Pistoning and Subsidence of Femoral Stems

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- Discussion:
    - by definition, is a decrease of more than 2-5 mm in the verticle distance of the femoral stem;
          - minor subsidence is to be expected in the first year or two following surgery;
          - more than 5 mm of subsidence is considered to be an indication of loosening;
    - generally measured from the tip of the greater trochanter to the junction of the neck and shoulder of the prosthesis;
          - alternatively it can be measured from the tip of the greater trochanter to the tip of the stem;
    - pistoning behavior can result from two sets of circumstances;
    - subsidence of the stem in the cement mass:
            - radiolucent line is seen between stem and cement at superolateral part
                  of the stem that is caused by incomplete encapsulation of cement or
                  loss of superomedial support and inadequate midlateral fixation;
            - stem is displaced distally, producing radiolucent zone &
                  punched out frx of cement near the tip of the cement mass;
            - in this case, tip of stem knocks off end of cement mantle & gives off
                  characteristic crack in cement at the stem tip is instantly recognizable;
    - subsidence of the cement mantle and stem:
            - radiolucent zone can be seen about entire cement mass, often w/ halo or
                  thin line of reactive sclerotic bone about radiolucent zone;

- Technique of Measurement:
    - subsidence is best measured on AP radiographs if the component has a trapezoidal
            shape in the coronal plane;
    - subsidence of 1 mm or less is within limits of measurement;
    - subsidence of cement mass & stem or migration of stem into cement
            mass resulting in more distal position of collar or platform in
            relation to proximal surface of cement & femoral neck;
    - subsidence may not be noted unless relationship of stem & cement mass
            to proximal femur is carefully evaluated in serial x-rays;
    - stem may subside in cement, in which case there is usually frx of
            cement near tip of stem, or entire cement mass & stem may subside;
    - other suggestive radiographic features include sclerosis of the medial calcar,
            and formation of distal bone plug (which may obliterate medullary canal);