called peri-implantitis, increasing the chance of long-term failures. long-term steroid use, osteoporosis and other diseases that affect the bones can increase the risk of early failure of implants.: 199 it has been suggested that radiotherapy can negatively affect the survival of implants. nevertheless, a systemic study published in 2016 concluded that dental implants installed in the irradiated area of an oral cavity may have a high survival rate, provided that the patient maintains oral hygiene measures and regular follow ups to prevent complications. biomechanical considerations the long-term success of implants is determined, in part, by the forces they have to support. as implants have no periodontal ligament, there is no sensation of pressure when biting so the forces created are higher. to offset this, the location of implants must distribute forces evenly across the prosthetics they support.: 15–39 concentrated forces can result in fracture of the bridgework, implant components, or loss of bone adjacent the implant. the ultimate location of implants is based on both biologic (bone type, vital structures, health) and mechanical factors. implants placed in thicker, stronger bone like that found in the front part of the bottom jaw have lower failure rates than implants placed in lower density bone, such as the back part of the upper jaw. people who grind their teeth also increase the force on implants and increase the likelihood of failures.: 201–208 the design of implants has to account for a lifetime of real-world use in a person's mouth. regulators and the dental implant industry have created a series of tests to determine the long-term mechanical reliability of implants in a person's mouth where the implant is struck repeatedly with increasing forces (similar in magnitude to biting) until it fails. when a more exacting plan is needed beyond clinical judgment, the dentist will make an acrylic guide (called a stent) prior to surgery which guides optimal positioning of the implant. increasingly, dentists opt to get a ct scan of the jaws and any existing dentures, then plan the surgery on cad/cam software. the stent can then be made using stereolithography following computerized planning of a case from the ct scan. the use of ct scanning in complex cases also helps the surgeon identify and avoid vital structures such as the inferior alveolar nerve and the sinus.: 1199 bisphosphonate drugs the use of bone building drugs, like bisphosphonates and anti-rankl drugs require special consideration with implants, because they have been associated with a disorder called medication-associated osteonecrosis of the jaw (mronj). the drugs change bone turnover, which is thought to put people at risk for death of bone when having minor oral surgery. at routine doses (for example, those used to treat routine osteoporosis) the effects of the drugs linger for months or years but the risk appears to be very low. because of this duality, uncertainty exists in the dental community about how to best manage the risk of bronj when placing implants. a 2009 position paper by the american association of oral and maxillofacial surgeons, discussed that the risk of bronj from low dose oral therapy (or slow release injectable) as between 0.01 and 0.06 percent for any procedure done on the jaws (implant, extraction, etc.). the risk is higher with intravenous therapy, procedures on the lower jaw, people with other medical issues, those on steroids, those on more potent bisphosphonates and people who have taken the drug for more than three years. the position paper recommends against placing implants in people who are taking high dose or high frequency intravenous therapy for cancer care. otherwise, implants can generally be placed and the use of bisphosphonates does not appear to affect implant survival. main surgical procedures basic implant surgical procedurean area with a single missing toothan incision is made across the gingiva, and the flap of tissue is reflected to show the bone of the jaw.once the bone is exposed, a series of drills create and gradually enlarge a site (called an osteotomy) for the implant to be placed.the implant fixture is turned into the osteotomy. ideally, it is completely covered by bone and has no movement within the bone.a healing abutment is attached to the implant fixture, and the flap of gingiva is sutured around the healing abutment. placing the implant most implant systems have five basic steps for placement of each implant:: 214–221 soft tissue reflection: an incision is made over the crest of bone, splitting the thicker attached gingiva roughly in half so that the final implant will have a thick band of tissue around it. the edges of tissue, each referred to as a flap are pushed back to expose the bone. flapless surgery is an alternate technique, where a small punch of tissue (the diameter of the implant) is removed for implant placement rather than raising flaps. drilling at high speed: after reflecting the soft tissue, and using a surgical guide or stent as necessary, pilot holes are placed with precision drills at highly regulated speed to prevent burning or pressure necrosis of the bone. drilling at low speed: the pilot hole is expanded by using progressively wider drills (typically between three and seven successive drilling steps, depending on implant width and length). care is taken not to damage the osteoblast or bone cells by overheating. a cooling saline or water spray keeps the temperature low. placement of the implant: the implant screw is placed and can be self-tapping,: 100–102 otherwise the prepared site is tapped with an implant analog. it is then screwed into place with a torque controlled wrench at a precise torque so as not to overload the surrounding bone (overloaded bone can die, a condition called osteonecrosis, which may lead to failure of the implant to fully integrate or bond with the jawbone). tissue adaptation: the gingiva is adapted around the entire implant to provide a thick band of healthy tissue around the healing abutment. in contrast, an implant can be "buried", where the top of the implant is sealed with a cover screw and the tissue is closed to completely cover it. a second procedure would then be required to uncover the implant at a later date. timing of implants