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Wheeless' Textbook of Orthopaedics

Above Knee Amputation Prosthesis



- 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 fromthe "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 up to 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 amputations.



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Friday, May 11, 2012 2:11 pm