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Biomechanics of Total Knee Biomechanics of Total Knee ReplacementsReplacements

By: Melissa Perri and Niki EfantisBy: Melissa Perri and Niki Efantis

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Anatomy of the KneeAnatomy of the KneeMajor BonesMajor Bones

http://www.northeastortho.com/patient_education/knee_replacement/index.html

::FemurFemurPatellaPatellaTibiaTibia

LigamentsLigaments::PosterierPosterier cruciatecruciate ligament (PCL)ligament (PCL)Anterior Anterior cruciatecruciate limagentlimagent (ACL)(ACL)Lateral collateral ligament (LCL)Lateral collateral ligament (LCL)Medial collateral ligament (MCL)Medial collateral ligament (MCL)

MenisciiMeniscii::Medial meniscusMedial meniscusLateral meniscusLateral meniscus

Other componentsOther components::ArticularArticular cartilidgecartilidgeSynovial LiningSynovial Lining

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Movement of the Knee JointMovement of the Knee JointMajor movements of the knee are flexion and extension; also someMajor movements of the knee are flexion and extension; also some rotationrotationshape of the articulating surfaces of the tibia and femur as welshape of the articulating surfaces of the tibia and femur as well as the 4 l as the 4 ligaments of the knee all play a role in movement of the kneeligaments of the knee all play a role in movement of the kneeRollback mechanism of flexion Rollback mechanism of flexion -- as knee bends in flexion femur glides over and rolls back on tias knee bends in flexion femur glides over and rolls back on tibiabia-- PCL prevents femur from gliding/rolling too far back on tibial PCL prevents femur from gliding/rolling too far back on tibial plateauplateau

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Unhealthy KneesUnhealthy KneesCONDITIONS:CONDITIONS:

1.) Arthritis1.) Arthritis

2.) Infection2.) Infection

3.) Injury3.) Injury

4.) Aging4.) Aging

5.) 5.) ArticularArticular Cartilage wearCartilage wear

6.) Insufficient Synovial fluid6.) Insufficient Synovial fluid

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Types of ArthritisTypes of Arthritis1.) 1.) OsteoarthritisOsteoarthritis

--at the age of 50; hereditaryat the age of 50; hereditary--Cartilage wears away, bone rubs on boneCartilage wears away, bone rubs on bone

2.) 2.) Rheumatoid ArthritisRheumatoid Arthritis--synovial membrane thickens and becomes synovial membrane thickens and becomes inflamed produces too much synovial fluid causing inflamed produces too much synovial fluid causing internal pressureinternal pressure--can cause cartilage loss and paincan cause cartilage loss and pain

3.) 3.) Post Traumatic ArthritisPost Traumatic Arthritis--Caused by serious knee injuriesCaused by serious knee injuries-- knee fracture or severe tears of ligaments causing knee fracture or severe tears of ligaments causing articulararticular cartilage damage over timecartilage damage over time-- limits knee functionlimits knee function

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Healthy Vs. Unhealthy kneesHealthy Vs. Unhealthy knees

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Who is a good candidate for Who is a good candidate for Total Knee Total Knee ArthroplastyArthroplasty??

--people with severe knee pain people with severe knee pain --Knee deformity (Knee deformity (egeg. Bow legged or knock kneed). Bow legged or knock kneed)--Inability to tolerate pain medicationsInability to tolerate pain medications--Failure of other nonFailure of other non--operative procedures such as cortisone injections and physical toperative procedures such as cortisone injections and physical therapyherapy

--Usually performed on patients 60Usually performed on patients 60--80 yrs of age. Can be performed on younger or older 80 yrs of age. Can be performed on younger or older patients but unlikely candidates.patients but unlikely candidates.

