Osteoarthritis
of the Knee Joint
Ali Legnos
Friction and Wear of Materials
Fall 2012
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1. Background
The knee joint is a subject of much biomechanical and tribological study. This joint
bears on average 3.9 times the individual’s body weight while walking on level ground1,
and with daily use and age, the lubricating cartilage and fluid protecting bones in the knee
can thin due to fatigue wear. When partial or complete of erosion of the cartilage occurs
and bare bones come into contact, a degenerative disease called Osteoarthritis will result.
Bone spurs can develop at the joint causing pain and can leave patients bedridden. There
is currently no cure for Osteoarthritis.2
Extreme cases of Osteoarthritis of the knee require Total Knee Replacements (TKRs), or
Total Knee Athroplasty (TKA), in which the damaged bone and cartilage is removed and
replaced by a prosthetic.
Prostheses are typically made of metal covered with
polyethylene plastic.3 A normal knee compared to a knee with TKR joint is shown in
Figure 1.
Figure 1 Normal Knee (Left) and Knee with TKR (Right)4
Understanding the friction, wear and lubrication of the knee joint is a complex problem,
requiring understanding of the anatomical make-up of the joint and what factors influence
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and are affected by friction and wear on the contacting surfaces. There is a plethora of
research on several aspects of the knee joints, including but not limited to its
biomechanics; friction, wear and lubrication properties; and prosthesis design and modes
of failure. The focus of this research is to provide an overview of the anatomy of the
knee joint and present a compilation of recent scientific and engineering conclusions
relating to a) factors that affect wear of the knee joint, and b) methods commonly used to
measure friction, wear and lubrication of the joint.
1.1. Knee Anatomy
The knee joint consists of the intersection of bones protected by lubricating films of
cartilage, and surrounded by a lubricating fluid called synovial fluid. A brief overview of
the anatomy of the knee is provided below.
1.1.1. Knee Bones
The knee joint is the intersection of three bones: the femur (thigh bone), tibia (shin bone)
and patella (knee cap), as shown in Figure 2. The tibiofemoral joint is a hinge joint
connecting the tibia to the femur. The patellofemoral joint is a saddle joint connecting
the patella to the front of the head of the femur bone.
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Figure 2 Normal Knee Joint, Side View5
1.1.2. Articular Cartilage
Articular cartilage is smooth tissue that covers bones where they intersect other bones at
joints. It functions as a low friction surface to protect the bones from the compressive
forces on the knee during joint movement. With use, articular cartilage can gradually
wear thin, decreasing the protective lubricating layer over the bone. In extreme cases,
articular cartilage will wear away completely at a given location on the joint and the
bones will come into direct contact, as shown in Figure 3. Since cartilage is not
regenerative, once it is worn away surgical options, such as inserting prostheses, are often
employed.6
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Figure 3 Knee Osteoarthritis7
The heads of the femur and of the tibia located at the knee joint are covered by a layer of
articular cartilage, as shown for a healthy knee in Figure 2 and in a knee with
Osteoarthritis in Figure 3. The underside of the patella is also protected by a layer of
cartilage. The tensile modulus of cartilage will decrease with age and wear, decreasing
the elasticity of the material in the plastic deformation regime. Wear will also lead to a
reorganization of fibers present in the cartilage, which can result in split-lines and wearlines on the surface of the cartilage.8
1.1.3. Synovial Fluid
Synovial fluid is a viscous substance located in synovial joints, such as the knee, that acts
as a lubricating agent to reduce friction. It is found in the cavity surrounding the heads of
the tibia and femur, and the patella, as shown in Figure 2. Synovial fluid has a high
concentration of hyaluronic acid, whose high molecular weight and long coiled molecules
give synovial fluid high elasticity and viscosity.
