Multiaxial Fatigue and Deformation: Testing and Prediction (ASTM Special Technical Publication, 1387)
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This comprehensive new ASTM publication examines state-of-the-art multiaxial testing techniques and methods for characterizing the fatigue and deformation behaviors of engineering materials. Multiaxial Deformation of Materials--investigates constitutive relationships and deformation behavior of materials under multiaxial loading conditions.
Fatigue Life Prediction under Generic Multiaxial Loads--examines the challenging task of estimating fatigue life under general multiaxial loads. Multiaxial Fatigue Life and Crack Growth Estimation--covers crack growth monitoring under cyclic mulitaxial loading conditions and determination of fatigue life. Multiaxial Experimental Techniques--explores state-of-the-art experimental methods to generate mulitaxial deformation and fatigue data to develop and verify both constitutive models used to describe the flow behavior of materials and fatigue life estimation models.
We will send you an SMS containing a verification code. Please double check your mobile number and click on "Send Verification Code". Enter the code below and hit Verify. Part-through Crack Fatigue Life Prediction. Sensory Evaluation of Appearance of Materials Stp Use of Computers in the Fatigue Laboratory - Stp Sampling and Analysis of Toxic Organics In.
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Astm Standards on Emergency Medical Systems. Scott A. Stephen J. Rods do not break as a direct result o f interconnection strength mechanisms. Indirectly, loose interconnections could lead to fretting corrosion, which could contribute to rod fractures. A fatigue test from standard F might indicate if fretting corrosion could be a problem, but it would still not directly test rod breakages.
Disconnection between the rod, bolt, nut, or screw would seem to be a failure o f the interconnection mechanisms. However, these failures can also be caused by improper installation o f the spinal assemblies themselves. Faraj etal. Figure 2, below, shows a rare problem but is another disconnection example.
The rod has slipped out of both screws presumably because the locking nuts o f the assembly were not properly tightened. ASTM F does not test for improper installation o f the device. However, since it cannot be determined retrospectively if correct surgical technique was followed during installation, the interconnection strength tests do at least give reasonable assurance Figure 2 - - There is an obvious disconnection between the rod and the pedicle screws.
The disconnection is attributed to insufficient tightening o f the lock nut and not due to any inherent interconnection failure o f the implant. Screw loosening from the bone is a more complex mechanism that may or may not be related to the screw itself but cannot be evaluated with interconnection strength testing. Loss o f correction is a complex phenomenon and its causes are not well understood. Is it indicative o f an interconnection strength failure?
We are not aware o f any study that correlates loss of correction to an interconnection deficiency. Loss o f correction in vivo is extremely difficult to measure accurately, and in many cases the loss may not even be a problem. Only 3 authors even mentioned it  and of those, only van Royen et al. Presumably it was not noticeable or noteworthy for any o f the other cases. They are all highly dependent on the surgeon and each individual case rather than the implant itself.
Still, comparing their incidence rates to those that might be considered more related to the implant itself can be informative and keep things in perspective by noting the small number o f occurrences. Examining the issue from another angle, if an implant's interconnection mechanism were to fail, could it even be detected or measured? Given the complex nature of measuring objects in 3 dimensions, it seems unlikely that one could detect a moderate change, especially one in vivo, corresponding to a failure mode in one o f the interconnection strength tests.
If the problem cannot be detected in vivo, and it does not seem to be contributing to any complications seen in the clinic, is it worth devoting resources to test? In a world of limited resources, it is an important question to ask. On the other hand, lack of evidence does not necessarily mean that no problem exists or that it should be ignored. For example, intuition suggests that some interconnection strength is required to avoid loss o f correction. If the interconnection strength were zero, the system would not be able maintain a surgical correction. The optimal required interconnection strength is unknown, and a wide range o f system stiffnesses have been found to be successful.
The clinical performance data have not shown direct evidence o f an interconnection mechanism failure. Even if such a failure were to be detected, it would still need to be determined if the failure were the cause of, the result of, or incidental to a symptomatic problem. Several authors cited examples where broken implants were discovered but the patient did not have any associated symptoms. For example, Boos et al.
However, all 12 were asymptomatic and their fusions appeared solid. Are asymptomatic failures a problem? One limitation o f this study is that the literature examines cases where patients have had their implants for several years. Therefore, complications from newer technologies such as polyaxial screws are not as well documented. The scope o f this paper was limited to examining evidence of clinical relevance for ASTM standard Clinical relevance is extremely important because pedicle screws must ultimately succeed within the patient to improve their quality o f life.
Creep-Fatigue Interactions - PDF
Therefore, it is an important consideration to keep in the overall picture e. This is not to say that the standard does not have merit as a development tool because, as stated earlier, the standard can be useful to compare new systems to other systems that have been shown JENSEN ET AL. However, "adequate strength" is such a nebulous and unknown quantity that more research needs to be done to define its optimal parameters. Currently, there is still not enough evidence in the literature to show that interconnection strength related to construct stiffness has a direct correlation in clinical success.
Conclusion The tests outlined in ASTM F do not adequately test for noteworthy failures seen in clinical practice as documented in the literature. In fact, their results may not have any bearing on the clinical outcomes of patients who use the spinal devices. The only evidence of possible interconnection failures in vivo have been cases where the rods disconnect from the pedicle screws, and these cases seem likely to be caused by complications seen only in the surgical environment and not in the lab. The failures do not seem to be inherent device failures that the current interconnection strength tests as written would detect.
Broken screws do seem to be a problem in vivo and should be addressed. More study into these occurrences could be beneficial. However, the interconnection strength guidelines o f A S T M F are not appropriate tests for these particular failures. There is still a question as to whether interconnection strengths might contribute to loss of correction.
However, the causes, possible correlations, and optimal strengths are unknown.
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More studies need to be done to understand the mechanisms for loss of correction or screw breakage more specifically. These studies should also suggest better test methods that correlate more directly to clinical problems. Otherwise there has not been any other documented direct evidence of an interconnection failure in vivo. Even if one were to be detected, it should be determined that the interconnection failure is not just incidental to or the result of a symptomatic clinical problem. References  Blumenthal, S. A meta-Analysis of Literature Carson, W.
The other used a fixed-free end equivalent assembly unilateral construct. The fixed-fixed assembly consistently produced higher fatigue strength, and a different failure mode with only one exception.
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Freebody diagram analysis of the fixed-fixed end rod-connector assembly revealed that the superior and inferior ends of the rod shared in resisting the applied load, which resulted in lower internal forces and moments than in the inferior rod end o f the fixedfree assembly. Lordotically contoured rods had lower flexion fatigue life than straight rods due to the residual tensile stress from contouring.
Changing ASTM F to fixed-free end assembly testing is recommended since the interconnection loading conditions created are "worst case" and are more clinically relevant particularly at ends o f constructs , and tests can be performed with bent or straight rods. Carson  performed interconnection flexion fatigue tests with both straight and lordotically contoured rods using the unilateral construct shown in Fig. Each end o f the unilateral construct is equivalent to a single fixed-free end assembly since the construct is symmetrical about a mid transverse plane.
Bent rods were used to determine their effect if any on the spinal instrumentation interconnection. Copyright9 by ASTM lntcrnational 63 www. The fixed-fixed rod end assembly consistently produced higher fatigue strength, and a different failure mode with only one exception at the lowest load.