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Keywords:

  • bone substitute;
  • cone beam CT;
  • dental implant;
  • GBR ;
  • graft material;
  • long term;
  • membranes;
  • RCT

Abstract

  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

Purpose

The aim of this controlled clinical study was to compare a polyethylene glycol membrane (PEG) used for bone regeneration of peri-implant defects to a collagen membrane with respect to implant survival rate, dimensions of buccal peri-implant bone and mucosa.

Materials and methods

Thirty-seven patients who received single tooth implants with simultaneous guided bone regeneration (GBR) in the posterior maxilla or mandible were enrolled in the study. Intra-operative heights of bone defects were assessed prior to bone augmentation. The defects were augmented with xenogenic bone mineral and randomly covered either with a porcine collagen membrane (control) or with a PEG membrane (test). Five years after implant placement, clinical evaluation and cone beam computed tomography (CBCT) scans were performed. Remaining height of bone defect, horizontal bone thickness, level of mucosal margin, and mucosal thickness were assessed in CBCT images. The difference of height of bone defect at implant placement and at 5-year follow-up was calculated. The differences between the two groups were analyzed using two-sided t-test and Mann–Whitney U-test.

Results

After 5 years, 32 patients could be included and exhibited an implant survival rate of 100% for both groups. The buccal vertical bone gain between implant placement and 5-year follow-up amounted at 4.3 ± 1.5 (SD) mm and 4.8 ± 2.6 (SD) mm for the control and the test group, respectively (P = 0.493). Neither the bone height nor the thickness reached statistical significant differences between the two groups. The distance between mucosal margin and implant shoulder resulted in 0.8 ± 0.7 (SD) mm in the control and 0.5 ± 0.8 (SD) mm in the test group (P = 0.198). The mucosal thickness reached 1.4 ± 0.5 (SD) mm in the control and 1.3 ± 0.3 (SD) mm in the test group (P = 0.715). There were no significant correlations between height of bone defect at baseline and at follow-up examination and between different 5-year parameters.

Conclusion

A polyethylene glycol membrane used for bone regeneration of peri-implant defects performed as successfully as a collagen membrane with respect to implant survival rate and dimensions of the buccal peri-implant bone and mucosa after 5 years.

The use of GBR to treat bony defects around dental implants has been extensively documented throughout the past decades (Fugazzotto et al. 1997; Zitzmann et al. 1997; Hammerle et al. 2002; Aghaloo & Moy 2007). Among all the available resorbable barrier membranes used for GBR procedures, membranes made of collagen have become the membrane of choice in many clinical situations (Zitzmann et al. 1997; Jung et al. 2003; Moses et al. 2005). A few years ago, a synthetic and resorbable membrane that can be directly custom made intra-operatively for an individual defect has been engineered (Lutolf et al. 2003; Jung et al. 2006, 2009b; Wechsler et al. 2008). This newly developed synthetic membrane is made of polyethylene glycol (PEG). PEG hydrogel has been shown to be highly biocompatible and was investigated in other medicine disciplines, for example, as a sprayable adhesion barrier (Vaage et al. 1997; Mettler et al. 2003).

In a recent randomized, controlled clinical trial, this PEG hydrogel used as a barrier membrane for GBR was compared with the standard collagen membrane in the treatment of bony dehiscence defects around dental implants after an observation time of 3 years (Ramel et al. 2012). It was concluded that the PEG membrane performed as successfully as the natural collagen membrane regarding implant survival, clinical soft tissue parameters, and marginal bone levels after a follow-up period of 3 years. Further long-term data are mainly available for collagen membranes with observation periods of at least 12 years (Jung et al. 2013a).

The criticism of these types of studies is the fact that the success of the GBR procedure is assessed through a two-dimensional measurement of the mesial and distal radiographic bone level at the implant site and further clinical parameters. However, in the majority of these GBR procedures, the bone augmentation was performed mainly on the buccal aspect of the implants. Hence, there is limited date available for long-term controlled clinical studies assessing the bone dimensions at the buccal aspect of the implants, which have been placed simultaneously with bone regeneration procedures. A very recently published prospective, cross-sectional study reported on the long-term outcome of implants placed simultaneously with GBR procedures. Stable peri-implant hard and soft tissues at the buccal aspect were reported after a follow-up time of 5–9 years (Buser et al. 2013).

