2. Macroarchitecture of dental implants / implant design
Dental implants come in a variety of designs that enable the individual patient situation as well as the preferences of the dentist to be considered. There is a choice between
- different implant lengths and widths
- a cylindrical or conical shape
- one-part or multi-part implant systems
- or a different number and arrangement of the thread grooves.
The choice of implant length is usually determined by the patient’s individual bone situation and the location of the inferior alveolar nerve in the mandible. In compromised bone situations, shorter or reduced diameter implants can be also be used to avoid bone augmentation or nerve lateralisation with the associated risk of complications. Shorter implants (< 10 mm) have been described as being more susceptible to failure (MISCH, 2005). Diameter-reduced mini implants (< 3 mm) initially have comparable survival rates to standard implants but the long-term survival rates of these implants have not yet been adequately demonstrated (BIDRA et al., 2013).
Both the implant diameter and the length and angle of the implant neck appear to have a critical influence on the loads that develop at the contact point between the cortical bone and the implant surface (bone-implant interface) (FAEGH et al., 2010). An increase in the implant diameter, a longer implant neck and a positive implant neck angle, that is, an increase in the diameter towards the implant tip, can reduce the loads developing between the two surfaces, for example (FAEGH & MUFTU, 2010). The choice between one-or two-part implant systems depends primarily on the patient’s individual indication. Two-part implant systems are made up of an implant screw that is inserted into the bone and is located beneath the gingiva and an abutment that acts as a connective element between the implant and the crown construction. With one-part systems, the implant itself includes a connection to the prosthetic restoration that protrudes above the gingiva after implant placement. One-part implants are therefore particularly suitable for cases in which immediate loading is desired, whereas in most cases two-part implant systems are exposed only after an initial healing phase and at this later point an abutment can be attached. Studies that compare the advantages and drawbacks of the two systems in terms of the influence of both systems on the surrounding hard and soft tissues show very different results in some cases (for a review: [PRITHVIRAJ et al., 2013]. While some studies have described less bone loss and a smaller reduction in the biologic width for one-part systems (HERMANN et al., 2001), other studies have found a lower success rate and an increase in bone resorption for one-part implant systems (OSTMAN et al., 2007; ZEMBIC et al., 2012). One reason for this discrepancy may lie in the fact that the implants used in the individual studies differ not only as to whether they are one-or two-part implant systems but also in terms of other properties which therefore makes a direct comparison between the studies difficult.
2.1 Biologic width
The biologic width mentioned previously is an increasing important dimension in the area of implant dentistry. While only the bony healing of the entire implant was usually noted in the past, improvements in implant survival rates have also increased the demands placed on the aesthetics, which are fundamentally determined by the surrounding soft tissue. The biologic width of natural teeth differs only slightly from the biologic width of implants. With natural teeth the area between the highest contact point of the gingiva to the tooth crown and the highest point on the alveolar bone is referred to as the dentogingival complex. This complex is made up of the sulcus (0.2–0.5 mm) and the biologic width, which can in turn be divided into the epithelial attachment (about 1 mm) and the connective tissue attachment (about 1 mm). This complex has a primarily protective function and is intended to effectively delineate the underlying tissue from the oral cavity by means of the connective tissue fibres and the epithelium (SICHER, 1959). The fibres of the connective tissue attachment on the natural teeth have a three-dimensional arrangement (FENEIS, 1952) along and across the tooth axis, whereas the connective tissue fibres of the biologic width of implants only run parallel to the longitudinal axis of the implant (BUSER et al., 1992; BERGLUNDH et al., 1991) (Fig. 1). It has been demonstrated that the height of the dentogingival complex of about 3 mm (GARGIULO et al., 1961) is relatively constant and with a stable dentogingival complex a change in the alveolar bone height leads to a corresponding change in the gingival height. For implant dentistry this means that a reduction in the crestal bone at the implant results in regression of the gingival height and thus an aesthetically unappealing exposure of the implant neck. With two-part implant systems, a dependency between crestal bone resorption and the location of the microgap has been observed in this context (HERMANN et al., 1997; HERMANN et al., 2000). This microgap develops between the implant shoulder and the attached abutment. The design of the implant shoulder and the seated abutment can alter the location of the microgap and thus directly influence the crestal bone resorption.
2.2 Platform switching
Platform switching is one method that takes into consideration the dependence of the crestal bone resorption on the location of the microgap to improve the marginal bone situation. A two-part dental implant is used with an abutment that has a smaller diameter. This displaces the microgap between the implant and the abutment horizontally from the outer wall of the implant to the centre of the implant. Several studies observed reduced marginal bone resorption in this context in both animal studies (BECKER et al., 2009) and in patients (ATIEH et al., 2010) which was lower the greater the size difference between the implant and abutment.
Various reasons have been proposed to explain the curbing of the marginal bone loss through platform switching.
- The space available for the tissue in the biologic width may be optimised by the platform switching (DEGIDI et al., 2008)
- The inflammatory connective tissue around the implant abutment connection is displaced horizontally to the centre of the implant (LUONGO et al., 2008)
- The area of maximum biomechanical load is displaced towards the implant axis (CHANG et al., 2010)
A systematic review by Annibali and colleagues (ANNIBALI et al., 2012) confirmed the reduced marginal bone resorption on dental implants. However, no noteworthy difference was demonstrated between implants with and without platform switching regarding the implant survival rates.