Between September 2017 and September 2020 all patients experiencing problems with their maxillary overdenture that were referred to the Department of Oral and Maxillofacial Surgery of the University Medical Center Groningen in Groningen, the Netherlands were screened if they were eligible for maxillary implant overdenture therapy. Patients that had insufficient bone volume to place at least four implants in the edentulous maxilla and were unwilling to be treated with reconstructive surgery, were asked to participate in this case series. To be able to participate, the bone volume in the anterior maxilla, as assessed on a cone beam computed tomography (CBCT), had to be sufficient for the placement of two implants. A participant had to be at least 18 years of age, fully edentulous for at least 1 year and did not have an American Society of Anesthesiologists score (ASA) of IV or higher [12].
In this study the participants were treated following an existing procedure and was, therefore, not considered research performed on test-subjects as meant in the Medical Research Involving Human Subjects Act (WMO) (MEC-reference M19.224998). The study was registered in the UMCG Trial Register (RR201900060). This study was conducted in accordance with the 2008 revised requirements of the Helsinki Declaration of 1975.
Surgical procedure
All the implants were planned in the canine region using 3D Virtual Surgical Planning using computer software (Proplan CMF software; Materialise, Leuven, Belgium) to ensure an optimised implant location from both a surgical and prosthetic perspective. The implant positions were transferred to surgical template using computer software (3-Matic Medical 11.0; Materialise, Leuven, Belgium; Fig. 1).
All the participants were treated under local anaesthesia (Ultracain® D-S forte, Sanofi Aventis, Gouda, Netherlands) by the same oral and maxillofacial surgeon (GMR). The implants were placed using the surgical template and consecutive diameter drill sleeves, following the manufacturer’s instructions. After removal of the template, two implants (Nobel Active NP 3.5 mm, Nobel Biocare®, Zurich, Switzerland) were placed with a minimum torque of 45 Ncm. Small bone dehiscences were covered with intra-orally harvested bone and a resorbable membrane (Bio-Gide®, Geistlich Pharma North America Inc., Princeton, USA). After the insertion of cover screws, the flap was repositioned and sutured. All participants received antibiotics (500 mg Clamoxyl, GlaxoSmithKline, Utrecht, the Netherlands) for 7 days, three times daily. The participants were instructed not to wear their conventional denture for 2 weeks and rinse their mouth with 0.2% chlorhexidine (Corsodyl, GlaxoSmithKline, Utrecht, the Netherlands). After 2 weeks, the sutures were removed and the conventional denture was relined with a soft reline (Soft-Liner, GC, Leuven, Belgium). After 3 months of osseointegration the implants were provided with healing abutments during a second surgical procedure, which was followed by the prosthetic procedure.
Prosthetic procedure
Using a stock metal tray (Schreinemakers; Clan Dental Products, Maarheeze, the Netherlands), a preliminary alginate impression (Cavex CA 37; Cavex Holland BV, Haarlem, the Netherlands) was made to enable the dental technician to fabricate an individual impression tray of acrylic resin (Lightplast base plates; Dreve Dentamid GmbH, Unna, Germany). After relining the rims of the individual tray with a thermoplastic material (ISO Functional; GC Europe A.G., Leuven, Belgium) and placing screw-retained impression copings to the implants, the final impression was made with a polyether material (Impregum F; 3 M ESPE, St. Paul, MN, USA). The vertical dimensions and intermaxillary relations were recorded with wax rims and a pin registration device. Acrylic resin teeth (Ivoclar SR Orthotyp DCL and Ivoclar VivodentPE, Ivoclar Vivadent AG, Schaan, Liechtenstein) were positioned for a trial arrangement following a bilateral balanced occlusion concept.
For the implant superstructure two matrix copings (Locator® RTX, Zest Dental Solutions, Carlsbad, California, USA) were placed into the overdenture’s base. All copings were initially provided with medium force nylon attachment caps, enabling up- or down-grading the retentive force if needed. For additional support, all the overdenture were designed with full palatal coverage (Fig. 2a–c). At overdenture placement, all the participants received hygiene instructions for their overdenture and superstructure and were scheduled for routine maintenance recalls. All prosthetic procedures were accomplished by one prosthodontists (HJAM).
Outcome measures
The study’s primary outcome measures were implant and overdenture survival. Secondary outcome measures were MBLC, clinical outcome measures (presence of plaque and calculus, mucosal health, bleeding on probing and PDC), masticatory performance, patient-related outcome measures (PROMs) and complications. Implant and overdenture survival, MBLC and clinical outcome measures were evaluated after 1 month (T1) and 12 months (T12) after placement of the prosthesis. Complications were recorded throughout the whole follow-up period. Masticatory performance and PROMs were evaluated prior to treatment (T0) and at T12.
Implant and overdenture survival
Implant survival was defined as the percentage of initially placed implants still present and immobile at T12. Implant mobility was assessed by performing a percussion test. Overdenture survival was defined as the percentage of overdentures still present at the follow-up evaluation.
