One-Year Multicenter Randomised Controlled Trial Comparing OT Equator® and Locator Attachments to Retain an Early Loaded Implant Overdenture on Two Implants

This investigation aimed to compare the effectiveness of the OT Equator® (Rhein, Bologna, Italy) and the Locator attachment systems used to retain early loaded implant-retained overdentures. This study was designed as a multicenter randomised controlled trial of parallel groups. After implant placement, the patients were randomised to receive OT Equator® attachments in the test group or Locator attachments in the control group. The outcome measures were implant and prosthetic success and survival rates, any biological and technical complication, marginal bone loss, patients' satisfaction, and periodontal parameters. Overall, 42 patients were consecutively enrolled and treated. One implant was lost in the control group, while no implants were lost in the test group. No prostheses failed in both groups. Only a few complications were experienced in both groups. The main was represented by loss of retention of the attachments (retentive caps). The OT Equator® attachment showed statistically lower periodontal parameters. In conclusion, both attachment systems were suitable for overdenture implant retention.


Introduction
Ageing is accelerating globally, resulting in an increased number of dental implants placed in patients who are elderly and who may require healthcare support due to systemic problems. Implant rehabilitation in geriatric patients is a viable treatment option with a high implant survival rate. However, a recent systematic review from the University of Geneva, Switzerland, clearly concluded that age alone should not be a limiting factor for dental implant therapy [1]. Substantial evidence supports predictable, long-term successful implant therapy in elderly patients in terms of implant survival rates, patient satisfaction, periodontal parameters, and complications. Tere are several evidencebased rehabilitation procedures for elderly patients, including, but not restricted to, dental implant therapy. As per the McGill consensus statement, implant-supported overdentures have been established as a conventional approach for prosthetic management of fully edentulous jaws, whether employing immediate or delayed loading procedures [2]. Clinical evidence has demonstrated that utilising twoto-four implants to support mandibular overdentures can efectively address the needs of individuals with missing teeth, ensuring good retention and support [3,4]. Te crucial factor in achieving favourable outcomes lies in the secure attachment of dental implant overdentures. Te McGill consensus statement states that technological advancements have signifcantly enhanced implants' afordability, efectiveness, ease of placement, and long-term success. Nevertheless, the rising expectations for improved aesthetics and functionality among patients and the challenges posed by atrophic conditions have made developing comprehensive removable solutions more challenging than in the past.
To face economically viable rehabilitation, implant overdentures have evolved, not only improving the aesthetics of the teeth but also supporting materials. Ageing signifcantly increased the microhardness values of resins, resulting in chromatic alteration [5]. For several years, retentive anchors with a titanium matrix and ball-type attachment systems were considered the better choice from a fnancial point of view, considering the initial low cost of the components and the reduced number of complications [6]. Nevertheless, attachment systems evolved from a ball type to a low-profle type, aimed at improving the retention and stability of overdentures. Among the latter, the OT Equator ® (Rhein'83, Bologna, Italy) attachment system is becoming a suitable attachment, ensuring good retention of the implant overdenture. Te main beneft of the OT Equator ® (Rhein'83 Bologna, Italy) attachment system is its reduced shape and volume, compared to the well-known Locator attachment (Zest Anchors LLC, Espandido, CA, USA) [7,8]. Moreover, the option for fxed and removable dentures makes this attachment unique [9,10].
Tis multicenter randomised controlled trial aimed to compare peri-implant tissue health, complications, and patient preference between implant-retained overdentures delivered on two unpainted implants with diferent attachment systems, Locator and OT Equator ® (Rhein'83 Bologna, Italy). Te null hypothesis is that there are no diferences in clinical outcomes.

