Extensive work by the Swedish orthopaedic surgeon P.-I. Branemark led to the discovery that commercially pure titanium (CPTi), when placed in a suitably prepared site in the bone, could become fixed in place due to a close bond that developed between the two (Fig. 2.1), a phenomenon that he later described as osseointegration (OI). This state has anatomical and functional dimensions, as it requires both a close contact between the implant and surrounding healthy bone and the ability to transmit functional loads over an extended period without deleterious effects either systemically or in the adjacent tissues. OI is not defined in terms of the extent of the bone-implant contact, provided that functional requirements are met and the tissues are healthy. Many of the factors that predispose to the development of OI are now known, and where these exist a successful outcome will probably follow the placing of a suitable implant. Similarly, failure is more likely where factors known to predispose to an unsuccessful outcome exist. Occasionally, implants fail for no apparent reason, sometimes in groups in one patient - the so-called 'cluster
Fig. 2.1 Close physical approximation between the surface of a dental implant and vital bone is a key structural characteristic of osseointegration, which also has important functional parameters.
phenomenon'. It is therefore important to advise patients that a satisfactory outcome cannot be guaranteed.
OI is currently viewed as the optimum implant-bone interface, without which success cannot be obtained, and great emphasis has been placed on its production and maintenance. Nevertheless, it is only one component of successful dental implant treatment and does not in itself prevent that treatment from failing. While the absence of OI is equated with treatment failure, its achievement does not guarantee success, which is dependent on the design and performance of the final prosthesis. This may be precluded by an inappropriately placed implant, even if it is integrated.
While the osseointegrated interface and associated soft-tissue cuff where the implant penetrates the oral mucosa are often thought of as dental analogues, they have a number of important differences. In particular, the interface is more rigid and less displaceable than the periodontal ligament, and behaves essentially elastically as opposed to the viscoelasticity of the periodontal ligament. The stability of the interface also precludes implant repositioning by orthodontic manoeuvres, but may permit dental implants to be used as anchorage for fixed orthodontic appliances. The osseointegrated interface is also associated with a slow rate of loss of crestal alveolar bone, typically less than 0.1 mm per annum after the first year of implantation. As a result, most implants can be expected to be functional throughout adult life.
Inflammation of the tissues around an endosseous implant is sometimes observed; it is described as peri-implant mucositis when it involves only the soft tissues and peri-implantitis where loss of the bone interface occurs. While the microorganisms associated with these lesions are similar to those seen in periodontal disease, it is currently unclear whether they cause the lesion or colonize the region subsequently.
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