Prosthetic Maintenance Assessment for Two Implant-Retained Overdentures Reinforced with PEKK versus Co-Cr Framework: A Randomized Controlled Clinical Trial

doi:10.21203/rs.3.rs-3892108/v1

Background: The primary challenge associated with implant-retained overdentures lies in the occurrence of denture fractures around the attachment region. Hence, it is recommended to enhance flexural strength through reinforcement frameworks. This study aimed to assess and compare the prosthetic maintenance of mandibular implant-retained overdentures reinforced with Co-Cr and PEKK frameworks.

Material and Methods: Twenty-four participants with completely edentulous maxillary and mandibular ridges were selected, and two implants were placed at the mandibular canine areas using a two-stage surgical technique with the help of a radiographic and surgical stent. After a three-month osseointegration period, ball attachments were installed. Participants were randomly assigned to two groups based on the prostheses received: Group Ireceived a mandibular implant-retained overdenture reinforced with a Co-Cr framework, while Group II received a mandibular implant-retained overdenture reinforced with a PEKK framework. Both groups were also rehabilitated with a maxillary removable complete denture. Prosthetic maintenance evaluations for mandibular implant-retained overdentures in both groups were conducted twelve months post-denture insertion. Categorical data were analyzed using Fisher's exact test, and results were presented as frequency and percentage values.

Results: Group II exhibited a significantly higher percentage of cases with screw looseness, denture relining, and tooth separation compared to Group I. Although Group II cases showed a non-significant increase in the percentage of insert wear and retention loss.

Conclusion: Within the limitations of this study, the findings suggest that Co-Cr, in contrast to PEKK frameworks, offers a more reliable reinforcement of the implant-retained overdentures.

Trial registration: The research was registered retrospectively and published on https://clinicaltrials.gov/ with the NCT number (NCT06219811) and registration date 23/01/2024.

Dental implants

Overdenture

Maintenance/standards

Computer-Aided Design-Computer-Aided Manufacturing

Dental implants are one of the important treatment modalities that is used nowadays to rehabilitate partially and completely edentulous patients [1]. The use of implant supported overdentures (ISODs), are currently a popular treatment option for patients who are fully edentulous. Enhanced function, stability, and retention especially in the mandible are among its advantages [2]. The two implants-retained overdenture is not an ideal treatment option as ISODs, but considering factors such as cost, clinical time, patient satisfaction, and increased retention of the mandibular overdenture, it might be considered acceptable for many patients. [3].

Different types of attachments, such as bar, ball, and magnetic attachments, together with telescopic crowns, were suggested to connect the overdentures to the dental implants. Ball attachments are the most simple and easy treatment used in implant-retained overdentures. In addition to that, they offer high retention, can be used with unparallel implants, and reduce the load transferred to the implants [4].

Polymethyl methacrylate (PMMA) as a denture base material is more susceptible to distortion during the mastication process due to its low mechanical strength. Denture fracture is one of the common complications with two implant-retained overdentures, especially at the anterior region, where stress is concentrated and typically occurs over the implant because the acrylic resin around the attachments is not thick enough. Denture fractures start with a crack's beginning and propagate along the whole denture base [5]. To enhance the flexural strengths and protect against implant overdenture fractures, denture bases can be reinforced. Additionally, it lessens denture base deformation and increases rigidity [6]. Manufacturing a metal framework to strengthen the denture base of overdenture can be considered an efficient and cheap procedure [7]. Nevertheless, the insertion of the metal frameworks into the denture base has several disadvantages, including being heavy, raising expenses, and making this course of treatment complicated. Additionally, because they are composed of alloys, allergies to the underlying tissues could develop [8]. The most common metal framework used to strengthen the denture base are cobalt-chromium (Co-Cr) and Nickel-chromium (Ni-Cr) alloys [8].

Polyaryletherketones (PAEKs) are a class of thermoplastic polymers with exceptionally high performance, which includes polyetheretherketone (PEEK) and polyetherketoneketone (PEKK). These semi-crystalline polymers are characterized for their excellent mechanical performance, which have attracted researchers and clinicians to investigate their application in several designs of dental prostheses, implants, and correlated items [9–11]. PEEK is characterized by high chemical and mechanical properties and is used as an alternative to metal in patients with metal allergies [11]. Despite the massive benefits of PEEK, it was found that the incorporation of PEEK framework into denture bases increased the stress transferred to the supporting structure more than Co-Cr alloy. This is due to the high deformation of PEEK material due to its low elastic modulus [12]. PEKK is a linear thermoplastic polymer formed consisting of successively connected benzene rings to which ether or ketone groups are added, and second ketone group offers improved mechanical and physical qualities [13]. Pekkton® ivory (Cendres + Métaux, SA, Switzerland), a product of PEKK has 80% higher compressive strength compared to un-reinforced PEEK. The incorporation of titanium dioxide (TiO2) in PEKK, increases the hardness, wear resistance and shock absorbance material [14].

