Implant prosthetics (part 1) – screwed or cemented?

Fig. 5: from left: 3D planning before implantation, construction design and finished data set before the superstructure is manufactured.

Andreas Kunz, Dr. Insa Herklotz

Screwed or cemented in implant prosthetics – a question of preference or are there case-related decision factors?

The type of connection in fixed implant restorations (implant prosthetics) can generally be divided into screw-retained and cemented suprastructures. The question for treatment teams is which individual factors are crucial for the choice of connection. When looking at the complication rates of fixed superstructures on implants, only 5% of prosthetic restorations are complication-free after 60 years. The majority of complications are of a technical nature such as fracture of the veneer, screw loosening, and screw and abutment fractures (Pjetursson et al 2004, Jung et al 2012).

Ready-made vs. individual

Biological complications such as peri-implantitis account for 9%. Sailer et al 2012 were able to show that screw-retained restorations in particular tend to have technical complications and that biological complications are more likely to occur with cement-retained restorations. In the case of cemented connections, cement residues remaining in the sulcus are seen as a possible trigger for peri-implant diseases (Wilson 2009). Many studies use prefabricated abutments for cemented restorations. A tooth-like emergence profile can often only be achieved with a low-lying crown edge (Fig. 1). Linkevicius was able to show that the increasing subgingival position of the crown margin is accompanied by increased remaining cement residue (Linkevicius et al 2013). In implant prosthetics, it is often difficult to individually adjust the interface between the prefabricated abutment and the crown horizontally and vertically. Unfortunately, based on the sales figures of well-known implant manufacturers, a proportionately large number of prefabricated abutments are still sold. Abutments that are individually adapted to the emergence profile enable an equigingival position of the crown margin. This makes it easier to remove the cement residue. At the same time, the peri-implant soft tissue is optimally shaped by the individualized abutment and a natural emergence profile of the crown is achieved (Fig. 2). The additional placement of a retraction cord before cementing makes it easier to control the removal of the cement residue.

Fig. 1 Prefabricated titanium abutments often do not allow for an equigingival position of the crown margin. Cement residue is difficult to remove.

Fig. 2: from left: Image 1 – exit profile before shaping. Image 2 – Horizontal dimension of the tooth to be replaced. Image 3 – Discrepancy between emergence profile and implant crown. Figure 4 – Abutment shaping of the emergence profile. Image 5 – Inserted prefabricated titanium abutment.

Abutment materials

Individual abutments should be made from a tissue-compatible and mechanically strong material. Good optical properties can also improve aesthetics. Welander et al 2008 showed poorer biological compatibility of abutments cast with gold compared to zirconium oxide and titanium. Scarano et al 2004 and Degidi et al 2006 confirm that zirconium oxide has equivalent or slightly better biological tissue compatibility than titanium. From today's perspective, titanium is still considered the gold standard when selecting abutment materials. Zirconia usually occurs in the connection as a hybrid abutment. Here, an individually manufactured zirconium oxide structure is glued to a ready-made titanium base (Fig. 3). Individual abutments can be manufactured precisely using CAD/CAM processes. They enable an equigingival position of the crown margin and thus a reduction in submucosal cement residues.

Fig. 3: Hybrid abutment crowns/bridges before bonding the titanium adhesive bases into the superstructures.

Cemented restorations (implant prosthetics)

Cemented, implant-supported superstructures are primarily used when the future screw channel is in an unfavorable position due to the prosthetically incorrect implant axis (Fig. 4). If the screw opening is outside the occlusal surface or, in the case of front teeth, in the incisal edges or labial area, an occlusally screw-retained implant crown is not indicated for definitive tooth replacement. Prosthetically oriented, pre-surgical implant planning can show at an early stage whether occlusal screw fixation of the restoration is possible. The development of the digital volume tomogram has enabled progress in 3D diagnostics and planning. Using 3D planning software, the emergence profile and screw channel openings can be planned preoperatively (Fig. 5). With the help of template-supported navigation methods, implant planning can be implemented intraorally with high precision. A prosthetically optimally placed implant is crucial for the final superstructure not only for functional and aesthetic reasons, but also for biological reasons. It enables the patient to optimally clean the restoration and thus ensures long-term stable soft tissue conditions.

Fig. 4: The position and orientation of the implants determine whether they are screwed or cemented. Bone availability and augmentation options influence the prosthetic reconstruction.

Fig. 5: from left: 3D planning before implantation, construction design and finished data set before the superstructure is manufactured.

Cemented implant crowns and bridges are easier to insert than screw-retained ones in terms of the approximal contact point. Due to the conical stump preparation (4-6 degrees) of the abutment, the direction of insertion has a higher variance (Fig. 6). Occlusally screw-retained superstructures in parallel-walled, internal rotation-protected implant systems have a small variance between the insertion direction of the approximal contact points and the insertion direction of the implant crown/bridge. With conical implant connections, the vertical end position is often only achieved with a defined torque. Adjusting an excessive interproximal contact point is time-consuming due to the repeated screwing and unscrewing of the crown.

Fig. 8 With occlusally screw-retained implant crowns, the soft tissue can be gradually shaped.

Fig. 7: By conical preparation of the abutments of a bridge reconstruction, divergences due to different angulation of the implants can be compensated for.

The disadvantage of cemented implant restorations is the inability to remove the crown. This is necessary in the event of screw loosening or fracture as well as ceramic fractures, so-called chipping, one of the most common technical complications of implant restorations (Schwarz et al 2012). Most of the time, these crowns then have to be trepanned and thus destroyed in order to reach the screw channel and be able to unscrew the crown. A study by Gracis et al showed that screw loosening occurred more frequently in implant systems with external rotation locking than in systems with internal rotation locking.

Decision factors “cemented” implant crown

Advantages

The disadvantages:

Read Implant Prosthetics (Part 2):

Further information is also available in the publication: Herzklotz I, Kunz A, Beuer F: Screwed or cemented – is that the question? Quintessence 2017;68(9):1-7