4. Aim
• Determine the relationship between
malalignment and the resulting knee joint
contact mechanics
• Stress cannot be measured in vivo and the use
of computational methods can yield useful
information about joint mechanics.
7. Constraint Definitions
Type Constraint
Perfect bonding Cartilage - Bones
Perfect bonding Ligaments - Bones
Perfect bonding Meniscal Horns – Tibia
Cartilage-cartilage and cartilage-
meniscus interfaces were modelled as
frictionless sliding
8. Boundary Conditions
Model validation, using 6 DOF robot:
1. Fix the proximal femur
2. Apply axial load at the distal tibia
3. Set constraints:
•Flx/ext constrained in sagittal plane
• Other degrees of freedom free
4. Apply different bending moments
9. Cadaveric study to validate
finite element models
Cadaveric specimen
in 6 degree of
freedom robot
10. Validation
Axial force: 374 N
Bending moment:
15 Nm varus
Axial force: 374 N
Bending moment:
0 Nm
Axial force: 374 N
Bending moment:
15 Nm valgus
A A A
PP P
MM
M
LL L
A A A
PP P
MM M
L
L L
a) In Vitro Results
b) Finite Element Results
Pressure (MPa)
16. Summary
• Minimum peak contact pressure occurred at an MAD of
-1.44 mm and a corresponding HKA angle of 0.5° valgus
• Minimum peak shear stress occurred at an MAD of 3.6 mm
and a corresponding HKA angle of 2.5° valgus
17. Discussion
• A computational model has been developed to
predict stress and force as a function of
malalignment prior to surgery
• Understanding the link between malalignment
and knee joint stress will help improve surgical
outcomes
19. • This research was gratefully funded by the Chelmsford
Medical Education and Research Trust
Acknowledgements
Notas do Editor
Thank you for introduction
Audience knows about OA and HTO
Current targets are…..
However…..
Therefore, the aim of this study is to…..
To create a 3D model of the knee joint we used MRI images where we selected the boundaries of each tissue in each slide of the MRI. Following those boundaries the 3D representation of each part was created.
Explain the model
We then created constraint definitions by bonding the ligaments, cartilage and meniscal horns to the corresponding bones.
Cadaver tests were carried out at HSS. Cadaver placed in a 6DOF robot
Stress distributions for different moments spanning from 15Nm valgus to 15Nm varus simulating the knee adduction moment. In-vitro and FE resutls have the same distributions.
Here you can see the medial and lateral pressure for each the FE and in-vitro model against bending moment. You can see that they corroborate very well. The full scale error is 6.67 and 5.94%. Therefore the model is 93% accurate.
After validation we applied loads generated within the knee during level walking. We applied forces at the end of weight acceptance as it simulates the first peak.
This graph shows the force distribution within the knee for both medial in red and lateral in blue against HKA angle and MAD. This graph represents the ideal force scenario within the knee and the surgeon can pick the corrpesponding HKA angle or MAD.
Similar for the pressure distribution. The intersection of the two graphs represents the ideal stress scenario within the knee. For this specific knee this was at an HKA angle of -0.5degree valgus. The surgeon could decide to slighly overcorrect the knee and can then pick the cprresponding HKA angle or MAD.
Same or shear stress
For this specific knee the minimum peak contact pressure …..