Biomechanics of Knee Replacement

Mujda Hakime, Paul Malcolm

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Table of contents

Knee Anatomy Movements of the Knee Knee conditions leading to knee

replacement Materials Alignment and Joint Loading Knee Implant failure Future development

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Knee Anatomy:

Bones of the knee Knee joint capsule Ligaments of the

knee Tendons of the knee Muscles around the

knee Knee Cartilage

(Menisci)

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Knee Anatomy: Bones of the Knee

Four Major Bones: Tibia Patella Femur Fibula

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Knee Anatomy: Ligaments of the Knee

Medial collateral ligament Lateral collateral ligament Anterior cruciate ligament Posterior cruciate

ligament Patellar ligament

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Knee Anatomy: Cartilage of the Knee Articular cartilage Lateral meniscus Medial meniscus

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Knee Anatomy: Muscles of the Knee

Quadriceps: composed of four major muscles in front of thigh which allows you to straighten your the leg from a bent position

Hamstrings: composed of 3 major muscles on the back of the thigh which allows you to bend your knee from a straight position

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Knee Anatomy: Knee joint capsule

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Knee Anatomy: Other features

Tendons Bursae Arteries and veins

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Movement of the Knee Knee Extension: The Normal range of knee extension is 0-10 degrees.

Knee Flexion: The Normal range of knee flexion is 135-150 degrees.

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Movement of the Knee

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Reasons for Knee replacement

Arthritis : Osteoarthritis, rheumatoid arthritis and traumatic arthritis

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Reasons for Knee replacement

Severe Knee pain limiting daily activity Knee pain even when resting Chronic knee inflammation and swelling Knee Stiffness Avascular necrosis (AVN) – results from an

inadequate supply of blood to the bone end inside the joint.

Malalignment of the knee joint: bowing in or out also known as knock-knees or bowlegs 15

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Fundamentals General idea is to replace the diseased surface with one

made of a synthetic material Joint is interfaced with the bone using one of two

methods; PMMA cement or bone ingrowth ACL is removed, sometimes PCL as well Plastic surface is inserted between femoral and tibial component, so weight is transferred metal-plastic-metal

Four Components of a Knee Replacement 1. Femoral component 2. Tibial component 3. Plastic insert 4. Patellar component Total or partial (sometimes called unicompartmental) Mobile-bearing or fixed-bearing regarding the plastic insert Cruciate-retaining (allows PCL to help support knee joint) Posterior-stabilized (when PCL has been removed)

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Design Challenges and Requirements

Lifespan – patient age vs. artificial knee durability

85-90% of replacement knees are still functional after 10 years, has a lot to do with experience of surgeon

Geometric design of the surfaces Interface between artificial joint and the

surrounding bone

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Materials and Their Properties Cobalt-chromium alloys are most commonly used since

they are tough, corrosion resistant and bio-compatible Metal components are anchored using PMMA cement,

which has good tissue compatibility Sometimes patients have an allergic reaction to the

nickel in the Co-Cr alloy, in which case titanium can be used

Polyethylene is usually used for the plastic spacer and allows for maximum joint mobility while minimizing surface wear (coeff. of friction 0.07, wear rate of 0.25mm/year)

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Gait Analysis 1. Knee flexion angle

about the axis from the lateral to medial condyle of the femur

2. Adduction moment about the posterior-anterior axis (line of progression)

3. Moment about the distal-proximal axis of the limb

(Deluzio et al. 708)

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Alignment The effectiveness of a knee

replacement greatly depends on having the proper alignment of the components, to maximize stability and decrease discomfort

“mechanical” alignment only considers the 2-d alignment of the knee, lining them up to give a 0-degree hip-knee-ankle angle

However, in reality only 2% of humans have this 0-degree setup naturally

“kinematic” alignment takes into account the 3-d alignment and has been clinically shown to result in better motion and patient satisfaction

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Tibiofemoral Joint Loadings Heel strike at 0, toe off at 100. Forces exerted on the knee joint can be up to 8x body weight walking downhill, and 4x body

weight while walking on a level surface These forces are difficult to measure, and are greater than designers originally calculated…

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Contact Area of Replacement Knees

in a natural knee, the average contact area ranges from 765mm2 to 1150mm2

Assuming a uniform load distribution and a load of 8x BW for a

70kg person, the stress on the cartilage of a healthy knee is less than 5MPa

The contact area of a typical knee prostheses anywhere from 80-300mm2, leading to a stress on the polyethylene inlay that could be up to 60MPa

The polyethylene inlay has a yield point of 20MPa Designers typically use 4x BW during the testing phase of a knee

replacement Shows the importance of being able to measure kinematics

accurately

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Knee Implant Failure Wear and loosening: Friction Infection Fracture Instability: A sensation of the knee “giving away” Stiffness: Loss of range Osteolysis: long term when tiny particles of

worn-out plastic or cement from the implant that may potentially migrate into the bone and cause localized damage to the bone

Thrombosis: blood cloth inside blood vessel obstructing blood flow through circulatory system

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Future Advances

Biologic Knee Replacement

DeNovo® ET Living Cartilage Implant

“smart” or “intelligent” orthopedic implants

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References “DeNovo® ET Living Cartilage Implant.” ISTO Technologies, Inc. Web. 8 October 2012. <http://www.istotech.com/denovo-et-orthopedics.html>

Fischer S., “Knee Injuries and Conditions.” UConn Health Care Center. Web. 8 October 2012. < http://uconnsportsmed.uchc.edu/injury/knee/total_knee.html> “Knee Joint Anatomy, Function and Problems.” Health Pages.org. 6 July 2010. Web. 8 October 2012. <http://healthpages.org/anatomy-function/knee-joint-structure-function-problems/#bones-of-the-knee> “Knee Anatomy.” Sportsinjuryclinic.net. Web. 8 October 2012. <http://www.sportsinjuryclinic.net/anatomy/knee-anatomy> Deluzio, Kevin, U. Wyss, P. Costigan, C. Sorbie, B. Zee. “Gait Assessment in unicompartmental knee arthroplasty patients:

Principal component modeling of gait waveforms and clinical status”. Human Movement Science 8 (1999): 701- 711.

“Kinematic vs. mechanical alignment: What is the difference?”. Orthopedics Today. 11 Oct. 2010.

<http://www.healio.com/orthopedics/knee/news/print/orthopedics-today/%7Bca0f9a6b-d1bf-4d84-8437-c324d5cc0ad9%7D/kinematic-vs-mechanical-alignment-what-is-the-difference>.

“Types of Knee Replacement Prosthetics”. Livestrong. 30 Mar. 2011. <http://www.livestrong.com/article/263154-types-of-knee-replacement-prosthetics/> “Joint Load Considerations in Total Knee Replacement”. M. Kuster, G. Wood, G. Stachowiak, A. Gachter. 1 Jan. 1997. <http://www.bjj.boneandjoint.org.uk/content/79-B/1/109.full.pdf>

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