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History of Knee ReplacementsHistory of Knee ReplacementsKnee surgery carried out by ancient EgyptiansKnee surgery carried out by ancient EgyptiansThe first knee replacement surgeries began just after WWII The first knee replacement surgeries began just after WWII In the late 1960In the late 1960’’s use of plastic on metal became more commons use of plastic on metal became more commonBy the 1970s engineers and scientists designed total By the 1970s engineers and scientists designed total condylarcondylar prosthesis prosthesis improved instrumentation used to install replacement during surgimproved instrumentation used to install replacement during surgery was developed ery was developed Modern designs compromise between stability and freedom of movemModern designs compromise between stability and freedom of movement while ent while trying to minimize stress in the joint.trying to minimize stress in the joint.

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Implant DesignImplant Design

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MaterialsMaterials1.) strong and high resistance to wear1.) strong and high resistance to wear2.) low coefficient of friction2.) low coefficient of friction3.) biocompatibility3.) biocompatibility

Metal components: titanium alloys or cobalt chromium alloyMetal components: titanium alloys or cobalt chromium alloy--durable, lightweight, inert in bodydurable, lightweight, inert in body

Plastic components: ultra high molecular weight polyethylene (UHPlastic components: ultra high molecular weight polyethylene (UHMWPE) MWPE) --used for its very low coefficient of frictionused for its very low coefficient of friction

MaterialsMaterials Coefficient of frictionCoefficient of friction

Tire/roadTire/road 11

Nylon/steelNylon/steel 0.20.2

PTFE/PTFE PTFE/PTFE 0.070.07

PTFE/PTFE (in water)PTFE/PTFE (in water) 0.040.04

CoCr/CoCrCoCr/CoCr (in water)(in water) 0.380.38

CoCrCoCr/UHMWPE ( in water)/UHMWPE ( in water) 0.040.04

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SurgerySurgeryDamaged portion of femur and Damaged portion of femur and tibialtibial bone are removed and bone are removed and replaced with prosthetic componentsreplaced with prosthetic components

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Alignment during surgeryAlignment during surgeryalignment is critical to ensure minimal stress and strain to thealignment is critical to ensure minimal stress and strain to the prosthesisprosthesismechanical axis is restored as much as possiblemechanical axis is restored as much as possibleSpecial instruments are used to ensure proper cutting and alignmSpecial instruments are used to ensure proper cutting and alignmentent

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BiomechanicsBiomechanicsKinematicsKinematics::

Walking cycleWalking cycle::represented by 2 stepsrepresented by 2 steps4 states: swing through, heel strike, weight 4 states: swing through, heel strike, weight

transfer, toetransfer, toe--offoffmotion of leg during each step has 2 phases: motion of leg during each step has 2 phases:

stance (support) phasestance (support) phaseswing (nonsupport) phaseswing (nonsupport) phase

Lubrication of kneeLubrication of kneeSwing phaseSwing phase: a thick film is pushed into space : a thick film is pushed into space

between cartilagebetween cartilageHeal strikeHeal strike: load on knee increases, so film : load on knee increases, so film

squeezed out to reduce thicknesssqueezed out to reduce thicknessToeToe--offoff:: load is maximum, but there is still film load is maximum, but there is still film

present to avoid surfacepresent to avoid surface--toto--surface contactsurface contact

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• Three Dimensional (3D) Static AnalysisThree Dimensional (3D) Static Analysis

• Force at the knee joint 10% into the Force at the knee joint 10% into the stance phase of climbing the stair.stance phase of climbing the stair.••Forces at knee are dependent on Forces at knee are dependent on position of the tibia.position of the tibia.

