to improve torsional rigidity and strength Problems:

Destroy intramedullary blood supply.

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Biomaterial applications in Internal FixationMaterial Properties Applications

Stainless steel Low cost, easy fabrication Surgical wire, Pin, plate, screw, IM nails

Titanium alloy High costLow density and modulusExcellent bony contact

Surgical wirePlate, screw, IM nails

Co-Cr alloys High costHigh density and modulusDifficult fabrication

Surgical wireIM nails

Polylactic acid Resorbable Pin, screw

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Failure modes of internal fixation devices

Failure mode Failure location Reason for Failure

Overload Bone fracture siteImplant screw holeScrew thread

Small size implantUnstable reduction

Fatigue Bone fracture siteImplant screw holeScrew thread

Small size implantUnstable reductionFracture

Corrosion Screw head-plate holeBent area

Different alloy implantOver tightening screwMisalignment of screwOver bent

Loosening Screw MotionWrong choice of screw type

BIOMATERIALSENT 311/4

Lecture 11 (cont)Orthopaedic Implant: Joint

replacementPrepared by: Nur Farahiyah Binti Mohammad

Date: 18th September 2008

Email : [email protected]

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1.0 Introduction

Total joint replacement An arthritic or damaged joint is removed

and replaced with an artificial joint called prosthesis.

Goal - to relieve the pain in the joint caused by the damage done to the cartilage.

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1.0 Introduction

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Joint degradation

Also called Osteoarthritis. Is the end stage of a process of destruction

of the articular cartilage. Results in:

Severe pain Loss of motion An angular deformity of the extremity

Cartilage unable to regenerate So, when exposed to a severe mechanical,

chemical or metabolic injury, the damage is permanent.

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Joint degradation

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Types of Total Joint Replacement (TJR)

Joint Type

Hip Ball and socket

Knee Hinged, semiconstrained, surface replacement, unicompartment or bicompartment

Shoulder Ball and socket

Ankle Surface replacement

Elbow Hinged, semiconstrained, surface replacement

Wrist Ball and socket, space filler

Finger Hinged, space filler

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Implant fixation method

Three type of methods of fixation:1. Mechanical interlock

This fixation achieved by press-fitting the implant by using PMMA bone cement

Bone cement Bone cement is a substance commonly used to hold

implants in bone and filling the space between the skeleton and the total joint device.

Often cement is used for hip replacement and knee replacement surgery.

Cement implies that the material sticks the implant into the bone

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In reality, bone cement should really be called bone grout.

The reason is that this material actually acts as a space-filler, to create a tight space for the implants to be held against the bone.

Bone cement does not stick substances together, rather it fills the void between the implant and the surrounding bone.

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On the X-ray pictures one sees the bone cement as a white layer around the shadows of the total hip device.

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The microscopic structure of bone cement is made by two substances glued together.

One substance are the small particles of pre-polymerized PMMA (PolyMethylMetaAcrylate), so called "pearls.

These pearls are supplied as a white powder. The other substance is a liquid monomer of

MMA(MethylMetacrylate). Both substances are mixed together at the operation table with added catalyst that starts the polymerization of the monomer fluid.

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Honeycomb structure gives the bone cement the ability to absorb downward (compression) loads.

An important characteristics in an otherwise brittle material.

the bone cement act mechanically as an shock absorber

(1) The unloaded phase: the bone cement net is regular, not deformed.

( 2) Load applied by body weight impact on the total hip (its femoral component): the bone cement net within marrow cavity deforms elastically, but does not brake and stays in contact with both total hip and skeleton.

( 3 ) When the load ceases the bone cement's structure returns to its original regular form.

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2. Biological fixation Which is achieved by using textured or

porous surface, that allow bone to grow into the interstices.

Porous in growth fixation Pore size range should be 100 to 350μm Pores should be interconnected with each other

with similar size of opening.

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3. Direct chemical bonding between implant and bone for example by coating the implant with

calcium hydroxyapatite, which has mineral composition similar to bone.

Material used as coating such as Bioglass and glass ceramic.

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Biomaterials for TJRMaterial Application Properties

Co-Cr alloy Stem, head (ball), Cup, porous coating, metal backing

Heavy, hard, stiffHigh wear resistance

Titanium alloy Stem, porous coatingMetal backing

Low stiffnessLow wear resistanceLight

Pure titanium Porous coating Excellent osseointegration

Tantalum Porous structure Excellent osseointegrationGood mechanical strength

Alumina Ball, cup Hard, Brittle, High wear resistance

Zirconia Ball Heavy and high toughnessHigh wear resistance

UHMWPE Cup Low friction, wear debris, low creep resistance

PMMA Bone cement fixation Brittle, weak in tensionLow fatigue strength

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Comparison between Specific Orthopedic Implant Prosthetic Materials

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Comparison between Specific Orthopedic Implant Prosthetic Materials

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Total hip replacement

Hip joint is a ball and socket joint Can be divided into 2 types:

1. Monolithic: Consist of one part, less expensive, less prone to corrosion

2. Modular: Consist of 2 or more parts , allow customizing of the implant during future revision surgery.

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Design aspect Prosthetic hip component are optimized to

provide wide range of motion to prevent dislocation.

