8Prevention of Hardware Complications
In order to avoid permanent deformation or fracture of the implant body, standard implant diameters and materials should be chosen that are optimized in terms of material characteristics and design. Reduced-diameter implants may play an important role in narrow ridges and narrow spaces. They provide easier and less expensive treatment options and lower morbidity compared to GBR procedures (Sohrabi and coworkers 2012). This also applies for short implants compared to sinus augmentation or nerve displacement procedures (Neldam and Pinholt 2012; Srinivasan and coworkers 2012). There is no evidence yet that so-called mini-implants can be recommended for definitive reconstructions (Bidra and Almas 2013).
When the envelope of indications is stretched, however, it is advisable to choose materials with improved physical properties but with an equivalent osseointegrative surface. The search for additional indications for implant-supported reconstructions led to the development of reduced-diameter implants manufactured from a titanium/zirconium alloy (Steinemann 1998; Gottlow and coworkers 2010; Thoma and coworkers 2011).
Ongoing clinical testing of mandibular overdenture cases with one reduced-diameter (3.3 mm) bone-level implant made of grade IV titanium and one implant with the same design but made of titanium/zirconium alloy has demonstrated very similar clinical performance (Al-Nawas and coworkers 2012). Osseointegration and stable tissue integration can be achieved with this new material while getting a chance of better biomechanical performance for fixed reconstructions (Barter and coworkers 2012; Chiapasco and coworkers 2012). This improved material has been integrated into a new set of implants by a leading manufacturer.
The search for attractive materials in esthetically demanding cases and for ways to avoid the use of different materials in a reconstruction/abutment/implant complex led to the development of zirconia implants. This material seems to osseointegrate in a manner similar to titanium implants (Gahlert and coworkers 2009; Bormann and coworkers 2012) but its physical properties are completely different. The clinical application of reduced-diameter zirconia implants has so far been unsuccessful (Gahlert and coworkers 2012). In addition, the one-piece implants in zirconia create unfavorable prosthetic conditions.
The design characteristics of the platform and neck region influence the load transfer from the reconstruction/abutment complex to the surrounding crestal bone, and the neck surface is in direct contact with the adjacent soft-tissue cuff. Attempts to improve the hardware were made (conical connection designs, platform switching, surface alterations), all aiming to provide the capacity to sustain and transfer the load and to interfere only minimally with the formation of a stable biologic width and crestal bone level.
Any maintenance procedures such as oral hygiene or professional supportive therapy should not damage the implant neck (or the abutment). This applies especially to the use of mechanical instruments, acids, abrasive substances, lasers, and the tips of ultrasonic devices. When performing aggressive interventions, such as cutting a non-retrievable cemented crown, caution is required not to damage the platform e.g. in tissue-level implants.
Damage unintentionally inflicted with a drill may be irreparable and will pose a problem for a precise fit of a future reconstruction. An individual conventional impression can be required after smoothening the affected platform when a new reconstruction is delivered.
If the three-dimensional position of the implant body (axis and platform) is unfavorable relative to adjacent and opposing structures, the prosthetic procedures will be more complex. The reconstruction may require esthetic, biological, or functional compromises—or the implant may be even non-restorable. Prosthetically driven implant planning and positioning is therefore mandatory. This includes the depth of the platform in relation to adjacent teeth, adjacent implants, and the opposing jaw; the buccooral position of the platform(s); the axis of the implant(s); and the relative positions of the platform and the planned reconstruction. This applies not only to fixed reconstructions but also to overdentures.
Improved connection designs have been developed to prevent hardware-related complications and failure, and the loosening/fracture rate has been reduced to acceptable levels (Theoharidou and coworkers 2008) compared to when oral implants were first introduced.
Clinicians can avoid problems by careful handling of the connection area, by creating an open access with healing screws and mucosa formers, and by using long-term provisionals for conditioning the emergence profile. Any foreign matter (tissue, graft particles, blood) should be rinsed off before connecting a component.
Finding the correct index position is very important. Radiographic controls of impression posts can assist in this, as can transfer keys for angled or modified abutments. The impression post must match the implant.
The abutment screw should not be pushed down into the connection. Rather, the index of the abutment base should first be found; only then is the abutment screw turned slowly and without tilting.
