Partially-guided Protocol for Maxillary Overdentures Retained by Ti-Zr One-piece Mini Implants

Design This is a single-center, single-group, prospective clinical trial testing a partially-guided protocol for placement of mini implants overdentures in the edentulous maxilla.

Study setting, trial registration, and funding This clinical trial will be conducted in the clinical setting of the School of Dentistry of the Federal University of Goias, Brazil. The research protocol will be previously submitted for ethical approval by the local Ethical Research Committee and will be registered prospectively at the ClinicalTrial.gov database before initiating patient recruitment.

Participants Eligible participants will be subjects with an edentulous maxilla, wearing a conventional complete denture. They will be selected from the list of patients who received conventional treatment at least six months previously. The possible conditions of the mandibular arch will include fully dentate subjects, partially edentate wearing fixed or removable prostheses, fixed or removable implant-supported dentures, or a shortened dental arch.

During the recruitment phase of the study, they will be informed about the treatment to be provided, details about the clinical trial, and the need for collaboration in the prescribed procedures and post-treatment follow-up visits over a minimum 1-year period. The selected participants will sign an informed consent form to participate in the study.

Inclusion and exclusion criteria We will select participants ≥ 60 years of age, with an edentulous maxillary arch, wearing a complete maxillary denture evaluated as sufficient for surgical planning. At the implant site level, they should present a healed edentulous maxilla (minimum of twelve weeks since the last extraction), minimal ridge dimensions of 5.4 mm (width) by 12 mm (height) in the anterior and premolar maxillary area, allowing the placement of one-piece implants with a 2.4 mm-diameter and minimum length of 10 mm. General health condition should be a physical status ASA1 or ASA2.

Excluded criteria will comprise the inability to perform adequate oral hygiene, incapability to provide written informed consent and compliance to the protocol, and any contraindication for oral surgery such as but not limited to, uncontrolled diabetes, immunosuppression, radiation, chemotherapy, or antiresorptive medication such as bisphosphonates, presence of dental, periodontal or peri-implant disease in the opposing arch, presence of a conventional complete denture in the opposing arch (mandible), presence of an insufficient partial removable denture or implant-overdenture in the opposing arch, which cannot be rendered sufficient by a reline, a heavy smoking habit with > 20 cig/d, reported severe bruxism or clenching habits, clinically present oro-facial pain, depression with a Geriatric Depression Scale above 12, xerostomia with a stimulated salivary flow rate inferior to 0.7 ml/min. At the implant site level, potential participants will be excluded if they present a ridge morphology type IV, defects requiring bone augmentation procedures, or unhealthy soft tissues.

Intervention This prospective clinical trial will be conducted on maxillary edentulous patients recruited for treatment with six mini implants (Straumann® Mini Implant System, Institut Straumann AG, Switzerland) for retention of a maxillary overdenture.

Surgical diagnostic will consist of a preoperative visit with a thorough examination of the patient's medical and dental history, and clinical and radiological assessment. This includes a preliminary OPT-tomography to determine ridge morphology and exclude patients with insufficient ridge dimensions before additional radiographic examinations. In case of a positive preliminary radiographical examination, the radiological analysis will be performed with a Cone Beam Computed Tomography (CBCT) radiograph of the maxillary arch. The existing denture will be marked with gutta-percha or composite spheres in the second premolar, canine and central incisor regions prior to image-taking for the purpose of spatial reference. The existing denture will be subsequently scanned with an intra-oral scanner (TRIOS; 3 Shape), and the STL file will be imported into an implant planning software (coDiagnostiX; Dental Wings GmbH) along with the DICOM files of the CBCT. The six mini implant protocol will be virtually planned for an even distribution of the implants bilaterally, with a minimum of 5 mm distance between contiguous implants. The available lengths of the mini implants are 10, 12, and 14 mm, and the appropriate length will be selected according to the bony anatomical situation. In addition, the distribution and axial position of the mini implants will be planned to achieve the best biomechanical distribution and the most parallel position between implants, perpendicular to the occlusal plane and path of insertion of the prosthesis.