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The molecular weight and
concentration of hyaluronic acid are responsible for the boundary layer phenomena at the
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synovial fluid-articular cartilage interface. It is the “stickiness” of the boundary layer that
is theorized as to why synovial fluid is an effective lubricant10, but overall, lubrication
measurements of synovial fluid are varied and poorly understood.11
2. Factors Affecting Knee Joint Wear
Understanding joint kinematics and the properties of bone, cartilage and synovial fluid
are crucial to understanding the tribological environment of the knee joint. Articular
cartilage and synovial fluid act as lubricants for the bone and achieve low friction in
healthy joints. Loading on the joint, including maximum and normal sustained loading
must be understood for the various conditions a human knee is subject to, and the
compressive and shear forces calculated appropriately. Williams et. al. summarized the
unique factors contributing to friction, wear and lubrication of a synovial joint, such as
the knee, in Figure 4.12
Figure 4 Key Challenges for Synovial Joint Tissue Engineering
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Several macro scale factors have been hypothesized to influence the location and degree
of wear on the knee joint, including age, sex, height, weight, genetics and positional
alignment of the leg bones.
2.1. Weight, Body Mass Index, Gender and Age
Several studies have been performed to correlate a patient’s weight, Body Mass Index
(BMI), gender and age to stress on the knee and the onset and severity of Osteoarthritis.
Being overweight and/or having an elevated BMI put an increased stress on the knee
joint, and are generally agreed to be a significant factor in the onset of Osteoarthritis of
the knee due to an increased load on the joint. An explicit BMI has not been identified as
a cut-off value to separate high-risk from low-risk individuals.13
Some studies have cited that females have increased wear of the knee joint, leading to
higher incidences of Osteoarthritis diagnoses than in men, and/or more severe symptoms,
while other studies have shown only weak correlations. One study noted that females on
average have greater knee flexion and knee extensor movement, which is a factor in the
onset of Osteoarthritis.14 Another study found that women showed worse Osteoarthritic
symptoms then men.15
2.2. Knee Bone Alignment
Studies have indicated that misalignment of the bones at the knee joint can lead to
accelerated wear on the joint. Ding et. al. found that the degree of tibial varus, or degree
of rotation of the tibia in the direction of the body’s centerline (i.e. bowlegged) with
respect to the femur, is highly correlated to knee joint wear.16 Srivastava et al. studied
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the correlation of tibial varus to articular cartilage wear. The surface geometry of sixteen
polyethylene tibial prosthetic inserts that had been removed from patients’ knees were
compared to unused prostheses using a laser displacement sensor with a resolution of 3
μm. Surgical techniques aim for tibial insert alignment at 90 degrees to the sagittal plant,
with a tibial varus of 2 to 3 degrees often achieved with surgical tools. The results
indicated a strong correlation between the tibial varus and wear volume of the prosthetic
insert, indicating twice the volumetric wear on prostheses with greater than 3 degrees of
varus compared to those with less than 3 degrees of varus.17
3. Wear Measurement Techniques
Observations of cartilage wear on the knee joint can be accomplished by direct
measurement using Magnetic Resonance Imaging (MRI) and x-rays.
India ink is
commonly used to measure the surface roughness of articular cartilage. Additionally,
material properties have been estimated for the bones, cartilage and synovial fluid, and
computer simulations have been developed to predict wear.
3.1. Magnetic Resonance Imaging
Direct measurement of an individual’s knee, including bone volume and cartilage
thickness, is readily measured using MRI. Subsequent measurements can indicate a loss
of cartilage volume, or the volumetric wear. Scales have been developed and published to
standardize the measurement of cartilage wear.
The Whole-Organ MRI Score
(WORMS), shown in Table 1, classifies the location and severity of cartilage wear on a
joint.18
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Table 1 Whole-Organ MRI Score (WORMS)
Connolly et. al. measured cartilage thickness on the tibia and femur in the tibiafemoral
joint and calculated average values.
