5.1 Joints of the Body 5.1 Joints of the Body

Transcription

5.1 Joints of the Body
• Synovial joints
– Diarthrodial joint (large motion)
– Stabilized and constrained by a fibrous
joint capsule, which may contain
connective ligaments.
– The
Th articulating
ti l ti
surfaces
f
off th
the b
bones are covered
d with
ith a thi
thin llayer off articular
ti l
cartilage
– Contains highly viscous synovial fluid, which is secreted by a thin layer of
synovial cells (synovium)
– Extremely low friction coefficient (μs <0.01, μk<0.003): Table 1.1
• Classification of synovial joints
1. Ball and socket joint: shoulder, hipÆ ball(head of the femur) and the socket
(acetabulum), kinematic constraints
2 Bicondylar joint: two curved condyles articulate against relatively flat surfaces
2.
on the mating bone, unstable, ex) knee
3. Multiple bone joint: multiple bones are closely fitted and bound together by
ligaments Combined motion makes large movement
ligaments.
movement, ex) wrist
wrist, ankle
94
• Synovial joints
1. Ball and socket joints
Hip Joint
Conforming contact b/w the femoral
head and acetabulum
(substantial
( b t ti l kinematic
ki
ti constraint)
t i t)
Shoulder Joint
Non-conforming contact b/w the
humeral head and the socket of glenoid
(little kinematic constraint Åaffected by
muscle and soft tissues
95
• Synovial joints
1. Ball and socket joints
- shoulder
Bones & articular
g
cartilage
+ ligaments
+ capsules
(synovial joint)
+ muscles
96
• Synovial joints
2.
-
Bicondylar joints
Two pairs of articulating surfaces
Relatively flat articulating surfaces
Little kinematics constraints
Greater range of motion in one plane
Unequal loads, may resist moment with little
activity of muscles or tissues
- Ex) knee joint
97
BIOME LAB
• Synovial joints
3. Multiple bone joints:
- Between ball socket and bicondylar joints
- Fitted and bound together by ligaments
- Combined motion makes large
g movement
Ex) Wrist: 8 carpal bones
Ankle: 7 tarsal bones
wrist
98
5.2 Joint Stability
Joint stability
the ability of a joint to maintain an appropriate functional position
throughout its range of motion.
The characteristics of stable synovial joints
1.
2.
3
3.
Joint contact occurs between surfaces, both of
which are covered with articular cartilage
There exists a unique position of equilibrium for any
loading.
Small changes in either the magnitude or direction of
the functional load do not lead to large changes in
the position of joint contact.
99
Ankle Sprain
발목 염좌 비디오
5.2 Joint Stability
Main mechanism for joint stability
1) Contact at the articular surfaces
• Primary mechanism of stability
• Passive mechanism
• Varying extent in all articulating diarthrodial joints
– Example
» Hip joint is very stable due to the geometric constraints
» Shoulder joint is lesser stable
» Contact forces are perpendicular to articulating surface
100
5.2 Joint Stability
•
Ex. Contact points and joint loading
•
•
•
Known: magnitude and orientation of GRF, orientation of muscle and contact force
Unknown: magnitude of muscle and contact force
FM 2 > FM 1
Answer>>
–
JJoint
i t contact
t t iis much
h greater
t when
h it acts
t att
point 2 than at point 1 as is the muscle
force
Thi strong
This
t
association
i ti
b
between
t
th
the llocation
ti
of the joint contact force and the magnitude
of the muscle force
This is a key attribute that needs to be
incorporated into design of artificial joints;
an artificial joint should be designed in a
way that
th t a llarge contact
t t fforce should
h ld nott
occur to prevent wear
Neuromuscular system has been tuned for
equilibrium
ilib i
Æ Disruption
Di
ti
h
harm th
the jjoint
i t
–
–
–
J 2 > J1
5.2 Joint Stability
2) Muscle action
• Voluntarily induced by active muscle contraction
• Equilibrium can be achieved and the point of contact controlled by
control of the muscle forces
3) Co-contraction of agonist and antagonist
• Co-contraction increases stability by increased compression
• Particularly important for frontal plane
• No protection to acute traumatic situation due to response delay (~20ms)
4) Ligament
•
•
•
•
Passive
Resistant to abnormal range of motion. Soft at normal range.
