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Stress Shielding from Femoral Components

- Discussion:
    - stress shielding is the redistribution of load (and consequently stress onthe bone) that occurs when the femoral head is replaced by the 
           femoral component of a total hip replacement;
    - stress on proximal 10 cm of femoral cortex is reduced, because much of the load bypasses this region and is carried in the metal stem to 
           the isthmus of the femur;
    - w/ extensively porous coated stems, upto 25-30% of femurs will demonstrate some stress shielding;
    - as noted by Bugbee et al 19997, significant stress shielding was present in 23% of 207 hips at 2 years (using the anatomic medullary 
           locking hip);

- X-rays:
    - atrophy of the proximal femur is substantially greater w/ stiff cementless femoral components than w/ cemented components.
    - for most pts this incr degree of local disuse osteoporosis has not yet had clinical consequences, but there are examples of severe disuse 
          osteoporosis in which much or all of proximal femoral cortex has been completely resorbed;
    - stress shielding of prox femur is more pronounced when a stem of large diameter has been used;
          - almost all femurs demonstrating moderate or severe proximal resorption involves stems of 13.5 mm or greater;
    - amount of stress shielding that is acceptable in clinical setting is difficult to determine;
          - fortunately a point of equilibrium is reached and bone loss does not appear to progress after a period of 2 years

Long term clinical consequences of stress shielding after total hip arthroplasty without cement.

Porous-coated hip replacement. The factors governing bone ingrowth, stress shielding, and clinical results.

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