Sample size determination and grouping
Sixty extracted human mandibular canine teeth with no signs of internal or external resorption were used. Mature apex and absence of carious lesions or root canal fillings were mandatory as well. Samples were randomized and allocated into three equal groups according to the material of the ball attachment used; eighteen teeth (N = 20) in each group. In group TI, Titanium ball attachments were used and served as a control group. However, in group PE, they were made of PEEK and in group PK they were made of PEKK. In each group, the teeth were divided equally into two subgroups (n = 10); subgroup T0 and subgroup T1. The tensile bond strength in subgroup T0 was measured before aging while in subgroup T1, it was measured after aging.
The sample size was determined in the light of the results published by Fuhrmann et al. and Benli et al. [15, 16]. The sample size was calculated based on 95% confidence interval and power 90% with α error 5% (G.power 3.19.2) Ethical approval was granted by the ethical committee in the author’s university (RecER022221). The extracted teeth were taken from the archives of the Oral and Maxillofacial surgery department in the author’s university.
Abutment teeth preparation
A single operator prepared the teeth and made the root canal treatment. The length of the roots was standardized to 15 mm using a diamond disc mounted in a low-speed hand piece under water coolant. The patency of the canal was validated using a Size # 10 K file (Mani, Utsunomiya, Japan). The working length was shorter 1 mm from the length obtained when the file tip just appeared at the apical foramen. Root canals were instrumented using ProTaper Gold rotary files (Dentsply, Maillefer, Switzerland) according to the manufacturer’s instructions up to # F5. The file had a 0.5 diameter tip and variable taper along its length. For irrigation between successive instrumentation, 3 ml of 2.5% NaOCl (Clorox, Egypt) was delivered at a rate of 3 ml/min using a disposable plastic syringe with 30G side-ended needle (Sung Shim Medical Co., Bucheon, Gyeonggi, South Korea) 2 mm short of the total working length. At the end of the preparation, each canal received the final flush protocol with 5 ml of 2.5% NaOCl for 1 min, then 5 ml of 17% Ethylenediaminetetraacetic Acid (EDTA,Meta Biomed, Korea) for 1 min and 5 ml of saline (Novartis, Egypt) for 1 min to remove the smear layer [25]. All root canals were obturated with the cold lateral compaction technique using gutta-percha and AD-seal sealer (Meta Biomed, Korea). The quality of the obturation was confirmed radiographically. After obturation, 3 mm of gutta-percha was removed from the canal, and temporary filling material (Cavit,3 M ESPE, St. Paul, USA) was placed. All roots were kept in 100% humidity at 37 °C for 7 days to ensure complete setting of the sealer [25].
Attachment fabrication
For group TI, ready made titanium ball attachments (9 mm length OT Pivot flex, Rhein 83, Italy) were purchased. The titanium attachment was scanned (DOF swing scanner, DOFlabs, Seoul, South Korea) and a standard tessellation language (STL) file was generated using a CAD software (Exocad Dental CAD, Exocad Inc. Darmstadt, Germany) (Fig. 1). The file was used for milling similar ball attachments made of PEEK (Brecam Biohpp,Bredent, Germany) that were used in group PE. Similarly, the STL file was used for milling the PEKK ball attachments (Pekkton Ivory, Cendres+Metaux Medtech, swizerland) that were used in group PK [16].
Root canal drilling and attachment cementation
After the 7-day setting period, preparation and cementation of the ball attachments were performed. A channel was created in each tooth for the ball attachments using the manufacturer’s drill (Mooser post reamer bur, Rhein 83, Italy). A single operator mounted the bur to a low speed handpiece to prepare the obturated canals to a length of 9 mm. The channels were rinsed with water and dried with paper points (Meta Biomed, Korea). EDTA was injected into the prepared space and left in place for 1 min to remove the smear layer. The canal space was then rinsed with saline and dried with paper points [25].