Important balance b/w laxity and allowable joint motion
Imbalance by trauma
trauma, disorder
disorder, replacement affect stability of joint
102
5.2 Joint Stability
• Stability is extremely important for the design of total joint replacement
- Integrity of arthroplasticity
- Correct alignment necessary
• Medially directed force (or varus /adduction
moment) in the knee joint
- Equilibrium by two supporting elements
- contact at curved medial tibial plateau
generate lateral force
- Stretch of lateral collateral ligament (fibula
collateral
ll t l liligament)
t)
• Medial contact force is higher than lateral
contact force during gait – more disease
on medial side
103
5.3 Joint Replacement
BIOME LAB
Joint Replacement
–
–
A surgical procedure where articulating surfaces are replaced to restore
function to joints that have become damaged and painful
Most common reason for joint replacement is Arthritis (osteoarthritis and
rheumatoid arthritis
Osteoarthritis
–
–
–
–
–
Affects
Aff
t iisolated
l t d jjoints
i t such
h as hip,
hi kknee, or elbow
lb
Causes
• Normal wear and tear during lifetime
• Secondary to joint trauma (sports injuries)
Destruction of the articular cartilage
The damage is restricted to the articular surface
• The bone tissue is generally normal
• Joint is provided good support for an implant.
g is still irreparable,
p
, but considerable research ongoing
g g
Articular cartilage
104
Current Techniques –Articular cartilage repair
Mosaicplasty
p
y
Autologous
g
Chondrocyte
y Implantation
p
Microfracture
Rheumatoid arthritis
–
–
–
–
Systemic disease that often affects multiple joints
Damages the articulating surfaces, bone tissue and soft tissue
Deformities and loss of function
Some types of fixation may be difficult to achieve in the rheumatoid patient,
and the prosthesis may have to provide functional constraints that are
unnecessary for patient with osteoarthritis
osteoarthritis.
105
•
품목번호
1
3010.01 0
B3010.01~02
인공발목관절
2
B3020.01~02
인공팔꿈치관절
3
B3030.01~02
인공손가락관절
4
B3040.01~02
인공엉덩이관절
5
B3050.01~02
인공무릎관절
6
B3060.01~02
인공어깨관절
7
B3070.01~02
B3070 01 02
인공발가락관절
품목명
8
B3080.01~02
인공손목관절
9
B3160.05~06
인공추간판
인공어깨관절
Shoulder
인공팔꿈치관절
Elbow
인공추간판
Disc
인공손목관절
Wrist
Usually, both surfaces of
a joint pair are replaced
Exception:
hemiarthroplasty of the
hip ~ natural socket in
th pelvis
l i iis retained
t i d
the
Hip
인공손가락관절
Finger
인공무릎관절
Knee
인공발목관절
Ankle
인공발가락관절
Toe
Artificial Joints (world market)
(Unit: Million Dollar)
Item
2001
2005
CAGR(%)
K
Knee
2 219
2,219
4 100
4,100
17 2
17.2
Hip
2,437
4,600
16.7
Sum
4,656
8,700
16.9
Artificial Joints (world market overview)
20,000
Million Do
ollar
•
No.
BIOME LAB
World:
16%
% increase p
per
year
Korea:
12% increase
15,000
10,000
5,000
0
2005년
2006년
2007년
2008년
2009년
2010년
인공무릎관절
4100
4551
5052
5607
6224
6909
인공무릎관절
4,600
5336
6190
7180
8329
9662
2002년 Biomet, Merrill Lynch, “Global Industry Analysts”
2005년 Stryker, “Market Overview”
2011년 KISTI “Artificial Joints World Market Overview”
Biomechanical Test Criteria for Artificial Joints
Design
Failure,
Fatigue
failure
(내구성)
Osteolysis
(골용해,
골흡수)
Coating
(표면처리)
Range of
motion
(운동범위)
Design
Successful surgery:
1.Artificial joint
2.Clinician
3.Patient
DesignBearing
Surface
Wear
(마모)
Successful total replacement designs involve both functional and
structural considerations.
