The temporary functional implants
The basic concept of biodegradable metal implants is providing a mechanical support to the healing process of an injured or diseased tissue and degrading away thereafter. Some clinical problems in cardiovascular, orthopedic, and pediatric care require only temporary support for eventual healing. A stented arterial blockage undergoes vessel remodeling, thus its permanent presence becomes unnecessary if not causing longterm disadvantages . A set of mini plate and screws for bone fracture fixation that degrades after a complete bone reconstruction will avoid local biomechanic disturbance and patient discomfort and nullify the need for a second surgery . Ideally, a biodegradable metal implant will have an optimum compromise between maintaining its mechanical integrity for a required period and progressing its degradation over time, as illustrated in Fig. 2.2.
Referring to Fig. 2.2, the inflammatory process starts rapidly within 1-7 days after fracture followed by soft callus formation in 2-3 weeks. Once the soft callus links fracture tips, the hard callus stage starts and lasts until the fragments are firmly united by new woven bone (3-4 months). Until this stage, the biodegradable metal plate must retain its mechanical integrity. The woven bone is then slowly replaced by lamellar bone through the osteonal remodeling process, which may take a few months to several years until the bone has completely returned to its original morphology [42,43].
Figure 2.2 Illustration of ideal compromise between mechanical integrity and degradation rate of a biodegradable metal bone implant.
In the case of stent applications, the mechanical integrity and degradation must suit the period of the vessel’s healing process. Inflammation occurs following the implantation of a stent for several days and is followed by a granulation period where endothelial cells migrate to cover the injured surface and the proliferation of smooth muscle cells (1-2 weeks). Furthermore, the remodeling period takes place with the deposition of extracellular matrix. The total time required for a stent to mechanically support the whole process has not been clearly defined, but a period of 6-12 months has been suggested [44,45].