Full‐zirconia single‐tooth molar implant‐supported restorations with angulated screw channel abutments: A 1‐year prospective case series study

Abstract Background Implant‐supported restorations in the posterior region are subjected to various complications that could be prevented by changing either the design or the material. Purpose The aim of this prospective case series study was to evaluate full‐zirconia implant‐supported restorations with angulated screw channel abutments in the molar region of the maxilla and mandible and their effect on hard and soft peri‐implant tissues, during a 1‐year follow‐up period. Materials and Methods Thirty consecutive patients with a single missing molar, sufficient bone height, and implant site free of infection were included. Each patient was to receive a parallel‐walled implant with conical connection according to a two‐staged surgical protocol. After 3 months, a full‐contour screw‐retained zirconia restoration with angulated screw channel abutment was provided. Clinical and radiographic examinations were performed 1 and 12 months after placement of the restoration. Patients' satisfaction was scored prior to treatment and after 12 months with the restoration in function. Primary outcome measure was success of the restoration. Results All patients could be evaluated after 12 months. Success of the restorations was 100%. From loading to the 12‐month follow‐up, the mean marginal bone loss was 0.16 mm (SD: 0.26). Mean scores for plaque, calculus, peri‐implant mucosa, bleeding, and pocket probing depth were low, depicting healthy peri‐implant conditions. Patients' satisfaction was high and had improved after treatment. Conclusion Full‐contour zirconia implant‐supported restorations with angulated screw channel abutments in the molar region have an excellent clinical performance after 1 year of function.


| INTRODUCTION
Dental implant survival in cases of replacement of single missing teeth in the posterior region of the maxilla and mandible is high. 1 However, implant-supported restorations are prone to a number of complications, such as loosening of abutment screws and cement retention, peri-implant mucosa infection due to cement remnants, and fracture of veneering ceramics. [2][3][4][5][6][7] Screw retention of implant restorations eliminates the risk of cement remnants and subsequent soft tissue complications. 8 Loosening of screws could be minimized by an internal conical connection between implant and abutment. 9 The possibilities for screw retention in the posterior region, however, can be hindered by off-axis inclination or position of the implant when the location of the screw access opening of the restoration would be interfering with occlusion, articulation, or marginal thickness of the restoration. This could be of functional or aesthetic concern or could weaken the restoration; therefore, as an alternative to the traditional straight screw channel, the angulated screw channel has been developed. 10 A recent prospective clinical study evaluating the retention of single restorations with angulated screw channel abutments has reported favorable results without major complications. 11 Zirconia-based implant-supported single restorations are rated as highly successful with a cumulative 5-year survival rate of 97.1%.
However, the most common complication reported is the fracture of the veneering material, especially in the posterior region. 7 The excellent mechanical properties of monolithic zirconia could help overcome this technical complication. 12,13 Nonetheless, there is only scarce clinical evidence in support of full-contour zirconia implant-supported single restorations, 7,14 while clinical evaluation of full-contour zirconia restorations with angulated screw channel abutments has yet to be published. Also, the material properties of the restoration itself may have an impact on bone surrounding the implant. With finite element analysis, it has been calculated that an occlusal material with a high modulus of elasticity, such as zirconia, barely dampens occlusal impact forces, thereby increasing its effect on the bone-implant interface. 15,16 Regarding soft tissues, it has been affirmed in systematic reviews that zirconia abutments have an excellent soft tissue response. 17,18 However, the impact of full-zirconia restorations on soft tissue response has been addressed only in a limited number of studies.
Therefore, the aim of this prospective case series study was to evaluate full-zirconia implant-supported restorations with angulated screw channel abutments in the posterior region of maxilla and mandible, their effect on hard and soft peri-implant tissues, and patients' satisfaction during a 1-year follow-up period.

| Patient enrolment
All patients referred to the Department of Oral and Maxillofacial Surgery (University of Groningen, University Medical Hospital, The Netherlands) from January 2016 till December 2016 for single-tooth implant therapy in the maxillary and mandibular posterior region were considered for inclusion. The following inclusion criteria were applied: • One missing tooth, being a first or second molar in the maxilla or mandible, with a minimum of 3 months of healing post-extraction; • Sufficient bone volume to insert a dental implant with a length of at least 7 mm; • Implant site free from infection; • Adequate oral hygiene (as expressed by modified plaque-index 19 and modified sulcus bleeding-index 19 ) • Sufficient mesio-distal, bucco-lingual, and interocclusal space for placement of an anatomic restoration; • Patient is capable of understanding and giving informed consent.
Patients were excluded from the experimental protocol when at least one of the following exclusion criteria was met: • Medical and general contraindications for the surgical procedures; • Presence of active and uncontrolled periodontal disease; • Probable bruxism, based on self-report and clinical examination, based on the consensus definition of Lobbezoo et al 20 ; • .Smoking: patient is declaring to be a smoker (and intends to continue) or has been smoking during the past 3 months; • History of local radiotherapy to the head and neck region.