–
–
–
–
–
–
Normall range off motion
N
ti
The strength of the prosthetic components
Damage to the articulating surfaces
The strength of the interfaces b/w the implant and the bone
The potential adaptation of the bone due to altered loading
Implanted components must be biocompatible
109
Function
• The function of joint involves force transmission coupled with kinematics
•
The motion of a natural joint is determined by the geometry of the joint
surfaces and others
–
–
The ligament
ligament, joint capsule
capsule, soft tissue structure muscle force
Prosthesis substitutes the function that was previously supported by ACL,
articular cartilage, and/or PCL
•
To design the articulating surfaces, joint loads must be known
•
Difficult to get precise values of joint contact forces because the system
is indeterminate.
•
The kinematics and the joint forces are coupled
coupled.
–
An interesting case in point is the knee, which is constrained by soft tissue
structures and substantial laxity.
110
•
The kinematics created during total joint replacement surgery can
influence the joint loads.
–
–
–
The location of the femoral head w
w.r.t
r t the femur
The location of the acetabulum w.r.t the pelvis
The relative motion between the femur and pelvis can be altered, both the
magnitude and direction of hip joint force may be affected
affected.
•
The distance between the origin and insertion of a muscle may be
lengthened or shortened when the geometry of the system is changed
changed.
•
This cause changes in muscle force capabilities because of the forcelength relationship.
111
Structure
• Total joint replacement fall into two classes
1) Replace only the articulating surface
2) Replace the surface and a substructure of a bone
1) Surface replacement
–
•
Acetabular cup(hip), glenoid(shoulder), femoral and tibial components
The goal is to transfer the joint loads to the underlying cancellous bone similar to
the way it is done in the normal joint
112
Structure
2) Surface and substructure of bone replacement
–
–
–
Femoral components for artificial hips
Humeral components for shoulder replacement
Loose hinges for elbow prostheses
Femoral
components
•
Humeral
components
Elbow
components
Components
off artificial
are fi
fixed
bone with
(bone
C
ifi i l jjoints
i
d to b
i h either
i h PMMA (b
cement) or cementless technique.
113
•
Thin layer of PMMA can be used to precoat portion of the fixation stem
–
–
–
•
Only a portion of the stem is bonded to the cement
The cement and precoat stem creates strong interface
The transition between the bonded and unbonded interface regions may be a
region of stress concentration
Without precoating, the cement fills the space between the implant and the
bone
–
–
–
The strengths
Th
t
th off th
the iinterface
t f
d
depends
d upon mechanical
h i l iinterlock
t l k
At the cement-bone interface, the PMMA interlocks with the roughness of the
bone
At the cement
cement-implant
implant interface,
interface grooves and undercuts or roughened surface
are often employed
114
•
The disadvantage of PMMA is low tensile fatigue strength.
–
–
–
•
The bone-implant system must be designed so that stresses in
the cement and the interfaces are within safe limits
Cement is the weak link in the bone-cement-implant system
New method: Cementless fixation
• Press-fit prosthesis
• Biologic fixation
Biologic fixation
–
–
–
Coat portions of the prosthesis with a porous layer
Layers of metal beads or mesh sintered
Ingrowth of the bone into the porous layer provides
l
long-term
t
fi
fixation
ti
–
Trabecular Metal (Zimmer)
115
The Composite structure: Load transfer
–
–
–
–
The new composite: consist of cortical bone, cancellous bone, implant,
cement etc
cement,
Important to understand load transfer b/w components
Sufficient strength & Biocompatibility
Fatigue problem for polymers
B
Bone
adaptation
d t ti
–
–
Bone remodels with environment loading changes
Implants share the load and the bone remodels. The bone remodels to meet the
requirements of new loading condition
E.g.) parallel load sharing : vicious cycle
Implant carries loadingÆ stiffness of bone decreases Æ relative stiffness
of implant increases Æ load carried by bone further decreases Æfurther
decrease in bone stiffness
116
Articulating Surfaces
–
–
–
–
–
Surface damage due to contact force is
of concern
The small particles, debris accumulates,
the biological response can lead to
i f ti
infection
and
d lloosening
i
The mechanism still unclear
Reduce stress, better material to resist
wear Æ design problem
Experiments- different designs
Fig S
Fi
Surface
f
d
damage off a polyethylene
l th l
tibial component
Modular Components
–
Modularity provides flexibility in surgery
Custom Design
–
Custom designed implant-correct congenital problems
problems, replace failed
prosthesis, or treat patient with bone tumors
117
Design of bone-implant system
z
g Performance ((in p
Factors Influencing
particular,, structural p
performance))
1) Implant, 2) Patient, 3) Surgical factors
–
Surgical instrumentation to aid surgeon to get consistent position and
orientation and good fixation Æ Engineer’s role
–
Variables associated with
p
((material,,
the implant
geometry, manufacturing,
etc)
Variations by experience of
surgeon
Variation in patients
–
–
Æ Robust-insensitive design to
variations
Figure: Factors influencing the design of a bone118
implant system
Evaluation of Implant Performance
–
Long-term follow-up studies, using statistical analysis
–
Survival analysis: probability of survival in some time post surgery. Little
information of design features on performance
–
Retrieval analysis: examination of device during revision surgery or autopsy.