The aim of this study is, therefore, to assess the buccal dimensions of peri-implant bone, the implant survival rate and the mucosal level of implants that have been placed 5 years ago with simultaneous GBR either with a polyethylene glycol or a collagen membrane in combination with deproteinized bovine bone mineral (DBBM).

Material and methods

  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

This study reports on the 5-year follow-up of a prospective, single-center, randomized, controlled clinical investigation, where the 6 months and the 3 years results have already been published (Jung et al. 2009a; Ramel et al. 2012). The clinical study protocol and all procedures and materials were approved by the local ethical committee of the Canton of Zurich and by the Swiss health authority before the start of the study (Ref. Nr. EK: KEK-ZH-Nr. 2011-0197/4). For performing the CBCT scans, an amendment had to be submitted to the local ethical committee of the Canton of Zurich and was subsequently approved. Informed consent was obtained from all patients before any study procedures were performed. The study was monitored by an independent study monitor to ensure consistency and accuracy.

Patients

The primary inclusion criterion was the necessity of at least one implant in the posterior mandible or maxilla with an expected osseous defect of at least 3 mm in vertical dimension. Thirty-seven patients fulfilled all the inclusion criteria, underwent comprehensive dental care, and were subsequently enrolled in the study. The randomization envelope was opened only after primary implant stability was reached and the site was assigned to the respective treatment modality (for more details see Jung et al. 2009a).

Patient assessments and follow-ups have been taken place at the Clinic of Fixed and Removable Prosthodontics and Dental Material Science, University of Zurich Switzerland.

Surgical procedure

The surgical procedure has previously been described (Jung et al. 2009a; Ramel et al. 2012). In brief, dental implants (Straumann Standard Plus Implants; Institut Straumann AG, Basel, Switzerland) were placed with simultaneous bone augmentation using a natural bone mineral of bovine origin (DBBM: BioOss Spongiosa Granules, particle size 0.25–1 mm; Geistlich Pharma AG, Wolhusen, Switzerland) and either a test or a control membrane:

Test group (PEG membrane):

  • The test device is a synthetic biodegradable barrier membrane (MembraGel, Institut Straumann AG). The membrane is applied in a liquid state directly intraoperative using a syringe and forms a hydrogel by a cross-linking reaction within approximately 90 s after application.

Control group:

  • As the positive control, a collagen membrane of porcine origin was used (BioGide membrane; Geistlich Pharma AG) following a standard procedure with fixation using resorbable tags (Resorpin; Geistlich Pharma AG).

Baseline parameter

After implant insertion and before bone augmentation, the bone defect was measured clinically using a periodontal probe (HuFriedy, Chicago, il, USA) to the nearest millimeter. Defect height at baseline (DHBL) was defined as the distance measured from implant shoulder to the first bone-to-implant contact (BIC) (Fig. 1a).

image

Figure 1. (a) Defect height at baseline from implant shoulder (IS) to first bone-to-implant contact (BIC), (b) remaining defect height after 5 years (c) Horizontal thickness of the buccal bone measured 1 mm apical to the BIC (d) Horizontal thickness of the buccal bone measured 3 mm apical to the BIC (e) Horizontal thickness of the buccal bone measured 5 mm apical to the BIC (f) Distance between the marginal mucosa level (MML) and IS (g) Distance from MML to BIC (h) Mucosa thickness 1 mm apical to the MML.

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Prosthetic reconstruction

After the re-entry procedure 6 months following implant placement, the soft tissues were allowed to heal for at least another 2 weeks before impression taking. Thereafter, fixed partial dentures were incorporated.

Five-year follow-up examinations

All patients were enrolled in a strict maintenance care program at the clinic during the entire study period according to the individual need of the patient. Clinical assessments were taken 5 years after implant placement. Form, color, and coverage of the peri-implant soft tissues were examined by visual assessment of the clinical appearance. Full-Mouth Bleeding Score (FMBS) and Full-Mouth Plaque Score (FMPS) were recorded (O'Leary et al. 1972; Lang et al. 1986).

