Research On Microtia Reconstruction With Nickel-titanium (NiTi) Shape Memory Alloy As Framework

Microtia, a smaller than normal and usually malformed auricle, affects about one in every 7000 to 8000 infants in the general population, occurring more often in right ears and males. The auricle is one of the most complex three-dimensional structures of the external body. The delicate structure of the auricle is defined by the convolutions of the underlying cartilaginous framework. Reconstruction of a congenitally absent ear is a great challenging task to the plastic surgeon. Factors contributing to this challenge include the thin skin and delicate framework. To restore an ear that looks like an ear, familiarity with the anatomical components of the normal auricle is imperative. Most ear reconstructive techniques have been derived from the formula that uses a framework placed beneath the skin to create an ear form.Numerous materials have been used to fabricate the ear framework, but autogenous costal cartilage is the most reliable and commonly used material for ear reconstruction of the framework, and has been proved to be the standard material. It has a demonstrated track record for durability and stability over time but is not without shortcomings. The complications evoked by costal cartilage harvest include the immediateproblems of the considerable pain and discomfort, even pneumothorax, and the delayed issues of chest wall deformity and scarring. Long-term complications in the reconstructed ear primarily relate to resorption of the cartilage framework, which may alter the shape and form of the auricular components.The use of an alloplastic auricular framework to reconstruct the microtic ear has been advocated to provide a more consistent aesthetic result and to avoid rib cartilage donor site morbidity. By using an alloplastic framework, reconstruction may begin at an earlier age because the size of the child's rib cartilage is not necessary.The initial aesthetic results of Silastic framework for auricular reconstruction were often excellent, and without donor-site deformity. However, long-term follow-up demonstrated spontaneous exposure of the implant with failure of the reconstruction in many cases. The use of silicone implants for ear reconstruction were abandoned.The use of porous polyethylene has been recently advocated by Reinisch. Porous polyethylene causes minimal tissue reaction, and its porous quality allows soft-tissue ingrowth, thereby providing greater stability. With technique modification, the performance of these constructs with good results were reported, but this technique remains to be proved safe and reliable.The potential to fabricate autologous parts has recently become ablooming field of scientific endeavor. Proponents for creating complex parts initially sought to render three-dimensional form through the process of molding tissue. Unfortunately, the constructs did not maintain their engineered shape over time because of the lack of a stable substructure. Although promising, microtia reconstruction using a tissue-engineered, prefabricated framework is not yet a practical option.Biomedical applications of Ni-Ti shape memory alloys have been extremely successful because of the functional properties of these alloys, increasing both the possibility and the performance of minimally invasive surgeries. The biocompatibility of these alloys is one of the important points related to their biomedical applications as orthopedic implants , cardiovascular devices , and surgical instruments.The aim of this study was to explore the possibility of nickel-titanium (NiTi) shape memory alloy stent in ear reconstruction in vivo. With this goal, two discrete experimental groups were formed, each comprising 6 New Zealand rabbits. Each animal underwent subcutaneous implantation with a NiTi alloy framework like human auricle under general anesthesia. In 6 animals, the implant was encased after skin expansion; in the other 6, it was directly placed. Implant vascularization was evaluated at months 1, 3, 6, 9,and 12 for histological analysis. The immunohistochemical methods were used to examine expression of VEGF and CD31 in tissue around the implant at 1,3,6,9 and 12 month after implantation,respectively. The NiTi alloy implant was examined microscopically with scanning electron microscopy. The fibrovascular ingrowth rate of implants is determined by bone scanning using 99mTc-PYP 1,3,6,9,12 months after implantation. The analysis of aspects related to the biocompatibility of the NiTi alloy framework is performed by assessing each of their elements in pig's hair, nickel and titanium, separately.One rabbit had exposure of the NiTi alloy framework due to overlying skin flap necrosis, but can be repaired with animal skin without the complete removal of the framework. All the other rabbits tolerated the implant well, and there were no complications. On histopathologic examination, fibrovascularization gradually increased over time. The implant harvested at 1 months showed only partial vascularization, and were completely vascularized at 3 months. The amount of VEGF-positive cells was markedly increased in 6 months and reached the highest at 3 month in the tissue. Compared with the other months , the CD31 positive endothelial cells at 3 month were significantly different. The over-expression of VEGF had a positive correlation of with the higher microvessel density(MVD) count.The results demonstrated that the fibrovascular ingrowth rate of implant assessed by 99mTc-PYP bone scan using ratios of 99mTc-PYP activity in placement regions is more high than the contralateral normal region.