Comparison of the Crestal Bone Loss Between Implant-Supported Prosthesis With Sinus Augmentation and Distal Cantilevered Implant-Supported Prosthesis Without Sinus Augmentation

Sinus augmentation is a surgical procedure that aims to obtain posterior alveolar bone support and sufficient crestal bone height for implant insertion. The technique increases bone volume in the maxillary posterior region with simultaneous or delayed dental implant placement. A successful sinus augmentation with delayed or simultaneous dental implant insertion shows good results in terms of implant stability and survival in the long term.¹  However, maxillary sinus augmentation involves elevation of the Schneiderian membrane and is associated with several complications, such as membrane perforation, bleeding due to arterial damage, and postoperative infection of the sinus or graft material.^2–4 

Cantilever fixed partial prostheses are restorations cemented on vital teeth or dental implants at 1 end while the other side is unsupported.^5,6  Fixed partial restorations with cantilevers are generally used to overcome the inadequacies in local conditions and anatomical obstacles, such as insufficient horizontal or vertical alveolar bone dimensions and pneumatized sinuses.⁷  Implant-supported fixed prostheses with cantilever extensions can be used in the posterior maxillary region with insufficient bone support to avoid surgical complications of sinus augmentation procedures, thus limiting the surgery to only dental implant osteotomy.

In the present study, a null hypothesis of observation of no difference in the mesial and distal crestal bone loss between dental implants loaded with a noncantilevered 3-unit fixed partial prosthesis (TUFPP) with sinus augmentation and distal cantilevered TUFPP in the maxillary posterior region is tested. The crestal bone loss between noncantilevered 2-implant–supported TUFPP with sinus augmentation and distal cantilevered 2-implant–supported TUFPP without sinus augmentation in the posterior maxilla with insufficient crestal bone at 6-, 12-, and 24-month follow-up controls is compared.

The study was approved by the local Clinical Research Ethics Committee with approval number 2018-313 and was performed under the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. All clinical and radiological data of the patients treated with 3-unit implant-supported fixed dentures between January 4, 2013, and January 3, 2018, were retrieved from the archives.

The inclusion criteria of the study were as follows: availability of panoramic radiographs recorded preoperatively and at 6-, 12-, and 24-month follow-up evaluations after loading; availability of intact clinical data of patients with 2-implant–supported distal cantilevered TUFPP in the posterior maxilla with insufficient alveolar bone (<5 mm) in the first molar region; and availability of intact clinical data of patients with 2-implant–supported TUFPP, where the posterior implant was inserted with simultaneous sinus augmentation in the first molar region. The exclusion criteria were as follows: patients with chronic systemic diseases, such as diabetes and hypertension, and those with implant-supported prosthesis with no opposing natural teeth.

The study patients were enrolled in 2 study groups. Group 1 consisted of patients with 2 implants: an anterior implant placed in the native bone and a posterior implant inserted with simultaneous sinus augmentation using a xenogenic bone graft to support a TUFPP (Figure 1). Group 2 consisted of patients with 2 implants inserted in the native bone to support a distal cantilevered TUFPP (Figure 2).

Sinus augmentation surgery using the lateral approach was performed by a single surgeon (Ö.D.). A lateral bone osteotomy was accomplished with surgical round burs under external irrigation, and the intact Schneiderian membrane was elevated with sinus elevators. An organic bovine bone (Bio-Oss, Geistlich Pharma AG) graft was placed in the surgical site with simultaneous dental implant insertion, and the wound was sutured with 3-0 silk sutures. The routine postoperative medication prescribed was antibiotic (amoxicillin + clavulanic acid, 1000 mg, 2 × 1), analgesic (naproxen sodium, 550 mg, 3 × 1), and oral rinse (chlorhexidine digluconate, 0.12%, 3 × 1).

The crestal bone levels at the distal and mesial aspects of each implant were measured at baseline and at 6-, 12-, and 24-month follow-up evaluations on panoramic radiographs (Planmeca ProMax, Planmeca). Panoramic radiographs were recorded with the same device, and the patient's head position was standardized with the cephalostat technique.⁸  Images of the panoramic radiographs were transferred to the software program CorelDraw 11.0 (Corel Corporation and Coral Ltd), and the measurements were obtained at ×20 magnification.

