3. Knee: A weight bearing joint
• Key Features
4 Ligaments alongwith meniscus
and muscle tissue provide stability
to knee
Condyles: Articulation with tibia
for flexion and extension
movements.
Patella: Articulation with distal
femur
Meniscus: Limits Medial Lateral
motion
4. Conditions leading to TKA
• Osteoarthritis
• Rheumatoid Arthritis
• Psoratic Arthritis
• Trauma
6. Design Input
• Polycurvature
• Stability: Hinge or Posterior Stabilized Knee
• Tibial Resection angle
• Key Measurements: AP, ML , Peg Height etc.
• Mechanical Axis Alignment
• Should conform to the distal femur with appropriate shape.
• Design Envelope(Knee)
7. Range of Motion
• Not a simple hinge
• Moves in all the three planes
• All modern total knee prostheses imitate at least
some of these complicated movements of the
natural knee joint
• ROM in healthy joint: 1350
of flexion
• Daily activities: 950
of flexion
8. Buechel-Pappas Primary Knee
Component
• Tricompartmental femoral component
• Anatomic Valgus angle
• Excellent bearing congruity with tibial component
• Accommodates Varus-Valgus & rotary motions.
• Increased contact congruity and range of contact congruity to
reduce stress and therefore wear.
• Loss of congruent contact takes place at 54 degrees of flexion when
the force on joint is less than body weight.
• Material Used: Titanium Alloy coated with TiN
10. Start an Assembly File
• Bring in the design envelope and place it in default
• Create a Part file for implant
Insert Co-ordinate Axis
and Datums for future
reference.
11. Create Datums and Co-ordinate Axis for Part
• Sketch the Design Box • Extrude the box
12. Box Cut(Chamfer Cuts)
• Anterior chamfer smaller than
posterior chamfer.
• Extrusion and removing
material.
Parallel to tibia so that
maximum force is
transferred to tibia
16. SWEEP CUT
Select Trajectory
Sketch Section
Or Save the Section(Sketch) file of
design Envelope and Bring it into
section sketch.
Select the Material to
be removed.
Patellar Flange
26. Pro- Mechanica: Finite Element
Analysis
• Divides the structure into n number of elements.
• Solves equation at nodes of these elements.
• If number of elements are high, more time to
solve the equations but results are accurate.
27. Main Consideration : Units
1 inch=0.0254 metre
1 lbf=4.448 Newton
Basically,
Lbf= Weight in Pounds*Acceleration
due to gravity
=0.4535kg*9.8m/s2
=4.448 Newton
=32.17 lbmft/s2
28. Worst-Case Scenario
Impact force
Force on one condyle
4 to 5 times Average weight
(190 lbf)
Therefore, force of 760 lbf is
applied to the medial condyle
since most of the weight passes
through it.
32. Materials Table
Ti6Al4V CoCr Mo
Density 4.43 g/cm3
0.31g/cm3
Poisson’s Ratio 0.342 0.31
Coeff of thermal
expansion
5.11*10-6 /
F 10*10-6
/K
Young’s modulus 165 ksi 30 msi
Tensile Strength, Yield 128 ksi 130 ksi
Tensile Strength, US 138 ksi 180 ksi
Fatigue Strength 34800psi 1E+7 -----
33. Preliminary Results(Before Filleting)
Von-Mises Stress high at sharp edges of
the magnitude of 180 ksi
Filleting reduces the magnitude of
stress by 40-50 ksi but still stress is
greater than Yield strength.
35. Results Table
ACTION Yield
Strength
Analysis
Result
Location Remarks
NO Filleting 128 ksi 180 ksi Sharp Edges High Stress
Concentratio
n
Filleting(o.12
5 in)
128 ksi 160 ksi Rounded
Edges
Stress
Decreased
Chamfer
Modification
128 ksi 125 ksi Edges Stress
Decreased
further
36. Fatigue Testing
• Cyclic loading and unloading
of the knee joint develops
stress which after
accumulating over a period of
time can lead to Fatigue
failure.
• From the graph, we can see
that Joint Force Weight is 3
times the body weight.
• In the FEA analysis for
Fatigue, we apply 450 lbf of
weight over one condyle.
http://www.endotec.com/pdf/B-P%20Femoral%20White%20paper.pdf
37. ….
• ASTM defines fatigue life as the number of stress cycles of a
specified character that a specimen sustains before failure of a
specified nature occurs.
• The maximum stress developed is less than Ultimate Tensile Stress
Values and may be also less than Yield Stress.
38. Stress developed in the non-filleting component is less than UTS of Ti6Al4V and
stress concentration at corners reduces even more with filleting.
The mechanical axis of the knee is a line extending from the center of the hip joint to the middle of the ankle joint. This line is practically perpendicular to the ground. Only when the mechanical axis passes through the center of the knee joint, the stresses on the knee joint surfaces are uniform in all areas of the joint and well balanced.
http://www.totaljoints.info/total_knee_operation.htm