Materials and Methods
Tis study was conducted following the principles outlined in the Helsinki Declaration of 1964 for biomedical research involving human subjects, as amended in 2013, and received ethical approval by the Ethical Committees of the Aldent University of Tirana (4/2018). Tis study was designed as a multicenter randomised controlled trial of the parallel group with two arms. Patients were enrolled and consecutively treated in eight European centers between December 2017 and November 2018. Each patient was provided with appropriate information regarding the study's nature. Written consent forms encompassing surgical and prosthetic procedures and clinical and radiological data utilisation were obtained from all individuals involved. Te present research has been registered in ClinicalTrials.gov (NCT03640910), and the manuscript was written according to the CONSORT guidelines.
Any healthy individual (ASA 1 and 2 classifcations; American Society of Anesthesiologists, https://www.asahq. org), aged 18 years or older at the time of enrollment, with complete edentulism in the mandible, or a failing dentition in the mandible, in need of an implant-retained overdenture, was considered eligible for the study. Te exclusion criteria are reported in Table 1.
Preoperative photographs, panoramic X-rays, and periodontal status were obtained for initial screening and evaluation. Hopeless teeth (in the mandible) were extracted three months before implant placement. All patients received a temporary complete removable denture before implant placement, according to functional and esthetic requirements. Nevertheless, if patients and clinicians accurately judged the complete removable denture, it was used as a temporary solution.
On the day of the surgery, a single dose of an antibiotic (two g of amoxicillin or 500 mg of azithromycin if allergic to penicillin) was administered one h before implant placement. Immediately before surgery, the participants rinsed with a 0.2% chlorhexidine mouthwash for one minute. Local anaesthesia preferred by the surgeon was administered. Te minimally invasive mucoperiosteal fap was elevated. Implants were placed in the mandible's interforaminal region (canine region) using a previously reported surgical approach [11]. Any brand of implants that provide either OT Equator ® (Rhein'83, Bologna, Italy) or Locator (Zest Anchors LLC, Espandido, CA, USA) attachments was used. Te preoperative radiographs and study models dictated the implant lengths and diameters. Implants were placed freehand with the aid of parallel implant pins.
After surgery, the patients were instructed to avoid any trauma at the surgical site, including brushing procedures. A postsurgical cold and soft diet was prescribed. Postoperatively, smokers were advised to abstain from smoking for two weeks. Detailed oral hygiene instructions were provided, including the recommendation of rinsing three times a day with 0.12% chlorhexidine. Analgesics, such as 600 mg of ibuprofen or similar alternatives, were prescribed as necessary. Sutures were typically removed within a timeframe of ten to fourteen days. According to a previously reported workfow, the prosthetic procedures began eight weeks after implant placement [11]. According to an early loading protocol, a new metal-reinforced, complete removable denture was delivered in both groups within four weeks after implant placement. After two to three days, the healing abutments were unscrewed, and the attachments were connected chairside to the new removable prosthesis. Te patients were randomised to receive OT Equator ®® attachments (Rhein'83, Bologna, Italy) in the test group or Locator attachments (Zest Anchors LLC) in the control group. Randomised attachment systems were placed and tightened according to the manufacturer's instructions.
In the test group (Figures 1-5), after gingival healing, the newest low-profle OT Equator ®® attachments were screwed onto the implants using the OT Equator ® square screwdriver, with a 22-25 N cm torque range. Te cuf heights ranged from 0.5 to 7.0 mm, depending on the size of the transition zone of each implant, easily measured using the colour-coded millimetre cuf height measurer gauge (Rhein'83, Bologna, Italy) after healing abutment removal. Afterwards, the needed space to accept the female housing steel cage was prepared in the ftting surface of the removable complete mandibular denture. Silicone protective discs (Rhein'83, Bologna, Italy) were placed over the OT Equator ® attachments. Extra-soft (yellow, 600 g) retentive caps were initially placed into the female steel housing, attached to OT Equator ® , and fnally fxed to the denture using self-cured acrylic resin. At the same time, the patient held the dentures in occlusion, directly chairside. After complete polymerisation, the denture was picked up, and silicone discs were removed. Acrylic excess was trimmed, and the denture was refned and polished. One month after the delivery of prostheses, the yellow retentive caps were replaced with a more robust type (pink, 1200 g).
In the control group ( Figures 6-10), the Locator attachment (Zest Anchors LLC) was screwed onto the implants using the Locator screwdriver (Zest Anchors LLC), with a torque range of 20-25 Ncm. Te cuf heights of 2.5 or 4.0 mm, depending on the size of the transition zone of each implant, were measured using the deep probe of the implant line after healing abutment removal. Afterwards, spaces to accept the female housing steel cage were prepared in the ftting surface of the removable complete mandibular General contraindications to oral surgery Pregnant or nursing Intravenous bisphosphonate therapy Alcohol or drug abuse Heavy smoking (≥20 cigarettes/day) Radiation therapy to the head or neck region within the last fve years Parafunctional activity Untreated periodontitis Psychiatric therapy or unrealistic expectations Immunosuppressed or immunocompromised Lack of opposite occluding dentition/prosthesis Acute infection in the area intended for implant placement Need for bone augmentation Poor oral hygiene and motivation (full-mouth bleeding on probing (BoP) and full-mouth plaque index [PI] higher than 25%) Patients participating in other studies, if the present protocol cannot be properly followed Patients referred only for implant placement or unable to be followed for three years Allergy or adverse reactions to the restorative materials   denture. Silicone protective white rings (Zest Anchors LLC) were placed over the Locator attachments. Passive black caps were used to load the attachment, attached to Locator, and fnally fxed to the denture using self-cured acrylic resin, while the patient held the dentures in occlusion, directly chairside. After complete polymerisation, the denture was picked up, and white rings were removed. Acrylic excess was trimmed, and the denture was refned and polished. Black caps were removed, and blue ones (6N) were mounted in the steel housing. One month after the delivery of prostheses, the retentive caps were replaced with a pink matrix (12N).
Occlusion was developed in both groups to deliver lingualised occlusion with balanced contacts during function, avoiding premature contact. Nevertheless, when the opposing arch was a completely removable denture, the over-jet had to be left purposely broad, from 2 to 5 mm, to avoid interferences during function. Domiciliary oral hygiene instructions were given to both groups. Tese depend on residual dentition. However, in all cases, instructions were given to clean attachments and prostheses. Follow-up visits were scheduled for occlusal adjustments and oral hygiene quality control every six months and every year for retentive cap replacement [11].         International Journal of Biomaterials periodontal parameters (bleeding index (BI) and plaque index (PI)): (i) An implant was deemed unsuccessful if it exhibited any form of mobility, determined by tapping or rocking the implant head using metallic instruments. In addition, progressive marginal bone loss, infection, or any mechanical issues that rendered the implant nonfunctional, despite maintaining stability within the bone, were also regarded as factors indicating implant failure. (ii) Te prosthodontic success of implant overdentures was assessed with the six-feld table analysis proposed by Payne and coworkers ( (vi) Te bleeding index and the plaque index were evaluated at four sites around each implantabutment interface at the baseline and one year after loading examination with a dedicated periodontal probe.

