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1 Basic Science ofBone andCartilage Metabolism
29
Dysplasias
Achondroplasia is the most common skeletal
dysplasia and the most common dwarng syndrome (Fig.1.44).
It follows an autosomal dominant inheritance
pattern, but the majority of cases arise via spontaneous genetic mutations. The genetic mutation
lies in the gene encoding broblast growth factor
(FGF) receptor-3, located on chromosome 4.
This genetic defect ultimately disrupts normal
endochondral bone growth and, therefore, results
in shortening of all bones that depend on this
mechanism for their growth. Classic manifestations along with short stature include the follow-
ing spinal deformities: thoracolumbar kyphosis,
foramen magnum stenosis, and lumbar spinal
stenosis, as well as a “champagne glass” pelvis—a pelvic outlet wider than it is deep
(Fig.1.45).
Bone dysplasias (intrinsic defects of bone
growth) are, as a general rule, genetic in origin
despite the fact that some of the milder (tarda)
forms may not be apparent until the child begins
growing.
Chromosomal Defects
Down syndrome is often characterized by
severe ligamentous laxity. This is the basis for
the numerous orthopedic conditions that are
typical in this group. Atlanto-axial instability,
at feet, patellar subluxation, bunions, and subluxation of the hips all point to the inability of
the ligamentous structures to stabilize joints.
Many of the chromosomal abnormalities
involve defects in mesoderm development,
which accounts for the common coincidence of
musculoskeletal, genitourinary, and cardiac
abnormalities.
Fig. 1.44 An achondroplastic dwarf. (a) Note the propor-
tionately shorter proximal limb segments compared to the
distal limb segment, with the hands only racing to the hip
region. (b) The proximal limb segments are proportionately shorter than the distal, with the hands reaching only
to the hip region. The legs are bowed (genu varum) and
there is marked lumbar lordosis with prominent buttocks
as a result of pelvic tilt. (From Orthopedic Surgery:
Principles of Diagnosis and Treatment, Figure11.57)
Fig. 1.45 The radiographic appearance of the pelvis of a
young boy with achondroplasia. The iliac bones are
rounded, and the acetabular roofs are horizontal. The sciatic notch is narrow and the acetabulae broad and at,
resulting from inadequate growth of “Y” cartilage in this
region. The shape of the pelvis itself has been described to
resemble a champagne glass, wider than it is deep. (From
Orthopedic Imaging: A Practical Approach. 7E. 2021.
Chapter 15, Figure33.33)
30
Fig. 1.46 Clubfoot deformity is associated with forefoot
supination, deep medial creases, and equinovarus of the
hindfoot. (From Orthopedic Surgery: Principles of
Diagnosis and Treatment, Figure11.177)
Congenital Deformity
The clubfoot deformity is the most common
musculoskeletal defect with an overall incidence
of 1in 1000 births (Fig.1.46). A genetic component to this condition is strongly suggested,
resulting in muscle contractures contributing to
characteristic deformities and ultimately bony
malalignment. Usually identied at birth, clubfoot is a generalized dysplasia of the mesenchymal structures (bone, ligament, muscle) of not
only the foot but truly the entire leg. In addition
to the genetic component, environmental (intrauterine position) factors have been implicated,
but their exact interaction remains unknown.
Miscellaneous
Neurobromatosis is another relatively common
(1 in 3000 live births) condition with multiple
classic orthopedic manifestations. Resulting
from an autosomal dominant mutation in the neurobromatosis- 1 gene on chromosome 17,
extremity deformities, spinal deformities, and
classic skin lesions result. Specically, anterolateral bowing of the tibia, pseudoarthrosis of the
M. J. Kelly and J. N. Delahay
bones of the forearm or leg, scoliosis, limb hemihypertrophy, skin ndings (cafe-au-lait spots and
axillary freckling) and the devious presence of
malignant nerve sheath tumors are seen.