LetLetR = ground reaction forceR = ground reaction forceQ = quadriceps (primary muscle acting on the Q = quadriceps (primary muscle acting on the sagittalsagittal plane)plane)L = L = ilioilio--tibialtibial band (band (varusvarus movement in frontal plane)movement in frontal plane)V = joint force, perpendicular to tibial surfaceV = joint force, perpendicular to tibial surfaceH = joint force, parallel to tibial surfaceH = joint force, parallel to tibial surface

VerticalVertical force balance, force balance, sagittalsagittal planeplaneR + L R + L coscos 14 + Q 14 + Q coscos 20 + H sin 8 20 + H sin 8 –– V V coscos 8 = 08 = 0

HorizontalHorizontal force balance, force balance, sagittalsagittal planeplaneL sin 14 + V sin 8 L sin 14 + V sin 8 –– H H coscos 8 8 –– Q sin 20 = 0Q sin 20 = 0

Moment balance, Moment balance, sagittalsagittal planeplane8L + 53Q 8L + 53Q –– 49 R + 19H = 049 R + 19H = 0

Moment balance, Moment balance, frontal frontal planeplane8Q 8Q coscos 21 + 47L 21 + 47L coscos 14 14 –– 42R = 042R = 0

Setting R equal to body weight, W:Setting R equal to body weight, W:V = 2.42WV = 2.42W H = 0.28WH = 0.28W

Resultant joint force = 2.44WResultant joint force = 2.44WQ = 0.70WQ = 0.70W L = 0.80WL = 0.80W

Sagittal Plane Frontal Plane

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Design ConsiderationsDesign ConsiderationsLoad TransferLoad Transfer::

-- components of prosthesis are designed to transfer joint loads dcomponents of prosthesis are designed to transfer joint loads directly to the irectly to the underlying underlying cancellouscancellous bonebone-- this property is similar to load distribution in a healthy kneethis property is similar to load distribution in a healthy knee

Engineering solutionsEngineering solutions::

design goaldesign goal: distribution of joint loads to: distribution of joint loads tounderlying underlying cancellouscancellous bone asbone asuniformly as possible over as large anuniformly as possible over as large anarea as possible.area as possible.

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b.) b.) Maximum contact area of plateauMaximum contact area of plateau

–– goal: goal: distribute load over as large an area as distribute load over as large an area as possiblepossible-- Cannot extend the plateauCannot extend the plateau-- Coverage of the entire plateau is importantCoverage of the entire plateau is important-- This minimizes stress on bone and ligaments as This minimizes stress on bone and ligaments as Stress = F/A Stress = F/A -- In a normal knee joint pressures vary from In a normal knee joint pressures vary from 3kg/cm^2 to 19.3kg/cm^23kg/cm^2 to 19.3kg/cm^2

a) a) Metal BackingMetal Backing::

AdvantagesAdvantages: : –– helps to distributed applied loads uniformly over a large area helps to distributed applied loads uniformly over a large area (mod of elasticity)(mod of elasticity)-- reduces maximum compressive stresses and increases maximum tenreduces maximum compressive stresses and increases maximum tensile stressessile stresses

DisadvantagesDisadvantages: : --UHMWPE has to be attached securelyUHMWPE has to be attached securely-- polyethylene must be thinner (creates more wear)polyethylene must be thinner (creates more wear)-- dimensional changes occur because of wear particlesdimensional changes occur because of wear particles

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Pegs and Screws for Support

ModelModel Central Peg Central Peg (A)(A)

Short Short Peripheral Peg Peripheral Peg or Screw (B)or Screw (B)

Bladed Bladed CruciateCruciateForm (C)Form (C)

Bladed Bladed Central Peg Central Peg

(D)(D)

Long Long UncementedUncemented

IntramedullaryIntramedullaryStem (E)Stem (E)

AdvantagesAdvantages

--Most widely usedMost widely used--Does not stress Does not stress protect the protect the proximal proximal canellouscanellous bonebone--Carries some Carries some varusvarus--valgusvalgusbendingbending

PegsPegs::--Improved rotational Improved rotational resistanceresistanceScrewsScrews::-- reduce reduce micromotionmicromotionand enhance and enhance ingrowthingrowth processprocess

-- Resistance to Resistance to bending, bending, rotation, or rotation, or shear stressshear stress

--Resistance to Resistance to bending, rotation, bending, rotation, or shear stressor shear stress

