## 1 Introduction  
Generally, a 1.5 mm of crestal bone loss around dental implants during the first year, followed by a 0.2 mm of loss in subsequent years, has been considered acceptable [1, 2].  
More recent studies have advocated that the maintenance of the initially achieved peri-implant bone level (BL) as coronally as possible is crucial for long-term success and good aesthetic results of implant therapy [3].  
Many variables are related to marginal bone loss, including surgical trauma, occlusal overload, peri-implantitis, micrograph, biologic width, and implant crest module [4, 5, 6].  
Some of the factors mentioned above are highly influenced by implant-related factors, such as the macrogeometry of the implant, the implant-abutment connection, and the position of the coronal rim in relation to the crestal bone [7, 8, 9, 10, 11]. However, whether an implant is a one-stage or two-stage implant does not seem to be of great significance [3, 12].  
The manner in which load stress is transferred to bone surrounding an implant is decisive when considering bone maintenance [13]. For this reason, several modifications have been introduced in the macro- and microgeometry of this type of fixture to enhance the rate and extent of osseointegration [14, 15], promote the preservation of peri-implant tissues [16, 17], and shorten the treatment time [7].  
The crest module is the coronal part of the implant that is closely related to the bone cortex and plays an important role in stress distribution. The crest module is designed with a smooth or rough morphology.  
Smooth modules are designed to be inserted in a supra-crestal manner to avoid the accumulation of dental plaque, and their infracrestal insertion favours bone loss [18; 19; 20; 21]. In contrast, modules with a rough surface have been shown to better distribute load stress on the bone cortex and are designed for infracortical use [22], although they may increase the difficulty of hygienic maintenance [23].  
Dental implant microthreads are an innovative solution to the problem of dental implants failing due to weak attachment. The microthreads are designed to be flexible and promote bone growth, creating an even stronger bond between the implant and the surrounding jawbone. This technology is becoming more popular among dental professionals as it has been proven to be an effective way to increase the success rate of dental implants.  
However, considering the onset of peri-implantitis, no difference has been found between implant collars with a rough or smooth surface [24; 25].  
Despite the literature on this topic, there remain gaps in the current research. For example, most of the research in this field has focused on finite element studies with limited research on the clinical impact in humans. It has long been proven that the presence of a rough surface or the presence of microthreads improves the distribution of mechanical stress, especially shear forces. However, we do not know the minimum number of spires required to achieve this effect, or the maximum number of spires above which no improvement would be noticeable. In addition, potential risks remain to be determined, especially during long-term follow-up periods. Therefore, more research is needed to determine the role of the number of microthreads and the potential risks of increased incidence of peri-implant disease and bone loss in humans.  
The number and distribution of microthreads in the crest module is a factor that has not been extensively studied. The purpose of the present study was to determine whether the presence of three microthreads and a 1 mm long neck versus the presence of nine microthreads and a 3 mm long neck makes a difference in the maintenance of bone over 5 years around implants subjected to the same surface treatment.  
The null hypothesis of the study is that there is no significant difference between the number of microthreads and bone level in implants followed-up for 5 years.