Summary
Many different pathologic states impact the skeletal system, whether they are primary or secondary. Bone has a limited number of ways of
responding to abnormal stimuli whether they are
chemical, mechanical, infectious, circulatory,
etc. In general, one can expect to see either bone
resorption or bone formation, either locally or
systemically, dominate the pattern. A working
knowledge of the normal usually allows the
observer to anticipate the response to many of
these pathologic processes.
In this regard, observing the changes that one
sees on standard imaging studies will often permit the development of a working differential
diagnosis. Using the basic seven disease categories and expanding each into a plausible list of
diagnoses should lead, given more data, to a
denitive diagnosis and hence appropriate
treatment.
Further Reading
Mescher AL, editor. Junqueira’s basic histology text and
Atlas. 16th ed. McGraw Hill; 2021.
Morcuende JA, Sanders JO.Chapter 1. Embryology and
development of the musculoskeletal system. In: Lovell
and Winter’s pediatric orthopaedics.
Compton JT, Lee FY. A review of osteocyte function
and the emerging importance of sclerostin. J Bone
Joint Surg Am. 2014;96(19):1659–68. https://doi.
org/10.2106/JBJS.M.01096. PMID: 25274791;
PMCID: PMC4179450.
Greenspan A.Orthopedic imaging: a practical approach.
7th ed. Wolters Kluwer; 2021.
Deyrup AT, Siegal GP.Practical orthopedic pathology: a
diagnostic approach. Elsevier; 2015.
Wiesel etal. Orthopedic surgery: principles of diagnosis
and treatment. Springer.
Langman’s medical embryology, 14th edn. 2018.
1 Basic Science ofBone andCartilage Metabolism
31
Rockwood and Wilkins fractures in children.
Rockwood and Green’s fractures in adults.
Bernstein J, editor. Musculoskeletal medicine. Rosemont,
IL: American Academy of Orthopaedic Surgeons;
2003.
Bogumill GP, Schwamm HA.Orthopaedic pathology: a
synopsis with clinical and radiographic correlation.
Philadelphia, PA: Saunders; 1984.
Buckwalter JA, Einhorn TA, Simon SR, editors.
Orthopaedic basic science: biology and biomechanics
of the musculoskeletal system. 2nd ed. Rosemont, IL:
American Academy of Orthopaedic Surgeons; 2000.
Deng X, Wu L, Yang C, Xu Y. Neuropathic arthropa-
thy caused by syringomyelia. J Neurosurg Spine.
2013;18(3):303–9. https://doi.org/10.3171/2012.11.
SPINE12860. Epub 2013 Jan 4. PMID: 23289508.
Marenzana M, Arnett TR.The key role of the blood sup-
ply to bone. Bone Res. 2013;1(3):203–15. https://
doi.org/10.4248/BR201303001. PMID: 26273504;
PMCID: PMC4472103.
Biomechanics andBiomaterials
DanielHampton andPatrickBurroughs
2
The topic of biomechanics within orthopedics
brings together physics, human biology, and
engineering within the musculoskeletal system to
describe how forces allow the human body to
move and interact with the world. When discussing the orthopedic principles of biomechanics
and biomaterials, it is important to rst begin
with a set of denitions that apply to commonly
used terms within this eld that are central to all
discussions involving physics and engineering.
Scalar and vector are quantities used to
describe the state of objects. Both scalar and vectors have magnitude, however, vector quantities
differ from scalar quantities because they have
both a magnitude and a direction. Mass is a scalar
quantity describing the amount of matter within
an object. In relation to orthopedics, mass is signicant in that it reects the inertia of an object,
and its resistance to acceleration or change in
movement. Displacement is a vector quantity that
denes the change in position of an object.
Velocity is the change in displacement of an
object in a direction, measured as distance over
time. Velocity has a direction and is therefore a
D. Hampton · P. Burroughs (*)
MedStar Georgetown Orthopedic Institute,
Georgetown University School of Medicine,
Washington, DC, USA
Department of Orthopedics, MedStar Georgetown
University Hospital, Washington, DC, USA
e-mail: Daniel.m.hampton@gunet.georgetown.edu;
Patrick.J.Burroughs@medstar.net
vector. Acceleration is the change in velocity of
an object over time, and is a vector quantity.
