5.6 Mechanical stability tests of model implants
Bone implants have to withstand considerable stress. Therefore an
essential test has to show whether the metal does suffer a reduction of
stability during the intended production of a prosthesis, or if it
maintains its strength. Heating for the purpose of binding a bioactive
layer must be kept under critical limits in order to preserve the
internal structure of the metal. The laser induced process of fixing the
bioceramic powder binds these small particles very effectively. A very
significant property of the ultra-short pulses has to be emphasized
here. Shorter laser pulses cause an ablation only in a very thin surface
layer. There is hardly any heat transfer into deeper layers underneath
the ablated volume. One sometimes talks about “cold” ablation. The
immediate surrounding does not suffer much heating.
It has to be investigated if the marginal (heat) influence of the
ceramic powder fixation to the metal has any negative effect on the
mechanical stability. One established method for studying the material
strength is the rotating bending fatigue test. This statistical method
applied to specially prepared samples (hourglass rods) is already
described elsewhere (Symietz, 2012). A brief outline of the experiment
and the results is given here. The samples were rods of medical grade
Ti6Al4V (Merete Medical GmbH, Berlin, Germany) as shown in Fig. 13. The
so called hourglass rods have a reduced diameter in the middle, where
they will break under too much stress. A set of twenty samples was
necessary to approach the critical mechanical stress in steps. The rods
were fastened into a device that allowed turning them around their
symmetry axis with a frequency of 73.7 Hz. For each run, lasting for
more than one day, a constant four-point bending moment was adjusted.
The thinner central section of each rod was exposed to alternating
positive and negative stress, with the maximum absolute value on the
surface of this middle part. This material wear caused by alternating
compression and tension simulates an ageing process of the metal. A
final number of 107 rotation cycles defines the
desired survival life time of the samples. This is a step by step
approach by adjusting a predetermined upper or lower stress level. The
previous result (survival = unbroken rod, failure = broken rod) dictates
the choice for the next experiment. Normally 20 rods are enough to find
a stress value around which the individual test runs level off.
The first twenty rods were references, untreated samples as delivered.
Two further sets of twenty each were tested. One type were rods being
dip coated with Ca10, laser treated and washed, just like the fixing
process described above. These samples need to remain as stable as the
untreated ones in order to prove an unchanged mechanical stability after
the laser induced ceramic fixation. Because of unavoidable statistical
uncertainty, another 20 rods were used, rods that had been laser treated
with the result of about 10 µm ablation depth as a “worst case”
scenario. The three different kinds of samples are pictured in Fig. 14.
The middle rod was never coated with the ceramic. Here the spiral trace
of the laser (three full turns with a simultaneous lateral shift) is
darker and wider than the fixed ceramic sample after washing on the
right. The pure laser treatment with detectable ablation is a much more
severe impact on the metal than the relatively gentle powder fixation.
Fortunately all three different kinds of rods brought the same results.
In Fig. 15 the stress amplitude each rod was exposed to during the
individual experiment is seen. One point in the diagram stands for one
sample being treated by the bending machine. The abscissa is the number
of cycles the rods had to endure until they break. Samples that survive
the treatment are marked on the right edge of the diagram, symbolizing
the end of the experiment after 37 hours. Rods break earlier under
higher stress (left points). The medium value of the stress amplitude is
505 MPa ± 5 MPa for the reference without laser treatment (Fig. 15,
left), 506 MPa ± 18 MPa for the laser ablation (Fig. 15, right), and 508
MPa ± 5 MPa for the ceramic fixation (Fig. 15, right). The differences
between the three types can be neglected, since the error margin is even
larger.
The really important result of the whole series is the proven fact that
one can rely on the data of the fixed ceramic layer. The laser fixing
process does not reduce the metal stability. This is even valid for the
“worst case scenario” of pure laser ablation.
The position where the rods broke apart seems to be independent of the
laser trace. This is illustrated by the example in the middle of Fig.
14, showing a thin dark crack about 2 mm above the zone of laser
ablation. This sample did not break completely, but most failure rods
broke into two pieces.