after extraction of teeth there are different approaches to placement dental implants after tooth extraction. the approaches are: immediate post-extraction implant placement. delayed immediate post-extraction implant placement (two weeks to three months after extraction). late implantation (three months or more after tooth extraction). an increasingly common strategy to preserve bone and reduce treatment times includes the placement of a dental implant into a recent extraction site. on the one hand, it shortens treatment time and can improve aesthetics because the soft tissue envelope is preserved. on the other hand, implants may have a slightly higher rate of initial failure. conclusions on this topic are difficult to draw, however, because few studies have compared immediate and delayed implants in a scientifically rigorous manner. one versus two-stage surgery after an implant is placed, the internal components are covered with either a healing abutment, or a cover screw. a healing abutment passes through the mucosa, and the surrounding mucosa is adapted around it. a cover screw is flush with the surface of the dental implant, and is designed to be completely covered by mucosa. after an integration period, a second surgery is required to reflect the mucosa and place a healing abutment.: 190–1 in the early stages of implant development (1970−1990), implant systems used a two-stage approach, believing that it improved the odds of initial implant survival. subsequent research suggests that no difference in implant survival existed between one-stage and two-stage surgeries, and the choice of whether or not to "bury" the implant in the first stage of surgery became a concern of soft tissue (gingiva) management when tissue is deficient or mutilated by the loss of teeth, implants are placed and allowed to osseointegrate, then the gingiva is surgically moved around the healing abutments. the down-side of a two-stage technique is the need for additional surgery and compromise of circulation to the tissue due to repeated surgeries.: 9–12 the choice of one or two-stages, now centers around how best to reconstruct the soft tissues around lost teeth. additional surgical procedures hard tissue reconstructionif bone width is inadequate it can be regrown using either artificial or cadaveric bone pieces to act as a scaffold for natural bone to grow around.when a greater amount of bone is needed, it can be taken from another site (commonly the back of the bottom jaw) and transplanted to the implant site.the maxillary sinus can limit the amount of bone height in the back of the upper jaw. with a "sinus lift", bone can be grafted under the sinus membrane increasing the height of bone. for an implant to osseointegrate, it needs to be surrounded by a healthy quantity of bone. in order for it to survive long-term, it needs to have a thick healthy soft tissue (gingiva) envelope around it. it is common for either the bone or soft tissue to be so deficient that the surgeon needs to reconstruct it either before or during implant placement.: 1084 xenogeneic collagen matrices are used for gingival augmentation after dental implantation. hard tissue (bone) reconstruction main articles: sinus lift and bone grafting bone grafting is necessary when there is a lack of bone. also, it helps to stabilize the implant by increasing survival of the implant and decreasing marginal bone level loss. while there are always new implant types, such as short implants, and techniques to allow compromise, a general treatment goal is to have a minimum of 10 mm in bone height, and 6 mm in width. alternatively, bone defects are graded from a to d (a=10+ mm of bone, b=7–9 mm, c=4–6 mm and d=0–3 mm) where an implant's likelihood of osseointegrating is related to the grade of bone.: 250 to achieve an adequate width and height of bone, various bone grafting techniques have been developed. the most frequently used is called guided bone graft augmentation where a defect is filled with either natural (harvested or autograft) bone or allograft (donor bone or synthetic bone substitute), covered with a semi-permeable membrane and allowed to heal. during the healing phase, natural bone replaces the graft forming a new bony base for the implant.: 223 three common procedures are:: 236 sinus lift lateral alveolar augmentation (increase in the width of a site) vertical alveolar augmentation (increase in the height of a site) other, more invasive procedures, also exist for larger bone defects including mobilization of the inferior alveolar nerve to allow placement of a fixture, onlay bone grafting using the iliac crest or another large source of bone and microvascular bone graft where the blood supply to the bone is transplanted with the source bone and reconnected to the local blood supply.: 5–6 the final decision about which bone grafting technique that is best is based on an assessment of the degree of vertical and horizontal bone loss that exists, each of which is classified into mild (2–3 mm loss), moderate (4–6 mm loss) or severe (greater than 6 mm loss).: 17 orthodontic extrusion or orthodontic implant site development can be used in selected cases for vertical/horizontal alveolar augmentation. soft tissue (gingiva) reconstruction main articles: gingival graft and subepithelial connective tissue graft soft tissue reconstructionwhen mucosa is missing, a free gingival graft of soft tissue can be transplanted to the area.when the metal of an implant becomes visible a connective tissue graft can be used to improve the mucosal height. the gingiva surrounding a tooth has a 2–3 mm band of bright pink, very strong attached mucosa, then a darker, larger area of unattached mucosa that folds into the cheeks. when replacing a tooth with an implant, a band of strong, attached gingiva is needed to keep the implant healthy in the long-term. this is especially important with implants because the blood supply is more precarious in the gingiva surrounding an implant, and is theoretically more susceptible to injury because of a longer attachment to the implant than on a tooth (a longer biologic width).: 629–633 when an adequate band of attached tissue is absent, it can be recreated with a soft tissue graft. there are four methods that can be used to transplant soft tissue. a roll of