transfemoral biomechanics

1. Biomechanics
Of
The trans-femoral Amputee
By Pheasa, T., B.PO
1
Transfemoral Prosthetic manual
Chapter 2

2. • If you have questions, write them down nicely.
we will discuss all questions at the end of the
session.
2

3. In TF case, knee stability can be achieved
by two ways
1. Voluntary – by action of muscles
2. Involuntary – by alignment and
mechanism of prosthetic knee
3
Knee stability in TF is achieved by what
means?

4. ACHIEVING KNEE…..
Look at these drawings below and find out which
one is the most stable?
4

5. Amputated gait
• The most unstable or dangerous phase of gait
for TF is during “heel strike”.
• Why?
– Think about where the GRF at this time point?
– What moment is created by the GRF?
– To which direction the shank tends to rotate?
5

6.  The GRF is behind the
knee, creating knee flexion
moment and the shank
tends to rotate forward until
it is checked by the floor.
This makes the knee to flex.
6
Heel Strike

7. AMPUTATED GAIT
• The patient needs to contract
his/her hip extensors against
the posterior wall of the
socket to bring the
prosthetic knee behind the
GRF.
• As the GRF stays in front of
knee, external extension
moment is created. This
makes knee stable
What the TF amputee should
do to gain knee stability?
7

8. • Knee is remained extension if
the TKA line is placed anterior
to the knee axis.
8
Mid-stance

9. • Before push-off, GRF still
locates in front of knee axis.
• To prepare for swing, patient
needs to flex the hip to move
the knee axis in front of the
GRF line.
• When knee flexion moment
is initiated, the swing starts
9

10. • Pendulum action will make the
shank swing forward
• The speed of pendulum depends
on how strong the patient is
flexing the hip extensors
• Mechanical parts inside the knee
control the speed of the swing
10

11. Mechanical axis & femoral axis
• Mechanical axis of the
lower limb measures 3
degrees to the vertical line:
– Center of femoral head
– Center of knee
– Midpoint of ankle
11

12. • The femoral shaft axis
measures 9 degrees to the
vertical line:
– the line running along the
mid shaft of the femur.
Mechanical axis and femoral axis
12

13. Differences in pelvis and trunk stability
• The advantage of this
natural alignment is
allow all hip abductors to
function normally:
– to prevent the pelvis from
dropping more than 5
degrees on the swinging
limb, and
– to reduces the lateral
movement of CoG during
mid-stance
13

14. Differences in pelvis and trunk
stability
• In the TF amputation, the insertion of adductor
muscles are lost, their effective moment arm becomes
shorter. They are unable to generate a large
counterforce to the abductor muscles in order to hold
the femur still in the adduction position, leaving the
femur floating the soft tissue of the thigh
14

15. Differences in pelvis and trunk stability
• As hip abductors are
concentrically
contracting to stabilize
the pelvis/trunk, it pulls
the femur laterally.
15

16. Differences in pelvis and trunk stability
• Failing to stop the femur
from moving laterally
reduces the abductors’
ability to prevent the
pelvis drop more than the
natural range.
16

17. Solution by the amputee
• The amputee will bend trunk excessively to
the lateral to elevate the pelvis and to shift the
COG back into the base of support.
17

18. Solution by P&O
• Provide the lateral support to
femur to hold it in adduction
angle.
• As the femur is adduction, it
provides 3 biomechanical
advantages:
1. Prevent lateral movement of
femur
2. Increasing the ability of hip
abductors
3. provide a narrow base
walking gait 18

19. Problem with a short stump
• For the short stump,
– Difficult to stabilize the pelvis
and trunk due to the short lever
arm of remaining femur
• The exerting pressure becomes
bigger
Patient probably walks with a
wide walking base and use
lateral trunk bending to reduce
the effort of the hip abductor.
• Lateral wall should be kept
higher
19

20. 20
Short stump
Long stump

21. Socket
forces in
coronal
plane
21

22. Socket forces in coronal plane –
during mid stance
• Pressure areas are
– Ischial Tuberosity
– Medial proximal
aspect
– Lateral distal aspect
22

23. 23
Socket forces in sagittal plane

24. 24
Socket forces during heel strike
• Pressure areas are
– (S) but not as large as at mid
stance
– (A) counterforce to maintain the IT
on the seat
– (P) due to the action of hip
extensors.

25. 25
Socket forces during mid-stance
• Pressure areas are
– (S) greater due to full weight
– (A) counterforce to maintain
the IT on the seat
– (P) due to the action of the hp
extensors even though the
knee stability is achieved by
the involuntary control (the
pressure is lesser than that at
heel strike)

26. 26
Socket forces during push-off
• Pressure areas are
– (S) becomes less
– (A) due to action of hip
flexors
– (P) maybe trying to stop the
prosthesis from rotating
backward

27. 27