The vertical distance from the crestal bone level to the marginal bone contact level of the implant was measured on ×20 magnified images by a single examiner. For intraobserver agreement, the examiner also measured the crestal bone level in 10 patients with a single-tooth implant-supported prosthesis who were not included in the study. A distortion coefficient was used to determine the bone levels. The true crestal bone loss value was calculated by multiplying the true implant width by crestal bone loss measured on the radiograph and dividing it by the implant diameter measured on the radiograph.

An independent statistician reviewed the methodology and results of the study. SPSS version 21.0 Statistical Software (IBM) was used for statistical analysis of the results. The Shapiro–Wilk test (P < .05) showed that the measurement scores were normally distributed. Three-way repeated measures ANOVA with 1-factor repetition was used to evaluate the effect of the prosthesis type (TUFPP and TUFPP with cantilever) on distal and mesial crestal bone loss in 6-, 12-, and 24-month follow-up evaluations. The Bonferroni test was used for post hoc evaluation.

Fifty-two patients (mean age: 57.9 ± 8.3) and 104 implants were included in the study. Among them, 22 patients (42.3%) were male, and 30 patients (57.7%) were female. Thirty patients were rehabilitated with TUFPP with sinus augmentation, and 22 patients received cantilevered TUFPP. Mean values of the recorded crestal bone loss in the distal and mesial aspects of the anterior and posterior implants in groups 1 and 2 are demonstrated in Figures 3 and 4, respectively. Anterior and posterior implants in groups 1 and 2 showed statistically significant distal crestal bone loss through all the follow-up periods (P < .05) (Table 1). There were significant differences in the distal crestal bone loss between the anterior and posterior implants in group 1 at the 6-month follow-up (P < .05) but not at the 12- and 24-month follow-ups (P > .05). There was no statistically significant difference in distal crestal bone loss between the anterior and posterior implants in group 2 at any follow-up evaluation (P > .05). There was no statistically significant difference in distal crestal bone loss between the anterior implants of both groups at the end of all follow-up evaluations (P > .05). Distal crestal bone loss was significantly increased in the posterior implants of group 1 when compared to the posterior implants of group 2 at the 6-month follow-up evaluation (P < .05).

Anterior and posterior implants of both groups showed significant differences in mesial crestal bone loss through all follow-up evaluations (P < .05), except between the 12- and 24-month follow-up evaluations of the group 2 anterior implants (P > .05). There was no significant difference in mesial crestal bone loss between the anterior and posterior implants of both groups in all follow-up evaluations (P > .05). There was also no significant difference in the mesial crestal bone loss between the anterior implants of both groups and between the posterior implants of both groups in all follow-up evaluations (P > .05) (Table 2).

Surgical bone augmentation is a procedure to correct vertical bone deficiencies and rehabilitate the posterior maxilla with dental implants. Although sinus augmentation is a well-known and predictable procedure, wound morbidity and surgical complications can be challenging in the dental implant rehabilitation of posterior maxilla. Implant-supported fixed partial prosthesis (ISFPP) with cantilevers can be used in the posterior maxilla to avoid the surgical complications of bone augmentation techniques.⁹  Implant-supported fixed prostheses with cantilevers provide a distal extension and reduce the number of inserted dental implants. Malo et al¹⁰  reported that ISFPP with a cantilever showed a 99% success rate and acceptable crestal bone loss at the end of a5-year follow-up period. A systematic review conducted by Zurdo et al¹¹  on the effect of cantilevers on the survival and complication rates of ISFPP observed that cantilever extensions with 1 tooth unit could be safely used with ISFPP. Furthermore, Storelli et al⁷  reported that 2 or more ISFPP with cantilevers demonstrated a high survival rate for dentures.

Some studies have suggested that eccentric and nonaxial forces exert a mechanical influence on the peri-implant area and that host bone produces a biological response in the form of crestal bone loss to these applied forces.¹²  Therefore, crestal bone loss in loaded dental implants is a predictive method to monitor the survival and success rate of implant-prosthodontic treatment. Halg et al⁹  compared the marginal bone loss between fixed partial prostheses with and without cantilever in the posterior region of both jaws and showed that the marginal bone loss was not significantly affected by the presence or absence of the cantilever. However, the cantilevered fixed partial prosthesis demonstrated more technical complications than the fixed partial prosthesis without cantilever. A similar study conducted by Aglietta et al¹³  reported no significant marginal bone loss between dental implants supporting single-unit crowns and fixed partial prostheses with mesial or distal cantilevers.

