1686-e_SL-PLUS MIA.indd

Discussion
In this investigation a torsional moment was applied along the longitudinal axis of the stems for
an isolated observation of rotational stability. This does not represent the actual physiological load
situation but is well suited to allow an isolated evaluation of the impact of the design differences
on the rotation stability and thus on the primary stability.
Both stem models, SL-PLUS MIA Standard and SL-PLUS Standard, were identically and neutrally
positioned in the FEM model. In both models the lateral cortical contact ends in the region of the
trochanter, representing an average clinical situation.
The virtually identical values for the ball head displacement taken as a measure of the maximum total
deformation of the bone implant system show that, in terms of the displacement caused by rotation,
the SL-PLUS MIA Standard stem is equivalent to the SL-PLUS Standard stem. The slightly lower displacement
of the SL-PLUS MIA Standard stem compared with the SL-PLUS Standard stem (0.5%) can be
attributed to the reduced weakening of the greater trochanter, leading to greater rigidity in this region.
Both systems show a primarily distally based load transmission to the cortical bone. The stress represented
as a measure of the anchoring forces is greater in the distal cortical bone than in the trochanter
region. In the SL-PLUS Standard stem these forces fall steadily above the second-lowest cross hole.
Further, in the proximal region the contact is primarily between the cancellous bone and the implant,
where the forces are lower due to the mechanical properties of the bone. The lateral shoulder of the
SL-PLUS Standard stem, which has been eliminated in the SL-PLUS MIA Standard stem in favor of
an implantation technique that preserves more tissue, has a relatively low impact on the rotational stability,
as it is mainly in the cancellous bone. This explains why, without the lateral shoulder of the
SL-PLUS Standard stem, the SL-PLUS MIA Standard stem is very similar to the SL-PLUS Standard stem
in terms of its rotational stability under application of a torsional moment.
Finally, it should be noted that, as a result of the different implantation techniques, the greater trochanter
is weakened to a greater extent with the SL-PLUS Standard stem than with the SL-PLUS MIA Standard
stem. As the greater trochanter is not weakened to the same extent as with the implantation of the
SL-PLUS Standard stem, this seems to compensate for the anticipated loss of rotational stability resulting
from the design of the SL-PLUS MIA Standard stem. Both bone-implant systems show identical
values for deflection during rotational loading on its own.
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