Long-term Dimensional Changes in Single Crowns Supported by Short (6 mm) Transmucosal Implants With Divergent or Convergent Neck Profiles in the Esthetic Zone. A Randomized Randomized Controlled Clinical Trial
Phase: NA
Conditions: Edentulous Alveolar Ridge
Sponsor: ARDEC Academy

 
Title: Long-term dimensional changes in single-unit crowns 
supported by short (6 mm) transgingival implants with a divergent 
or convergent neck profile in the esthetic zone. A randomized 
controlled clinical trial. 
 
 
 
 
 
 
 
NCT: xxxxxx 
 
 
 
 
 
 
 
 
Study Protocol date: January 1, 2023 
 
 
 
 

Long-term dimensional changes in single-unit crowns supported 
by short (6 mm) transgingival implants with a divergent or 
convergent neck profile in the esthetic zone. A randomized 
controlled clinical trial. 
 
Summary 
The objectives of the present randomized clinical trial are to evaluate the clinical and 
radiographic changes over time of hard and soft tissues around implants with a 
convergent or divergent collar. Twenty volunteer participants with two edentulous 
regions in the maxillary esthetic zone (from the right second premolar to the left 
second premolar) will be included in the study. Two implants will be randomly 
installed, one with a convergent collar and one with a divergent collar. After 3 months 
of healing, individual crowns will be installed. At each visit the following parameters 
will be evaluated: plaque index, probing depth, bleeding on probing, recession of the 
mucosal margin, intraoral radiographs, likewise control CBCT and impressions will 
be performed. The visit will take place at 6 and 12 months and then annually for a 
minimum of 3 years. Changes at the level of the marginal bone will be assessed over 
time on the radiographs. Dimensional changes will be clinically evaluated during 
control evaluations and later through digital impressions. 
 
 
Keywords: Peri-implant marginal bone; digital impression; bone volume; bone 
loss; mucosal recession 
 
 
 
 
 
 
 

Introduction 
The use of 6 mm long implants to support single crowns or fixed dental prostheses 
(FDP) has become a widely used approach in the rehabilitation of the posterior 
region of both jaws. This method allowed the installation of implants without 
performing bone augmentation procedures, decreasing the number of surgeries, 
time spent on treatment, and costs. Several recent reviews have evaluated the 
results of short implants. In fact, short implants are a predictable choice, showing 
fewer biological complications and less marginal bone loss than longer implants, 
according to a comprehensive review and meta-analysis (Ravidà et al. 2019a). 
however, the length of implants in the maxilla, but not the mandible, had a substantial 
effect on the implant survival rate, according to a second thorough research with 
meta-regression analysis (Ravidà et al. 2019b). A different review and meta-analysis 
of randomized controlled studies found that the outcomes were similar but that short 
implants had a greater incidence of biological complications than longer implants. 
(Bitaraf et al. 2019). In addition, another comprehensive review with meta-analysis 
that included 10 randomized controlled studies came to the conclusion that after 1–
5 years of functioning, short implants (6 mm) showed greater variability and a greater 
risk of failing than longer implants (>6 mm). Several studies have been performed 
with short transgingival implants with diverging profile necks supporting single 
crowns (Rossi et al. 2018; 2016) or bridges (Rossi 2017; Storelli 2018). 
In a prospective study involving 21 patients (Agustín-Panadero et al. 2019), two one-
piece transgingival implants, one with a converging collar and the other with a 
diverging collar, were installed in the posterior regions of the maxilla or mandible. 
After three months, the implants were loaded with fixed prostheses. Subsequently, 
after 24 months of operation, less peri-implant bone loss was found in implants with 
convergent collars (0.29 mm) compared to those with a divergent collars (0.60 mm). 
In a human histology study, platform-switch tapered abutments were shown to have 
circular fibers. It is speculated that these fibers could contribute to the stabilization 
over time of peri-implant marginal soft and hard tissues (Rodríguez et al. 2016). 
The characteristic of the abutment/neck surface also influences the level of the 
marginal bone if it is placed subcrestally . (Welander et al. 2009) In an experimental 