Marginal bone level change
Intra-oral radiographs were made at T1 and T12 to assess the MBLC. The radiographs were standardised with the paralleling extension-cone technique using a corresponding system (RINN, Dentsply, Elgin, IL, USA). The solitary attachments were digitally cropped from the digital radiographs, aiding in blinded assessment. Using computer software (Biomedical Engineering, UMCG, the Netherlands) the marginal bone level change was then assessed, utilising the implant diameter (3.5 mm) for measurement calibration and using the neck of the implant as a horizontal reference line. Measurements were done by one examiner (HJAM) at the mesial and distal side of the implant and from the neck of the implant to the crestal bone, perpendicular to the reference line. This measurement method was evaluated by Telleman et al. [13] with a Cronbach’s alpha of 0.867 which is similar to an almost perfect agreement. MBLC was defined as the difference in bone height between the measurements made at T1 and T12.
Clinical outcome measures
Measurements were done by one examiner (PO). Probing depth (PD) was measured the mesial, vestibular, distal and oral site of each implant using a manual periodontal probe. The distance between the marginal border of the mucosa and the tip of the periodontal probe was noted as probing depth. PDC was calculated by subtracting the measurements of T1 from T12.
The presence of plaque was assessed using the index described by Loë and Silness [14], ranging from 0 to 3, corresponding with no plaque detection (0); plaque accumulation after probing (1); visible plaque detection (2); and an abundance of visible plaque (3).
The presence of calculus was scored 0 or 1, corresponding with the absence (0) or presence of calculus (1), respectively.
The health of the peri-implant mucosa was assessed using the modified Löe and Silness index [14], ranging from 0 to 3, corresponding with normal mucosa (0); mild inflammation with slight oedema and redness (1), moderate inflammation with oedema, redness and glazing (2); and severe inflammation with marked redness, oedema and ulceration (3).
Bleeding on probing was assessed using the Mombelli et al. index [15], ranging from 0 to 3, corresponding with no bleeding (0); isolated bleeding (1); confluent bleeding along the mucosal margin (2); and heavy or profuse bleeding (3).
Masticatory performance
To objectively measure masticatory performance, the mixing ability test (MAT) was performed. For the MAT each participant was asked to chew 20 strokes on a prefabricated paraffine wax tablet with a red and blue layer. Chewing the tablet gradually decreases the spread of blue and red colour intensities, representing the masticatory performance. To prepare the chewed tablet for analysis, it was heated to 28 °C and compressed to a thickness of 2.0 mm using a hydraulic hand press at 50 bar. Both sides of each tablet were then optically scanned using a high quality scanner (Epson V750, Long Beach, California), resulting in an image with a spread of blue and red colours [16]. Using computer software (Adobe Photoshop CS3; Adobe, San Jose, California) the mixing ability index (MAI) was obtained by measuring the intensity distributions of the red and blue colours of the combined images [17]. The MAI ranges from 30 (badly mixed) to 5 (a theoretically perfect mix).
Patient-related outcome measures
The PROMs were assessed using validated questionnaires, i.e., the chewing ability questionnaire (CAQ) [18]. the denture complaints questionnaire (DCQ) [19] and the Dutch version of the Oral Health Impact Profile 49 questionnaire (OHIP-49NL) [20]. The CAQ was used to test masticatory ability. The participants were asked to rate their ability to chew nine different foods on a three-point scale, e.g., good, moderately, or bad. The foods were divided in three categories, e.g., soft foods (boiled vegetables, crustless bread, minced meat), tough foods (crusty bread, steak, Gouda cheese), and hard foods (apple, carrot, peanuts). The DCQ consists of 54 questions, divided in six categories, addressing functional problems of the lower denture, the upper denture, general functional complaints, denture aesthetics, facial aesthetics, and accidental lip, cheek, and tongue biting (‘neutral space’). Questions could be answered on a four-point scale, ranging from 0 (no complaints) to 3 (severe complaints). At the end of the questionnaire, the participant is asked to rate the overall denture satisfaction on a ten-point scale, ranging from 1 (very bad) to 10 (excellent). The OHIP-49NL questionnaire consists of 49 questions, divided in seven categories, i.e., functional limitation, physical pain, psychological discomfort, physical disability, psychological disability, social disability and handicap. Questions could be answered on a five-point scale, ranging from 0 (never) to 4 (very often).
Complications
Complications were scored throughout the whole follow-up period. Examples of complications were loosening or fracture of denture teeth, replacement of nylon caps, and adaptation of the denture edges because of pressure ulcers.
Statistical analysis
Implant and overdenture survival, MBLC, clinical parameters and complications were presented as descriptive statistics. Continuous data (MAI and PROMs) were tested for normality using the Shapiro Wilk test and analysing Q–Q-plots. In case normality could be assumed, these data were further analysed using the paired samples t test, if not, than the Wilcoxon Matched Pairs Signed Ranks test was used as a non-parametric alternative. A p value of < 0.05 was considered statistically significant. All analyses were performed with SPSS 23.0 software (SPSS, Inc, Chicago, Illinois).