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.