Original study - ZZI 04/2011

Implant-retained prostheses: ball vs. conus attachments – A randomized controlled clinical trial

The round heads employed in our study have now been described by numerous authors as sufficient retention elements for mandibular overdentures [6, 7, 10, 14, 20, 40].

Double crowns have hitherto been used mainly as dental retention elements for the restoration of partially dentate jaws using telescopic overdentures. This type of restoration has been well investigated [9, 27, 33, 35, 42]. The survival rate of tooth-borne telescopic overdentures is between 90 % and 95.1 % after four and 5.3 years respectively [19]. Resilient telescopic crowns have been described in clinical studies as a promising treatment option for implant-retained overdentures [15, 20]. One study showed lower prosthodontic maintenance with resilient telescopic crowns compared with ball heads [20]. This contrasts with the results of the present study in which the prosthodontic maintenance was roughly equal with both types of reconstruction. This may be attributable to greater sensitivity to technique with the non-resilient conical crowns used in this study compared with resilient telescopic crowns.

The SynCone conical crowns in the present study have previously been described as retention elements for the immediate restoration of edentulous jaws with four implants [8, 21]. The advantages compared with directly splinted retaining elements such as bar constructions are the improved oral hygiene capability and the less elaborate dental laboratory work as the matrices can be polymerized directly in the patient [8, 21, 43].

Promising long-term results were apparent in an in-vitro study of SynCone abutments. Over a simulated period of five years, the adhesive force of the SynCone abutments was stable [43]. This in-vitro study contrasts with the present clinical study, where adjusting the retention of the prefabricated conical crowns proved problematic. Loss of denture retention occurred frequently so that new matrices had to be polymerized in. Excessive denture retention was also observed often. This also required further work, which in turn contributed to the prosthodontic maintenance.

When the prosthodontic maintenance for the two investigated abutments is considered, it is apparent that numerous corrections were necessary with both the ball heads and prefabricated conical crowns. With the ball heads, the loss of denture retention was the most frequently encountered problem. Retention had to be increased by the dentist by activation of the matrices. If this was not possible, the matrices had to be exchanged. The problem of loss of retention when ball heads are employed has also been described by other authors [4, 14, 18, 32]. So far, there are no clinical studies for SynCone abutments used with two implants in the mandible for retention of an overdenture.

The need for relining was greater in the ball head group in the second year of the study than in the first and was generally higher than in the prefabricated conical head group. The manner of construction of the ball heads can result in greater loading of the distal denture bearing area compared with the conical crowns, which might explain the necessity of relining.

Retention problems occurred with the prefabricated conical crowns. In the first year of the study, too weak and too strong retention were both found. Appropriate adjustment of retention was difficult as it was not possible because of the construction to create greater or weaker retention by activation or deactivation of the matrices. In the second follow-up period, the number of subjects with prefabricated conical crowns whose overdenture retention was too weak increased markedly. It was therefore decided to exchange the abutments for ball heads in some of the patients and withdraw the patients from the study.

In the first year of follow-up, one abutment fracture occurred in the prefabricated conical crown group. The possible reasons for this might be excessively strong retention and associated excessive force when removing the overdenture or highly tilting forces due to incongruence between the denture base and the denture bearing area. It should be noted that the risk of fracture is greater with rigid attachment elements like such conical crowns than with resilient retention elements or those that can swivel around a point or axis. In patients who generate high forces, e. g., due to bruxism, SynCone abutments should therefore not be used with mandibular overdentures attached to only two implants on account of the high bending load and the long level arm.

The null hypothesis could not be rejected for the peri-implant soft tissue parameters of probing depth (PD), BOP, modified plaque index (mPlI), gingival index (mGI), the horizontal bone loss (HBL) and prosthodontic measurement. No significant differences were found between ball heads and prefabricated conical crowns for the various parameters. These clinical results are in agreement with those of other studies, in which no differences in peri-implant parameters were found between the different attachment systems [6, 14, 26, 30, 34].

With the prefabricated conical crowns, the rates of bone loss after 24 months were 0.4 mm distally and 1.0 mm mesially. It is striking that the mesial horizontal bone loss is greater than distal horizontal bone loss with both investigated abutment types. The background to the greater mesial bone loss is that the small test implants between the study implants were removed at the exposure operation. For this, the soft tissue had to be mobilized more widely in the mesial region of the study implants than in the distal region. This might have caused the greater mesial bone loss.

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