Best source for evaluating in vivo performance, Failure mode identified.
–
Preclinical evaluation before new design usage: in vitro experiments,
simulation studies,, in vivo animal studies,, and in vivo human studies
–
New designs are tired by a few skilled surgeons and multicenter trials
before general usages. FDA procedures
119
5.4 Structural Analysis: Bone-implant
BIOME LAB
Beam theory
–
–
–
To determine the stresses in the structure as a function of geometry, loads,
and material properties of the structure
One-material beam or composite beam
Plane sections before loading remain plane after loading:
axial loading Æ strain is constant over the cross section
bending Æ strain varies linearly from the neutral axis
Fig. 5.2 Proximal femur with prosthesis Æ
composite beam: fixation stem, cement
(interface layer), bone
Fig. 5.1 Long bone with a fracture
fixation plate Æ composite beam
5.4.1 Symmetric Beams
1) One-Material Beams: Axial Loading
Fig. 5.4 Equilibrium
P =
Fig. 5.3 Deformation of axially
loaded beam
ε x = lim
PQ → 0
P *Q * − P Q
PQ
σ x = Eε x
S in ce ε x is co n stan t (C ), σ x = C E
∫
σx =
A
σ xdA =
P
A
∫
A
C E dA = C EA
: ax ial stress
CE =
P
A
ΔL
P
E =
L
A
PL
ΔL =
AE
121
2) One-Material Beams: Bending
Fig. 5.6 Equilibrium
Fig. 5.3 Deformation of a beam in
pure bending
M
( ρ − t)dθ − ρ dθ
t
= −
ρ dθ
ρ
ρ : rad iu s o f cu rvatu re o f th e n eu tral ax is
z
= − ∫ σ x td A =
A
E
ρ
∫
A
t 2 dA
Mt
I
A t n etu ral ax is, fo r p u re b an d in g
ε x = lim
σx = −
σ x = Eε x
P = − ∫ σ xdA = 0
PQ → 0
A
122
3) Three-Material Beams: Axial loading
3 component: boneB , cement (interface layer)C, fixation stemP
εx = C
εx = −
co n stan t (ax ial lo ad in g )
t
ρ
(b en d in g )
σ i = E iε i
From Equilibrium condition
P =
∫
AB
σ xdA +
∫
AC
σ xdA +
∫
AP
σ xdA
P = C ( E B A B + E C AC + E P A P )
σj =
Figure Three-material beam consisting of
bone, cement, and prosthesis
E jP
E B A B + E C AC + E P A P
F o r n -d im en sio n al b eam ,
σj =
Ej
P
n
∑E
i =1
i
Ai
W h ere, P to tal lo ad actin g o n th e cro ss sectio n
123
4) Three-Material Beams: Bending
3 component: boneB , cement (interface layer)C, fixation stemP
M
= − ∫ σ x td A −
z
AB
∫
AC
σ x td A −
∫
AP
σ x td A
T h e resu ltan t fo rce at cro ss sectio n is zero fo r p u re b an d in g
P =
∫
M
=
z
σj =
AB
σ xdA +
1
ρ
∫
AC
σ xdA +
∫
AP
σ xdA = 0
(E B I B + EC IC + E P I P )
− E jMt
E B I B + EC IC + E P I P
The location of the neutral axis for a three-material beam
yˆ =
y B E B A B + y C E C AC + y P E P A P
E B A B + E C AC + E P A P
y is th e lo catio n o f th e cen tro id al ax is o f th e i th m aterial
124
EX. Two-material beam
Consider a two-material beam consisting of bone1 and stainless steel2 subject to axial
loading P = 1 0 0 0 N
E1 = 1 7 G P a
E2 = 200G Pa
A 1 =560m m 2
A 2 =50m m 2
What is the portion of the load carried by component 1 (bone)?
