implant, healing typically takes several weeks or months before the implant is fully integrated into the bone. First evidence of integration occurs after a few weeks, while more robust connection is progressively effected over the next months or years. Implants that have a screw-root form design result in bone resorption followed by interfacial bone remodeling and growth around the implant. Implants using a plateau-root form design (or screw-root form implants with a wide enough gap between the pitch of the screws) undergo a different mode of peri-implant ossification. Unlike the aforementioned screw-root form implants, plateau-root form implants exhibit de novo bone formation on the implant surface. The type of bone healing exhibited by plateau-root form implants is known as intramembranous-like healing. Though the osseointegrated interface becomes resistant to external shocks over time, it may be damaged by prolonged adverse stimuli and overload, which may cause implant failure. In studies done using "Mini dental implants," it was noted that the absence of micromotion at the bone-implant interface was needed to enable proper osseointegration. It was also noted that there is a critical threshold of micromotion above which a fibrous encapsulation process occurs, rather than osseointegration. Other complications may arise even in the absence of external impact. One issue is growth of cement. In normal cases, absence of cementum on the implant surface prevents attachment of collagen fibers. This is normally the case due to the absence of cementum progenitor cells in the area receiving the implant. However, when such cells are present, cement may form on or around the implant surface, and a functional collagen attachment may attach to it. Advances in materials engineering: metal foams Since 2005, a number of orthopedic device makers have introduced products with porous metal construction. Clinical studies on mammals have shown that porous metals, such as titanium foam, may allow formation of vascular systems within the porous area. For orthopedic uses, metals such as tantalum or titanium are often used, as these metals have high tensile strength and corrosion resistance with excellent biocompatibility. The process of osseointegration in metal foams is similar to that in bone grafts. The porous bone-like properties of the metal foam contribute to extensive bone infiltration, allowing osteoblast activity to take place. In addition, the porous structure allows for soft tissue adherence and vascularization within the implant. These materials are currently deployed in hip replacement, knee replacement and dental implant surgeries.