The post of each attachment was blasted with 50um aluminum oxide particles (BEGO sandblaster, BEGO Bremer GMBH, Germany) for 15 s at 0.25 MPa and 2–3 bars [14, 15, 20, 26, 27]. After sandblasting, the samples were dried with oil-free compressed air. The attachments in group TI were primed using Z-prime plus (Bisco Inc, United States of America) [26]. The attachments in the three groups were then inserted and cemented with self-adhesive resin cement (G-CEM, GC, Tokyo, Japan) following the manufacturer’s instructions. The attachments were inserted to the depth of the prepared channels with finger pressure then the excess cement was removed and light curing then followed. One trained operator cemented all the samples.
Mechanical aging
Model creation for mechanical aging
For mechanical aging, an educational mandibular completely edentulous cast (Ramses edentulous cast; Ramses medical products, Cairo, Egypt) was used in this study. A waxed-up mandibular denture was made for this cast. The cast was then scanned (DOF swing scanner; DOFlabs, Seoul, South Korea) and a standard tessellation language (STL) file was generated. The cast with the overlying waxed up denture was then scanned and the STL file was generated. Both STL files were superimposed to determine the position of the mandibular canines’ sockets in the virtual model on the CAD software (Exocad Dental CAD; Exocad Gmbh, Darmstadt, Germany). Mandibular right and left acrylic canine teeth (Ramses medical products, Cairo, Egypt) were scanned and the STL file was generated. This STL file was then used for subtraction of the mandibular canines from their corresponding sockets in the previously scanned mandibular model. A space of 0.25 mm was left between the inner surface of the socket and the canine root surface simulating the periodontal membrane space. A 2 mm layer thickness was removed from the scanned model crest representing the mucosal layer that was added later [28]. The design of the virtual model was checked and the STL file was sent to the additive manufacturing device (ULTRA 3SP; EnvisionTEC Inc, Michigan, United States of America). The printed model was then duplicated into 15 similar acrylic resin models (HUGE Dental Material CO, Shandong, China) that were randomized and allocated into the three groups of the attachments used in the current study.
Mucosa and periodontal ligament simulation
The abutment teeth were ditched on their labial and lingual surfaces and then placed in their corresponding sockets in each model after injecting a light bodied rubber base material (AFFINIS™ light body, Coltene whaledent Inc., Altstätten, Switzerland) for periodontal ligament simulation [29]. The simulated periodontal ligament was then secured in place with a thin film of cyanoacrylate adhesive (Amir Alpha; Amir Alpha Co., Cairo, Egypt).
For standardized mucosa simulation among the models, an acrylic template was used. A base plate wax of thickness 2 mm was added to the crest and slopes of the mandibular residual ridge in the printed model. Duplication of the model then followed to produce a stone model. The stone model was then placed in the vacuum press machine (Yates Motloid, United States of America) and an acrylic sheet (Bio-Art Equipamentos Odontologicos Ltda, Brasil) was pressed over it. The acrylic template was then trimmed and tried on the other models to check the fit. A light bodied rubber base impression material (AFFINIS™ light body, Coltene whaledent Inc., Altstätten, Switzerland) was dispensed in the acrylic template and seated over each model for mucosa simulation [28]. The simulated mucosa was then secured in place with a thin film of cyanoacrylate adhesive (Amir Alpha; Amir Alpha Co., Cairo, Egypt) (Fig. 2a).
Overdenture fabrication and determination of the geometric center
An overdenture was fabricated for the printed model and duplicated into 15 similar overdentures (Fig. 2b). For locating the geometric center on the printed model, two lines parallel to each other were determined; the first one passed through the apices of the retromolar pad and the second one passed through the incisal edge of the mandibular incisors. The midpoint on a third line that bisected the model and was perpendicular to the two previous lines represented the geometric center of the model [30]. A modeling wax sheet was then shaped in the form of a plate that was 10 mm in anterior–posterior dimension, 2 mm thickness and joining the occlusal surfaces of the teeth on both sides of the arch. The wax plate was then placed on the overdenture of the printed model so that its center was coincident with the geometric center of the arch. In the center of the wax plate, a recess was made to accommodate the tip of the load applicator in the chewing simulator machine. The wax plate was then invested (Bego bellavest®, Bego Gmbh, Bremen, Germany) and cast into a metal plate (Wironit, Bego Gmbh, Bremen,Germany). The metal plate was repositioned on the printed model in the previous predetermined position of the wax plate. An acrylic template was then made in the same way as that made for mucosa simulation in the current study. The acrylic template was then used to replicate the same position of the metal bar in the other models. The metal bars were positioned in place using autopolymerizing acrylic resin.