study in dogs (Hermann et al. 2011), one-piece implants with a neck featuring a 
rough surface had less marginal bone resorption compared to similar implants with 
a smooth neck. Furthermore, surfaces with a one-dimensional pattern seem to favor 
cell migration and phenotype modification (Doyle et al. 2009). 
In the aforementioned mentioned study (Agustín-Panadero et al. 2019), the 
dimensional changes of the alveolar process on the two implants with different neck 
conformation at long term were not evaluated. In another clinical study (Szathvary 
et al. 2015), a new method was applied to elaborate three-dimensional changes of 
the alveolar process in implants installed immediately after long-term tooth 
extraction, which provided interesting and reproducible data. 
However, a randomized controlled clinical trial comparing the clinical, dimensional, 
and radiographic results of short (6 mm) one-piece transgingival implants with a 
converging collar with microthreads or a diverging polished collar in esthetic regions 
is still lacking. 
Therefore, the aim of the present study is to assess the long-term osseous and peri-
implant soft tissue changes as well as the success rate of short implants (6 mm) with 
a converging collar with microthreads or a diverging polished collar placed in the 
esthetic zone of the maxilla. 
 
Description of the problem 
The literature frequently discusses how both hard and soft tissue around implants 
decreases over time. A convergent collar's conformation provides for better control 
of the conformation of the buccal portion of the crown in addition to providing a wider 
base of connective tissue than a non-convergent collar. It has been hypothesized 
that the crown's shape may enable the preservation or even the growth of the peri-
implant mucosa. 
 
General objective 
The overall objective of this randomized clinical trial is to assess long-term soft and 
hard tissue changes around implants with a convergent or divergent collar. 
 

Specific objectives 
To assess changes in marginal soft tissue levels, marginal bone levels, and bone 
volume around implants with long-term converging or diverging collars. 
 
Material and methods 
The protocol will be submitted to the approval of the research and ethics unit of the 
Faculty of Stomatology of the National University of Trujillo and the Declaration of 
Helsinki on medical protocols and ethics will be applied. Patients will be informed 
about the procedures and possible complications and will be asked to sign the 
informed consent. The Consort checklist (http://www.consort-statement.org/) will be 
followed for this study. This RCT (Randomized Clinical Study) will be registered with 
ClinicalTrials.gov (https://clinicaltrials.gov/) to receive the registration number. 
 
Study Population 
In this randomized clinical trial, the recruitment of patients, surgeries and follow-ups 
will be carried out at the facilities of the Moche Stomatology Clinic- Faculty of 
Stomatology of the national university of Trujillo.  
The inclusion criteria are as follows:  
1. Presence of at least two edentulous areas in the esthetic region of the maxilla 
(from right to left from second premolar to premolar, preferably first premolars and 
incisors) 
2. alveolar bone ≥ 8 mm in height and ≥ 5 mm in thickness assessed on CBCT. 
3. Age of ≥ 21 years 
4. Need for an implant-supported prosthetic restoration  
5. Be in good general health with no contraindications to oral surgical procedures. 
6. Not be pregnant. 
7. Patients who agree to participate in the study and sign the informed consent. 
The exclusion criteria are as follows:  
1. presence of any uncontrolled systemic disease. 
2. History of past or ongoing chemotherapeutic or radiotherapeutic treatments. 
3. Heavy smokers (>10 cigarettes per day). 

4. Previous bone regeneration procedures in the area of interest. 
 
implants used 
Two one-piece transgingival implants, one with a convergent hyperbolic collar 
(PRAMA, Sweden & Martina, Due Carrare, Padua, Italy), and one with divergent 
profile collar (TG, Sweden & Martina, Due Carrare, Padua, Italy) with ZirTi surface 
(Caneva et al. 2016) will be used. The length of the intraosseous portion is 6 mm. 
The convergent hyperbolic collar (PRAMA, Sweden & Martina, Due Carrare, Padua, 
Italy) implant (Agustín-Panadero et al. 2019) will have a 2.8 mm long neck, 
characterized by a 0.80 mm cylindrical path and a 2.0 mm high convergent 
hyperbolic coronal portion. The entire surface of the collar contains parallel micro-
threads (Ultrathin Threaded Micro-surface - UTM). The divergent profile collar (TG, 
Sweden & Martina, Due Carrare, Padua, Italy) implant has a 2.2mm high polished 
neck. 
 