Statistical Analysis.
A priori sample size calculation was performed online (https://clincalc.com/stats/samplesize. aspx) based on a previous preliminary report [15], given: alpha 0.05, beta 0.2, and power 0.80. Twenty centers were involved with six patients each to improve the sample size by at least one-third. Of these, three patients were to be treated with two implants and OT Equator ® (Rhein'83), and the same number of patients were to be treated with two implants and two Locators (Zest Anchors LLC). Te total sample size was to be 44 patients for each group. Data were planned to be collected 1, 3, and 5 years after loading. Eight computer-generated restricted randomisation lists were created. Te randomisation codes were enclosed in sequentially numbered, identical, opaque, sealed envelopes. Envelopes were opened successively after implant placement. One investigator, not involved in the study, was aware International Journal of Biomaterials of the randomisation sequence and could have access to the randomisation lists, which were stored on his passwordprotected laptop.
Statistical analysis was developed to fnd diferences between groups. Data were recorded in a spreadsheet (Numbers for Mac OS X). A statistician with expertise in dentistry analysed the data using the same software. Descriptive analysis was conducted using mean ± standard deviation with a 95% confdence interval (CI) for numerical parameters. Fisher's exact test was employed to compare the proportions of dichotomous outcomes, such as implant failures, prosthesis failures, and complications. Unpaired sample t-tests were utilised to compare the means at the patient level for continuous results, including OHIP, marginal bone loss, BOP, and PI. All statistical analyses were conducted at the patient level, and a signifcance level of 0.05 was applied.

