- Discussion:
- it is important that knee joint is extended throughout stance phase;
- this means that the components are assembled so that a straight line extended from
the "trochanter" will pass anterior to the knee and through the ankle;
- friction is adjusted to help control the amount of knee flexion and to help decelerate the shank;
- mechanical friction provides a constant resistance to flexioin and extension;
- fluid friction provides cadence dependent resistance;
- Biomechanics:
-
frontal plane:
- provide mediolateral stability of the pelvis during midstance on prosthetic side;
- conserve energy by minimizing the lateral displacement of center of gravity during gait;
- prosthesis is aligned so that a varus moment is generated about hip joint during midstance (lateral wall
of the socket is placed in adduction);
- ischium is considered a fulcrum & foot is aligned under fulcrum;
-
saggital plane:
- prosthetic socket is usually flexed, which allows the contra-lateral normal limb to take a normal step;
- placing knee center of rotation posterior to the weight line allows control in stance phase but makes
flexion difficult (involuntary control);
- w/ knee center of rotation anterior to weight line, flexion is made easier, but at the expense of control (voluntary control);
- the ideal comprimise is to have the plumb line fall from the greater trochanter directly through the axis of the knee;
-
hip flexion contracture:
- w/ hip flexion contracture, bore of socket should be flexed 5 deg more than the presented contracture;
- short residual limbs, which are more likely to present with flexion contractures, can often be accommodated;
- hip flexion contractures and insufficient anterior socket support can lead to excessive lumbar lordosis (compensatory);
-
rotational alignment:
- typically the prosthesis is placed in 5 deg external rotation;
-
outset - inset:
- w/ regard to medial-lateral placement, should be placed directly under the ischium or upto
1 inch lateral to the ischium;
- generally, the AK foot is never inset relative to the ischium;
- Specific Prosthetic Characteristics:
-
types of sockets:
-
quadrilateral socket:
-
cad cam socket:
-
suction contour suspension:
- provides an air tight seal via a pressure differential between the socket and atmosphere;
- AKA suspension - quadrilateral sockets where the posterior brim abuts the ischial tuberosity are the classic;
- this design makes it difficult to keep the femur in adduction
-
types of AKA knees:
-
prosthetic feet:
- AKA Prosthetic Problems:
- excessive prosthetic length and weak hip abductors or flexors can lead
to circumduction, vaulting, and lateral trunk bending;
- excessive friction in the knee joint can lead to circumduction as well;
- inadequate prosthetic knee flexion can lead to terminal knee snap;
- short AKA:
- requires a softer heel cushion, than longer AKA;
- medial whip:
- heel in, knee out can be caused by a varus knee, or excessive external rotation of
the knee axis, or muscle weakness;
- lateral whip:
- is caused by valgus knee, internal rotation at the knee, and weakness;
Does socket configuration influence the postion of the femur in above knee amputaitons.
FA Gottschalk et al.
J. Prosthet Orthop.
Vol 2. 1989. p 94-102.
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