Force is the vector quantity that changes an
object either in shape or position. Forces have a
variety of effects on an object, or body, depending on the composition of the material, the vector
of the force, and the relative environment that is
interacting with the body. Classically, force is
measured and represented as the ability to accelerate an object of known mass.
When determining the effects of a force on a
body, it is important to determine the composition of the object, and if it can be considered to
behave as a rigid body or deformable body.
Within rigid bodies, the particles within the
object do not change their position relative to one
another while forces are being applied. With
deformable bodies, the particles change their
position relative to one another [1]. These
changes may affect the shape of an object (lengthening a tendon under tension, for example) or its
volume. Furthermore, this deformation can be
characterized as elastic or plastic.
When materials undergo elastic deformation
after a force is applied, it means that the material
will return to its original position after the
deforming force is removed. When a material
undergoes plastic deformation, its shape has
changed permanently, and that change in shape
will remain after the force is removed. Creep is a
specic term to describe plastic deformation that
can be observed or measured after a deforming
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
W. F. Postma et al. (eds.), Essentials of Orthopedic Surgery,
https://doi.org/10.1007/978-3-031-66215-7_2
33
34
D. Hampton and P. Burroughs
force is applied over a period of time. The amount
of creep an object experiences will increase as
time increases [1].
Compression, tension, shear, rotation, or bending are all forces that have specic effects on bones
and orthopedic implants and must be considered
when discussing biomechanics. Compressive
forces act parallel to the surface of a bone or
implant and make the matter within that object
more compact. When bones fail in compression,
resulting fractures include buckle fractures in
pediatric patients or a fracture with an associated
buttery fragment. Tension forces also act perpendicular to the surface of an object but act to pull an
object apart with collinear forces acting in opposite directions. Bones fail in a transverse pattern
when they are under tension. A shear force is an
additional external force described in orthopedics
that acts on the surface of a bone or object at two
points which are eccentrically aligned. Bones are
capable of resisting signicantly higher compressive forces than shear forces [2, 3].
In addition to the forces already described,
moments are vector quantities that cause rotation
or bending about a single point, which is the pivot
or axis. Moments have an associated moment
arm dened by the distance between the axis of
rotation or bending of the object and the point at
which the force is applied. Moment is closely
related to torque, which refers to a specic
moment that results in rotation.
Within orthopedics, kinematics describes the
motion of joints in the human body. Simple
descriptions of these relationships include diagrams of static equilibrium. In order to describe
joint kinematics, free body diagrams are drawn,
with vectors representing force generated by
muscle, moments describing limbs, and weight
of objects in motion. When an object is not in
motion, or is not undergoing linear or angular
acceleration, it can be described as in a state of
equilibrium. When objects are in a state of equilibrium, the summation of the separate vectors
from forces acting on the object equals zero.
Within Biomechanics, statics refers to forces that
result in equilibrium, whereas dynamics refers to
the study of forces that result in acceleration or
rotation of an object.
For the sake of this discussion, Newton’s three
laws of motion govern the basic principles of
mechanics that will be applied to understand biomechanics within orthopedics. Newton’s rst law
states that an object will not change velocity, or
will remain at rest, until it is acted upon by an
outside force. Newton’s second law describes
how the motion of a body is affected by an external force, stating that the resulting acceleration of
an object as a force is applied is proportional to
the sum of the force vectors acting on the object,
and inversely proportional to the mass of the
object. Newton’s third law states that every
action, or force, on a body, has an equal and
opposite reaction [1, 3].