Results
Each center was supposed to enrol six patients, but after the study began, it was noted that only eight out of twenty centers could enrol patients. Overall, sixty-three patients were screened for eligibility, but only 42 participants were consecutively enrolled in the trial by the eight participating centers. In particular, Dr. Tallarico (Rome), Dr. Cristache (Bucarest), and Dr. Casucci (Siena) recruited six patients, Dr. Montanari Marco (Forlì), Dr. Scrascia (Taranto), Dr. Ferrari (Bologna), and Prof. Rodriguez (Pavia) recruited fve participants, and Dr. Xhanari (Tirana) recruited four participants. However, no patients dropped out after randomisation. Reasons for not including the 21 excluded patients are reported in Table 3.
Te primary baseline patients' and implant characteristics of the 42 patients that were randomised are presented in Tables 4 and 5. Tere were no apparent signifcant baseline imbalances between the two groups, except for more female and younger patients in group one (test group).
Implant Failures. At the one-year follow-up, one implant failed in the control group at center seven, while no implants were lost in the test group. Te diference was not statistically signifcant (P � 0.4286). Te patient lost the implant in position 33 six weeks after implant placement. Te implant was replaced six months later with no other complications/ failures. In the meantime, the patient wore the prosthesis attached to only one implant.
Prosthesis Failures. At the one-year follow-up, no prostheses failed in both groups (P � 1.0).
Complications. At the one-year follow-up, three difculties were experienced in each group. A comparison of implants and prosthesis failures and complications is reported in Table 6. Te diference was not statistically signifcant (P � 1.0). Te prosthesis was broken ten months after its delivery in the test group at center one. It was repaired chairside in 30 minutes. At centers two and seven, patients each showed an early loss of retention of the caps replaced chairside in fve minutes. In the control group, at centers two and seven, three patients (one at center two and two at center seven) showed an early loss of retention of the caps replaced chairside in fve minutes.
A comparison of the mean marginal bone loss, OHIP, mean BI, and PI are reported in Table 7. Tere was a statistically signifcant diference only for periodontal parameters with a lower value for OT Equator ® (Rhein'83).

Discussion
Both attachment systems provide successful results when comparing main and test groups, with no statistically  [17].

