and maintenance is well within statistical variance for this type of procedure. more often, osseointegration failure occurs when a patient is either too unhealthy to receive the implant or engages in behavior that contraindicates proper dental hygiene including smoking or drug use. long term the long-term complications that result from restoring teeth with implants relate, directly, to the risk factors of the patient and the technology. there are the risks associated with appearance including a high smile line, poor gingival quality and missing papillae, difficulty in matching the form of natural teeth that may have unequal points of contact or uncommon shapes, bone that is missing, atrophied or otherwise shaped in an unsuitable manner, unrealistic expectations of the patient or poor oral hygiene. the risks can be related to biomechanical factors, where the geometry of the implants does not support the teeth in the same way the natural teeth did such as when there are cantilevered extensions, fewer implants than roots or teeth that are longer than the implants that support them (a poor crown-to-root ratio). similarly, grinding of the teeth, lack of bone or low diameter implants increase the biomechanical risk. : 27–51 finally there are technological risks, where the implants themselves can fail due to fracture or a loss of retention to the teeth they are intended to support.: 27–51 long-term failures are due to either loss of bone around the tooth and/or gingiva due to peri-implantitis or a mechanical failure of the implant. because there is no dental enamel on an implant, it does not fail due to cavities like natural teeth. while large-scale, long-term studies are scarce, several systematic reviews estimate the long-term (five to ten years) survival of dental implants at 93–98 percent depending on their clinical use. during initial development of implant retained teeth, all crowns were attached to the teeth with screws, but more recent advancements have allowed placement of crowns on the abutments with dental cement (akin to placing a crown on a tooth). this has created the potential for cement, that escapes from under the crown during cementation to get caught in the gingiva and create a peri-implantitis (see picture below). while the complication can occur, there does not appear to be any additional peri-implantitis in cement-retained crowns compared to screw-retained crowns overall. in compound implants (two stage implants), between the actual implant and the superstructure (abutment) are gaps and cavities into which bacteria can penetrate from the oral cavity. later these bacteria will return into the adjacent tissue and can cause periimplantitis. criteria for the success of the implant supported dental prosthetic varies from study to study, but can be broadly classified into failures due to the implant, soft tissues or prosthetic components or a lack of satisfaction on the part of the patient. the most commonly cited criteria for success are function of at least five years in the absence of pain, mobility, radiographic lucency and peri-implant bone loss of greater than 1.5 mm on the implant, the lack of suppuration or bleeding in the soft tissues and occurrence of technical complications/prosthetic maintenance, adequate function, and esthetics in the prosthetic. in addition, the patient should ideally be free of pain, paraesthesia, able to chew and taste and be pleased with the esthetics. the rates of complications vary by implant use and prosthetic type and are listed below: single crown implants (5-year) implant survival: 96.8 percent crown survival: metal-ceramic: 95.4 percent; all-ceramic: 91.2 percent; cumulative rate of ceramic or acrylic veneer fracture: 4.5 percent peri-implantitis: 9.7 percent up to 40 percent peri-implant mucositis: 50 percent implant fracture: 0.14 percent screw or abutment loosening: 12.7 percent screw or abutment fracture: 0.35 percent fixed complete dentures progressive vertical bone loss, but still in function (peri-implantitis): 8.5 percent failure after the first year 5 percent at five years, 7 percent at ten years incidence of veneer fracture at: 5-year: 13.5 to 30.6 percent, 10-year: 51.9 percent (32.3 to 75.5 percent with a confidence interval at 95 percent) 15-year: 66.6 percent (44.3 to 86.4 percent with a confidence interval at 95 percent) 10-year incidence of framework fracture: 6 percent (2.6 to 9.3 percent with a confidence interval at 95 percent) 10-year incidence of esthetic deficiency: 6.1 percent (2.4 to 9.7 percent with a confidence interval at 95 percent) prosthetic screw loosening: 5 percent over five years to 15 percent over ten years the most common complication being fracture or wear of the tooth structure, especially beyond ten years with fixed dental prostheses made of metal-ceramic having significantly higher ten-year survival compared those made of gold-acrylic. removable dentures (overdentures) loosening of removable denture retention: 33 percent dentures needing to be relined or having a retentive clip fracture : 16 to 19 percent history greenfield's basket: one of the earliest examples of a successful endosseous implant was greenfield's 