Now that the basic terms of biomechanics have
been covered, they can be applied to the eld of
orthopedics. In this chapter, we will cover the
mechanics of classic mechanical levers with common motions performed by humans in their activities of daily living. To accomplish this, free body
diagrams of the limb or motion in focus will be
illustrated. In the example of a human hip, as demonstrated in Fig. 2.1, the joint reaction force is
demonstrated in the hip and the force of the abductors, mass of body, and relative lengths from the
human center of gravity to the femoral head, and
from femoral head to greater trochanter must be
accounted for. Other forces and measurements
within Fig. 2.1 include the weight of the system
Fig. 2.1 Biomechanics of the human hip
2 Biomechanics andBiomaterials
[W] which is equal to the weight of the human
body minus the weight of the leg (in this case, the
left leg). The force [H] is the force supplied by the
abductors of the hip, and [JRF] is the joint reaction
force experience within the hip joint. The distance
A is measured from the center of rotation of the hip
(femoral head) to the insertion of the abductors on
the greater trochanter. The distance B is from the
center of gravity of the body to the center of rotation of the hip (femoral head). Given these measurements, we can solve for the joint reaction force.
In Fig.2.1, the weight [W] is 800N, the distance A is 16cm (0.16m), and the distance B is
8cm (0.08m). With the system at equilibrium,
the force of the hip abductors is equal and
opposite to the weight of the body. Therefore, the
torque experienced at the center of rotation of the
femoral head can be represented with the following equations, which allow us to solve for the
force [H] for the abductors.
[W] (0.16m)−[H] (0.08m)=0
[800N] (0.16m)=[H] (0.08m)
128Nm=[H] (0.08m)
[H]=1600N
35
Fig. 2.2 Biomechanics of the human elbow
brachialis to maintain exion will always be
greater than the system weight of the forearm and
hand. In Fig.2.2, the system weight of the forearm and hand is 40N, with the center of gravity
18cm (0.18m) from the elbow joint. The force
applied to ex the elbow [B] is directed from the
brachialis insertion which is 6cm (0.06m) from
the elbow joint. In a static system in equilibrium,
one can solve for the force applied by the brachialis and the joint reaction force (JRF) experienced at the elbow joint.
In addition, the joint reaction force can be calculated with the following equation:
[JRF]−[H]−[W]=0
[JRF]−[1600N]−[800N]=0
[JRF]=2400N
This means that in the example provided by
Fig. 2.1, the joint reaction force at rest is three
times the patient’s bodyweight.
In Fig.2.2, a free body diagram is applied to
represent the biomechanics of the elbow joint.
The elbow is a class 3 lever, and in class 3 levers,
the load and the force (effort) are on the same
side of the fulcrum, meaning that the force to
maintain 90 degrees of exion at the elbow is
between the fulcrum (the radiocapitellar/ulnohumeral joint) and the system weight of the forearm
and hand. In addition, the distance from the fulcrum to the effort is always shorter than the distance from the fulcrum to the center of gravity of
the forearm. As a result, the force, or effort of the
−([B] (0.06m))+[40N] (0.18m)=0
[B] (0.06)=7.2Nm
[B]=120N
When solving for the joint reaction force at
the elbow, and assuming a system at equilibrium,
the (1) force applied at the joint, the (2) force of
the brachialis in maintaining exion at the elbow,
and the (3) weight of the forearm and hand sum
to 0.
[JRF]+[B]−40N=0
[JFR]=40N−120N
[JFR]=−80N, this force vector is negative, indi-
cating that it acts in a direction opposite to the
weight of the forearm and hand, which is intu-
itive when looking at the free body diagram.
The topic of biomaterials begins with the fundamental qualities of bones, ligaments, and tendons that make them effective structures to
support the human skeleton and permit locomo-
36
D. Hampton and P. Burroughs
tion. The study of biomaterials within orthopedics also includes other materials, organic and
inorganic, that are used to create implants which
are commonly used in orthopedic applications. In
the same way that biomechanics was reviewed
within this chapter, one must rst dene terms
closely related to biomaterials to begin examination of this subject. Each of these materials will
be described using a consistent set of terms that
effectively describe the qualities of these materials as they apply to orthopedic applications.
Stress is the amount of force applied to an
object, divided by the area that the force is applied
over. Stress is measured in Newtons per square
meter. Strain, on the other hand, is a unitless measure of a distance a material deforms divided by
its original length. Young’s Modulus of elasticity
is a quantitative measure of a material’s stiffness
and ability to resist deformation when a tensile
force is applied to it [2, 3]. A material’s Young’s
modulus is represented by the initial slope of the
stress vs. strain curve (Fig.2.3).