Cone beam computed tomography evaluation

To improve the visualization of the peri-implant soft tissues within the CBCT, a thin layer of radioopaque flowable composite resin (TetricEvoFlow; Ivoclar Vivadent AG, Schaan, Fürstentum Lichtenstein) was applied on the soft tissue around study implants and light cured.

Cone beam computed tomography imaging was acquired with a 3DExam CBCT scanner (KaVo Dental, Biberich, Germany). The scan was made with the following technical parameters: 120 kV acceleration voltage, 5 mA beam current, field of view (FOV) diameter of 16 cm, FOV height of 4 cm, 360° rotation and voxel size of 0.125 mm.

For the measurements within the CBCT, the bucco-oral central section perpendicular to the implant axes has been used (OsiriX Imaging Software, Geneva, Switzerland) (Fig. 2).

image

Figure 2. Cone beam computed tomography (CBCT) imagine illustrating the bucco-oral central section perpendicular to the implant axes. Measurements have been carried out by calculating the distance between the implant shoulder and the most coronal point of the first bone-to-implant contact.

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The following parameters have been assessed:

  • DH5y: Defect height after 5 years represented by the distance between the implant shoulder (IS) and the first BIC (Figs 1b and 2)
  • HT1, 3, 5 mm: Horizontal thickness of the buccal bone measured 1, 3, and 5 mm apical to the BIC (Fig. 1c, d and e)
  • MML-IS5y: Distance between the marginal mucosa level (MML) and the implant shoulder (IS) (Fig. 1f)
  • MML-BIC5y: Distance from MML to BIC (Fig. 1g)
  • MT5y: Mucosa thickness 1 mm apical to the MML (Fig. 1h)

All measurements have been performed by one single blinded person, which was trained and calibrated by an experienced person. In CBCT sections with unclear situations or measurements, the case needed to be reviewed with an experienced person until an agreement has been reached.

Statistical analysis

The data have been analyzed with SPSS Version 20 (SPSS Inc., Chicago,IL, USA). The assumption of normality of pooled data was checked by the application of Kolmogorov–Smirnov and Shapiro–Wilk tests. The data were summarized by mean values, standard deviation (SD), median, interquartile range, minimum and maximum values as well as 95% confidence interval.

The paired t-test together with 95% confidence interval was applied for the evaluation of the differences between the mean values of the test and the control groups. For those variables, which did not show a normal distribution the non-parametric Mann–Whitney U-test was applied.

For both study groups, the associations between MML-IS5y and DH5y as well as between MML-IS5y and MT5y were analyzed using Spearman and Pearson correlation.

Results of statistical analysis with P < 0.05 were considered statistically significant.

Results

  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

Five-year follow-up examinations

After 5 years, 33 of the 37 patients could be recalled representing 89.2% of the original study population. Fifteen patients were women and 18 men. Four patients (10.8%) have moved away or not wanted to participate in the study anymore. One patient had to be excluded for the analysis due to a movement artifact during the scanning process. Finally, 32 CBCT data sets (17 control and 15 test) could be analyzed. The median age at the time of re-examination was 53 years (range 37–77 years) for the control group and 59 years (range 28–85 years) for the test group.

After 5 years, both groups exhibited an implant survival rate of 100%.

Clinical examinations

The patients were all well maintained over the past 5 years and showed good level of oral hygiene. The full-Mouth Bleeding Score (FMBS) and Full-Mouth Plaque Score (FMPS) did not show any difference between the groups at the 5-year follow-up. The oral soft tissues at test and control sites showed a normal form and a pink color at the evaluation dates.

Cone beam computed tomography and clinical measurements

All results from the CBCT and clinical measurements are displayed in Table 1.