--Appropriate for Appropriate for sinkagesinkage of tibial of tibial componentcomponent-- transfers load transfers load directly to lateral directly to lateral cortexcortex-- suitable for suitable for patients with bony patients with bony defectdefect

DisadvantagesDisadvantages

-- Unsuitable for Unsuitable for uncementeduncementedapplicationapplication

PegsPegs::--Minimal support to Minimal support to varusvarus--valgusvalgus bendingbending--Sometimes stress Sometimes stress protect the proximal protect the proximal cancellouscancellous boneboneScrewsScrews--pressure is localizedpressure is localized

--Instrumentation Instrumentation must be must be accurateaccurate

--Instrumentation Instrumentation must be accuratemust be accurate

-- Invasive techniqueInvasive technique

Fixation:

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Prosthetic ModelsProsthetic Models3 widely used types:3 widely used types:

1.) 1.) CondylarCondylar replacementreplacement

-- Simplest formSimplest form

--Kinematics are similar to that of the normal of kneeKinematics are similar to that of the normal of knee

--Main problem is wear in polyethyleneMain problem is wear in polyethylene2.) 2.) Hinged or linked prosthesisHinged or linked prosthesis

-- very constrained designvery constrained design

--much of the force transferred to fixation stems and much of the force transferred to fixation stems and surrounding bonesurrounding bone

--mostly used for older people with serious knee mostly used for older people with serious knee instability,damaged ligaments and bone lossinstability,damaged ligaments and bone loss

3.) 3.) A/P stabilized prosthesisA/P stabilized prosthesis

--PCL is removedPCL is removed

--Post in Post in tibialtibial plateau prevents femur from rolling too far backplateau prevents femur from rolling too far back

--In addition to wear in polyethylene disc, post is also In addition to wear in polyethylene disc, post is also susceptible to wearsusceptible to wear

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Testing ProcedureTesting Procedure• Two types of failure modes:

•Fatigue•Wear-deformation

•Testing for 10 years of usage takes approximately 3 weeks

•Axial loading: offset leading could lead to failure of metallic parts, excessive deformation of plastic

•Shear force: only 10% of vertical force component, lead to severe plastic wear through metal tray

•Combination: combined stresses can be much higher than for one load condition alone.

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Problems Arising from ImplantsProblems Arising from Implants

Adverse Effects of Any SurgeryAdverse Effects of Any Surgery

ScarringScarringNerve injury (temporary/permanent numbness in localized area)Nerve injury (temporary/permanent numbness in localized area)Blood vessels injuryBlood vessels injurySlow healing (especially for diabetics, or patients with longSlow healing (especially for diabetics, or patients with long--term steroid use)term steroid use)InfectionInfection

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Adverse Effects of TKAAdverse Effects of TKAInfectionInfection

Symptoms may appear early after surgery, or may Symptoms may appear early after surgery, or may not appear for months, or even years after the not appear for months, or even years after the operationoperationInfection of the Infection of the hematomahematoma (collection of blood)(collection of blood)

StiffnessStiffnessusually caused by scar tissue formed near the usually caused by scar tissue formed near the prosthesis, limiting the range of motionprosthesis, limiting the range of motionPreventive methods: use of Constant Passive Motion Preventive methods: use of Constant Passive Motion (CPM) machine and physical therapy(CPM) machine and physical therapy

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LooseningLoosening

Fibrous Tissue:Fibrous Tissue: a soft fibrous tissue a soft fibrous tissue develops permitting more relative motion develops permitting more relative motion between implant and bone loadingbetween implant and bone loading

Bone necrosis (death)Bone necrosis (death)

Mechanical damage during surgeryMechanical damage during surgery

Wear debris:Wear debris:Foreign body wearForeign body wearSubSub--surface fatiguesurface fatigue

Mechanical loosening from fatigueMechanical loosening from fatigueDebondingDebonding: metal stem from cement, : metal stem from cement, and cement from boneand cement from bone

