골흡수와 인공결함 개선을 위한 저 탄성계수와 낮은 자화율을 갖는 지르코늄 합금 연구

Abstract

ABSTRACT

Research on Zirconium alloys with low elastic modulus and low magnetic susceptibility for improving bone resorption and susceptibility artifacts

Chang-Yong Lee D.D.S., Ms.D. Advisor : Prof. Ko Yeong-Mu D.D.S., Ph.D. Department of Dentistry, Graduate School of Chosun University

The objective of this study is to fabricate Zr-Cu alloys with low elastic modulus and low magnetic susceptibility for dental biomaterials. Many metals and alloys, such as gold, stainless steel, Co-Cr alloys, and Ti-based alloys are now commonplace within the oral implant industry. Stress shielding results in the reduction in bone density as a result of the removal of typical stress from the bone by an implant. In general, the bone in a healthy person will remodel in response to the loads it is placed under. Therefore, if the loading on a bone decreases, the bone will become less dense and weaker because there is no stimulus for continued remodeling that is required to maintain bone mass. Although Ti-based alloys have been widely used as implant components and devices, its elastic modulus is much higher than that of natural human bone. Serious damage may be easily caused in the human body when the modulus of implant materials does not match the natural bone due to the stress shielding effects. Therefore, in recent, persistent efforts have been done to obtain low elastic modulus of biological hard tissue materials to effectively transfer stress to the surrounding bones. Recently, magnetic resonance imaging (MRI), which is a medical imaging diagnosis tool used in radiology to investigate the anatomy and function of the body, has become an important diagnostic technique in orthopedics and brain surgery. Dental implants are used in the reconstruction of prosthetic defects in the oral cavity and in modern surgical methods in traumatology. These appliances present a concern from the viewpoint of the patient’s safety and quality of MRI data. Metallic implants, such as stainless steel, Co-Cr alloys, and Ti alloys, become magnetized in the strong magnetic field of the MRI instrument and produce artifacts (magnetic susceptibility artifacts) on the image. The areas that cause the artifacts on images are closely related to the magnetic susceptibility. This susceptibility artifact is caused by metals, such as Ti, Co, and stainless steels, inside the body. Therefore, metallic implants with a low magnetic susceptibility are preferable for surgery performed with the assistance of an MRI. Recently, Zr-based alloys have attracted interest as biomaterials due to their excellent mechanical properties and low magnetic susceptibility. The magnetic susceptibility of Zr-Nb was half that of Ti-6Al-4V. In addition, the magnetic susceptibility of Zr-Mo was reported almost one-third that of commercially pure Ti and Ti-6Al-4V alloy. However, there are limited reports concerning metallic biomaterials with low magnetic susceptibility for dental applications. The magnetic susceptibility of the Zr-Cu binary alloys was extremely low, approximately 10-7; this level is approximately one order less than that of pure Zr and other commercialized Ti-based metallic biomaterials. The Zr-Cu binary alloy exhibited moderate compressive strength (1261-1565 MPa), yield stress (432-595 MPa), favorable elongation (14-34%), high elastic energy (7.2-19.3 MJ/m3) and low elastic modulus (20-28 GPa). Consequently, Zr-Cu binary alloys have the potential to be used as biomaterials with nullifying magnetic properties for magnetic resonance imaging diagnosis and a good combination of mechanical properties indicates them potential biomaterials for biological hard tissue materials

---------------------------------------------------------------------------- Keywords : Dental biomaterials, Elastic modulus, Magnetic susceptibility, Zirconium alloy, Stress shield|결 론

치과용 생체재료로써 큰 사용가치가 있는 재료인 지르코늄 합금은 기존의 상업용 생체재료와 비교하였을 때, 매우 낮은 자화율을 가지며, 인체의 뼈와 유사한 수준의 탄성계수를 갖는다. 생체재료에 대하여 자기적 및 기계적 특성을 평가하기 위하여 Zircaloy4-xCu 합금 시편을 설계 및 제작하여 다음과 같은 결과를 얻었다.

1) Zircaloy4-xCu 합금의 자화율이 매우 낮은 수준 (10-6cm3g-1) 임을 확인하였으며, 이는 기존의 상용 생체재료로 널리 쓰이는 Ti합금, Co합금 등의 금속소재에 비해 약 1/10 수준의 낮은 자화율로 MRI 진단시 자화율 인공결함(susceptibility artifact)을 방지하는 것이 가능하게 된다. 2) Zircaloy4-xCu 합금의 압축강도는 1261-1565 Mpa이며 각각의 시험편에서 20-28 GPa에 해당하는 영률을 나타내었다. 이러한 결과는 인체의 뼈와 유사한 수준의 탄성계수 (15-30 GPa)이므로 응력차폐 (stress shield) 효과에 대한 극복이 가능할 것으로 판단된다. 3) Zircaloy4-xCu 합금에서 미세조직적으로 기지부에는 a-Zr과 계면에는 Zr2Cu 조대한 공정상을 확인 하였으며, Cu의 첨가에 따라 금속간 화합물인 Zr2Cu 공정상이 증가하고 15%Cu 첨가하면 약 40%의 면적 분율로 Zr2Cu 공정상이 증가하였다. 4) Cu를 첨가한 합금에서는 압축강도와 연신률 그리고 인성이 감소하였으나, 항복강도와 경도는 증가하였다. 이는 기지인 a-Zr에 상대적으로 경한 Zr2Cu 공정상의 증가에 따른 결과이고 인장하중이 작용 시 a-Zr과 Zr2Cu 계면이 취약해져 균열이 발생하고 연신율과 인성이 감소하였다. 5) Cu의 함량이 증가함에 따라 탄성에너지는 증가하고 10%Cu에서 최대치를 나타내었으나, 인성은 5%Cu 이후 급격하게 감소하였다. 이러한 탄성에너지의 증가는 항복강도의 증가 때문이며 인성치의 감소는 Zr2Cu 공정상 증가로 인한 a-Zr 기지와 Zr2Cu 계면에서의 파단 때문이다.