Table 1. Results at baseline and at 5 year cone beam computed tomography (CBCT) examination
Parameters (mm)DHBL (IS-BICBL)DH5y (IS-BIC5y)∆DHBL-5yHT 1 mm (IS-BIC + 1 mm)HT 3 mm (IS-BIC + 3 mm)HT 5 mm (IS-BIC + 5 mm)MML-IS5y (Mm-IS)MML-BIC5y(Mm-BIC)MT5y (Mm  + 1 mm)
  1. DHBL, Defect height at baseline from implant shoulder (IS) to first bone-to-implant contact (BIC); DH5y, remaining defect height after 5 years; ∆DHBL-5y, Change in defect height from baseline to 5 year examination; HT1 mm, Horizontal thickness of the buccal bone measured 1 mm apical to the BIC; HT3 mm, Horizontal thickness of the buccal bone measured 3 mm apical to the BIC; HT5 mm, Horizontal thickness of the buccal bone measured 5 mm apical to the BIC; MML-IS5y, Distance between the marginal mucosa level (MML) and IS; MML-BIC5y, Distance from marginal mucosa level (MML) to BIC; MT5y, Mucosa thickness 1 mm apical to the marginal mucosa level; SD, Standard deviation; 95% CI, 95% Confidence interval; Min, Minimum; Max, Maximum; IQR, Interquartile range.

  2. a

    t-test result.

  3. b

    Mann–Whitney U-test result.

Control group (n=17)
Mean6.5062.2414.2652.0292.7063.0530.8313.0751.407
SD1.49020.91311.54631.08611.42021.57640.72910.78530.4949
95% CI5.740, 7.2721.772, 2.7113.470, 5.0601.471, 2.5881.976, 3.4362.242, 3.8630.443, 1.2202.657, 3.4931.133, 1.681
Median5.8002.1004.0001.9002.4002.7000.8502.9501.400
Min4.81.01.40.60.40.6−0.72.00.6
Max9.84.86.84.45.36.12.14.52.4
IQR2.50.52.71.72.12.31.01.40.7
Test group (n=15)
Mean7.6752.7534.7801.5202.4473.0070.4543.3151.346
SD2.06161.27442.58081.04691.40761.83240.81001.05420.3455
95% CI6.576, 8.7742.048, 3.4593.351, 6.2090.940, 2.1001.667, 3.2261.949, 4.065−0.036, 0.9432.678, 3.9521.137, 1.555
Median8.3002.5004.8001.4002.3002.5500.4003.2001.400
Min4.81.0−0.50.40.70.8−1.31.60.9
Max10.85.38.83.96.18.22.35.32.0
IQR3.00.93.81.51.82.00.91.10.5
P-value0.070a0.176b0.507a0.188a0.609a0.941a0.198a0.619b0.715a

At baseline, the mean value of clinically evaluated vertical bone defects (DHBL) amounted to 6.5 ± 1.49 mm in the control group (n = 17) and to 7.7 ± 2.06 mm in the test group (n = 15) with no statistical significance difference.

After 5 years, the control group revealed a remaining bone defect height (DH5y) of 2.2 ± 0.9 mm and the test group of 2.8 ± 1.3 mm without statistical significant differences (P = 0.399). The distribution of the remaining defects from <1 mm to more than 3 mm is similar within the test and control group (Table 2). The buccal vertical bone gain between implant placement and the 5-year follow-up can be calculated by the difference of the buccal bone defect height at baseline (DHBL) and at the 5-year follow-up (DH5y). Therefore, the vertical bone gain (∆DHBL-5y) after 5 years amounted to 4.3 ± 1.5 mm for the control and to 4.8 ± 2.6 mm for the test group, again without statistical significant differences (P = 0.493).

Table 2. Distribution of remaining defect heights in mm after 5 years of guided bone regeneration (GBR) (Test a, control b)
Defect height 5 year (mm)Number of patients%
a)
0, ≤1211.8
>1, ≤2635.3
>2, ≤3635.3
>3317.6
Total17100.0
b)
0, ≤116.7
>1, ≤2426.7
>2, ≤3746.7
>3320.0
Total15100.0

With respect to the thickness of the buccal regenerated bone (Fig. 3), the control group revealed 1 mm below the first bone contact (HT1 mm) a thickness of 2.0 ± 1.1 mm and of 1.5 ± 1.0 mm in the test group. At HT3 mm, the thickness amounted 2.7 ± 1.4 (SD) mm in the control group and 2.4 ± 1.4 (SD) mm in the test group. On 5 mm below the crest (HT5 mm), the control group demonstrates a value of 3.1 ± 1.6 (SD) mm and of 3.0 ± 1.8 (SD) mm in the test group. None of the parameters revealed any statistical significant differences between test and control.

image

Figure 3. (a) test group (b) control group.