The elasticity of an object refers to the material’s ability to return to its original dimensions,
length, width, and depth, after a compressive or
tensile force causes it to lengthen or shorten
(Table2.1; Fig.2.4).
The yield strength of an object is the force,
represented by the rst peak, relative to the Y-axis
of the stress vs. strain curve, where a material’s
properties change from elastic to plastic. After
that point, the material is irreversibly deformed
and will not return to its original dimensions. For
typical metals, the yield strength is reached when
a material has undergone a strain of 0.2% [3].
The ultimate strength, sometimes referred to
as the tensile strength, is the maximum tensile
force that a material can withstand before breaking, and is represented by the highest point on the
stress vs. strain curve. The nature of the distance
between the yield strength and the ultimate
Table 2.1 Elastic modulus of common orthopedic tissues and biomaterials
Material Elastic modulus (GPa)
Ceramic 300
Cobalt chrome 230
Stainless steel 200
Titanium 100
Cortical bone 20
Trabecular bone 10
Bone cement 3
Polyethylene 1
Cancellous bone 0.4
Tendon/ligament 0.3
Cartilage 0.02
Yield Strength
Stress
Young’s Modulus
0
Fig. 2.3 Stress vs. strain curve
Stress vs. Strain
Ultimate Strength
Strain
Relative Values of Young’s Modulus in Orthopaedics
Strain
Stress (Pa)
ome)
2 Biomechanics andBiomaterials
Fig. 2.4 Relative values
of Young’s Modulus.
The pneumonic
CAST-Bone is helpful to
remember the
decreasing relative value
between ceramic, cobalt
chrome (alloy), stainless
steel, titanium, and
cortical bone [4]
37
1
2
3
4
5
6
7
1. Ceramic
2. Alloy (cobalt chr
3. Stainless Steel
4. Titanium
5. Cortical Bone
6. Bone Cement
7. Polyethylene
8. Cancellous Bone
9. Te ndon/Ligament
Cartilage
10.
8
9
10
strength for two materials can be used to describe
an additional material quality, whether they are
brittle or ductile. Ductility refers to a material’s
ability to undergo plastic deformation without
failure [5].
When comparing two materials, because the
more ductile material can endure more deformation prior to breaking, and therefore tolerate a
greater strain, there is a longer distance between
yield strength and ultimate strength on the x-axis
of the stress vs. strain curve (Fig.2.3). Conversely
a brittle material will deform to a lesser extent,
and have a shorter distance between these two
points on the x-axis of the stress vs. strain curve.
In truly brittle materials, such as polymethylmethacrylate, the cement used in total joint
arthroplasty, the stress strain curve travels directly
to the ultimate strength, without a period of plastic deformation.
Aside from ultimate strength, materials often
undergo fatigue failure, which refers to fracture,
or failure of a material after cyclic loading of a
force that is less than the ultimate strength [1].
The toughness of a material is represented as
its ability to endure strain and is represented by
the area under the stress vs. strain curve. In some
materials, including organic materials such as
bones, tendons, and ligaments, the rate, or time
period over which the tensile force is applied,
affects the materials strain behavior in the
response to a given stress. Materials with variable stress vs. strain curves depending on the
rate of an applied force are referred to as viscoelastic [1, 3].
In addition to the importance of a materials
properties, its shape and construction are critical
to its performance under load. For example, the
bending rigidity of a long object with a rectangular cross-section is calculated differently than the
bending rigidity of a cylinder. With a rectangular
object, the rigidity is proportional to base of the
rectangle multiplied by the height cubed,
Rigidity=(base×height3)/12. In comparison, the
bending rigidity of a cylinder is proportional to
the radius raised to the fourth power [3]. This is
an important concept when considering the difference in rigidity of common orthopedic
implants such as intramedullary nails. The bending stiffness of a plate used in a plate and screw
construct is proportional to the third power of its
thickness.
