Superelasticity of NiTi Ring-Shape Springs induced by Ageing for Cranioplasty Application

The aim of this work was to improve the superelastic behavior of ring-shape springs by increasing and stabilizing the strain range of the stress plateau for long term distraction in cranial bone modeling. Since 2002, the authors started the application of titanium-nickel rings to model the cranium. After the sparing excision of cranium vault sutures in the shape of the letter “H” the compressed ring is given in the sagittal axis oval shape and in this form it is fixed to osseous margins. The ring expansion simultaneously broadens and shortens the cranium  vault. The rings formed from a straight superelastic wire flattened to an ellipse, does not show the presence of a typical force plateau but rather a pseudoelastic loop during loading-unloading. Based on the idea that superelasticity in more complex ring-shape springs may be induced by precipitation process, the further studies were carried out on alloys with higher nickel contents (51.06 at% Ni). The formed rings were welded and aged at optimal temperature and time. The improved superelastic behavior during compression and unloading the rings were obtained by introducing small deformation by drawing the quenched wires before forming the rings and ageing. The TEM studies allow to conclude that the ageing of rings results in hardening the parent phase by  precipitation of the coherent Ni₄Ti₃ particles , which are the source of internal stress and sites of preferable nucleation of the martensite. The very positive clinical reshaping by long-term distraction with the superelastic ring-shape springs were achieved for the young children-under one year and less spectacular in the group of older children.