Original study - ZZI 04/2011

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


Prosthesis incorporation

After a healing period of at least two months, the implants were exposed and provided with healing abutments. About four weeks after implant exposure, an open impression of the implants was taken with a custom tray, transfer posts (DENTSPLY Friadent, Mannheim, Germany) and a polyether impression material (Impregum Penta, 3M Espe, Neuss). Fabrication of working models and registration templates was followed by facebow transfer and centric bite registration. A wax-up of the teeth (Vita Physiodens, VITA Zahnfabrik, Bad Säckingen) was then tried in the mouth. After checking fit, function and appearance, the metal frameworks of the prostheses were constructed in the next step. The prostheses were then transferred to acrylic (Aesthetic Autopolymerisat, Candulor AG, Wangen, CH). After insertion of the ball head abutments (Ankylos Ball Attachment, DENTSPLY Friadent, Mannheim) (Fig. 1) or the prefabricated conical crowns (SynCone Abutments 4°, DENTSPLY Friadent, Mannheim) (Fig. 2) in the implants, the matrices were polymerized into the metal framework spacers with a self-curing resin (Pattern Resin LS, GC Europe, Leuven, Belgium) after prior Rocatec treatment in the patient directly.

The patients were randomized to the two groups. Randomization was carried out by a statistician not involved in the study using a sealed envelope. The envelope with information about the abutments to be employed was opened by a dental nurse on the day of final incorporation. Blinding of the investigators was not possible because the prosthetic reconstruction was visible.

The patients were given precise oral hygiene instructions after the prostheses were fitted.


Clinical evaluation

The patients were investigated with regard to different peri-implant soft tissue parameters on the day of prosthesis incorporation and after twelve and 24 months. The probing depth (PD) in mm, the Mombelli modified plaque index mPlI (grade 0?III; grade 0 = no plaque, grade 1 = non-visible thin film of plaque, which can be detected by scraping the tooth surface with a probe, grade 2 = visible plaque, grade 3 = massive plaque deposits which fill the interdental space) [28], the bleeding index BOP (bleeding on probing, 0 = no bleeding, 1 = bleeding) [1] and the Mombelli modified gingiva index mGI (grade 0–III; grade 0 = no inflammation, grade 1 = slight change in color and surface, grade 2 = erythema, edema, gingival bleeding on probing, grade 3 = marked erythema and edema, spontaneous gingival bleeding, ulceration) [29] were recorded. The probing depth and bleeding index were measured in mesial, distal, vestibular and lingual positions. The means of these four measurements were obtained and used for statistical analysis.


Radiographic evaluation

To evaluate horizontal bone loss (HBL) of the implants, orthopantomographs were taken after implantation, when the prosthetic restoration was fitted (baseline) and after twelve and 24 months. The marginal bone level was measured mesial and distal to the implants. The upper edge of the implants was used as reference point for the marginal bone measurement. The marginal bone levels were recorded for the baseline and 1-year and 2-year follow-up. Using the real and digital implant length, the real distance of the implant shoulder from the first bone-implant contact was calculated using a rule of three. This was then compared with the measurements at the previous examinations (baseline) and the real difference in mm was calculated. The difference between the two values then gives the bone loss. The digital orthopantomographs were measured using computer software (measurement module, Dental Vision, Computer Forum GmbH, Elmshorn, Germany) on a 17-inch monitor (Neovo DR-17 model, Dürr Dental AG, Bietigheim-Bissingen, Germany). The monitor resolution was 1280 x 1024 pixel. Magnification was adjusted to provide optimal measurement conditions.


Patient-centered evaluation

The patients were asked about their satisfaction with the new implant-retained mandibular restoration at the different annual follow-ups. A distinction was made simply between satisfied and dissatisfied. Satisfaction was not divided into different sub-variables. Any necessary prosthodontic interventions were also performed and documented.


Statistical analysis

A linear mixed model with a random effect was adapted (“subject = patient”). For each soft tissue parameter and horizontal bone loss parameter, the continuous “response” variable was modeled as a linear function of the abutment type and time
(0 years, 1 year and 2 years). In addition, the corresponding time-type interaction was modeled as an explanatory variable. Variance components were used as covariant structures. Paired differences of the means (least square means) were calculated. All calculations were performed with the statistical software SAS System Version 9.1 using the “PROC MIXED” procedure. The statistical significance level was set at p ? 0.05. Subgroup analysis was performed according to Tukey-Kramer with adjusted p values.


Patient data

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