BKA Prosthesis



- See: Prosthetic Feet:

- PTB sockets:
    - provide some weight bearing support in the area of patella tendon and medial tibial flare;

- BKA Suspension:
    - important design considerations for both sockets include
          - support (pressure distribution)
          - control (based on limb socket interface)
          - suspension (socket and/or corsete)
          - alignment (angular and linear)
    - total contact socket contour suspension is commonly used & can be one of two designs: patella
            tendon bearing (PTB) or patella tendon supporting (PTS);
    - ineffective suspension will manifest as pistoning during swing phase of gait;
    - PTS sockets
         - cover the condyles of the femur and have a high anterior wall enclosing the the patella;
         - provides more support anteriorly & add improved stability & suspension;
    - PTB w/ supracondylar cuff:
         - advantages include a kinesthetic hyperextension stop;
         - disadvantages include restriction in sitting, does not provide maximum M-L stability, and poor cosmesis;
         - addition of a supracondylar wedge of a flexible material to a PTB socket gives more
               stability by providing locking fit over the condyle (these may be fixed or removable);
    - PTB w/ sleeve suspension:
         - advantages include excellent suspension, conceals prosthetic trimlines, available in latex, neoprene ect.
         - may cause dermatologic problems;
         - neoprene sleeves can also be used to provide additional skin protection (especially for diabetics);

- Liners:
     - Soft PTB sockets are most commonly prescribed, especially with bony or scarred residual limbs, peripheral vascular disease, 
           volumetrically unstable residual limbs;
     - Hard sockets may be preferred in warm, humid climates;
     - Pylete:
     - TEM:

- Standard Prosthetic Alignment: (corsetless prosthesis)
    - knee flexion moment (and posterior foot placement):
         - typically, the BKA prosthesis is placed in 12-14 deg of flexion;
         - increases effective heel lever, producing knee flexion from heel strike to foot flat;
                - encourages knee flexion between strike and foot flat;
         - reduces length of keel encouraging knee flexion from midstance to heel off;
         - places the quadriceps muscle under slight stretch at heel strike, which improves stability and control;
         - encourages "roll over" between mid stance and heel off;
         - limits recurvatum (hyperextension) forces during mid-stance to terminal stance phase of gait;
         - tends to load more pressure tolerant areas of the proximal tibia;
         - note that when a BKA prosthesis with corsette is required, flexion is removed from the socket, and the prosthesis is translated 
                posteriorly;
         - excessive knee flexion (at heel strike) may result from:
                - heel too firm (knee flexes when the heel is fully compressed);
                - foot too posterior
                - foot too dorsiflexed
                - interface too flexed
                - forward placement of socket
                      - causes increased knee entension
                      - patellar pain
                      - instability is not a problem;
    - varus alignment:
         - initial foot placement is 12 mm inset in relation to midpoint of interface;
         - narrows the base of support
         - smooths horizontal displacement of center of gravity
         - makes for efficient gait;
         - helps load pressure tolerant areas (medial tibial condyle, lateral fibular surface);
         - excessive varus moment may be caused by:
                - foot positioned too medially;
                - M-L interface is too large;
                - there is laxity of the lateral collateral ligament;
         - insufficient varus moment may be exhibited by:
                - foot positioned too laterally;
                - loading of pressure sensitive areas (such as a load on the fibular head);
                - in coronal plane fulcrum is about the patellar tendon;
                - gait is excessivel widened;
                - excessive medial shoe wear;
    - translation of socket:
         - lateral translation:
                - excessive translation causes valgus knee strain;
         - medial translation: (excessive translation causes:)
                - varus strain on knee;
                - due to 3 point bending, expect increased pressure distal-laterally and proximal-medially;
         - anterior translation:
                - slight anterior translation may be useful to minimize the tendency for knee hyperextension at heel strike (instead encourages knee 
                       flexion), assists w/ roll over between mid stance and heel off;
                - knee but excesssive cause knee to buckle into flexion at heel strike;
         - posterior translation:
                - tends to keep knee in hyperextension during stance phase;                
                - when a thigh corsette is used, the socket must be displaced posteriorly;

- Skin Tolerance:
    - pressure tolerant areas:
           - patellar ligament
           - anterior compartment
           - medial flare of the tibia
           - distal end
           - shaft of the fibula
           - gastroc
           - popliteal fossa
    - pressure sensitive areas:
           - anterior distal tibia
           - fibular head
           - crest of tibia
           - peroneal nerve
           - distal cut fibula
           - lateral tibial condyle

- Common Prosthetic Problems:
    - pistoning:
          - pistoning during swing phase of gait is usually caused by ineffective suspension system;
          - pistoning during the stance phase is due to poor socket fit or volume changes in stump (may require a change in stump sock 
                thickness);
    - pressure related pain or redness should be corrected w/ relief of the prosthesis in the affected area;
    - foot related problems:
          - too soft a foot results in excessive knee extension, while too hard a foot causes knee flexion and lateral rotation of toes;
          - when a BKA prosthesis w/ corsette is required, then a softer heel cushion is required;
    - too much knee hyperextension: (at heel off);
          - foot too anterior or plantar flexed;
          - insufficient flexion of the socket or posterior displacement of the socket;
          - too soft heel cushion (knee flexes when the heel is fully compressed);
          - too long a keel of a SACH foot



An alternative bent-knee prosthesis.

The preformed socket and modular assembly for primary amputees.

A plaster-pylon technique for below-knee amputation.

Prescription options for the below knee amputee. A review.

Amputations and Artificial *Limbs--Symposium:* New Developments in Recreational Prostheses and Adaptive Devices for the Amputee.

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Ambulation levels of bilateral lower-extremity amputees. Analysis of one hundred and three cases.

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Lower-extremity amputation with immediate postoperative prosthetic placement.

Rehabilitation after lower limb amputation: a comparative study of above-knee, through-knee and Gritti-Stokes amputations.



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

Last updated by Data Trace Staff on Friday, May 11, 2012 2:40 pm