1686-e_SL-PLUS MIA.indd

White Paper for Rotational Stability
*smith&nephew
SL-PLUS MIA
Cementless Hip Stem System

SL-PLUS MIA
Building on the long-term success of the SL-PLUS hip stem, the SL-PLUS MIA hip stem has been
developed for minimally invasive surgical procedures. The design and surgical technique of this
stem allow implantation with reduced lateral bone resection, preserving the tendons on the greater
trochanter. The anchoring technique of the modified, dual-taper straight stem in the region of the
lateral shoulder in the diaphysis is fully maintained (fig. 1, 2). The aim of this paper is to investigate
the impact of the modified shoulder on the rotational stability of the SL-PLUS MIA Standard stem
compared with the SL-PLUS Standard stem.
Due to the complex geometric conditions and biomechanics, the finite element (FEM) method is
frequently utilized (3, 4, 5, 6, 7) to calculate the load distribution, displacement and the system
performance in general. This method is particularly suitable for comparative calculations to evaluate
various designs.
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SL-PLUS MIA
Materials and methods
This analysis compares the SL-PLUS Standard stem size 6 and the SL-PLUS MIA Standard stem size 6
in terms of their primary rotational stability using FEM calculations. A numerical 3D model of the femur
was used to represent the bone. The cortical and cancellous bone were modeled separately (please
refer to table 1 for material properties) and the hip stems to be studied were virtually implanted into
the model bone. In both cases the size 6 stem achieved good cortical contact in the virtually rasped
bony bed of the distal femur (fig. 1C) thereby reflecting the immediate postoperative situation.
Cortical fixation
of the stem
A B C
Fig. 1: Implanted hip stem in the bone model SL-PLUS MIA Standard (A), and SL-PLUS Standard (B). In the semi-transparent
model the cancellous bone is shown in orange. Figure C shows a cross section through the femur.
The SL-PLUS MIA Standard stem is less prominent in the proximal/lateral femur than the SL-PLUS Standard
stem (fig. 1). In accordance with the clinical procedure, care was also taken to ensure that both stems
were primarily fixed into the cortical region of the diaphysis. Due to the different implant techniques,
the greater trochanter major is weakened to a greater extent with the SL-PLUS Standard stem than with
the SL-PLUS MIA Standard stem (fig. 2).
Fig. 2: Proximal view of the femur with the SL-PLUS MIA Standard stem (left) and the SL-PLUS Standard stem (right).
The difference in the trochanter weakening caused by the implantation techniques can be clearly seen.
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The 3D geometries were imported into a finite element program for further processing and loading
(pre- and post-processing program MSC Patran, network with approximately 60’000 tetrahedron
elements). Friction at the implant/bone surfaces of the anchoring was established with a friction
coefficient of 0.5.
Elasticity module Poisson ration Element types Source
Cortical bone 25’000 N/mm 2 0.3 Tet-4, 4 mm [1]
Cancellous bone 1’500 N/mm 2 0.3 Tet-4, 2 mm [1]
Titanium 110’000 N/mm 2 0.3 Tet-4, 1 mm [2]
Table 1: Material properties for the FEM calculation.
Based on Bergmann et al. [8] a torsional moment of approximately 16 Nm was applied as a load
along the longitudinal axis of each stem. The bone was secured distally at the isthmus. The displacement
of the ball head center, representing rotational stability in this model, was calculated,
as was the interface load transmission in the region of the implant/bone interface.
Mz = 16 Nm
A
B
C
Fig. 3: FEM models of the femora with SL-PLUS MIA Standard (A), and SL-PLUS Standard (B). A torsional moment (C) was
applied along the longitudinal axis of the stem.
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SL-PLUS MIA
Results
1. Displacement of the overall system
The maximum displacement values of the ball head center were approximately 0.211 mm for the
SL-PLUS MIA Standard stem and approximately 0.212 mm for the SL-PLUS Standard stem (differ-
ence = 0.5%).
2. Interface load transmission
The stress distribution represented in the transverse levels along the prosthesis stem shows that
the load in the diaphyseal cortical anchoring for both stems leads to higher distal forces (fig. 4). In the
proximal region, cortical loading is reduced for both stem designs when compared to the diaphysis.
Section 9 Section 9
Section 1 Section 1
Fig. 4: Stress distribution in the implant (SL-PLUS MIA Standard left, SL-PLUS Standard right) and bone, as seen in the
transverse plane in the anchorage zones. High stress = red, low stress = blue.
Going from distal to proximal the pattern of stress distribution is virtually identical for both stems.
The maximum stress below the greater trochanter is marginally higher with the SL-PLUS Standard
stem. The volume of cancellous bone in the trochanter only slightly affects the rotational stability
of the SL-PLUS Standard stem in levels 6–9 in comparison to the stress in the cortical levels 1–5.
SL-PLUS MIA Standard – SL-PLUS Standard
Distribution of friction
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
1 2 3 4 5 6 7 8 9
Distal to proximal cross section
–––– SL-PLUS MIA Standard
–––– SL-PLUS Standard
Fig. 5: Distribution of stress along the lateral edge for the
SL-PLUS MIA stem and the SL-PLUS Standard stem.
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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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SL-PLUS MIA
References
A. Rotem
Effect of implant material properties on the performance of a hip joint replacement.
J of Medical Engineering, Vol 18, Nr 6, p 208–216, Taylor&Francis 1994
M. Ungethüm, W. Winkler
Metallische Werkstoffe in der Orthopädie und Unfallchirurgie
Georg Thieme Verlag Stuttgart, New York, 1984
P. J. Prendergast, D. Taylor
Stress analysis of the proximal-medial femur after total hip replacement
Journal of Biomedical Engineering, Vol. 12, p. 379–382
Butterworth & Co Ltd, 1990
T. Vail, R. Richard, D. Theodosis
The effect of hip stem material modulus on surface strains in human femora
Journal of Biomechanics, Vol. 31, p. 619–628, 1998
M. Akay, N. Aslan
Numerical and experimental stress analysis of a polymeric composite hip joint prosthesis
Journal of Biomedical Materials Research, Vol. 31, p. 167–182
John Wiley & Sons Inc. 1996
E. Savidis, F. Loer, O. Werner
The significance of the torque loading of the total hip prosthesis
Biomechanics, Basic and Applied Research, p. 267–272
G. Bergmann, R. Kölbel, A. Rohlmann
Martinus Nijhoff Publishers, Dordrecht, Boston, Lancaster, 1987
R. R. Tarr, I. Clarke, T. Gruen
Predictions of bone cement failure criteria: Three dimensional finite element Models versus clinical reality
of total hip replacement
Finite Elements in Biomechanics, p345–359
P. C. Johnson, J. F. Gross, J. Wiley & Sons Inc., New York, 1986
G. Bergmann et. Al.
Hip contact forces and gait patterns from routine activities
Journal of Biomechanics Vol. 34, p. 859–871, 2001
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