The evaluation of implant survival and the complication rate of mandibular implant-retained overdentures are the most important factors in determining prosthetic survival [15, 16]. There are two common types of complications in implant prosthesis: biologic and prosthetic. The biologic complications refer to the disorders in the implant function that disturb the supporting peri-implant tissues resulting in the mucositis or peri-implantitis in severe conditions, the technical complications refer to subjecting the implant, implant parts and/or the superstructures to a damage such as the screw loosening, attachment wear, fracture to part of the prosthesis or some of its teeth [17]. In this context this study aims to evaluate and compare prosthetic maintenance of mandibular implant-retained overdentures reinforced with Co-Cr and PEKK framework.

The null hypothesis set for this clinical trial is that there will be no differences between mandibular implant-retained overdentures reinforced with Co-Cr and those reinforced with PEKK frameworks regarding prosthetic maintenance.

Ethical approval

The study was approved by Faculty of Dentistry, Ain Shams University (number of approvals: FDASU-Rec PC 122352) The follow-up period was twelve months after clinical functional use of two implant- retained overdentures.

Study design

The study is a randomized clinical trial. The flow of the participants during the study is shown in the CONSORT 2010 flow diagram. (Figure 1)

Sample Size

A power analysis was designed to have adequate power to apply a statistical test of the null hypothesis that there is no difference between different tested groups regarding prosthetic maintenance. By adopting an alpha (α) level of (0.05), a beta (β) level of (0.2) (i.e., power=80%), and an effect size (ω) of (0.646) calculated based on the results of a previous study [18]; the total required sample size (n) was found to be (20) cases (i.e., 10 cases per group). Sample size was increased by (20%) to account for possible dropouts during follow-up intervals to be (24) cases (i.e., 12 cases per group) Sample size calculation was performed using R statistical analysis software version 4.3.2 for Windows [19].

Patient Selection

A total of twenty-four completely edentulous patients were selected from an outpatient clinic of the prosthodontics department at a dental institution. Patients were asked about their personal, medical, and dental history. The patients were selected according to the following inclusion and exclusion criteria:

Inclusion criteria

Patients will be gathered according to the selected inclusion: only male patients were included in this study, age range between 50 and 70 years old, completely edentulous patients, systemically free from any disease that contraindicates implant placement, A minimum of 12 millimeters of bone height and 6 mm of bone width at canine areas diagnosed from the pre-operative Cone Beam Computerized tomography (CBCT) and sufficient inter-arch space (12-15 mm) to accommodate the reinforced mandibular implant-retained overdentures [20,22].

Exclusion criteria

Patients will be excluded according to the following criteria: heavy smokers, patients with uncontrolled metabolic disorders such as diabetes mellitus, bone or mucosal diseases, parafunctional habits, irradiated patients [22], and conditions that might complicate the treatment, such as a severe gag reflex or limited mouth opening.

Clinical Examination

Extraoral and intraoral examinations were done for the patients to detect any abnormalities in the Temporomandibular joint (T.M.J.) and any bony abnormalities or tissue abnormalities present. The patient’s existing, removable complete dentures were examined for retention, vertical dimension, centric occluding relation, and interarch distance, and then they were used as radiographic stents by using Gutta-purcha as radiopaque markers that were fixed on the buccal and labial surfaces of the denture, starting from the occlusal surface until the lower border of the stent. If the old removable complete dentures were not perfect or not present, a new one will be fabricated. CBCT (i- CAT Vision®, Imaging Sciences International, Hatfield, PA, USA) was performed while the patient was wearing a radiographic stent in centric occluding relation to the opposing maxillary complete removable denture. Implant width and length was calculated and planed at the canine area.

Randomization technique

The participants were randomly allocated into two groups (I or II) by a practitioner who wasn’t involved in the study treatment process. The randomization process was made by using research randomizer (https://www.randomizer.org/) where it produces a list of a randomly generated codes which was printed and placed in an opaque and sealed envelopes. The randomization list was stored on a password-protected laptop and the practitioner was the only person who have the access to it. Patients and clinical practitioners were blinded to the type of prostheses at all study times (Double blinding technique). Patients were asked to choose one envelope, and the practitioner was asked to determine the group to which the patients were assigned.