| SURGICAL AND PROSTHETIC PROCEDURES
The surgical and prosthetic treatments were performed at the Department of Oral and Maxillofacial Surgery, University Hospital Groningen. One oral surgeon, experienced in implant dentistry, executed the surgical treatments and two experienced prosthodontists performed restorative procedures. All laboratory procedures have been carried out in a single dental laboratory.

| Surgical procedure
At the time of intervention, there was a healed site with a missing tooth for more than 3 months. There was enough bone to reach primary implant stability. One hour preoperatively antibiotic prophylaxis (2 g amoxicillin or, if allergic to penicillin, 600 mg clindamycin) was given and a 0.2% chlorhexidine mouthwash (two times daily for 10 days) was prescribed for oral disinfection. The surgical procedure was performed under local anesthesia. A parallel-walled implant with a TiUnite surface and conical connection (NobelParallel CC, Nobel Biocare AB, Goteborg, Sweden) was placed, according to the manufacturers' protocol. Implant diameter was 4.3 mm and length varied from 8.5 to 13 mm, dependent on available bone height at the implant site. A cover screw (Nobel Biocare AB) was placed, and the wound was closed. One week after implant placement, a follow-up visit was scheduled for suture removal and review of the healing process. After 3 months, the implant was uncovered and a healing abutment (Healing Abutment CC RP, Nobel Biocare AB) was installed. Immediately after placement of the restoration, thorough oral hygiene instructions were given to all patients.

| OUTCOME MEASURES
Primary outcome measure was success of the restoration, ascertained following modified United States Public Health Service (USPHS) criteria, being a composition of the outcomes: fracture of framework, loosening of restoration, wear facets, catching of probe at margin, anatomical shape, mismatch in color shade, cementation gap on radiograph, and patient satisfaction. 21,22 Clinical and radiographic evaluation was performed 1 month and 1 year after placement of the restoration. The following assessments were made: • Assessment of bleeding tendency with the modified Sulcus Index 19 ; • Assessment of peri-implant inflammation with the Gingival Index 23 ; • Presence of calculus • Probing pocket depth: measured to the nearest millimeter using a manual periodontal probe (Williams-Sulcus color-coded probe, Hu-Friedy, Chicago, Illinois). Probing of the implant was performed at four sites (mesial, distal, buccal, and lingual/palatinal); • Restoration survival; • Complications related to the restoration; • Patients' satisfaction. Patients were asked to complete a questionnaire prior to implant placement and 1 year after placement of the restoration.
With regard to the radiographic evaluation, radiographs were taken 1 and 12 months after restoration placement using a parallel technique, with an X-ray holder for periapical radiographs. The radiographs were analyzed using dedicated computer software to perform linear measurements on the digital radiographs. The calibration was carried out in the vertical plane for each radiograph by using the known length of the implant and distance between several threads. This calibration ensured a correct measurement. 24  The patients' satisfaction was recorded by means of a questionnaire asked to be filled out by the patients before surgery and 1 year after restoration placement. The nonvalidated questionnaire, previously published by Telleman 2013, 25 comprised of questions or statements to be answered on a 5-point rating scale ranging from "very dissatisfied"/"not in agreement" (score 1) to "very satisfied"/"in agreement" (score 5). The topics addressed were related to aesthetics and appearance, function (chewing), sense ("feeling like natural teeth"), speech, and self-esteem. Furthermore, patients were asked to mark their overall satisfaction concerning their dental situation at time of enrolment and at the 1-year evaluation on a 10-point rating scale from 0 to 10, in which 10 is the highest satisfaction score.

| STATISTICAL ANALYSIS
One observer was responsible for the collection and analysis of all the data. The worst score of the clinical and radiographic parameters evaluated per implant was used in the data analysis. Data were presented as frequencies. Differences in patients' satisfaction between pretreatment and 1-year follow-up were tested with the Wilcoxon signed rank test. Analysis was done with PASW Statistics 23.0 (SPSS Inc., an IBM Company, IBM Corporation, Chicago, Illinois). In all tests, the significance level α was set to 5%.

| RESULTS
All consecutive patients eligible to join the study on the basis of the inclusion and exclusion criteria agreed to participate in this study. A total of 30 patients (10 males and 20 females, mean age 53 years, range 27-83 years) who were to receive 30 implants were included.
Patient characteristics are depicted in Table 1. All patients completed the 1-year evaluation period. Implant and restoration survival were 100% at the 12-month evaluation.
The mean scores of the indices for plaque, calculus, gingiva, and bleeding were very low, indicating favorable results ( Table 2). The mean probing depth was 1.7 mm (SD 0.8 mm) at the 1-year follow-up.
Patient's satisfaction had significantly improved at the 1-year evaluation (P < .001). Mean presurgical overall score was 6.1 ± 0.7 at a T A B L E 1 Baseline characteristics of study group scale of 1 to 10, and after treatment, the mean score was 9.0 ± 0.8.
Feelings of shame because of visibility of being partially edentulous decreased (P < .001). Patients were significantly more satisfied about the ability to chew after restoration (P < .001) ( Table 4). The quality of the restoration was assessed according to modified USPHS criteria.
All applicable parameters were without concerns and favorable, resulting in a 100% restoration success (Table 5). Zirconia is presumed to be highly biocompatible with a potential for soft tissue attachment. 26