Aging and chewing simulation
For aging, the chewing function in addition to overdenture insertion and removal were simulated in the current study. Each overdenture was inserted and removed 5000 times by one operator to simulate patient’s insertion and removal [31]. For chewing simulation, the mounting ring of the chewing simulator (CS-4.4; SD Mechatronic, Germany) was painted with Vaseline. Each model was then placed so that the load applicator was positioned in its predetermined position in the metal bar. The model was then secured in place with autopolymerizing acrylic resin (HUGE Dental Material CO, Shandong, China). The setting parameters of the chewing simulator were adjusted (60 mm/sec, 5 mm vertical path, 0.5 mm horizontal path, 1.6 Hz frequency, 68.6 N).The machine chambers were filled with artificial saliva prepared in the pharmaceutical industry lab in the author’s university according to the composition of (Glandosane®; Fresenius Kabi Ltd, Germany). Bi-axial cyclic loading of 1,200,000 cycles at room temperature were applied to each model simulating five years of service (Fig. 3). The teeth were then removed from the models for tensile bond strength testing.
Samples preparation for testing
After a period of 2 weeks storage in distilled water at room temperature, the samples were placed in a standardized acrylic mold. To ensure parallel alignment of all the samples in their molds, the top part of the attachment was placed in line to the analyzing rod of a dental surveyor (Ney Surveyor, NeyTech, United States of America) by using sticky wax (Dentax ElKods,Cairo,Egypt) so that the long axis of the abutment was coincident with that of the analyzing rod (Fig. 4a). The vertical arm of the surveyor was then lowered to place the sample in the mold filled with the acrylic dough till the acrylic level just covered the shoulder of the sample coronal part (Fig. 4b). Placement of the samples in the molds was done by one trained operator.
The samples were mounted in the Universal Testing Machine (Lloyd LRX; Lloyd Instruments Ltd., Fareham, UK) for the pull out test. They were placed parallel to the loading direction and the machine grips grasped the attachment head. A constant loading rate of 0.5 mm/min was applied until failure was achieved. The load of failure was confirmed by a sharp drop in the load deflection curve recorded (Fig. 4c). Nexygen data-analysis software (Lloyd Instruments Ltd., Fareham, UK) was used for data display in Newtons [26, 27, 32, 33]. The recorded values were then divided by the bonding area to calculate the tensile bond strength value [16].
Failure mode analysis
Failure mode analysis was determined by microscopic examination of the samples surface. Examination was done under 65X stereomicroscope (Olympus SZX16, OLYMPUS, Tokyo, Japan). Failures were classified into 1 of 3 possible categories: (1) adhesive failure either between the attachment and the resin cement or between the resin cement and root dentin (2) cohesive failure within the resin cement (3) mixed adhesive and cohesive failures [32, 34]. Failure mode was analyzed by one calibrated operator.
For blinding of the operators during tensile bond strength measurement and failure mode analysis, the samples were coded with secret codes by a colleague who decoded them after measurement.
Statistical analysis
Data were checked using Kolmogrov-Smirnov test and showed normal distribution. One way ANOVA test followed by PostHoc Tukey test were used for intergroup comparisons. Paired T test was used for comparisons of the tensile bond strength pre and post aging in the same group. The significance level was set at P < 0.05 in all tests. Statistical analysis was performed using the statistical package for social sciences (version 21.0 SPSS Inc; IBM Corporation, Chicago, IL, USA).