Sol>
17
σ1 =
S tress in co m p o n en t 1,
1 0 0 0 = 0 .8 7 M P a
1 7 (5 6 0 ) + 2 0 0 (5 0 )
N o te th at th e stress in co m p o n en t 1 can b e w ritten as
P
σ1 = 1
A1
F ro m w h ich w e see th at
P1
E P
,
= 2 1
A1
∑ E i Ai
F ro m σ
j
=
Ej
∑E
i =1
i =1
an d th en o b tain
P1
=
P
E 1 A1
2
∑E
i =1
i
Ai
=
P
n
1 7 (5 6 0 )
= 0 .4 9
1 .9 5 2 × 1 0 4
125
i
Ai
5.4.2 Case Studies: Compression analysis of the vertebral body – load
sharing between cortical and trabecular bone
•
Engineered structures: uniform materials, mathematically well defined shapes
(e.g. planes of symmetry)
•
Bones are not engineered structures
– Irregular shape Å complex adaptation & genetic programming
– Heterogeneity across the population
– How to generalize results from a single bone to entire population?
•
Modeling issue
– Balancing the need for fidelity with the available resources
•
Osteoporosis
– Geometric and material
properties
– Radiographic diagnosis,
preventive action or
treatment
Fig 5.12 T-10 vertebral body
Left) 54 yr-old, Right) 82 yr-old
126
5.4.2 Case Studies: Compression analysis of the vertebral body – load
sharing between cortical and trabecular bone
Spine fracture
The most prevalent in osteoporotic fractures, 700,000 annually in the U.S.
By a permanent deformity (not by a complete separation of bony fragments)
•
The etiology of spine fractures
–
Wedge fractures
• isolated overload
• cumulative damage
• longer-term creep or fatigue
• the relative role of the cortical shell versus trabecular centrum
i lload
in
d sharing
h i
Æ which
hi h b
bony compartment
t
t needs
d tto b
be monitored
it
dd
during
i
screening for osteoporosis or targeted for drug treatment
127
Fig 5.13 Basis for classification
Wedge fracture is the most common in the
elderly. Biconcave and crush fractures are
more common in younger individuals (due to
high-speed
high
speed impact)
Fig
g 5.14 Vertebral body,
y 60μm
μ resolution.
The shell is a fused set of trabeculae than a
continuous thick cortex
128
How much load does the cortical shell carry?
ÆSimple model :Two spring in parallel
(cortical bone + trabecular bone)
- Cross section: circular
- From direct measurement (Thickness
of cortical bone: 1/3~1/2 mm)
- Due to dimension, material properties
are uncertain (different from uniaxial test
using large and uniform specimen)
–
Fig
g 5.15 Circular cylindrical
y
model of the
vertebral body.
Uniform deformation of δ
Uniaxial compressive behavior of the vertebral body
•
•
•
•
Uniform displacement δ
The strain ε=δ/H
Compressive force P = Pc+Pt
A
Assuming
i
lilinear elastic
l ti b
behavior
h i
129
Pc
E c Ac
=
P
E c A c + E t At
w h ere
E i : elastic m o d u lu s
Ai : cro ss-sectio n al area
•
The role of the trabecular density (ρ) ,
cortical shell thickness (t), cortical shell
elastic modulus (Ec)
A ssu m e th at E t = m ρ
Pc
1
=
=
E t At
P
1+
1+
E c Ac
Ec
1
mρ
At =
{(1 + 2 t / D ) − 1}
2
T yp ically , t ≈ 0 .3 5 m m , D ≈ 3 0 m m
Pc
=
P
1+
Fig
y
model of the
vertebral body.