Study design 
The study is a split-mouth randomized controlled trial. One implant with a converging 
collar with microthreads (PRAMA) or a diverging polished collar (TG) will be installed 
in each patient according to the site randomization in the esthetic region of the upper 
jaw (between the second premolars). It will be restored with individual crowns after 
three months of healing. Both implants will be placed with the rough margin ~1 mm 
subcrestally. 
 
Sample size 
To calculate the sample size, data from a clinical study (Agustín-Panadero et al. 
2019) were previously evaluated. In that study, a difference in marginal bone loss of 
0.31 mm was found between the two implant groups. Using a type I error probability 
of 0.05, with a power of 0.9, and an estimated standard deviation of the mean 
differences of 0.35, fifteen patients are needed to reject the null hypothesis that this 
differencein response is zero. Taking into account possible dropouts or 
complications, twenty patients are considered as an adequate sample. 

Randomization and allocation concealment 
Randomization will be performed electronically by one of the authors, who will not 
participate in the surgical procedures. Treatment assignments will be sealed in 
opaque envelopes that will be opened during the surgery. The treatment assignment 
will be disclosed to the surgeon after elevation of the alveolar mucosal flaps and 
prior to the preparation of the recipient sites. 
 
Clinical procedures 
The clinical procedures will be performed by an expert clinician. After the local 
anesthesia will be applied, the mucoperiosteal flaps will be elevated and the alveolar 
bone will be exposed. The preparation of the sites will be prepared according to the 
manufacturer's instructions. Both implants will be installed with the coronal margin 
of the rough surface approximately 1 mm deeper (subcrestal). Healing caps of 
sufficient length will be placed over the implants to ensure non-submerged healing. 
Sutures will be performed to adapt the flaps around the healing screw. Except for 
complications, antibiotics will not be provided. The sutures will be removed after 
approximately 1 week. 
After three months of healing, digital impressions will be taken, and patients will be 
provided with a unique zirconia crown. 
 
Maintenance 
After surgery, patients will be administered pain relievers if necessary. Mouth rinses 
with 0.12% chlorhexidine three times a day until the suture is removed, which will be 
done after 7-10 days. All the participants will be enrolled in a maintenance program 
throughout the study. All participants will be followed for at least 3 years and data 
will be reported annually. 
Clinical, radiographic and digital evaluations 
- 
At the time of the prosthesis installation (baseline time) the depth of the 
probing will be recorded. Clinical photographs, standardized intraoral 
radiographs, CBCT, and digital impressions will be taken. 

- 
After 6 months, the plaque index, bleeding on probing, and probing depth will 
be evaluated. Clinical photographs, standardized intraoral radiographs and 
digital impressions will be taken. 
- 
The plaque index, bleeding on probing, and probing depth will be evaluated 
annually. Clinical photographs, standardized intraoral radiographs, CBCT, 
and digital impressions will be taken. 
 
Calibration for assessment and blinding procedures 
Analysis of the radiographic images will be performed by an experienced evaluator, 
unaware of treatment group assignment, although implant types will be recognized 
on the images. For three-dimensional image analyses, the evaluator will be blinded 
to the treatment allocation. 
 
Imaging Analysis 
Bone tissue levels will be assessed on both intraoral radiographs and CBCT images. 
The peri-implant volumes will be evaluated on the three-dimensional images 
obtained from digital impressions. 
 
Data analysis 
The main study variables will be the dimensional changes over time in the height of 
the peri-implant bone tissues, evaluated in the intraoral radiographic images, CBCT, 
and the peri-implant volumetric changes over time evaluated in the three-
dimensional images obtained from the digital prints. Clinical data will be used as 
secondary variables. Additionally will be evaluated the implant surivival and success 
rate at 1y and 3y follow up and the criteria will be if there was no persistent and/or 
irreversible signs or symptoms such as pain, infection, neuropathies, or paresthesia, 
no peri-implant infection with suppuration, no mobility, and no continuous 
radiolucency around the implant. (Buser D et al. 2002) 
The differences between groups will be evaluated by applying the Wilcoxon test. The 
significance level will be set at α =0.05. 
 

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