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

References

• Does socket configuration influence the postion of the femur in above knee amputations.