Dental Implants for the general dental practitioner. Part 1: overview and assessment

Implant fixture

Endosteal implant fixtures of a root form design are the most commonly used dental implant and will be the focus of this article. The fixture is the surgically placed component that interacts directly with the bone. Implant fixtures can vary greatly depending upon the brand and the system used, but all aim to osseo-integrate (direct contact between living bone and implant¹⁷) to provide a stable foundation for a dental prosthesis. The main properties of implant fixtures that can vary are discussed below and overviewed in Table 2.

Implant abutment

This is the transmucosal component. Some systems include fixtures that come as a solid unit (one-piece implant) with the abutment already attached, as seen with mini-dental implants (MDIs); however, most have a separate abutment component (two-piece implants). The abutment can either allow healing and sculpting of the soft tissues (healing abutment), or connect the implant fixture to the prosthesis. Selection of the correct abutment depends upon the implant fixture and its prosthetic connection, the position of the implant fixture and the planned prosthesis. Implant abutments can be made from titanium, zirconia, gold or polyether ether ketone (PEEK), and can come separately or as part of the implant prosthesis. The main types of abutment are shown in Table 3.

A concept known as platform switching may be used where the implant abutment used is narrower than the implant platform, with an inward positioning of the implant-abutment junction.²⁶ This concept is radiographically represented in Figure 2. The term ‘platform matched’ means that the implant and abutment are the same width. It has been shown that inwardly repositioning the implant–abutment junction results in less marginal bone loss than in implants with matched implant-abutment platforms.²⁶ It is proposed that this inward repositioning of the implant–abutment junction increases the distance between the inflammatory cell infiltrate that can develop and the marginal bone level.

Implant prosthesis

This is the component that replaces the missing dentition (and sometimes soft tissue), aiming to restore both aesthetics and function for a patient. There is a plethora of implant prosthesis designs. They can be roughly categorized into the following: fixed implant prostheses, such as a single crown; a short-span bridge; a long-span bridge (including full arch); or a removable prosthesis, such as an overdenture.

Fixed-implant restorations can be either cement or screw retained. Cement-retained restorations involve the abutment being screwed to the implant fixture (and the screw then covered with cotton wool or PTFE to protect it), and then a crown cemented onto the abutment (Figure 3), in a similar fashion to conventional crowns. As the abutment is often milled, a minimal taper can be produced giving excellent retention and resistance form in ideal situations, with use of a temporary cement for ease of retrievability. Care must be taken to ensure that all excess cement is carefully removed after cementation because there is evidence linking this to peri-implant disease.^27,28

Cement-retained restorations are often used to overcome the aesthetic difficulties encountered when the angulation of the implant fixture determines that the screw access will emerge through the labial surface of the definitive restoration (Figures 3 and 4).

Screw-retained implant restorations involve the dental prosthesis being screwed into the abutment or the implant fixture (Figures 3 and 5). They offer the advantage of easier retrievability in comparison to cement-retained restorations owing to access to the prosthetic screw being maintained. Once the restoration is seated and the prosthetic screw torqued correctly, a protective layer, such as cotton wool or PTFE tape is placed over the screw itself prior to restoration of the access hole with either composite or a temporary filling material. This helps to prevent damage to the screw should removal of the restorative material be necessary in future.

Modern advances in implant dental technology, have facilitated the correction of non-ideal implant angulations with the use of angled screw channel restorations. These restorations allow up to 25 degrees of angulation correction of the screw access hole (Figure 6). Screw-retained prosthetic options can now be offered where, in the past, only cement-retained options would have been feasible.

Evidence suggests that although there is no difference in survival rates between cement- and screw-retained restorations, complications are more common with cement-retained prostheses.²⁹ Many factors need to be considered when deciding upon the use of screw-versus cement-retained restorations. For the authors, screw-retained restorations are the preference where possible given their ease of retrievability.

Removable prostheses can be retained using either ball abutments, locator abutments, magnets or bars (Table 2). Again, there is a large variety of components available for retention of implant-supported removal prostheses, but in general, the denture houses the female component of the system, while the male component that inserts into the female component is supported by the fixture (Figure 7).

The number of implants used to retain a removable prosthesis may vary between cases, but it is widely accepted that two implants are required to support a lower complete overdenture as the gold standard for the edentulous mandible.³⁰ For a removable overdenture in the edentulous maxilla, between four and six implants are recommended for support.³¹

Clinical assessment

At each of a patient's recall visits, a detailed clinical examination should be undertaken, which is outlined below. The recall interval should be tailored specifically for each patient, taking into consideration their individual risk factors for disease progression.³⁵ These risk factors are often changeable over time and therefore need to be reassessed at each recall, so that timings can be adjusted as necessary. Clinical assessment of dental implants should be undertaken at least once a year.³⁶

For screw-retained restorations, the integrity of the screw access restoration should be checked. Further occlusal assessment may also be necessary if complications have arisen to identify any contributing unfavourable occlusal forces.

Radiographic assessment

Radiographic assessment is a necessary adjunct to a clinical examination to aid in monitoring dental implants, as well as assisting in making a diagnosis if complications arise (Figures 9 and 10). A baseline radiograph should be taken upon fitting of the definitive prosthesis to ensure accurate seating, and to serve as a baseline reference in monitoring alveolar bone levels.³⁷ This should be taken by the restoring dentist and a copy should be sent to the GDP on discharge of the patient.¹¹

A further radiograph at 12 months is considered essential³⁸ to assess for changes in the bone due to functional remodelling or inflammation within the first year. It is dependent upon local guidelines as to whether this radiographic examination is completed by the restoring clinician or requested to be completed by the patient's primary care dentist.

There is no consensus as to the appropriate radiographic monitoring protocol of dental implants following the first 12 months in function. It is therefore necessary to correlate the clinical findings to influence when a radiograph is required.³⁷ For example, if the clinical findings indicate peri-implant disease or mobility, then a radiograph would be indicated.³² The Association of Dental Implantology suggests a radiographic review protocol every 2 years (after the first year in function), unless there are signs or symptoms of disease warranting investigation sooner.³⁹

This will often be the responsibility of the GDP and the British Society of Periodontology and Implant Dentistry (BSP) expect that the general dentist should be screening for implant complications, even if they have not placed the implants,¹² allowing them to refer onwards if necessary. Good communication will be required between the GDP and the placing and/or restoring practitioner to determine who will be providing ongoing monitoring to prevent either duplication or a lack of follow-up.