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The distance between mucosal margin and implant shoulder (MML-IS5y) resulted in 0.8 ± 0.7 (SD) mm at the control group and in 0.5 ± 0.8 (SD) mm at the test group (P = 0.198). When it comes to the distance between the mucosal margin and the BIC (MML-BIC5y), there was also no statistical significant difference (P = 0.619) between the two groups revealing a value of 3.1 ± 0.8 (SD) mm for the control group and of 3.3 ± 1.1 (SD) mm for the test group.

The mucosal thickness reached 1.4 ± 0.5 (SD) mm in the control group and 1.3 ± 0.3 (SD) mm in the test group. There were no statistical significant differences between the mucosal parameters (P = 0.715).

Discussion

  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

The present randomized, controlled clinical study demonstrated high implant survival rates and stable buccal regenerated bone and soft tissue 5 years after GBR around dental implants using either a synthetic PEG gel membrane or a standard porcine collagen membrane covering DBBM. CBCT and clinical measurements documented similar amounts of vertical bone height and bone thicknesses at the buccal aspect of the control and the test implants.

Both groups revealed an implant survival rate of 100% after 5 years. This is similar or higher compared with other studies evaluating implant in regenerated bone after at least 5 years. In a recent long-term clinical study, 222 implants in 58 patients could be followed for 12–14 years (Jung et al. 2013a). One of the groups used the same GBR materials as in the presents control group. They revealed a cumulative survival rate of 91.9% at the 12.5 years follow-up examination. Another 5-year clinical study with a split-mouth design demonstrated implant survival rates in sites with simultaneous implant placement and GBR of also 100% (Benic et al. 2009). A systematic review on bone augmentation procedures in localized peri-implant defects reported implant survival rates ranging from 93% to 100% after 12–60 months following implant loading (Jensen & Terheyden 2009).

One criticism of the majority of long-term studies is the fact that the success of the GBR procedure is indirectly determined by the height of the mesial and distal bone level at the implants measured on two-dimensional radiographs. It would, therefore, be of high scientific and clinical interest to assess the stability of the regenerated bone buccal to the implant site where the regeneration has taken place. This study represents so far the only RCT evaluating the efficacy of the bone regeneration procedures 5 years after implant placement with simultaneous bone augmentation. The mean distance between the implant shoulder and the first BIC was 2.24 mm for the control and 2.75 mm for the test implants after 5 years. Taking into account that the presently used soft tissue level implants have a polished neck of 1.8 mm, the effective buccal bone loss in relation to the structured SLA surface is 0.44 mm for the control and 0.95 mm for the test implants. With an overall buccal bone loss of <1 mm after 5 years, both GBR procedures can be considered very stable. Only one patient of the test group revealed a buccal bone loss to a level of 0.5 mm below the BIC before the bone augmentation procedure. In this particular case, a postoperative infection with a fistula was reported. The generally consistent buccal bone level in the present study is in contrast to a recently published clinical trial (Benic et al. 2012). In this study, single-tooth implants were immediately placed into fresh extraction sockets of anterior and premolar jaw regions with a simultaneous GBR procedure. The infrabony defects and dehiscences were grafted with DBBM and covered with a collagen membrane without over-augmenting the buccal bone plate. After 7 years, the soft and hard tissues were evaluated using CBCT scans in a very similar way as in the present study. The results revealed that in one-third of the sites, there was almost no buccal bone detectable on the CBCT, whereas in the other two-third, the buccal bone level covered the entire rough implant surface. Another clinical study with 40 patients did an assessment of anterior single tooth implants with conventional CT (Nisapakultorn et al. 2010). They revealed very similar results in terms of vertical defect height, however, with a much shorter observation period of only 12 months.

