Endosseous dental implants are titanium fixtures that are placed in edentulous ridges to serve as support for fixed or removable dental prostheses used to restore dentition.
Endosseous dental implants
There are many commercial brands of dental implants that may vary slightly in design, surface treatment, and other qualities.
The following are among the most used commercial brands; the variety of products offered by manufacturers can vary in implant design, surface treatment, and many other features that can enhance implant quality for any particular case.
Branemark System™, MKII And KIV (External Hex Connection)
Nobel Speedy™ Groovy (External Hex Connection)
Branemark System™ Zygoma (External Hex Connection)
Nobel Active™ (Internal Conical Connection)
Nobel Replace™ (Tri-Channel Connection)
Replace™ Select (Tri-Channel Connection)
NobelSpeedy™ Replace (Tri-Channel Connection)
Standard (Soft Tissue level)
Standard Plus (Soft Tissue level)
Tapered Effect (Soft Tissue level)
Tapered Screw-Vent Family
Tapered Swiss Pluss
Spline Reliance™ Ha Cylinder
Nanotite Certain Prevail (Internal Connection)
Osseotite Certain Prevail (Internal Connection)
Osseotite XP © Certain (Internal Connection)
Full Osseotite Certain (Internal Connection)
Full Osseotite XP Certain (Internal Connection)
Nanotite (External Connection)
Osseotite (External Connection)
Osseotite XP (External Connection)
Full Osseotite (External Connection)
Full Osseotite XP (External Connection)
The implant body is the component that is placed in the bone (endosseous component). These are screw-shaped fixtures that are inserted into tapered holes previously prepared with corresponding drills provided by manufacturing companies. Many designs also have a tapered shape that creates a wedging effect as the implant body is seated.
The single-part design penetrates mucosa from the time of placement.
In a multipart design, various mechanical linkages are used to facilitate the joining of the different components and mechanical integrity of the joint. A hexagonal socket provides resistance to rotation, while a tapered joint provides resistance and seal.
Placed immediately after the implant body placement, the cover screw is screwed on top of the implant where the implant body–abutment joint is during the healing phase of the implant.
Once the implant has properly osseointegrated to the surrounding bone, the implant is exposed and the cover screw is removed. The healing abutment is screwed in its place to project through the mucosa and to maintain patency of the mucosal penetration.
The transmucosal abutment is the link between the implant body and the prosthesis, usually made of titanium. It can be provided in various heights and can be manufactured or prefabricated.
Titanium is the material of choice, as it has good mechanical properties. Titanium is corrosive-resistant and biocompatible. When in contact with oxygen, a 2- to 10-nm oxide layer is formed on the surface, which has been described as osteoconductive. Titanium alloys have been found to have better strength (Ti-6Al-4V).
Candidates for dental implants and mini-implants include partially and totally edentulous patients with proper bone height and width for implant placement.
An implant should have at least 1.5-2 mm of bone between the body of the implant and an adjacent tooth. At least 3 mm should separate two implants. If there is not enough bone height or width, bone grafting to the site can be considered.
Absolute contraindications (medical-local) include the following:
Poor oral hygiene and patient compliance
Active infection or other pathology at the implant site
Medical conditions that prevent safe surgery
Relative contraindications (medical-local) are as follows:
Type 2 diabetes mellitus (not well controlled)
Significant tobacco consumption
Severely resorbed ridges
History of radiation to implant site
Inferior alveolar nerve and maxillary sinus position
Active periodontal disease
Parafunctional dental habits
Unbalanced jaw relationship
Clinical Trial Evidence
In the 1960s, Branemark placed titanium oculars in the femurs of rabbits; these osseointegrated in the femur after a period of healing.  Studies showed that direct anchorage of implants to bone could be achieved and more importantly elucidated how this osseointegration could be predictably reproduced. He further pursued its clinical application.
In 1965, a two-stage implant technique was introduced. First, the implant was placed surgically in the bone with the provision of soft-tissue coverage to allow the implant to properly integrate with the surrounding bone for a period of time without being subjected to a workload. Once the appropriate interval had elapsed without any signs of complication (eg, infection, soft tissue dehiscence), the implant was uncovered during a second surgical procedure and allowed to be loaded. This was an improvement on all previous implant designs.
Since Branemark’s studies were performed, implants have been widely studied and accepted as a predictable treatment for restoring dentition.
Albrektsson et al (1988) performed a multicenter study evaluating the survival rate of osseointegrated implants at various intervals. They reported success rates for implants placed in the mandible of 97.3% after 1 year, 96% after 3 years, and 92.8% after 5 years. For the maxilla, they reported 93.1% after 1 year, 89% after 3 years, and 100% after 5 years. 
Adell et al (1990) reported on a long-term study of osseointegrated implants placed in edentulous patients. More than 95% of maxillary placed implants had continuous prothesis stability at 5 and 10 years and at least 92% for 15 years. For the mandible, they reported 99% for all intervals. 
In their relatively recent study, Charyeva et al evaluated the long-term survival and success rates of dental implants and reported 96% survival and 94.3% success rate after a follow-up period of up to 6 years. 
Totally edentulous patients
Removable prostheses are used when regular dentures are not stable enough. The use of implants as support for the dentures provides better stability.
Fixed prosthesis: In patients who fail to adapt to regular or “classic” dentures owing to lack of support and retention, implants are a viable option to provide high stability, cosmesis, and proper function.
