Morse taper implant macrodesign, loading protocol and site of installation – retrospective study of 5,601 implants

favored implant maintenance. Conclusion: It can be concluded that Morse taper implants present a long-term survival rate that can be lowered by excessive torque, as well as by the diameter of the implant.


INTRODUCTION
Treatment with osseointegrated implants was described in the 1960's by Brånemark and his team, who recommended that the procedure be done in two stages, and with healing periods of 3 to 6 months 1 . However, other researches 2,3 have demonstrated the possibility of even faster and predictable surgical and prosthetic restoration. In consequence, the use of osseointegrated dental implants became a common practice in oral rehabilitation, being confirmed by the high efficiency in the restoration of single, partial, or complete edentulous arches 4,5 . Nevertheless, in spite of the success of osseointegrated implants, problems both mechanical and biological, such as loosening of the abutment screw and bacterial microleakage have been documented [6][7][8][9][10] .
It has also been recommended, with the objective of maintaining peri-implant bone tissue, the utilization of prosthetic components with a diameter reduced in comparison to the implant, also known as platform switching. Lazzara, Porter 23 hypothesized that less bone resorption occurred the farther the infiltrated inflammation area was from the crest. The clinical results and high predictability of Morse taper implants associated with platform switching abutments have been widely studied 15,24,25 .
With respect diameter of the implant, some are the advantages cited of using wider implants as more bone to implant contact, bicortical engagement and reduction in abutment stresses 26 . But in a systematic review the authors observed that the survival rate of small-diameter implants appears to be similar to the regular ones 27 . Considering length, Olate et al. 28 reported that early implant loss might be related to length, but not diameter.
Dental implant survival rate may also be related to the quality and quantity of bone. In a systematic review, the authors observed that the survival rate of dental implants inserted in low density bone was lower than in better quality bone 29 . But this worst result can be minimized with the choice of implant geometry in areas with low-density bone 30 .
The long-term implant-supported prosthetic rehabilitation monitoring is extremely important in evaluating parameters that could interfere in the success of the treatment. Nevertheless, studies on success and survival rates of dental implants are complex because they´re influenced by a large number of variables 31 . So, the aim of this cross-sectional study was to evaluate the relationship of macrodesign parameters, area of installation, insertion torque and loading time of Morse taper connection implants with its long-term stability.

MATERIAL AND METHOD
This cross-sectional study evaluated the long-term survival rate of Morse taper implants with a target population of patients who were rehabilitated through the installation of Morse taper implants at the ILAPEO School. Guidelines proposed by the STROBE Declaration (Strengthening the Reporting of Observational Studies in Epidemiology) were followed. This study was approved by the Research Ethics Committee at the International University Center -UNINTER -under the protocol number 1,484,665.
The sample consisted of consecutive rehabilitated patients between the years 2006 and 2012 in Implantology courses of Ilapeo College. The inclusion criteria were patients who had had Morse taper implant(s) supporting single, partial or full-arch prostheses during the aforementioned period. Incomplete medical records that did not have all the information necessary to complete the study were excluded.
After obtaining the list of the patients that were in accordance with the inclusion criteria listed above, the desired data were collected from their medical records through a software program called Google Form based on Cloud Computing.
The following variables were considered: Exposure variables: Related to the implant -diameter, length, body and apex shape (conical or cylindrical), insertion torque, and loading protocol (immediate or not); Related to the patient -rehabilitation location (anterior or posterior region), and area (maxilla or mandible); Outcome variables: Implant loss; Follow-up period.
A statistical analysis was done using Stata software 14.0 (StataCorp LLC, College Station, Texas, EUA) with an accuracy ratio of 95%. An independent statitian revised the study.

RESULT
A total of 5,601 implants (Neodent, Curitiba, Brazil) installed in 1,142 patients were included in the study. The mean follow-up period was 37.54 months (maximum 159 months). Three hundred and eighty-five medical records had inconsistencies (missing information) and were excluded.
The survival rate was 98.38% (5,510). Ninety-one implants suffered stability loss after being in function.
Considering data related to the installation location and area, 51.21% were in the maxilla. Data related to design variables and insertion torque are shown in Table 1. Of the 5,601 implants observed in this study, 1,090 were installed with immediate loading (19.46%). Only 91 implants were lost during this study and 0.27% (3 implants) had been submitted to immediate loading.
A logistical regression test was applied to evaluate which variables influenced the implant survival rate. Parameters such as: region, body, apex shape and length did not show any significant influence on implant loss ( Table 2).  An increase in torque and diameter both had a direct influence on implant loss. On the other hand, immediate loading was inversely proportional implant loss, that is, it favored implant maintenance (Table 3).  19 . This data demonstrates that a Morse taper platform, at the least, is equal to other types of platforms that have been on the market for some time (Table 4). The geometry of the implant-abutment area seems to be a factor in influencing the transmission of stress and tension in the area around the implant. Nevertheless, according to the literature both non-conical and conical connection systems show comparable successful results, such as implant survival, absence of radiolucency around the implant in after 1-year, maximum bone resorption of 0.2 mm after 1 year, the absence of mobility of single-tooth implants, no signs of infection, pain, or any pathology in progress 14,15,17,[32][33][34] . In the present study body and apex shape did not influence the survival rate of the dental implants, maybe it´s related to the adequate selection according to quality and quantity of bone available.
However, evidence demonstrates that Morse taper systems seem to be a better alternative in terms of bacterial sealing, resistance to abutment movement, improved biomechanics in relation to microgaps, more impediment to torque loss compared with other systems, as well as high resistance to fatigue and flexion 16,18,32 .
In accordance with the data generated by this research, the macrodesign most used was cylindrical implants (70.33%) with the apex shape following the same dimensions. The diameter most commonly used was 3.5 mm (30.17%) and 3.75 mm (39.73%) with a mean length of between 11 mm (14.69%) and 13 mm (29.39%). This demonstrates a tendency to use implants with a smaller diameter more than in other research which used a diameter of 3.5 mm (78.60%) with a length of 11 mm 33 . The logistical regression analysis showed that body and apex shape designs and length, had no significant statistical influence on implant loss. Diameter significantly influenced the survival rate of the dental implants, which is not in accordance with a systematic review that reported that the survival rate of small-diameter implants appears to be similar to the regular ones 27 .
Regarding immediate loading, 19.46% (1,090) of the implants were immediately loaded and of theses only 3 failed. Therefore, this study showed statistically significant difference and exemplifies that immediate loading, when well indicated and respecting all appropriate criteria and recommendations, could be beneficial for the maintenance of implant osseointegration. One of these recommendations is related to the final torque of the implants, of which, this study found evidence that a torque of 41-50 Ncm (16.62%) as a mean torque was appropriate for immediate loading.
As this study only used information available from patient medical records, there could have been some bias in the results due to problems with inconsistencies in filling in the medical records and data interpretation of the same. Also, it was not possible to radiographically evaluate bone level in view of the non-standardization of the exams.
It is important to note that the data in this research was collected from an academic institution and that almost all the Implantology procedures were carried out by students with differing levels of apprenticeship.

CONCLUSION
It is possible to conclude that rehabilitation using Morse taper type platforms has a high survival rate, and that the variables: immediate loading, torque and diameter affected long-term stability. Implants from 3.5 mm to 4.0 mm in diameter and 8.0 mm to 11.0 mm in length were the most commonly used, which shows a tendency to use smaller implants.