Transcript Slide 1
Categories of Biomaterials Bioinert materials: do not interact with biological systems. bone screws and plates, knee prostheses and tooth implants Bioactive materials are durable materials that can bind chemically with the surrounding bones and in some cases even with soft tissue Biodegradable materials degrade on implantation to the body. Desirable is that the material degrades at the same rate at which the host tissue regenerates Different Bioceramics • Carbon, Yttria Stabilized Zirconia • Articulating surfaces in joint replacements • high surface finish and its excellent wear resistance, alumina is often used for wear surfaces in joint replacement prostheses. Such applications include femoral heads for hip replacements and wear plates in knee replacements. • Bone Spacers • Porous alumina may also be used to replace large sections of bone that have been removed for reasons such as cancer. These may take the shape of rings that are concentric around a metallic pin, inserted up the centre of the remaining bone itself. The porous nature of these implants will allow new bone to grow into the pores, effectively using the alumina as a scaffold for new bone formation. • Dental Applications • Bioactive glass, Glass Ceramics, Hydroxy Apatite • Hydroxy apatite, calcium phosphates Problems: • The method of fixation has suffered from limitations, both theoretical and real. – Bone cement can fragment and the formation of particles at the cement – bone interface can result in osteolysis and loosening. – Osteolytic reactions have also been related to wear debris – Differing metabolic activities • Solution – The bioactive properties of ceramics such as hydroxyapatite, some calcium phosphates and various types of bioactive glass, are well known. – When placed in bone tissue, these materials promote bone formation, and bond to bone at various rates. – plasma-sprayed hydroxyapatite coatings. Calcium phosphate ceramics • Include several materials which differ not only in their chemical composition, but also in their specific surface area, crystal structure and macro- and microporosity. • There are differences due to variations in the calcium to phosphate ratio; tricalcium phosphate, hydroxyapatite and tetracalcium phosphate have Ca/P ratios of 1.5, 1.67 and 2 respectively, and there are other materials with ratios in between these • Plasma spraying can have a profound effect on the chemical and physical characteristics of the deposited coating and few commercial coatings are alike. – Fortunately, the compositional and structural changes which result from the spraying usually enhance the bone-forming properties of hydroxyapatite, but at the price of increasing the rate of progressive resorption of the coating with time. This would not necessarily be detrimental if the function of the coating was simply to stimulate bone formation How do Bioactive and Resorbable Bioceramics help bone formation • The bioceramic provides the right environment for the new bone to grow into. • They also have a special chemical composition that allows a type of cell called osteoblasts - responsible for bone production - to attach to the ceramic’s surface, and start generating new bone. The interconnected tiny holes within the bioceramic structure facilitate the proliferation of the cell network, and the growth of the bone, within the synthetic scaffold. • The calcium content of the bioceramic provides the inorganic component that new bone requires to develop its mineral-like structure. • The complex processes underlying bone’s astonishing capability to regenerate are only partially understood. • Each new finding in our research throws ten new questions that need an answer, and so on. This is proving to be the major impediment to the further development of bioceramics. At the moment, they are improving only slowly, by trial and error: the researchers slightly change the properties of the material, and then study the biological response to the synthetic material. • • • • • Calcium Phosphate Bioceramics There are several calcium phosphate ceramics that are considered biocompatible. Of these, most are resorbable and will dissolve when exposed to physiological environments. Some of these materials include, in order of solubility: Tetracalcium Phosphate (Ca4P2O9) > Amorphous calcium Phosphate > alphaTricalcium Phosphate (Ca3(PO4)2) > beta-Tricalcium Phosphate (Ca3(PO4) 2) >> Hydroxyapatite (Ca10(PO4)6(OH)2) Unlike the other calcium phosphates, hydroxyapatite does not break down under physiological conditions. In fact, it is thermodynamically stable at physiological pH and actively takes part in bone bonding, forming strong chemical bonds with surrounding bone. This property has been exploited for rapid bone repair after major trauma or surgery. While its mechanical properties have been found to be unsuitable for load-bearing applications such as orthopaedics, it is used as a coating on materials such as titanium and titanium alloys, where it can contribute its 'bioactive' properties, while the metallic component bears the load. Such coatings are applied by plasma spraying. However, careful control of processing parameters is necessary to prevent thermal decomposition of hydroxyapatite into other soluble calcium phosphates due to the high processing temperatures.