Fixation - Orthopaedic Biomaterials in Research and Practice - page 324

a value for the elastic modulus of bone and using its actual dimensions.
Then, a cemented femoral prosthesis was inserted and the experiment
was repeated. The results are shown in Figure 13.5. (Weightman per-
formed the experiment with an actual load of 2 kN and calculated results
for a 4 kN load. His results have been rescaled for a 1715 N load, cor-
responding to 2.5 times body weight for a standard 70 kg patient. Other
minor adjustments have been made to the figure.)
Several initial points are obvious. The insertion of the cemented
stem generally reduces outer fiber stresses proximal to the stem tip and
increases them distal to the stem tip. The reduction of stress in the calcar
area is especially noticeable and led to hot debates about the relation-
ship between this reduction and calcar resorption of the role of medial
prosthesis collar-calcar contact in load carriage and stress distribution.
These are still not fully resolved and are complicated by considerations
of the role played by surgical interference with the blood supply to the
proximal femur. However, this overall pattern of stress reduction after
prosthetic implantation is called stress protection or stress shielding and
is generally appealed to explain postimplantation remodeling leading to
osteoporotic or absent cortical and cancellous bone.
However, there are several less obvious points that should be made.
At location 1 , the cut edge of the calcar, the integrity of the bone, as a
cancellous-supported cortical shell, has been lost and previous normal
stresses could not be sustained without either hoop failure (longitudi-
nal fracture) or transverse buckling. The catastrophic consequences of
such failure may be appreciated by reference to the differences in areal
moment of inertia between a complete and a split cylinder (see Table 2.4).
Thus, the goal of restoring normal stresses in this area cannot be safely
achieved after partial or total head-neck removal. This will always be
the case, independent of fixation techniques, and has led to recent device
design proposals that retain the femoral neck. Others have included stem
designs with a collared portion in hopes of transmitting more natural
stresses to the cut section of the neck. However, in many cases, stress
shielding may not be avoided because the collar itself shields the sur-
rounding bone.
At location 2 , within the boundary of the cancellous bed (superior
to line C - C ′), both the stem and the bone are carrying load, with load
transfer increasing as one moves distally. A great deal has been said
and written about the need to equalize elastic moduli to reduce relative
stem-bone motion, that is, to have the stem modulus be as close to that
of cortical bone as possible:
E S = E B
where the subscripts S and B refer to the stem material and bone, respec-
tively. However, this proposal is in error. The stem and the bone are both
subject to a bending moment at this point. For there to be no relative
motion between stem and bone, the appropriate condition is
M SF /( E SF ∙ I SF ) = M B /( E B ∙ I B )