Partially edentulous patients
Implant placement is becoming the standard of care among patients with single edentulous sites, since it avoids the loss of tooth structure of the adjacent teeth that a classic partial fixed prosthesis (bridge) requires. In addition, implants can be used as abutments to restore large edentulous spaces without the need to alter the adjacent teeth, so they can be used as support for partial fixed prostheses.
Success criteria for dental implants comprise the following determinants  :
The resultant implant support does not preclude the placement of a planned functional and esthetic prosthesis that is satisfactory to both patient and the clinician
There is no pain, discomfort, altered sensation, or infection attributable to the implants
Individual unattached implants are immobile when tested clinically
The mean vertical bone loss is less that 0.2 mm annually following the first year of function
After the surgical procedure is complete, several steps need to be taken in order to ensure that the risk of implant failure and other complications are minimized, as follows:
A radiograph of the site is recommended to evaluate the positioning of the implant.
Instruct the patient to maintain a liquid to soft diet for 5-7 days following the implant placement.
Implants should be allowed to heal for 4-6 months before loading to allow for osseointegration.
Implant mobility: If the implant is mobile immediately after placement, this can be a sign of poor bone quality and quantity.
Postimplant pain: If pain occurs immediately after surgery, it can be a sign of nerve involvement, inflammation, or thermal trauma. If it occurs later, periimplant infection or excessive pressure from the temporary prosthesis may be the cause.
Paresthesia: This results from trauma to the nerve (more common in the mandible). It usually subsides when no direct injury to the neurovascular nerve bundle has occurred.
Infection: Infections are rare.
Fractures: A fracture of the implant body can be associated with high occlusal overloading, external trauma, or significant horizontal bone loss. Fracture of the screws may be related to overload, poorly aligned components, poor component fit, or excessive or inadequate tightening torque. Attempts to remove the screw should be made without damaging the implant body.
Disturbance of surrounding anatomical structures: Mandibular fractures, nasal floor/sinus perforations, cortical plate perforations, and devitalization of adjacent teeth can be avoided with adequate planning and proper surgical technique.
A study investigated if the patient can have a role in reporting early peri-implant complications. This study demonstrated that by using validated questions, an educated patient can perceive peri-implant health/disease. The authors also add that this can play a role in the early diagnosis of peri-implant complications. [6, 7]
Exposed implant during healing time: This can increase the risk of implant failure and may indicate excessive pressure from temporary prosthesis. It can also result from poor design of the surgical flap or tension in flap closure. The patient should be instructed to clean thoroughly around the area, and the restorative dentist should relieve any excessive pressure from temporary prosthesis.
Failure to integrate: This is characterized by loose implants, typically associated with predisposing causes.
Periimplant mucositis: This characterized by inflammation of soft tissues adjacent to implant. This can result from mechanical irritation or a lack of sufficient keratinized tissue (eg, calculus, foreign bodies within the periimplant sulcus, poor fit between abutment and implant body). The usual treatment involves removal of the irritant; the implant can be taken out of function by removing the restoration placed above the implant out of occlusion until there is resolution. A soft-tissue graft is often needed to ensure that keratinized tissue surrounds the implant.
Periimplantitis: This is characterized by loss of bone-implant contact due to infection. The condition can present with or without pain and is usually diagnosed by clinical examination along with radiographic evaluation. It can be caused by local or systemic problems such as infection, tobacco use, diabetes (poorly controlled), mechanical irritation, and mechanical overloading of the implant, among others. Treatment usually involves the removal of mechanical and bacterial cause. If enough bone has been lost, the implant may need to be removed.
Fracture or distortion of prosthetic components: This can be caused by occlusal overload and/or inadequate construction/fit of the prosthetic components.
Thread exposure: This is associated with superficial implant placement, marginal bone loss, and thermal trauma. It can affect aesthetics and may necessitate covering surgically or with a soft-tissue graft. See the image below.
Risk Factors Associated with Implant Failure
Successful outcome of dental implant therapy and long-term survival of implants are realistic expectations if certain surgical and prosthodontic requirements are met. However, failures still occur due to variety of reasons and certain conditions are associated with an increased risk. Among these, patient related risk factors are more pronounced. The correlation between many of these factors and implant failure has been the subject of several studies.
Chen et al  evaluated the association between several patient-related factors and implant failure and reported higher risk in patients with a history of smoking and/or radiotherapy. The same study however, failed to identify a significant correlation of diabetes or osteoporosis and implant failure. Likewise, Liddlelow and Klineberg  previously stated that only inconclusive evidence exists that diabetic patients have a higher failure rate. A common finding in these studies is the association of increased risk of dental implant failure and history of radiotherapy. Ihde et al  previously reported that implants placed in irradiated bone exhibited a 2-3 times greater failure rate compared to non-irradiated bone with doses above 50 gy having a significantly higher failure rate. Moreover, they suggested that failure is more likely when implants are placed in the maxilla. The association between osteoporosis and the risk of dental implant failure was not found to be significant as reasonable success was observed in osteoporotic patients treated with adapted preparation and extended healing times.
In their recent review, Chrcanovic et al  suggested that implants in heavy smokers and patients with low bone quality and quantity at the implant site are correlated with increased failure rate whereas other local or systemic factors including compromised medical status and periodontal condition are weakly associated with implant failure. Liddlelow and Klineberg similarly found only weak evidence that supports the theory that history of periodontitis is associated with an increase in peri-implant disease.
Taking these findings into consideration, it can be concluded that dental implants can safely be placed in patients with systemic comorbidities if these conditions are taken into account and properly addressed. However, particular caution should be exercised when managing patients with a history of radiotherapy or heavy smoking.