Outcomes

1. Accuracy of implant placement The primary outcome will be the accuracy of the implant position using the partially-guided protocol. Assessment of the accuracy of the methods will be based on the measure of the difference between the actual and the planned position of the mini implants, based on the measurement of (1) the global deviations at the coronal and apical regions of the mini implants, (2) the vertical deviation, and (3) the apical angle deviation. The global deviation is defined as the 3D distance between the coronal/apical centers of the planned and placed implants. The vertical deviation is the distance between the coronal center of the longitudinal axis of the planned implant and a plane parallel through the coronal center of the placed implant. The angle deviation will be measured as the angular divergence between the longitudinal axes of the planned and actual position of the mini implants. For image assessment, post-operative CBCT will be performed six weeks after surgery using the same CBCT machine and protocol used for the pre-operative CBCT. The images of the pre- and post-operative CBCTs will be superimposed using the automated surface best fit matching with the iterative closest point algorithm in the treatment evaluation mode of the coDiagnostiX® software. The individual deviations for the six inserted mini implants will be measured (in millimeters) at the limit between the machined and SLA surface of the mini implant, and at the apex between the planned and actual implant positions. The divergence of the implant axis (in degrees) will also be recorded. Six separate superimposing scans of the CBCT image will be obtained for each of the mini implants inserted in each patient, and measurements of deviations and angulations will be done separately.
2. Treatment intercurrencies and complications Any complications associated with the surgical procedures, or deviations from the prescribed protocol due to surgical intercurrences will be recorded. These include failed drilling of the planned implant site, need to raise a flap for adequate positioning of the needle drill, fracture of the bone walls, insufficient insertion torque (< 10 Ncm), early implant failure, and others.
3. Patient-reported outcomes (PROs) Overall impacts of treatment on oral health-related quality of life will be assessed using the Oral Health Impact Profile instrument for edentulous subjects (OHIP-EDENT). This is a shorter version of the 49-item OHIP, and the validated Portuguese translation of the questionnaire will be utilized. Denture satisfaction will be evaluated by using the Denture Satisfaction Index (DSI), a 10mm visual analogue scale (VAS)-based questionnaire. This index evaluates ease of cleaning, general satisfaction, speech, comfort, aesthetics, stability, chewing ability, function and the general oral condition.
4. Chewing efficiency Chewing efficiency will be evaluated with a two-color mixing ability test, where participants are given a two-colored chewing gum and are asked to chew the specimen for 20 chewing cycles. The resulting bolus is evaluated visually on a reference scale and opto-electronically using the Variance of Hue (VOH).
5. Maximum voluntary bite force (MBF) The Gnathodynamometer DMD® (Kratos Equipamentos Industriais Ltda., Cotia - SP, Brazil) is utilized. It is a digital force gauge with an 8.6mm thick bite element. MBF is assessed bilaterally between the upper and lower first molar. The participant's task is to bite as hard as possible on the force gauge, but to stop clenching when she/ he starts feeling uncomfortable. The peak force of three recordings carried out on each side will be used for analysis.
6. Implant Survival and Success Implant Survival and Success rate will be defined as followed: I - Success (optimum health) II - Satisfactory survival III - Compromised survival IV - Failure (clinical absolute failure) Any of following: Additionally, fractured implants will be considered as failures, except fracture of the implant apex during implant placement.
7. Prosthetic Success and Survival During the prosthodontic examination, the implant-supported prostheses will be examined for any complications according to the 2015 ITI Treatment Guide: A failure will be defined as an event leading to the loss of the reconstruction and need for the fabrication of a new overdenture.
8. Clinical Peri-implant Measurements At 4 sites per implant (mesial/distal/buccal/oral), the following clinical parameters will be evaluated at baseline and each follow-up visit: Plaque index and bleeding on probing; Pocket probing depth; width of the keratinized tissue around implants.
9. Peri-implant bone height Intra-oral radiographs are exported and four parallel lines are drawn corresponding to the shoulder and the apical end of the implant on. One line each is placed at the shoulder and apical end of the implant and the other two at the defined bone level on the mesial and distal aspect of the implant. Knowing the implant length and using the implant-shoulder as a reference point, bone levels can be calculated through a simple rule of three with NIH Image J V1.44.
10. Implant Stability Primary implant stability will be assessed by measuring the final and peak insertion torques, and the primary and secondary implant stability will be assessed using the Periotest® instrument.

The Periotest® hand piece (Periotest®, Siemens AG, Bensheim, Germany) will be used with the head of the patient stabilized so that the implants were perpendicular to the floor, and the Periotest will be held parallel to the floor. The test point will be in the most gingival region of the Optiloc attachment, and each measurement will be performed 3 times and the average value will be recorded for each implant.

Sample size:

The sample size for this study will include 10 participants. A 20% increase in sample size will be adopted to account for patient dropout during the 1-year follow-up, and with no relevant impact on the study power. Therefore, the final sample will comprise of 12 participants, and 72 units at the implant-level.

Data analysis:

Descriptive statistics and bivariate tests for independent groups will be used to analyze differences between groups, according to the data distribution patterns. Subsequently, a Linear Mixed-Effects Model (LMM) regression will be used to test the effect of independent variables (anatomic and functional features) on the post-surgical accuracy of the implant position. This multi-level approach will be used because of the clustered effect of multiple implants within patients.