Must enable implants to support loads Proper femoral neck length will decrease

bending stress

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If femoral stem is designed with sharp corner: bone in contact with sharp corner may necrose and resorb.

Replaceability: possible to remove one part without removing the other.

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Hip joint replacement

Component or Total Hip joint:

1. Femoral component Stem –Neck Ball/head

2. Acetabular component Cup

Backing Insert

Femoral component

Acetabular component

stem

neck

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Schematic representation

1. Cortical bone2. Trabecular bone3. Bone cement4. Femoral stem4 (a). Backing of

acetabular cup5. Insert of acetabular

cup

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Materials used for each component of THR

Femoral component Acetabular componentStem Ball/head Cup BackingCo-Cr alloy

Titanium alloy

Co-Cr alloy

Alumina

Zirconia

Co-Cr alloy

UHMWPE

Alumina

Metal

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Possible combination of THR

Femoral component Acetabular component

Fixation Stem Head Cup Backing Fixation

PMMA

Bone ingrowth

Press fitting

Co-Cr alloy

Titanium alloy

Co-Cr alloy

Alumina

Zirconia

Co-Cr alloy

UHMWPE

Alumina

Metal

None

PMMA

Screw or press fitting

Bone ingrowth

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1. STEM Titanium alloy Advantages:

Excellent corrosion resistance Highly reactive material Lowest rate dissolution

Disadvantages: Wear Generation of fine wear particles:

inflammation and implant loosening

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2. HEAD Alumina: elicit minimal response from

host tissue Advantages:

High Wear resistance Reasonable fracture toughness Extremely stable (undergo little

physical/chemical deformation)

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Disadvantages: Degradation in-vivo:WEAR

Weakens the material Change shape that may effect fuction Produce biology active particles Low creep resistance: can influence the

behaviour of joint

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3. CUP UHMWPE Advantages:

Tough inert Disadvantages:

Wear debris cause inflammatory reaction

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Most frequent problems

1. Infection2. Wear3. Migration and failure of implants4. Loosening

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WEAR

Degradation in vivo Wear debris produced by load bearing

and motion of the prosthesis 150 000 particles generated each step Cells from the immune system of the

host, such as macrophages, will identify the wear particles as foreign matter and initiate a complex inflammatory response

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WEAR

Problem that will be occur as a result of wear are: Rapid focal bone lose (osteolysis) Bone resorption Loosening Fracture of bone

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Shoulder Joint Replacement

Humerus fractures -caused by a direct blow or by a fall or osteoporosis (thinning of the bones).

the ball is removed from the top of the humerus and replaced with a cobalt chrome or titanium implant. This is shaped like a half-moon and attached to a stem inserted down the center of the arm bone.

The socket portion of the joint is shaved clean and replaced with a plastic socket that is cemented into the scapula.

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Elbow Joint replacement

Humeral component replaces the lower end of the humerus in the

upper arm. has a long stem that anchors it into the

hollow center of the humerus.

Ulnar component replaces the upper end of the ulna in the

lower arm. has a shorter metal stem that anchors it into

the hollow center of the ulna.

The hinge between the two components is made of metal and plastic.

The plastic part of the hinge is tough and slick. allows the two pieces of the new joint to glide

easily against each other as you move your elbow.

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Knee Joint Replacement

Femoral component upper part of a knee system made of a strong polished metal. covers the end of the femur

Patellar component replaces the kneecap in the center

of the knee.

Tibial component covers the top end of tibia covered with a metal tray topped with a disk-shaped

polyethylene insert (sits on a highly polished surface and rotates around a conical post)

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Knee Joint Replacement

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Ankle Joint Replacement

Tibial component replaces the socket portion of

the ankle (the top section) metal tray is attached directly

to the tibia bone plastic cup fits onto the metal

piece, forming a socket for the artificial ankle joint.

Talus component replaces the top of the talus. made of Titanium fits into the socket of the

tibial component

Documents

BIOMATERIALS ENT 311/4 Lecture 11 Orthopaedic Implant: Internal Fixation Prepared by: Nur Farahiyah Binti Mohammad Date: 15 th September 2008 Email : [email protected]