Group I rehabilitated with a mandibular implant-retained overdenture reinforced with the Co-Cr framework.

Group II rehabilitated with a mandibular implant-retained overdenture reinforced with the PEKK framework.

Surgical procedures

All patients signed informed consent before the start of surgical procedures, which explains all the surgical and prosthetic steps. The patient was asked to rinse with chlorohexidine mouthwash (hexitol, Kahira Pharma and Chemical Co., Cairo, Egypt) before surgery by two days, three times per day, and to take a prophylactic amoxicillin-clavulanate antibiotic (Augmentin 1 g, GlaxoSmithKline Pharmaceutical Nigeria Ltd., Lagos, Nigeria) one hour before surgery. The radiographic stent was modified to be a surgical stent to locate the site of implants by creating two holes in the canine area. Mental and lingual nerve blocks (LIGNOSPAN®, Septodont, United Kingdom) were given bilaterally. A periodontal probe was used to mark a bleeding point at the proposed site of two implants. An intraoral crestal incision was made between the marked bleeding points and just behind it by using a surgical blade 15 with the buccal realizing incisions for an easy release of the flap without laceration. A complete mucoperiosteal flap was reflected buccally and lingually for complete exposure of bone. Sequential drilling at the planned implant site and parallelism were checked by using paralleling pins. The two implants (SuperLine, Dentium, Katella Avenue, USA) with a size of 3.6 mm x 12 mm were inserted at a lower torque of 35 Ncm [23]. Single interrupted sutures were preformed to close the surgical flap by using resorbable sutures.

All participants were instructed to take prophylactic antibiotic amoxicillin-clavulanate antibiotic every twelve hours for seven days. An analgesic drug (Ibuprofen 600 mg, Knoll AG, Ludwigshafen, Germany) was instructed to the patient every eight hours for three days to control inflammation and pain after surgery. In addition to that the patient was instructed to do strict oral hygiene measures and to rinse with chlorhexidine mouth wash starting from the day after the surgery three times daily for seven days.

A follow-up visit was done one week after the surgery for removal of sutures, and for relining of relived removable complete denture with soft liner (MUCOPREN^® SOFT, Kettenbach GmbH & Co. KG, California, USA) opposite to the surgical site.

Prosthetic procedure

After three months of osseointegration, patients were recalled, and healing abutments were reconnected to the dental implants by using the previous stent for relocating of site of implants. After one week, a preliminary maxillary and mandibular impression was taken by using irreversible hydrocolloid (CA37, Cavex Holland BV, Haarlem, Netherlands). A maxillary and mandibular special tray was constructed using auto-polymerizing acrylic resin (Acrostone Cold Cure, Acrostone Dental & Medical Supplies, Cairo, Egypt) with two holes opposite to the mandibular dental implants over the study cast. Secondary impression was performed by border molding of a special tray using green stick compound (Hiflex Green Sticks, Prevest DenPro, USA) and maxillary secondary impression with medium rubber base impression material (Thixoflex M; Zhermack, Badia Polesine, Italy). Afterwards, long impression copings were connected to the dental implants, and open tray mandibular impressions were performed with medium rubber base impression material, implant analogues were attached to the copings before pouring of master cast. Ball abutments were connected to the implant analogues in the master cast, and occlusion blocks were fabricated on the master cast. A face-bow record (Bio-Art Elite Face Bow, Bio-Art, São Carlos, Brazil) was taken for mounting the maxillary cast on a semi-adjustable articulator (A7 Plus Articulator, Bio-Art, São Carlos, Brazil), and an interocclusal record using the wax wafer technique (CAVEX SET UP WAX, CAVEX, Haarlem, Netherlands) was performed for the mounting of the lower cast. Setting-up of acrylic teeth, waxing-up, and try-in were performed.

Framework construction

Metal housings were connected to the ball abutments on the master cast, then it was scanned with a desktop scanner (Medit IdenticaT500, MEDIT, Seoul, South Korea) with and without the trial denture base to obtain a standard stellation file (STL). STL files were overlapped and transferred to the designing software (Exocad DentalCAD 2.4 Plovdiv, exocad GmbH, Darmstadt, Germany) for the design of frameworks, according to the group. All undercuts were blocked on the virtual model. The desired framework was outlined and placed in the virtual model to cover the crest of the ridge with space over the metal housings and with 1mm and 2mm thickness for the Co-Cr and PEKK frameworks, respectively according to the recommended minimum thickness of each from the previous studies [24,25]. Finally, a 2mm relief was designed under the frameworks, and then it was checked and smoothened from all surfaces. (Figure 2)

The STL file was then imported into the milling machine (K5, vhf camfacture AG, Ammerbuch Germany) to begin the framework milling according to the group.

