The above findings could be explained as follows: under a small cyclic strain, a fatigue crack was formed at a later part of its life. Spending 50% of fatigue life under the small cyclic strain caused < 50% of accumulated damage, and no crack was initiated. Conversely, fatigue crack initiation occurred early under a large cyclic strain. The crack continued to grow even under a smaller strain. In other words, the smaller strain would cause more harm if applied after a large cyclic strain than when it was applied to the pristine instrument. Comparing the two batches of files, PT F2 demonstrated prolonged lives of 120% and PF #25 .04 maintained even longer lives of 140%. This can explain why the smaller strain formed with the insertion angle A20° can affect PT F2 more than PF #25 .04.<br />Prior studies showed that instrument fatigue life may degrade after clinical use. We still do not know the exact cause underlying this phenomenon, and sometimes it is attributed to surface defects formed during repeated use. By employing the two-step testing, the current study threw more light on the possible mechanisms. On the basis of our results, we suggest that after operating in a severely curved canal, the file should not be used again.<br />The <a href='http://www.e-64d.com/forum.php?mod=viewthread&tid=2614&extra=
'>scanning cytoskeleton </a> microscopy study showed a typical phenomenon of cyclic fatigue similar to that described in a previous study. There were crack initiation, fatigue propagation, and final ductile fracture in both single- and two-step fatigue tests. In the single-step fatigue test, the area of fatigue striation was larger in the lower strained condition (A20°) than in the higher strained condition (A40° and A60°). A larger area of fatigue striation corresponded to a longer fatigue life in this study. The final ductile fracture might happen due to excessive load and strain on the damaged file. The more curved canal applied a higher strain and a larger load on the file. Therefore, <img src="http://farm5.static.flickr.com/4332/37209354325_b1ab4cefd8.jpg
" align="left" width="231" style="padding:10px;"/> both files had lower fatigue lives in the curved simulated canals (A40° and A60°).<br />In the two-step fatigue test, the three fatigue fracture patterns were similar to those in the single-step test. The areas of fatigue striation presented in the two-step conditions of (A60°, A20°) and (A40°, A20°) were larger than those in conditions of (A20°, A40°) and (A20°, A60°), respectively. Compared with the single-step fatigue test, the areas of fatigue striation presented in the two-step conditions of (A60°, A20°) and (A40°, A20°) were similar to those in the single-step test condition of A20°. In the reverse condition, the fracture patterns in two-step test conditions of (A20°, A40°) and (A20°, A60°) were similar to those in the single-step test conditions of A40° and A60°, respectively. These findings indicate that the development of fatigue striation and final ductile fracture was highly correlated with the second step of the two-step fatigue test. However, the first step of fatigue test could cause the initiation of crack. Actually, crack initiation is the crucial step influencing the fatigue life. In the less strained condition A20°, the damage accumulated slowly and thus prolonged the time for the occurrence of a crack. Therefore, fatigue lives were longer in two-step fatigue test conditions of (A20°, A40°) and (A20°, A60°) than in the single-step fatigue test conditions of A40° and A60°, respectively.<br /><br><br />Acknowledgments<br /><br><br />Introduction<br />Growth disorders, associated with <a href='http://en.wikipedia.org/wiki/Growth_hormone
'>growth hormone (GH)</a> in children, include growth hormone deficiency (GHD), idiopathic short stature (ISS), Noonan syndrome, Prader–Willi syndrome, and Turner syndrome. Among these, ISS is defined as a height less than third percentile or two SDs (standard deviations) for age, sex, and population without evidence of nutritional, systemic chronic disease, endocrine, and chromosomal abnormalities. The cause of ISS may be gene mutations and deletions in the SHOX gene for children, the prevalence has been estimated at 1–5%. In addition, a karyotype should be considered in girls with no underlying specific cause of ISS to rule out Turner syndrome.