Ac =
π
4
π
4
D2
( D + 2t )2 −
π
4
D2
1
mρ
0 .0 5 E c
130
Pc
=
P
1+
1
mρ
0 .0
05 Ec
ca se 1 . E c ≈ 1 7 0 0 0 M P a , m ρ ≈ 3 0 0 M P a
m ρ / E c = 0 .0 1 8
(E t /E c ≈ 0 .0 1 8)
Pc / P ≈ 7 4 p ercen t
ca se 2 . m ρ / E c = 0 .55 (E t /E c ≈ 0 .5)
5)
Pc / P ≈ 9 p ercen t
Fig
y
model of the
vertebral body.
The relative stiffness of the cortical to
trabecular bone strongly affect the mechanical
behavior of the vertebral body.
131
5.5 Total Hip Replacement
BIOME LAB
Goal of joint replacement
1) Relieve pain, 2) Improve function, 3) Longevity
Hip: a ball and socket joint
- Provide 3 rotation (flexion-extension, abduction-adduction, internal-external
rotations of the femur w
w.r.t
r t the pelvis) and transmitting loads
- Component fixed by bone cement or bone ingrowth/ongrowth to the
prosthesis via porous or coating
- Common structural failure
①
②
③
fracture of the fixation stem in the central zone
interface failure in the proximal/distal zones
bone resorption due to stress-shielding
•
Functional and structural issues of THR
-
How changing the centers of the articulating surfaces w.r.t the pelvis and the
femur affects the forces transmitted across the joint
Approach to study THR can be applied to other joints
-
132
Kinematics and Loads
–
–
–
–
–
THR alters kinematics of the proximal femur to the pelvis.
Th goall iis to position
The
ii
the
h components to retain
i the
h normall
kinematic and structural relationship. Muscle to operate at the
same point on its length-tension curve and maintain normal
muscle
l excursion
i
Reconstruction of normal anatomical geometry
Neck of femoral component may impinge
on the rim of the acetabular component
For patient with abnormal joint due to
disease or damage, new-positioning is
necessary
134
•
The stiffest path carries the greatest load. Dense
cancellous bone in the proximal femur
–
–
The center
Th
t region:
i : apparentt d
density
it 2 ti
times,
elastic modulus 4 times
A stiff inner core and outer layer
Δ C =Δ O =
PC L
P L
= O
E C AC EO AO
the length and areas are the same,
Fig 9.2 A dense region of
cancellous bone transfer load from
the surface of the head to the
dense cortical bone at the calcar
PC E C
=4
=
PO EO
The stiff core carries 80% of the total load.
- The line of action of joint contact force lies
along the region of dense cancellous bone
Fig 9.3 Cross-section of a 135
hollow
cylinder with a stiff core
5.6 Total Knee Replacement
•
•
•
•
•
•
BIOME LAB
In the knee, role of soft tissues is very important, it is less-conforming joint and
constrained by the soft tissues
3 component: femur
3-component:
femur, tibia
tibia, and patella
The motion of patella is guided by the trochlear groove and the action of the
quadriceps
M i l ACL and
Mainly
d PCL
CL provide
id anterior-posterior
i
i constraint.
i
MCL and
d LCL provide
id
varus-valgus constrains.
ACL may provide tibial internal rotation and valgus rotation constraints
Damage to the soft tissue may
cause secondary injuries to
other joint structure and alter
joint kinematics
136
5.6.1 Knee Function
1) kinematics
•
•
•
•
•
•
6 DOF motion, primary motion is flexion-extension
Hyperextension ~ 5°, Full flexion ~ 160°
Tibial axial rotation is least in full extension, greater in flexion
Valgus-varus
Valgus
varus rotation may produce lift-off.