For Group I it was milled from polymethylmethacrylate (PMMA) (PMMA disk, YAMAHACHI, Japan), afterwards it was casted from Co-Cr alloy by conventional casting technique [26]. (Figure 3)

For Group II the framework was milled from PEKK blanks (Pekkton®ivory, Cendres + Métaux SA, Biel/Bienne, Switzerland).

Processing of Dentures

The frameworks were checked on master cast (Figure 4&5) ,and intraorally to check adaptation, then the maxillary and mandibular overdenture were processed using heat-cured acrylic resin (Vertex Rapid Simplified, Vertex Dental, Soesterberg, Netherlands) via a conventional long polymerization cycle, and it was finished and polished in the usual manner.

Delivery and pick-up of dentures

The denture was checked for retention, stability, and support, and necessary adjustments were performed. Ball abutments were connected to the dental implants. Afterwards, the undercuts were blocked out, the block-out shims were inserted, and the metal housings were connected intraorally to the abutments. The housings were directly picked up to the fitting surfaces of the overdenture using auto-polymerizing acrylic resin (DuraLiner II Denture Reline, Reliance Dental Manufacturing LLC, United States). The patients were instructed to bite in a centric relation until the setting of the acrylic resin. Excess acrylic resin was removed, and the denture was finished, polished, and checked again intraorally. (Figure 6) The patients were instructed to perform strict oral hygiene measures.

Evaluation of prosthetic maintenance

Evaluation of prosthetic maintenance for mandibular implant-retained overdentures in both groups was made after twelve months of denture insertion. The following aspects were inspected regarding screw (screw loosening, screw fracture), attachments (wear, distortion, fracture, replacement), and overdentures (reline, remake, fracture, teeth wear, teeth separation, or fracture) [23].

Statistical Analysis

Categorical data were presented as frequency and percentage values and were analyzed using Fisher's exact test. The significance level was set at p<0.05 within all tests. Statistical analysis was performed with R statistical analysis software version 4.3.2 for Windows[19].

Summary statistics and intergroup comparisons of prosthetic maintenance. are presented in (Table 1) and in (Figure 7). For ball abutments, abutment and screw fractures were not found in all cases in both groups. However, significantly higher percentage of cases had screw looseness in group (II) (p=0.027) and the odds ratio was (13.20). For housing and nylon inserts, higher percentage of cases in group (II) had insert wear and loss of retention, yet the difference was not statistically significant (p>0.05). In addition, all cases in both groups had nylon insert replacement after one year and didn't have housing fracture. For the overdenture, higher percentage of cases in group (I) had denture fractures, while higher percentage of cases in group (II) had teeth wear, yet in both instances the difference was not statistically significant (p>0.05). Nevertheless, significantly higher percentage of cases in group (II) had their dentures relined and had teeth separations (p<0.05) and the odds ratios were (9.00 and 15.00) respectively.

The main problem with implant-retained overdentures is denture fracture around the attachment area. Consequently, it is advisable to reinforce the denture to enhance its flexural strength. [5,27]. Ball attachment was used in this study as the common complication associated with implant-retained overdentures with ball attachment is denture base fracture. As a result, denture base strengthening may be recommended to improve the fracture resistance [28,29].

The null hypothesis of this study was rejected as there was a significant difference between implant-retained overdentures reinforced with Co-Cr and PEKK frameworks regarding some points like screw loosening, denture relining, and teeth separation.

Virtual design of the frameworks PEKK and Co-Cr frameworks were performed to standardize the thickness of frameworks. Moreover, in group (I), Co-Cr frameworks were milled first from PMMA, then casted by the conventional casting technique. This approach was adopted because milling precious metals is costly and can accelerate wear on milling burs [30].

Abutment screw loosening had a significantly higher percentage in group (II) than group (I). Although PEKK possesses shock-absorbing properties, it was observed that over time, it transmitted more stresses to the supporting implant than Co-Cr. This phenomenon is attributed to the fact that PEKK frameworks tend to undergo permanent deformation under dynamic stresses Top of Form[13,31]. Furthermore, a study discovered that a PEKK framework exhibited lower stress transmission to the implant and simulated adjacent tissue under dominant compressive stress. However, this stress increased when the dominant stress was tensile in nature Top of Form[32]. Moreover, in another study, it was found that the increased rigidity of the metal framework led to a more even load distribution [31].

