Biodegradable Hard Tissue Implants

Aging population and decreased physical activity due to increased life standards are two prevalent and inevitable factors that cause decrease in bone mineral mass, bone quantity, and muscle strength in the population. These consequences increase the incidence of bone fracture throughout the life of individuals. Although the bone has a great regenerative capacity compared to most other tissues or organs in the body, a proper healing of the bone requires appropriate alignment and fixation of fractured fragments throughout the process. There are different techniques and tools to provide bone substitutes with those properties. Most of the available fixation tools are made from non-eroding metals due to their inherent stiffness and toughness, the properties necessitated by the load bearing function of the skeletal system. Ideally, however, an implant should be temporary and be eliminated from the body as soon as its function is no longer necessary due to potential risks like late stage infection, bone resorption or immune reactions. For bone implants, due to the need for stabilization of fixation devices to the surrounding bone using screws or nails, removal operations may cause severe morbidity to the newly repaired fracture site. Another equally important problem with use of metal fixation devices is their superior mechanical properties that outweigh those of bone, lead the newly forming bone tissue not to be subjected to mechanical stimulation, which is a necessary requirement for bone forming machinery. Considering these problems, different biodegradable or bioerodible materials were suggested to be used in the production of temporary bone fracture fixation devices. This paper reviews the developments and trends in the field of biodegradable hard tissue implants, available materials, and their suitability to the host bone tissue.