This study demonstrated not only a stable vertical level of the buccal bone but also a quite thick regenerated buccal bone with mean values ranging from 1.5 mm to 2 mm at 1 mm apically to the first bone contact (HT1 mm) and from 3.0 mm to 3.1 mm at 5 mm apically to the crest (HT5 mm). This study proved considerably thicker buccal bone compared with the previously described CBCT examination evaluating immediate implants with simultaneous GBR procedures after 7 years (Benic et al. 2012). The investigators reported on a total of 14 patients revealing an average buccal bone thickness of 0.4 mm ranging from 0 mm to 2.1 mm. As mentioned before, in one-third of the sites almost no buccal bone was radiographically detected. One possible explanation for the differences in buccal bone thickness between the two studies could be the difference in time points of implant placement after tooth extraction. The present study investigated delayed implants, whereas the other study has assessed immediate implants, which possibly reveal more hard tissue alterations after implant placement (Botticelli et al. 2004; Jung et al. 2013b). Other factors explaining the discrepancies between the results of the studies might be the differences in the GBR procedure, in the CBCT scanning protocol and in the apico-coronal location of the measurement.

Further, clinical studies have analyzed the thickness of the buccal bone around natural teeth in different areas of the jaw (Vera et al. 2012; Zekry et al. 2013). They showed mean thicknesses of 0.83 mm in the anterior maxillary area and 1.13 mm in the premolar area (Vera et al. 2012). Rarely, a width of 2 mm was yielded with a generally increased thickness toward the posterior region. Hence, this present study revealed that the regenerated buccal bone after 5 years even exceeded the amount of bone thickness generally found around natural teeth.

From an esthetic point of view the most important factor is the buccal level of the soft tissue. In all, but two patients (one control and one test) the soft tissue covered the implant shoulder by 0.5–0.8 mm revealing no unesthetic exposure of the metal crown margin after 5 years. In a recent longitudinal study, it was documented that 51% of the crown margins were visible and were located in a supramucosal position after a mean follow-up time of 12.5 years (Jung et al. 2013b). The soft tissue dimensions of the present study are in line with a previous CT study including 40 implants sites in the anterior area (Nisapakultorn et al. 2010). In contrast to the present investigation, the study by (Nisapakultorn et al. 2010) showed a correlation between the mucosa thickness and the level of the musosa.

The present RCT reveals some methodological limitations. One is that titanium implants are causing artifacts within a CBCT scan, and therefore, it might be questionable whether the peri-implant tissue can be assessed by this technique (Draenert et al. 2007; Razavi et al. 2010; Schulze et al. 2010; Benic et al. 2013). This would mean that the amount of mineralized tissue, which can be detected around dental implants in CBCTs is rather underestimated. Hence, the fact that a buccal bone plate was visible at all of the implants after 5 years seems to be strong finding, pointing out the effectiveness of GBR around dental implants. The other limitation of the study is that clinical intrasurgical measurements have been compared with CBCT measurements. However, different studies have proven a sufficient accuracy of linear measurements of dento-facial structures within CBCTs (Suomalainen et al. 2008; Lamichane et al. 2009). An in vitro experiment with fresh pic jaws showed that the clinical measurements of the peri-implant bone dimensions differed in <0.2 mm to the CBCT measurements (Mengel et al. 2006). For further improvement of the accuracy, a small voxel size and a small “FOV” focusing on the area of interest has been recommended (Molen 2010). Therefore, the present study used the highest resolution of the CBCT machine with a voxel size of 0.125 mm and a scanning time of 26.9 s. To overcome the risk of movement artifacts with such a long scanning time, every effort was performed to stabilize the patient's head. Nevertheless, one patient had to be excluded due to a movement artifact. A further limitation is the fact that only one single-blinded person (DS) performed the measurements. However, all the measurements have been further checked and discussed within a team of 2–3 persons (RJ, GB, DS).

Conclusion

  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

It can be concluded that a polyethylene glycol hydrogel membrane performed as successfully as a natural collagen membrane in regenerating peri-implant bone defects after a follow-up period of 5 years. CBCT and clinical measurements documented high-implant survival rates, stable peri-implant soft tissues and similar amounts of vertical bone height and bone thicknesses at the buccal aspect of the control and test implants.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

The authors express their special thanks to Malgorzata Roos, Department of Biostatistics, University of Zurich, Switzerland for the statistical analysis of the present RCT. This study has been supported by the Clinic of Fixed and Removable Prosthodontics and Dental Material Science of the University of Zurich, by a research grant of the Swiss Society of Reconstructive Dentistry (SSRD) and by a research grant of the Institut Straumann AG.

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  1. Top of page
  2. Abstract
  3. Material and methods
  4. Results
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. References
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