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AdvancementsAdvancementsMinimally invasive surgery (MIS) of the knee is a new development

Advantages: - needs only a small incision-better cosmetic results

Disadvantages: -surgeon has limited visibility when placing components

As a result Computer Aided Orthopedic Surgery (CAOS) is becoming more common-uses infrared cameras, sensors and tracking devices a patient specific digital image of the knee is createdComputer tracks surgeon’s instruments and provides information about where to

cut bone forproper alignment

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Mobile bearing prosthesis – UHMWPE discs slide along inside of tibial plate

- increases contact area at all points of flexion and extension

TypeType Contact area (square Contact area (square mm)mm)

StableStable 200200--300300

MobileMobile 10001000--15001500

increased wear of polyethylene due to increased sliding since

Volume of wear (V) ∝ applied load (W) and sliding distance (x)

V = k(Wx)/3H

where k = wear coefficient

H = hardness/softness of material

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ReferencesReferencesA Patient's Guide to Total Knee Replacement Surgery A Patient's Guide to Total Knee Replacement Surgery http://http://www.healthpages.orgwww.healthpages.org/AHP/LIBRARY/HLTHTOP/TKR//AHP/LIBRARY/HLTHTOP/TKR/Biomechanics of Artificial JointsBiomechanics of Artificial Jointshttp://www.engin.umich.edu/class/bme456/artjoint/artjoint.htmhttp://www.engin.umich.edu/class/bme456/artjoint/artjoint.htmKnee Implants (American Academy of Orthopedic Surgeons)Knee Implants (American Academy of Orthopedic Surgeons)http://http://orthoinfo.aaos.org/fact/thr_report.cfm?thread_idorthoinfo.aaos.org/fact/thr_report.cfm?thread_id=279&topcategory=knee=279&topcategory=kneeTotal Knee Replacement (American Academy of Orthopedic Surgeons)Total Knee Replacement (American Academy of Orthopedic Surgeons)http://http://orthoinfo.aaos.org/booklet/view_report.cfm?Thread_IDorthoinfo.aaos.org/booklet/view_report.cfm?Thread_ID=9&topcategory=Knee=9&topcategory=KneeKnee Replacement SurgeryKnee Replacement Surgeryhttp://orthopedics.about.com/cs/kneereplacement/a/kneereplacemenhttp://orthopedics.about.com/cs/kneereplacement/a/kneereplacement.htmt.htmCorinCorin Group PCLGroup PCLhttp://http://www.corin.co.uk/p_kneereplacements.htmlwww.corin.co.uk/p_kneereplacements.htmlTotal Knee Replacement Patients (VIDEO). Total Knee Replacement Patients (VIDEO). National Association of Orthopedic Nurses, 1988.National Association of Orthopedic Nurses, 1988.KrackowKrackow, K.A. “The Technique of Total Knee , K.A. “The Technique of Total Knee ArthroplastyArthroplasty”. C.V. ”. C.V. MosbyMosby Company, Missouri, Company, Missouri, 1990.1990.DumbletonDumbleton, J.H. “, J.H. “TribologyTribology of Natural and Artificial Joints”. Elsevier Scientific Publishiof Natural and Artificial Joints”. Elsevier Scientific Publishing ng Company, New York, 1981.Company, New York, 1981.MorreyMorrey, B. “Joint Replacement , B. “Joint Replacement ArithroplastyArithroplasty”. Churchill Livingstone, New York, 1991.”. Churchill Livingstone, New York, 1991.DiAngeloDiAngelo, D.J. “Biomechanics of the Knee Joint: High , D.J. “Biomechanics of the Knee Joint: High TibialTibial OsteotomyOsteotomy Surgery”. McMaster Surgery”. McMaster University, 1993.University, 1993.Petty, W. “Total Joint Replacement”. W.B. Saunders Company, 1991Petty, W. “Total Joint Replacement”. W.B. Saunders Company, 1991..

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