Material can also be described as isotropic or
anisotropic. Isotropic materials behave in the
same way when a force, such as compression or
tension, is applied, independent of the direction of
the force or orientation relative to the material.
Metal alloys are generally understood to be isotropic materials. Bone and ligaments, on the other
38
D. Hampton and P. Burroughs
hand, are anisotropic materials. The mechanical
properties of anisotropic materials vary as force
vectors are applied in different directions through
the material [1]. Bones, ligaments, and tendons
are composed of collagen brils, and the mechanical properties of these tissues depend on the orientation of the collagen brils. Wood is another
common example of an anisotropic material, due
to the mechanical properties of the wood being
dependent on the direction of the applied force
relative to the grain of the wood.
Organic materials refer to the vast spectrum of
matter which is carbon-based and is either found
in the natural environment or human-engineered.
For this chapter, organic material primarily refers
to type one collagen in bone and gives bone its
exibility. Inorganic materials, on the other hand,
are not carbon-based, and are not the primary
molecules or byproducts of living systems. For
this chapter, in reference to biomaterials as it
applies to orthopedics and bone composition,
inorganic materials are the calcium and phosphate salts that lend bone its stiffness. Inorganic
material also refers to the polymer, ceramic, and
metal alloys that implants used in orthopedic surgery are composed of.
Polymers are synthetically created chemical
compounds composed of identical, repeating
units, or “mers”, that have a carbon backbone.
Polymers are covalently bonded to one another,
and the repeating units can be formed into long
structures, such as chains or sheets. Polyethylene
is a common polymer. Relative to other materials
used in orthopedics, such as metal alloys, polymers exhibit increased exibility and improved
resistance to corrosion, however, have decreased
strength.
Polymers are commonly used in total joint
arthroplasty, reducing friction and improving ear
characteristics between the tibial and femoral
components in a total knee arthroplasty, or as the
portion of the acetabular component in total hip
arthroplasty that articulates with the ceramic (or
cobalt chromium) femoral head. The polymer
chosen for these applications is UHMWPE, Ultra
High Molecular Weight Polyethylene, which has
been used for decades in total joint arthroplasty.
Sheets or rods of UHMWPE are formed into their
desired shape by extrusion or compression molding [6]. At body temperature, UHMWPE is
between its glass transition temperature and
melting point, allowing it to exist in partially
crystalline state and demonstrate desired mechanical properties of high wear resistance, strength
as well as resistance to fatigue [6]. UHMWPE is
sterilized by various means, including gamma
irradiation in air or inert gas, or in ethylene oxide
gas [3, 5]. Gamma irradiation is known to
increase cross-linking, which enhances resistance to wear while also increasing the brittleness
of the polymer and therefore the potential for
propagation of fatigue cracks [6].
Brittleness also occurs in UHMWPE due to
oxidation, which is a by-product of the irradiation
process and generation of free radicals. Currently,
there are processes to add Vitamin E to the polymer implants in addition to the cross- linking treatment to potentially mitigate the effects of
oxidation and reduce brittleness, but at this time,
long-term outcomes of Vitamin E are pending [6,
7]. The other mechanism to improve oxidation in
UHMWPE is melting the polymer after irradiation, which effectively reduces free radical levels,
but at the same time decreases the crystalline
structure of the polymer, which has a detrimental
effect on wear properties [7]. Therefore, in polymer processing, there exists a balance between
maintaining crystalline structure and removing
free radicals. To optimize outcomes in these two
domains, UHMWPE undergoes annealing to
below melt point, which removes free radicals
while having a less deleterious effect on crystalline structure than melting [5, 6].