1913 implant system while studying bone cells in a rabbit tibia using a titanium chamber, branemark was unable to remove it from bone. his realization that bone would adhere to titanium led to the concept of osseointegration and the development of modern dental implants. the original x-ray film of the chamber embedded in the rabbit tibia is shown (made available by branemark).panoramic radiograph of historic dental implants, taken 1978 sapphire blade type implants used in the past there is archeological evidence that humans have attempted to replace missing teeth with root form implants for thousands of years. remains from ancient china (dating 4000 years ago) have carved bamboo pegs, tapped into the bone, to replace lost teeth, and 2000-year-old remains from ancient egypt have similarly shaped pegs made of precious metals. some egyptian mummies were found to have transplanted human teeth, and in other instances, teeth made of ivory.: 26 etruscans produced the first pontics using single gold bands as early as 630 bc and perhaps earlier. wilson popenoe and his wife in 1931, at a site in honduras dating back to 600 ad, found the lower mandible of a young mayan woman, with three missing incisors replaced by pieces of sea shells, shaped to resemble teeth. bone growth around two of the implants, and the formation of calculus, indicates that they were functional as well as esthetic. the fragment is currently part of the osteological collection of the peabody museum of archaeology and ethnology at harvard university. in modern times, a tooth replica implant was reported as early as 1969, but the polymethacrylate tooth analogue was encapsulated by soft tissue rather than osseointegrated. the early part of the 20th century saw a number of implants made of a variety of materials. one of the earliest successful implants was the greenfield implant system of 1913 (also known as the greenfield crib or basket). greenfield's implant, an iridioplatinum implant attached to a gold crown, showed evidence of osseointegration and lasted for a number of years. the first use of titanium as an implantable material was by bothe, beaton and davenport in 1940, who observed how close the bone grew to titanium screws, and the difficulty they had in extracting them. bothe et al. were the first researchers to describe what would later be called osseointegration (a name that would be marketed later on by per-ingvar brånemark). in 1951, gottlieb leventhal implanted titanium rods in rabbits. leventhal's positive results led him to believe that titanium represented the ideal metal for surgery. in the 1950s research was being conducted at cambridge university in england on blood flow in living organisms. these workers devised a method of constructing a chamber of titanium which was then embedded into the soft tissue of the ears of rabbits. in 1952 the swedish orthopaedic surgeon, per-ingvar brånemark, was interested in studying bone healing and regeneration. during his research time at lund university he adopted the cambridge designed "rabbit ear chamber" for use in the rabbit femur. following the study, he attempted to retrieve these expensive chambers from the rabbits and found that he was unable to remove them. brånemark observed that bone had grown into such close proximity with the titanium that it effectively adhered to the metal. brånemark carried out further studies into this phenomenon, using both animal and human subjects, which all confirmed this unique property of titanium. leonard linkow, in the 1950s, was one of the first to insert titanium and other metal implants into the bones of the jaw. artificial teeth were then attached to these pieces of metal. in 1965 brånemark placed his first titanium dental implant into a human volunteer. he began working in the mouth as it was more accessible for continued observations and there was a high rate of missing teeth in the general population offered more subjects for widespread study. he termed the clinically observed adherence of bone with titanium as "osseointegration".: 626 since then implants have evolved into three basic types: root form implants; the most common type of implant indicated for all uses. within the root form type of implant, there are roughly 18 variants, all made of titanium but with different shapes and surface textures. there is limited evidence showing that implants with relatively smooth surfaces are less prone to peri-implantitis than implants with rougher surfaces and no evidence showing that any particular type of dental implant has superior long-term success. zygoma implant; a long implant that can anchor to the cheek bone by passing through the maxillary sinus to retain a complete upper denture when bone is absent. while zygomatic implants offer a novel approach to severe bone loss in the upper jaw, it has not been shown to offer any advantage over bone grafting functionally although it may offer a less invasive option, depending on the size of the reconstruction required. small diameter implants are implants of low diameter with one-piece construction (implant and abutment) that are sometimes used for denture retention or orthodontic anchorage.