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Press Fit Femoral Stem: Proximal Fixation



- Discussion:
    - press fit femoral stem main discussion
    - coating press fit femoral stem;
    - to maintain bone stock, proximal stress transfer must be a prominent feature of any cementless implant system;
    - proximal part of femur is often exposed to high longitudinal strain, & bone in this region is equipped for high lonitudinal deflection;
    - stem shape:
           - curved-stem prostheses:
                   - considered anatomic, because they fit to the anatomic bow of the proximal femur;
           - extensively coated stems with straight geometry
                   - when using straight stems, surgeon machines the curved bony anatomy to fit the prosthesis;
           - proximally coated stems with tapered geometry
                   - tapered stem achieves 3 point fixation as an effect of a straight stem being inserted into a femur with an anterior bow;
                   - rough porous surface bites into the proximal viso-elastic femur;
                   - references:
                         - Cementless double-tapered total hip arthroplasty in patients 75 years of age and older.
                         - Why a Taper?  

    - porous coating:
           - most cementless stems are porous in their proximal 1/3 only (or coating above the level of the lesser trochanter), especially when a 
                 stiff stem is to be used, (in order to avoid bone loss);
           - this design stategy allow bone ingrownth area is the only poriton of stem that transfers axial loads


- Stiffness of Stem:
    - flexiblity of the stem is important for achieving proximal loading of the femur;
    - effects flexible stems:
           - proximal micromotion of the stem is larger when flexible rather than rigid stems are used;
           - distal migromotion is smaller with flexible than w/ rigid stems;
           - more stress is transferred proximally by using a more flexible stem, but when a large amount of stress is transferred proximally,
                   proximal fixation is comprimised;
                   - the amount of proximal load transfer must not exceed the capacity of the proximal implant fixation to reduce proximal micromotion;
           - by using a flexible composite stem, stress shielding in the calcar region under axial load is significantly reduced but not elminated;
           - abnormally high hoop strain in the proximal femur may occur from torsional loading thru an intnramedullary stem;
                   - flexible stems will worsen the torsional strain which can cause loosening and thigh pain;
    - effects of rigid stems:
           - rigid metal stems cause significant stress shielding in proximal region of the femur under axial load;
           - when conditions are not optimal for proximal fixation (such as in revision THR, w/ proximal deformity, or osteoporosis), a relatively rigid
                   stem is required to avoid over-loading proximal fixation and to avoid creating excessive proximal micromotion

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