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The term osseointegration as it is applied to endosseous dental implants is a direct functional and structural link between living bone and a load-bearing implant.1 It describes the state of integration between osseous tissues and the titanium surface of a dental implant to solidly incorporate the dental implant to the bone with no apparent soft-tissue interface.2 Various researchers have proposed criteria to help a clinician gauge an implant outcome. They include an unattached implant being immobile when tested clinically; no evidence of peri-implant radiolucency present when assessed on an undistorted radiograph; mean vertical bone loss less than 0.2 mm annually after the first year of service; no persistent pain, discomfort, neuropathy, paresthesia, or infection attributable to the implant; esthetically and phonetically implant design that is pleasing to the patient and dentist; and healthy periodontal parameters such as lack of bleeding on probing.3-5
Implant failure refers to the performance of an implant when it falls below an acceptable clinical level based on qualitative measurements.6 This definition encompasses a spectrum of clinical situations, which include surviving, ailing, failing, and failed. A surviving implant is still in the patient’s mouth but fails to meet the criteria of success.7 However, an ailing implant refers to a clinically stable implant that is affected by bone loss and pocket depth.8 The failing implant is characterized by bone loss of more than 0.2 mm after the first year of loading, pocket depths exceeding 5 mm, bleeding on probing, infection, and indication that bone-loss patterns are progressing irrespectively of therapy without mobility.6,9 Clinical parameters used to assess the state of a failing implant include progressive marginal bone loss, progressive marginal infection, and progressively increasing pocket-probing depth.10 A failed dental implant is defined by a loss of osseointegration, including a thin peri-implant radiolucency, implant mobility, and a complete loss of the bone-to-implant interface.10 The treatment is removal,10 and implant failure may be slow and progressive.
Endogenous and Exogenous Factors
Endogenous and exogenous factors may contribute to implant failures. Endogenous factors are systemic and local factors that include uncontrolled diabetes mellitus, Crohn’s disease, and osteoporosis.11-13
Bone quality and quantity are more examples of endogenous factors. Researchers have classified bone quality from Type I to Type IV.14,15 Type I is the densest, comprised primarily of cortical bone, and is found primarily in the anterior mandible. Type II is in the posterior mandible and Type III is in the anterior maxilla. Type IV is the most porous, composed primarily of cancellous bone, and found primarily in the posterior maxilla. Type IV bone is associated with the highest dental implant failure rate.14,15
Another endogenous factor, smoking, has been associated with higher implant failure rates, complications, and altered peri-implant tissue conditions.16,17 This has been seen particularly with heavy smokers compared to nonsmokers.18
Occlusal overload can also adversely affect dental implants, resulting in marginal bone loss and implant failure.10,19 This also raises concern as it relates to prosthetic design, especially with cantilever restorations and when forces are applied off the long axis of the implant. These failures may also be associated with mechanical failures such as fracture of the implant fixture or the prosthetic screw. Such failures are often associated with occlusal overload.
Having active periodontitis or a previous history of periodontitis are other endogenous factors that may contribute to implant failure.20,21 An increased risk for cross infection between natural teeth with periodontitis and sites receiving dental implants is possible; thus, successful treatment of periodontitis prior to the placement of implants is imperative.
Exogenous factors are related to operator or biomechanical issues that may contribute to implant failure. Operator experience and surgical trauma are examples of exogenous factors.10 In general, operator experience and implant failure rates have an inverse relationship. This typically coincides with surgical trauma, which can include overheating of the bone during osteotomy preparation. Overheating may occur as a result of lack of coolant, use of dull burs, or drilling at a rate that is too high. Overheated bone will lead to thermal necrosis in which delayed death of osteocytes may occur weeks after surgical injury.22
Another exogenous factor is lack of primary stability at the time of implant placement. Some reasons include poor osteotomy preparation and surgical trauma during an extraction. A lack of primary stability is often associated with immediate implant placement.23 Micromotion during implant healing may also adversely affect osseointegration, resulting in implant failure. Most often, micromotion is associated with loading of the implant during healing.24
Exogenous factors associated with biomaterials, implant characteristics, and prosthetic design of the restorative case may also contribute to implant failure. In general, the rougher implant surfaces fair better than machined, smoother surfaces.25 Often, anatomy dictates length and width of the implant surgical fixture selected for a case. Consideration should be made for using a wider implant when the implant is shorter in length to facilitate a greater potential surface area for osseointegration.26
Early and Late Failures
Implant failures may be attributed to biologic, mechanical, or iatrogenic causes, or as a result of a lack of patient adaptation.27 Biologic failures may either be considered early or late.10 An early failure happens prior to occlusal loading of the implant and is due to the implant’s inability to establish osseointegration.10 Early failures are more common, occurring weeks or months of implant placement. Such outcomes are usually the result of host-related systemic factors that impair healing, iatrogenic causes related to poor surgical technique, poor bone quality, smoking habit, premature traumatic loading, poor prosthetic design, and infection.28
Infection can jeopardize implant success. Poor surgical technique leading to bacterial contamination of the dental implant fixture is one potential source. Another is contamination from a periodontally or endodontically affected tooth.29,30 Prior to placement, the clinician should address and treat all infections to increase the likelihood of success.
Peri-implant disease describes a microbial plaque-induced inflammatory process, which may be proceeded by occlusal overload.10 It affects the soft tissue and bone around an osseointegrated dental implant.10 Without treatment prior to placement, peri-implant disease may result in further bone loss and implant failure.10 When it is of bacterial origin, the condition is called infectious peri-implantitis. The microflora is similar to that of periodontitis.31,32 Clinical signs of peri-implantitis include swelling, fistula formation, suppuration, mucosal dehiscence, and osteomyelitis.10
A late failure occurs after occlusal loading, occurring 1 year or more after placement.33 These failures are usually due to occlusal overload or peri-implantitis33 and are related to the inability to maintain osseointegration.10 One form of peri-implantitis is a result of occlusal issues. This is known as retrograde peri-implantitis, which may be the result of off-axis loading, excessive occlusal loading, traumatic occlusion, or premature occlusal forces being applied to the implant.31,32 The microflora associated with retrograde peri-implantitis is similar to the flora associated with a periodontally healthy periodontium.31,32 The clinical and radiographic presentation on retrograde peri-implantitis is initially a lack of soft-tissue involvement or bleeding with a periapical bone resorption.31,32
Implants may fail due to a lack of patient adaptation.27 Examples would include an esthetic or phonetic failure in which the patient or the dentist is unsatisfied with the result. These issues are typically related to poor surgical placement and prosthetic design and restorative complications.
Dental implant therapy is a treatment option that should be considered during prosthetic treatment planning. Defined criteria denote success in dental implant therapy. However, some circumstances lead to implant failures. Implant failures may be due to endogenous or exogenous factors. Dental implant failures may also be classified as biologic, mechanical, iatrogenic, or as a result of poor patient adaptation. As practitioners, it is our duty to be as knowledgeable about those circumstances that can impact the success of the care we render to our patients.
About the Author
Dr. Glascoe is an associate professor in the Department of Periodontics and Preventive Services at Howard University College of Dentistry, Washington, DC.
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