Success
No evidence of retreatment except for accepted maintenance (includes patrix activation/repair/replacement, matrix activation/repair/replacement, and asymptomatic peri-implant/interabutment mucosal enlargement not requiring excision). Tere was a limit of two replacements of either patrix or matrix in the frst year and fve replacements in 5 years and one reline of the overdenture base in fve years Survival Survival: patient could not be examined directly, but the patient or another clinician confrms no evidence of retreatment except that described for a successful outcome Unknown Unknown (lost to follow-up): the patient could not be traced; surviving or successful implant overdenture removed to allow-to provision of a new overdenture, e.g., conversion to another overdenture design with additional implants or a fxed implant prosthesis using the same or additional implants International Journal of Biomaterials Table 3: Patients not included with reasons [14].
Center 1 (n � 9) 1 patient refused implants; 3 patients requested fxed restorations; 5 patients required implant-supported hybrid overdentures Center 2 (n � 4) 2 patients were treated just 5-6 months ago; 2 patients required fxed restorations Center 3 (n � 4) 4 patients were not included. Tey required fxed restorations, and 4-6 implants were inserted Center 4 (n � 0) None Center 5 (n � 2) 1 patient refused implants; 1 patient required fxed restoration Center 6 (n � 0) None Center 7 (n � 2) 2 patients refused implants Center 8 (n � 0) None  Lately, more and more completely edentulous patients with atrophic mandible or maxilla require fxed rehabilitations. Nowadays, overdenture retained by implants is one of the best solutions to achieve an optimal masticatory and phonetic function and satisfy esthetic requests. In the present randomised multicenter controlled trial, two diferent attachment systems for overdenture were evaluated. In particular, the implant and prosthesis success and their survival rate, the biological and technical complications, the marginal bone loss, and the quality of life were assessed by the Oral Health Impact Profle (OHIP-19) questionnaire. As shown by Khalid et al. [16], an improvement in the patient's outcomes after implant therapy was estimated, independent of the type of implant attachments. Tese data agree with the data from the present study, where the OHIP was signifcantly reduced in both groups.
Unfortunately, several centers failed to enrol patients. Nevertheless, 42 patients were fnally enrolled and randomised. Te enrolled patients were treated in eight diferent clinical centers and divided into two groups depending on    Table 5, except for more females and younger patients in the test group. In the present study, and previous report, it seems that sex did not afect the results [15]; however, it appears that female patients required functional and esthetic oral rehabilitation more than male patients. On the contrary, clinical evidence documented sex diferences in oral health. Women's oral health declines more rapidly than men's. Te results of the present study partially agree with those of previous research, reporting successful outcomes in both groups [15,17], with no signifcant diferences, except for lower periodontal parameters in the test group. In fact, regarding 42 patients, no prostheses failure happened, and one implant failure was reported. Te main complication, equally presented in both groups, was the loss of retention of the prosthesis due to wear of the retentive caps. Also, the literature is described as a common mechanical problem, regardless of the type of attachment used [1,15].
Previously, Nieves Mínguez-Tomás et al. [18] evaluated, in vitro, the retention capacity of Locator (Zest Anchors LLC) and OT Equator ® (Rhein'83 Bologna, Italy) attachments, concluding that both systems had similar retention values. Te complication experienced in both groups was the wear of the retentive caps that required their replacement. As reported in the literature [19], most technical difculties may occur in the frst year. Despite this being a standard feature for overdenture, in our research, both groups experienced early needs for retentive caps replacement. Te authors believe that the incorrect insertion of the overdenture may explain this problem. Re-explaining the attachment method and changing the matrix with a more retentive one were enough to accomplish the problem.
At one year follow-up, no statistically signifcant difference was found for MBL between the groups. Te only meaningful diference was lower bleeding and plaque indices in the test group. Identical results were found in a preliminary multicenter retrospective analysis on implant overdentures conducted by Tallarico et al. [15] after fve years of examination. Te attachment height from the gingival margin was controlled by selecting the attachment height in both groups. Te correct height of the attachments was needed to ensure their cleanliness, notably easier brushing, and the proper connection with the female portion. Even if successful results were found in both groups, the OT Equator ® attachments have a low vertical profle of only 2.1 mm, making them the most miniature attachment system on the market. Due to the narrow diameter of the OT Equator ® attachment, a possible explanation for better periodontal results in the test group could be the platform switching efect that could be obtained even in smaller diameter implants, contributing to marginal soft tissue maintenance [11-14, 16, 20]. Nevertheless, several confounding factors exist. For the latter, further studies are needed to confrm this hypothesis.
In the present study, all the implants were early loaded. Further investigation should be conducted to evaluate the performance of attachment systems in case of the immediate loading procedure [21], also assessing risks and benefts, including patients' satisfaction. Another study [22] presents a randomised controlled trial comparing the outcomes of two-implant-and three-implant-supported overdentures for edentulous mandibles. Te study evaluated implant and prosthetic success rates, complications, marginal bone loss, patient satisfaction, and peri-implant tissue health. A total of 34 patients were enrolled in the trial, with 14 in the threeimplant group and 20 in the two-implant group. Two implants failed in the three-implant group at the one-year follow-up, while no implants were lost in the two-implant group. Tree complications were experienced in the twoimplant group, and one occurred in the three-implant group. However, the groups had no statistically signifcant diferences in any of the evaluated outcomes.
No prosthesis failure was registered, and only one early implant failure occurred in the control group. Te implant was reinserted without further problems. Te main limitation of the present study was the small sample size because most of the involved centers needed to recruit patients; moreover, the status of opposing dentition must be described in detail. Chewing efciency may afect the wear of retentive elements up to the loss of retention of an implant-retained overdenture.

Conclusions
With the limitation of the present multicenter randomised controlled trial, successful results were found in both groups, even if statistically signifcant better periodontal parameters were found in the test group. Tis abutment (OT) type shows signifcant clinical and biomechanical advantages compared to similar abutments. Indeed, the smaller footprint from a spatial point of view is a good advantage for the clinician and the dental technician. Furthermore, more excellent retention guarantees reliability and predictability of oral rehabilitations. Further studies with larger sample sizes and longer follow-ups are needed to confrm the results of this preliminary report.

Data Availability
Data are available on request to the corresponding author.

Conflicts of Interest
Te authors declare no conficts of interest.