| BUECHEL-PAPPAS™
INTEGRATED HIP REPLACEMENT SYSTEM |
Femoral
Stem Design Rationale
Titanium
Alloy with UltraCoat® Provides Superior Mechanical and
Biological Compatibility
The
greater flexibility of titanium alloy allows a greater share
of the load to be carried by the bone than by the stiffer
Co-Cr alloy. Thus, titanium alloy provides superior mechanical
compatibility by reducing stress protection of bone. Further,
titanium alloy is more biocompatible than Co-Cr alloy whose
major components can be carcinogenic. Finally, titanium alloy
is stronger than Co-Cr alloy in both fatigue and yielding
resistance. Thus, except for the inferior abrasion resistance
of titanium alloy, it is superior to Co-Cr alloys for use
in implants.
UltraCoat®
finished stems provide enhanced biocompatibility. TiN is inert
in vivo, it shields the surface of the implant, particularly
the porous coated region with its high surface area, against
metallic ion release. The extreme hardness, and abrasion resistance,
of TiN ceramic coatings should eliminate the metallosis observed
in both uncoated titanium and Co-Cr alloy prostheses. This
is because much less abrasion debris are generated, and the
debris are inert and consists of nontoxic Ti and N ions. As
a result, UltraCoat® finished titanium stems represent
the most mechanically and biologically compatible femoral
stems available.
Retrieved
specimens of stems with modular heads have shown a disturbing
degree of corrosion at the taper connection interface. This
corrosion is the result of micromotion in the interface (fretting
corrosion) and is present in both mixed and similar metal
combinations.
Much
of the micromotion in this interface is the result of excessive
tolerances used in the manufacture of the tapers. The orthopaedic
implant industry generally uses tolerances which are an order
of magnitude greater than specified for normal machinery applications.
To minimize this micromotion Endotec employs the same close
tolerances used in the machinery industry.
The use
of UltraCoat® ceramic coating on the modular connection
taper on the neck of the femoral stem, when used with an UltraCoat®
finished titanium head, virtually eliminates corrosion at
this interface since TiN ceramic coating and titanium are
extremely resistant to fretting corrosion.
Neck
Aligned with Peak Load Vector Optimizes Neck Function
The
line of action of the joint reaction force vector varies with
activity and phase. During the peak load phase of normal walking
the vector is at an angle of about 148° to the axis of
the stem. This phase produces the highest stresses and loads
in the femoral component and bone.
A femoral
shaft to neck angle of about 135° is optimal for the human
femur, producing compressive stresses in the neck at peak
loads with a bias medially for transfer of load through the
calcar to the femoral shaft. This angle is, however, not optimal
for a metal prosthetic neck. With the neck diameters used
for femoral stems, this neck angle produces tensile stress
in the lateral side of the neck which can lead to fatigue
failure and increases the compressive stress in the medial
side of the neck. Thus, neck diameters of 12mm and greater
are typically used with 135° angle necks.
Aligning
the neck with the peak load vector has several advantages:
It eliminates tensile stresses and minimizes compressive stresses
in the neck at this critical loading phase. This allows the
safe use of a much smaller diameter neck providing an increased
range of motion reducing the potential for impingement and
dislocation.
Changing
effective neck length by use of modular heads does not alter
joint biomechanics during this phase.
Bipolar cups with positive eccentricity become aligned with
the neck maximizing the effective ROM of the device.
The Buechel
- Pappas Femoral Stem System positions the head at the normal
head location but medializes the distal neck junction to achieve
a neck to stem angle of 148°, thereby aligning the neck
with the peak load vector. A neck diameter of 9mm is used.
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Proportional
Sizing and Cylindrical Geometry Reduce Torsional Restriction
and Thigh Pain
Proportional proximal sizing in 1mm increments provides a
close proximal fit and maximum proximal flare for torsional
resistance to load. Torsional resistance is provided by the
oval cross-section of this proximal flare and by the collar.
Finite element analysis demonstrates the important role the
collar plays in resisting torsional loads. Thus, torsion loads
are transferred proximally, thereby avoiding torsional stress
protection of the more distal regions and thus, transferring
load more naturally than stems which resist torsional loads
in the shaft. Such distal torsional restriction may contribute
to the high incidence of thigh pain associated with designs
that transfer load distally. Clinical experience with the
Buechel - Pappas Femoral Stem indicates a relatively low incidence
of thigh pain which may be attributed to the lack of distal
load transfer
Collar
Slots Ease Removal
The use of lateral slots in the collar of the Buechel - Pappas
Femoral Stem provides access to the anterior and posterior
porous coating interfaces allowing resection of the interfaces
in these regions if removal of the stem is required. Now since
the lateral porous coating interface is always accessible,
and since there is little medial coating, the femoral component
can be removed with little bone loss. A threaded removal hole
is provided in the superior aspect of the stem allowing insertion
of an impact-type removal instrument.
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