The wear of the nylon insert was high in both groups with an insignificant difference, and this was proven by the previous studies that due to cyclic and dynamic loading, cleansing of the denture, and the presence of saliva that led to the swelling of the insert, all of this increased the wear of the nylon insert over time [33-35].

The number of dentures that needed relining was high in both groups. This is often attributed to increased load on the distal flange of mandibular implant overdentures, which tends to induce bone resorption as a local factor. However, implants may contribute to preventing resorption in the adjacent bone [36]. The results of this study showed a significant increase in denture relining in group (II). This can be attributed to the permanent deformation of PEKK under load, causing flexure in the denture base and subsequently increasing the load on the distal portion of the residual ridge. [13,30].

Teeth separation was significant in group (II); which can be attributed to the increased thickness of the PEKK framework, as suggested by prior research. This increased thickness resulted in a reduced remaining space for acrylic resin and teeth, likely contributing to the observed teeth separation [21,24,25,30].

Finally, the imitations of this study are that a larger number of patients are needed to be included in this study and that the investigation time need to be more than twelve months.

Within the limitations of this study, Co-Cr frameworks provided a more reliable reinforcement of the implant-assisted overdenture bases as compared to PEKK frameworks.

CBCT Cone Beam Computerized tomography.

ISODs Implant supported overdentures.

PMMA Polymethyl methacrylate.

Co-Cr Cobalt Chromium.

Ni-Cr Nickel-chromium.

PAEKs Polyaryletherketones.

PEEK Polyetheretherketone.

PEKK Polyetherketoneketone.

TiO₂Titanium dioxide.

T.M.J Temporomandibular joint.

STL Standard stellation file.

Ethics approval

This research was approved by Faculty of Dentistry Ain Shams University Research Ethics (number of approvals: FDASU-Rec PC 122352).

Consent of publication

Not Applicable

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing of interest

The authors declare that they have no conflict of interest.

Funding of the Study.

This study was self-funded.

Author’s contribution

Study Design: Omnia M. Refai, Heba A. Salama.

Methodology, Investigation, Formal analysis: Omnia M. Refai, Heba A. Salama.

Writing – original draft preparation: Omnia M. Refai

Writing – review and editing: Omnia M. Refai, Heba A. Salama.

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Table 1 Summary statistics and intergroup comparisons of prosthetic maintenance.

Parameter	Complication	n (%)		χ²	p-value	Odds ratio (95% CI)
Parameter	Complication	Group (I) (n=12)	Group (II) (n=11)	χ²	p-value	Odds ratio (95% CI)
Ball abutment	Abutment fracture	0 (0.00%)	0 (0.00%)	NA	NA	NA
	Screw loosening	1 (8.33%)	6 (54.55%)	5.79	0.027*	13.20 (1.24:140.69)*
	Screw fracture	0 (0.00%)	0 (0.00%)	NA	NA	NA
Housing and Nylon Insert	Nylon Insert wear	6 (50.00%)	9 (81.82%)	2.56	0.193	1.09 (0.02:59.4)
	Nylon Insert replacement	12 (100.00%)	11 (100.00%)	NA	NA	NA
	Loss of retention	5 (41.67%)	9 (81.82%)	3.88	0.089	6.30 (0.93:42.73)
	Fracture of housing	0 (0.00%)	0 (0.00%)	NA	NA	NA
Overdenture	Denture reline	4 (33.33%)	9 (81.82%)	5.49	0.036*	9.00 (1.29:63.03) *
	Denture fracture	4 (33.33%)	1 (9.09%)	1.98	0.317	0.20 (0.02:2.16)
	Teeth wear	2 (16.67%)	3 (27.27%)	0.38	0.640	1.88 (0.25:14.08)
	Teeth separation	0 (0.00%)	4 (36.36%)	5.28	0.037*	15.00 (0.7:319.54)
	Teeth fracture	0 (0.00%)	0 (0.00%)	NA	NA	NA

NA: Not Applicable, CI: Confidence interval, *Significant (p<0.05).

No competing interests reported.

Prosthetic Maintenance Assessment for Two Implant-Retained Overdentures Reinforced with PEKK versus Co-Cr Framework: A Randomized Controlled Clinical Trial

Status:

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Abstract

Figures

Introduction

Materials and Methods

Results

Discussion

Conclusion

Abbreviations

Declarations

References

Tables

Additional Declarations

Status:

Version 1