In multi-unit reconstructions, it is advisable to activate the screws alternatingly and in increments until the final seating is achieved with the torque ratchet.
The torque recommended by the manufacturer was optimized for the respective material and design combinations and must be respected.
The design of an abutment can determine where an abutment screw will fracture in case of excessive force onto the superstructure/implant complex—preferably in an area that provides good access and visibility. In these cases, the fragment can easily be removed with a probe and rotational movements. However, an abutment screw fragment may also be completely blocked in the connection, or restricted access to a fragment may limit its controlled removal. The connection area should not be damaged while removing fragments. The microscope may provide a direct view of the area to help locate the problem so the fragment can be removed with an endodontic explorer. Any drilling or retapping should only be performed with a precise guide (from the manufacturer’s service set).
It is a clinical reality that not only impression posts but also abutment screws may be unintentionally exchanged. This could happen at the laboratory or at chairside. For example, the use of an abutment screw with a conical head base may result in fracture of a ceramic abutment. The threads of the abutment screw must match the bore in the implant, so the use of original abutments is advisable.
The axis of the screwdriver has to be positioned along the long axis of the implant/abutment to find the correct insertion position. When tightening down a crown, the contacts with adjacent teeth or implant-supported crowns should not be so tight that they block the screw movement, as otherwise a fracture in the ceramic veneer or damage to the bore/thread might occur. The abutment screw should not be forced downwards. The correct index should be first located and the reconstruction/abutment held in the insertion direction. The head of the abutment screw must be protected, preferably with Teflon, underneath a screw-canal restoration.
With respect to the correct handling of the components, it also seems important to provide the clinician/technician with assistance related to logistics, information on new products, instruction, training, logical (e.g. color-coded) step-by-step procedures, exchangeable components, and, in case of problems, assistance in the form of a service set and a well-organized complaint system.
These issues relate not only to the implant and to prosthetic components but also to the auxiliary parts, such as impression posts, analogs, registration posts, and the screwdrivers and torque ratchets.
The abutment body must match the implant configuration. Narrow, regular, and wide designs should not be mixed; third-party abutments do not match the original connection precisely.
Economic pressure to produce and deliver implant-supported reconstructions at a reduced price may lead to the acceptance of alternative solutions involving third-party abutments (i.e. abutments made by a different manufacturer than the implant) available on the market. Limited access to equipment and halting investment in laboratory equipment could also result in the selection of third-party abutments. The designs of screw joints such as those at the implant/abutment interfaces should be matched carefully because the biomechanical properties mainly depend on factors such as materials, tolerance, connection designs, and preload (Dixon and coworkers 1995; Gratton and coworkers 2001; Khraisat and coworkers 2002; Meng and coworkers 2007; Lee and coworkers 2010).
The aims of a recent in-vitro study (Gigandetand coworkers 2014) were:
1.To test the in-vitro mechanical resistance of three original implant/abutment interfaces and to compare these original interfaces to two combinations of third-party abutments and one of the original implants.
2.To test the influence of geometric discrepancies at the interfaces between the implants and the original/third-party abutments by assessing rotational misfit.
3.To assess and compare failure modes.
Small-diameter implants were chosen to test a high-risk condition compared to implant/abutment connections of standard dimensions. The null hypothesis was that there would be no significant difference in the mechanical characteristics between the original and third-party interfaces.
Third-party abutments differed in the design of the connecting surfaces, shape, dimensions, and material and had a higher rotational misfit. All these differences resulted in unexpected failures and may have had an adverse effect on clinical handling. More clinical studies testing the failure and complication rates of reconstructions on original and third-party connections should be performed; however, at this point it is recommended to use abutments by the original implant manufacturer when restoring implants.
A recent clinical study related to the survival of alumina-reinforced zirconia abutments examined the long-term risks associated with third-party interfaces between implants and abutments (Kim and coworkers 2013). A total of 213 patients had received 611 externalhex implants and 328 fixed reconstructions. Implants from five manufacturers were placed. The ceramic abutments, however, were produced by a single manufacturer. Posterior single-unit reconstructions (n=101) demonstrated an increased and clinically unacceptable rate of technical failures. The third-party abutments on the five implant systems were not stable enough to provide satisfactory function in the posterior area.