Ceramic, the next material covered in this text,
has been used as an orthopedic implant, and more
specically as a bearing surface for femoral head
in total hip arthroplasty, since the 1970s. The
design of ceramic implants in total joint arthroplasty has improved in an iterative fashion, and the
latest generation of ceramic has several desirable
mechanical properties, including hardness, resistance to wear and scratches, wettability, inertness,
and biocompatibility [1]. Ceramics are extremely
hard but brittle materials; their main drawback as a
bearing surface in total joint arthroplasty is that
there is a higher risk of fracture when compared to
2 Biomechanics andBiomaterials
39
a metal implant. If fracture does occur, the comminution of the ceramic head and subsequent
retention of ceramic fragments increases wear,
osteolysis, and likelihood of reoperation [3, 5].
When used in combination with a polyethylene
liner, there is less wear in ceramic on polyethylene
implants when compared to metal on polyethylene
[5]. In addition to Alumina (Al2O3), zirconia
(ZrO2) is ceramic that was historically used as an
orthopedic implant [3]. Zirconia has increased
fracture resistance and higher strength when compared to alumina, however, poorer wear properties, roughening of the bearing surface, and
manufacturing issues resulted in zirconia being
passed over as an implant material [1, 5].
Metals are composed of individual elements
aligned in an organized, crystalline structure that
provides each particular metal with consistent
characteristics with respect to their ductility and
strength, as well as their ability to conduct electric current. Two elemental metals used in orthopedics include titanium and tantalum. Titanium is
resistant to corrosion due to its ability to readily
form a titanium oxide and can be used for xation of fractures that do not experience high
loads. Tantalum is chosen in orthopedic implants
for its ability to promote ingrowth of new bone,
supporting solid xation of the implant [1, 3].
In addition to the use of elemental metals, these
individual metals may be combined in the form of
alloys, allowing their properties to be blended to
achieve a desired effect. This is particularly relevant as it applies to surgical implants. This chapter
is not all-inclusive but will address several of the
more common alloys used within orthopedics.
Stainless steel is a common metal alloy, with
varying compositions of iron, carbon, nickel, and
chromium. Carbon adds strength to steel at the
expense of increased brittleness. Chromium is
added to stainless steel to form an oxide that resists
corrosion, and nickel increases both the alloys’
corrosion resistance, ductility, and its ability to be
welded or formed into useful structures [3]. In particular, an alloy of stainless steel—A316L, is used
in the eld of orthopedics [2, 3]. This alloy is chosen due to its relatively high chromium content
and subsequent resistance to corrosion [3].
Titanium is an element of important and common utilization within orthopedics due to its
strength, relatively lower density, and resistance
to corrosion. Although pure titanium nds limited use within orthopedics [1], titanium alloys
that combine varying amounts of aluminum and
vanadium have applications in multiple implants,
such as intramedullary nails for fractures involving the tibia or femur. In this application, the aluminum and vanadium lend the titanium alloy
increased strength and ductility [1, 3].
Alloys of cobalt and chromium are valued for
their high strength and longevity. They are common in implants used in total joint arthroplasty.
In these use cases, the cobalt chromium alloy is
expected to repeatedly resist high loads for several decades. Cobalt alloys may include small
amounts of carbon and molybdenum to improve
ductility and strength [1].
Corrosion refers to the chemical degradation or
dissolving of a material [8]. There are various
types of corrosion, and within orthopedics, there
are specic instances where these types of corrosion are most prevalent. Corrosion is signicant in
orthopedics primarily for two reasons. First, there
is subsequent weakening of the implant, increasing the risk of failure. In addition, corrosion
releases metal ions into both the local environment
and systemically within the human body. These
ions promote in inammatory changes that damage tissue and can weaken the interface between
the implant and the bone itself [1, 3].
To reduce the incidence of corrosion among
metal implants, many have an intentional thin
oxide coating that is resistant to further chemical
change within the body. However, in instances of
pitting corrosion, that oxide has been worn away,
and is generally seen in orthopedic implants
made of stainless steel, whereas titanium alloy is
not prone to pitting corrosion [1, 3, 8].
Galvanic corrosion refers to the degradation
that occurs between two different metals when
there is an electric potential that exists between
them, with one metal acting as an anode and the
other a cathode. Galvanic corrosion requires the
metals to exist in an electrolyte solution, which
describes most environments within the human
body. In galvanic corrosion, as electrons ow