| ANCA-Fit
The oblique grooves in the proximal section
of the stem ensure firm bone anchorage and stable secondary
fixation. Furthermore, since they are at 45ø, approximately
the same as the trabecular bone of the Greater Trochanter,
it guarantees an even load transmission over the entire trochanteric
area.
The surface of the proximal section of the
stem has a high degree of roughness (Ra +/-6 microns) and
is coated with hydroxiapatite (thickness 80 microns). In this
way, the hydroxyapatite will stimulate formation of new bone
which will become firmly anchored to the rough surface of
the stem.
Two-thirds of the stem length has a lower
surface of approximately 2 microns. This is a realistic compromise
between the need for good bone anchorage without impairing
the distal structural strength of the stem.
A major problem with non-cemented hip stems
is pain generated by movement of malposition of the distal
tip. The design of the Ancafit stem, with a proximal section
that guarantees excellent femoral cavity filling, reduces
micro-movement between bone and prosthesis. It has been kept
very short, thinned down in the distal sectiona dn with a
blunt tip. All these elements are designed to prevent unnecessary
contact of the distal tip with the bone cortex, especially
anteriorly.
The anatomical design of the shaft is based
on the experience gained with two anatomical prosheses: the
original An.C.A. and the G.S.P.. The former implant was implanted
since 1985 with excellent osteo-integration reported. The
incidence of shaft mobility was less than 2% (k) at 8 years
follow-up. Despite these positive results, the original An.C.A.
stem often suffered from the problem of stress-shielding caused
by the rigidity of the stem made from a chromium/cobalt alloy.
The GSP stem, used since 1987, in addition to validating clinically
the use of modular
necks, confirmed the advantages offered by stems in titanium
alloy. Reduced availability of sizes sometimes made a selection
and correct press fit insertion into the diaphyseal cavity
difficult.
Much research was undertaken on the morphology
of the femoral diaphysis, also using modern statistical methods
such as clusters that can classify the variables of the human
femur into signiticantly distinct sets on the basis of their
characterising elements. Based on this research, 8 dimensions
were chosen to permit adjustement to the most common anatomical
variants. The Ancafit prosthesis is available in 8 sizes left
and right. The design of the stem was optimised with the help
of the computer, where the ratios between the size of the
proximaland distal sections minimise risk of fracturing the
femoral metaphysis whilst ensuring excellent torsional stability.
These designs were fully endorsed by a rigorous range of bench
tests.
The stem was fully tested for endurance and
strength. In addition to exceeding the specifications of ISO
7206/3 standards, surface roughness was also optimised. If
increased surface roughness corresponds to higher mechanical
stress resistance by the bone to proshtesis contact, we must
equally bear in mind that this is counterbalanced by a decrease
in stem endurance.
After in-depth experiments, the surface of
the stem was finished with two rougness levels: higher on
the thicker proximal section (where good osteo-integration
is required and the risk of stem fracture is lower) and lower
on the distal section.
The use of modular
necks with the Ancafit proshesis is unquestionable a significant
advantage as the surgeon is able to correct offset geometry
and leg length per-operatively.
Here, precisely bacause of the risk of fretting
corrision (wear caused by micro movement of contiguous metal
surfaces) at the neck/stem interface, a lengthy and complicated
series of experiments was carried out. The results confirmed
the reliability of the modular
neck design proposed with the Ancafit prosthesis. The
metal cone of the modular
neck is neither smotth nor polished. In fact, its surface
is appropriately fine machined to create spiral grooving.
The crest of these grooved deform when the ceramic head is
inserted and this ensures a continuous metal-ceramic contact
surface to prevent stress risers and the potential for head
fracturing.
For more information on the Ancafit prosthesis
such as brochures, articles, video etc., please contact
Ortotech directly.
  
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