lift off. Valgus-Varus
Valgus Varus rotation is constrained
by collateral ligament and muscle contraction
Anterior-posterior translation is constrained by cruciate ligament and the
geometry
g
y of condyles,
y ,p
plateaus,, and menisci
Injury results in loss of ligament function
- ACL rupture: anterior tibial translation increases
- PCL rupture: posterior tibial translation increases, which reduce the quad
moment arm and decrease the range of motion of the knee
137
5.6.1 Knee Function
2) Loads and Moments
•
•
•
•
Femoral-tibial contact force ~ several times of body weight
during functional activities
Muscle forces, cruciate and collateral ligaments, menisci provide
stability
Sliding and rolling-tibiofemoral motion
Patella-femoral contact
138
5.6.2 Knee Structure
•
Load transfer from the joint surfaces to the cortical bone in the proximal tibia has
been studied the most, because tibial loosening of prosthetic components has
been a big problem
•
Figure 10.6 The cancellous bone texture is oriented in the proximal-distal
direction. The density seems to decrease from proximal to distal
•
Proximal region: load is mostly carried by cancellous bone
•
From proximal to distal, the cancellous bone becomes less dense and the outer
shell becomes thicker(stiffer). As the cancellous bone disappear, outer shell
(di
(diaphysis)
h i ) carries
i th
the entire
ti lload.
d
CrossFig 10.6 Cross
sectional view of the
proximal tibia. Regions
of dense cancellous
bone transfer the
condylar forces distally
to the cortical shell
139
5.6.3 Knee Replacement
•
•
•
•
•
•
TKR is more complex than THR, because soft tissues provide most of kinematic
constraints.
Prosthetic component provides substitute function for the ligaments that are lost
in surgery
Reconstruction of normal range of motion and geometry
Replace only surface (distal femur and proximal tibia)
Canlellous bone in the proximal tibia and surface damage in polyethylene
component
Hi h contact stress and
High
dd
debris
b i iincrease risk
i k off llate lloosening
i
140
1) Types of Replacements
•
Unicondylar replacement (lateral or medial replacement only)
•
Bycondylar replacement
1. Unconstrained type (mobile-bearing)
2. Semi-constrained type
- PCL substituting design (more consistent performance)
- PCL retaining design (mostly
(
flatter tibial surface, less conforming
contact, better kinematics)
3. Constrained type
- Single hinge axis Æ not common: the axis of rotation moves during
g increases
flexion. Fixed axis takes all load. Loosening
141
2) Function
•
•
Joint motion controlled by joint load, articulating surfaces, soft tissues
Symmetric or unsymmetric design
•
1.
Two types of contacts
The surfaces of the femoral and tibial
components are both toroidal
The lateral-medial geometry of the femoral and
tibial articulating surfaces is highly conforming
2.
•
-
sym
unsym
Lift-off
Varus or valgus moment
S iff articulating
Stiff
i l i
surface
f
Flat surface ~ extreme edge loading, increase risk of failure and wear on
polyethylene
142
3) Structure
•
•
•
•
-
Compressive force on femoral component along the femoral axis
Force on the tibia moves w.r.t. the tibial plateau during flexion-extension. More
loosening in tibial component.
j structural p
problems
Three major
1. component failure (design problem, stress concentration)
2. fixation failure (porous coating degrades fatigue strength)
3 surface damage
3.
Most common failure: crushing the cancellous bone under tray
Stiffest and strongest subcondral cancellous bone is removed
R
Remaining
i i
cancellous
ll
b
bone must carry lload.
d E
Endosteal
d
lb
bone shell
h ll
in this region is thin.
The amount of bone resected should be minimized
143
3) Structure (continued)
•
Tibial tray with a central large peg or smaller fixation
peg beneath the medial and lateral plateau
•
yp
y y
y y
y
polyethylene
or p
polyethylene
with metal tray
Entirely
•
Fixation on the tibia
1 PMMA – bone cement
1.
2. Porous coating-cementless
144
BIOME LAB
4) Long-Term Clinical Evaluation
•
•
Survival analysis
– success rate
Retrieval analysis
g y
– revision surgery
- relationship between design and failure
- failure mode analysis with particular reasons
Total knee design issues (1~3:structural)
1.
2.
3.
4.
5.
Kinematics
Contact stress-wear
Fixation
Ligament tension
Patient Function
145

5.1 Joints of the Body 5.1 Joints of the Body

Transcription

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