THR: Cement Degradation & Fragmentaton

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- Discussion:
    - aging of cement results in decreased elastic modulus and flexural and
          compressive strength, and increased creep deformation;
    - cracks occur as a result of creep deformation through voids in cement;
    - w/ poor cement technique and varus position of stem, x-ray evidence of
          loosening may be seen, and the pt may have pain within 6-12 months
          or sooner after surgery;
    - fibrous tissue membrane with some areas of fibrocartilage forms at
          cement bone interface and in the first 6 months after surgery may
          produce radiolucency, a dark line, upto 2 mm wide;
    - thin dense layer of bone is often formed on outer edge of fibrous
          membrane, and this produces a thin white line in the x-ray
          on the outer border of the radiolucent area halo effect;
    - this may occur in one or more zones about the cement mass;
    - fracture of the cement mass, most commonly near the tip of the stem;
    - fragmentation of cement, especially between the superomedial aspect
          of the stem & femoral neck;
    - radiolucency about part or all of the cement mass; scallopingg of
          endosteal surface is highly suggestive of infection;
    - migration distally of the cement mass (subsidence) as determined by
          relationship of its superior aspect to cut edge of femoral neck;
    - areas of rarification in the cement mass;
    - large cystic areas may be produced, especially near the tip of stem;
    - these frags may produce local tissue reaction, causing further
          destruction of bone, especially along the medial cortex;