Adjusting the Subsurface Properties of Biocompatible Magnesium-Calcium Alloys as Absorbable Implant Material by Machining Processes

摘要

Several million people suffer bone fractures caused by accidents or diseases annually. The number of fractures caused by age-related diseases such as osteoporosis will even increase in the industrial nations within the next years due to the demographic change. Many of those complex fractures have to be surgically fixated by internal bone implants. Traditional methods of osteosynthesis or osteotomy use permanent metal implants e.g. bone screws and bone plates made of steel or titanium alloys. This internal fixation involves several disadvantages. Conventional metal implants are too stiff, the Young's modulus of cobalt-chromium or titanium alloys ranges between 100 and 200 GPa, the corresponding value of cancellous bone is below 60 GPa. This leads to a stress-shielding of the fractured bone. Rigid metallic fixation deprives the bone of its normal stress, which suppresses the bone remodeling and therefore the healing process of the fracture may be impaired. Consequently, delayed unions of bone fragments, non-unions and refractures after metal removal are reported. Accordingly permanent metal implants have to be excised. Usually, metal implants should be removed 1 or 2 years after the first operation, only in patients older than 60 years it is acceptable to leave metal in situ. Therefore, another surgical intervention is necessary with all its personal, medical, social and economical consequences and costs [1-5]. Biodegradable implants, which dissolve in the human organism, represent an appropriate solution. Polyglycolide (PGA), polylactide (PLA) and polydioxanone (PDA) are synthetic biodegradable polymers that have been clinically used as surgical materials during the last decades. However, these materials exhibit insufficient mechanical properties, when used as bone implants with conventional geometries [1, 2, 6].