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Bone Scan: in THR



- See: Total Hip Replacement Menu:
          - General Discussion of Bone Scans:

- Normal THR:
    - normal study (minimal reative bone formation) may be expected from months to a year after placement;
    - area of lesser trochanter to midshaft of prosthesis usually becomes normal in about 6 months;
    - area around acetabulum, greater trochanter, and tip of prosthesis may show increased activity for up to 2 years, but on average shows
            normal findings at about 1 year in the majority of patients;

- Non Cemented THR:
      - evaluation of of porous coated prosthesis may require a different interpretation from cemented THR;
      - bone & fibrous tissue ingrowth into porous acetabular & intertrochanteric surfaces may cause findings unrelated to complications;
      - with loosening there is little inflammation and, therefore, the first two phases of the bone scan are normal;
      - even culture of material taken at operation, which microscopically shows presence of infected material, fails to grow microorganisms in 
              up to 10% of cases;

- Infection:
    - infection shows a more diffuse pattern of abnormality occurring along prosthesis shaft and is usually positive on the 2 early phases;
    - low grade infection may consist of minimal hyperemia & may show a mild delayed uptake on bone scan which is difficult to distinguish 
           from loosening;
    - false positive results may occur in upto 40% and false negatives in 5-10%;
    - Indium-111:
           - more specific for making definitive dx of infection but may be less sensitive