MRI of 20 patients were drawn in CAD and
analyzed. The study concluded that the average femoral articular cartilage thickness is
2.75 mm and the average tibial articular cartilage thickness is 2.76 mm, and showed a
strong correlation between cartilage thickness and the patients’ height and weight. 19
Similarly, Iranpour et. al. generated CAD models of 37 patellae with articular cartilage
based on x-ray images. They found the average patellar thickness to be 2.5 mm.20
3.2. Surface Geometry Using India Ink
With age and wear, the tensile strength of articular cartilage of the knee experiences a
decrease in tensile strength, as much as 65% between the ages of 24 to 90. Cartilage
deterioration results from a degradation of extracellular matrix components and
modification of the collagen network through crosslinks. 21 Covering the surface of
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articular cartilage with India Ink is a common method of visually inspecting the cartilage
surface to assess the extent and location of wear. An individual particle of India Ink is
approximately 40 nm in diameter, and particles clump in groups approximately 100 nm in
diameter.
These large diameters do not permit the India Ink from penetrating and
permanently staining healthy cartilage, but staining may occur in protein deficient areas
of the cartilage. The amount of light reflection and surface pattern are used to visually
inspect the location and amount of cartilage erosion.22
3.3. Modeling Techniques
Data gathered from physical models, either using India Ink or MRI images, and cadavers
is often modeled using CAD and analyzed using finite element methods. Such models
are often used to measure contact stresses and strains, which are difficult to measure in
vivo. Computation simulations are often employed as a low cost and non-invasive
option.23
Li et. al. tested a cadaver of the femur and patella bones and their articular cartilage, and
generated a CAD model of the cadaver geometry, as shown in Figure 5.
Both
experimental setups underwent the same series of loading to simulate wear due to fatigue,
and it was found that wear depths measured in the computer simulation were within 0.01
mm accuracy to the cadaveric experiment. An experimental limitation was identified as
the difference in wear between living articular cartilage and dead cartilage on the
cadaver. Archard’s wear law is appropriate for use on the dead cartilage, but the need for
a validated wear law for cartilage was expressed.24
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Figure 5 Li et. al. Cadaver and CAD Models Used to Compare CAD Model Accuracy25
4. Conclusions
Understanding the anatomical and physiological aspects of the knee joint is imperative to
improving the diagnosis, and possible prevention or cure of Osteoarthritis. There are
several factors, many currently disputed in the scientific community, affecting the
amount and location of wear on the tibia, femur and patella, including age, sex, weight,
genetics, alignment of the knee bones, etc. Understanding these factors will help doctors
diagnose severe cartilage wear prior to a diagnosis of Osteoarthritis, and hopefully better
prepare the population to prevent or cure the disease.
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Graeme A.; Stachowiak, Gwidon W.; Gachter, Andre; University of Western Australia,
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2 “Osteoarthritis”, Mayo Clinic,
http://www.mayoclinic.com/health/osteoarthritis/DS00019
3 “Knee Joint Replacement”, Biomet,
http://www.biomet.com/patients/knee_replacement.cfm
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osteoarthritis”; Ding, C.; Cicuttini, F.; Jones, G.; Osteoarthritis and Cartilage, Volume
15, Issue 5, May 2007, Pages 479-486; http://www.sciencedirect.com.colelibprxy.ewp.rpi.edu/science/article/pii/S1063458407000106
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21 “Age- and site-associated biomechanical weakening of human articular cartilage
of the femoral condyle”; Temple, M. M.; Bae, W. C.; Chen, M. Q.; Lotz, M.; Amiel, D.;
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22 “Introduction to Bioengineering”; Fung, Yuan Cheng; Chien, Shu; p. 163
23 “Dynamic Simulation of Knee Joint Contact During Human Movement”, Yanhong,
Bei, University of Florida, Ph. D. Dissertation, 2003,
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Michele; D’Lima, Darryl D.; Sah, Robert L.; Fregly, Benjamin J.; J Biomech, 44(8),
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25 “Computational Wear Simulation of Patellofemoral Articular Cartilage during In
Vitro Testing”; Li, Lingmin; Patil, Shantanu; Steklov, Nick; Bae, Won; Temple-Wong,
Michele; D’Lima, Darryl D.; Sah, Robert L.; Fregly, Benjamin J.; J Biomech, 44(8),
1507-1513, May 2011, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3119794/
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