- 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? Mallory et al. J Bone Joint Surg Am.2002; 84: 81-89.
-
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;