A bone replacement process around the inserted dental implants occurs after sinus augmentation, and these dental implants naturally become more vulnerable to occlusal forces when compared to those inserted in the native bone. Therefore, a distinct increase in the crestal bone loss may be expected in the early period after loading. Few studies have investigated the crestal bone changes associated with dental implants inserted with/after sinus augmentation procedures. In a study by Kim et al,¹⁴  dental implants inserted after sinus augmentation with 2 different bone grafts showed no significant difference in marginal bone loss. Similarly, Ungor et al¹⁵  found no significant differences in the marginal bone loss around dental implants placed in maxillary sinuses augmented with a putty and powder form of demineralized bone matrix. In the present study, sinus augmentation was performed with 1 type of bone graft. Dental implants placed in the native bone in both groups showed less crestal bone loss than the implants inserted with sinus augmentation at the 6-month follow-up evaluation but not at the 12- and 24-month follow-up evaluations.

Reports indicate that the implant closest to the cantilever extension withstands greater concentrations of stress, leading to crestal bone loss implant failure.^16–18  According to a study by Kim et al,¹⁹  cantilevered fixed dental prostheses seemed to present with significantly more crestal bone loss than fixed dental prostheses without cantilever in the posterior mandible. In the present study, posterior implants of 2-implant–supported distal cantilevered TUFPP showed more distal bone loss than the posterior implants of 2-implant–supported TUFPP with sinus augmentation at the 6-month follow-up evaluation. However, there were no significant differences at the 12- and 24-month follow-up evaluations. The early distal bone loss in the posterior implant of the 2-implant–supported TUFPP with sinus augmentation could be due to the physiological bone remodeling after sinus augmentation. Crestal bone levels could have stabilized in the subsequent control periods, as the graft would have differentiated into mature lamellar bone with adequate capacity to withstand occlusal forces and provide biomechanical integrity. Posterior implants showed more distal bone loss than the anterior implants in the 2-implant–supported TUFPP of the sinus augmentation group at the end of the 6-month follow-up (P < .05). This observation could be a result of implant instability due to bone remodeling in the sinus. Anterior and posterior implants in both groups revealed no significant changes in the mesial crestal bone, suggesting that TUFPP with or without cantilever showed similar biomechanical behavior in the mesial aspect of the implants despite the distal tipping effect of the occlusal forces in prostheses with cantilever extension.

The limitations of the study were as follows:

1. Although the prosthetic units were designed by the same technician, the lengths of the bridge and cantilever pontics could not be standardized due to limitations of the dental laboratory technology. This constraint may have led to uneven loading of the pontics in both groups.

2. In long-term partial edentulism, patients develop unilateral chewing habits on the side with the remaining natural teeth. This masticatory pattern may persist after rehabilitation with dental implants and contribute to loading challenges, leading to premature crestal bone loss.

3. Oral hygiene is essential for dental implant maintenance. Crestal bone levels are affected by the oral hygiene status. There was no record of the oral hygiene status of the patients included in the study. Negligence of oral hygiene may result in the accumulation of bacterial biofilm under the pontics of the fixed partial prostheses and cantilevers and initiate crestal bone loss. Determination of the periodontal status with relevant indices would have been useful to monitor the cause-and-effect relationship between crestal bone loss and oral hygiene.

4. The current study was designed as a retrospective cohort study with convenient sampling instead of a prospective randomized manner. First, the topic of the current study is a specific issue, and there will be ethical problems, including some patients in a group with sinus surgery and some in a group without sinus surgery prospectively. Second, there was a need for anecdotal evidence on the comparison between noncantilevered 2-implant–supported TUFPP with sinus augmentation and distal cantilevered 2-implant–supported TUFPP without sinus augmentation on the maxillary posterior region to provide a base and support for future randomized prospective studies. In that sense, a cohort pilot study was appropriate for this purpose.

The mesial and distal crestal bone loss in relation to anterior and posterior implants in 2-implant–supported TUFPP with sinus augmentation may be comparable to that of anterior and posterior implants in 2-implant–supported distal cantilevered TUFPP in the medium term. There may also be significant differences in crestal bone loss between the anterior and posterior implants of both prosthetic systems in the medium term. Further prospective randomized studies with larger sample sizes should be designed to compare the performance of the 2-implant–supported cantilevered TUFPP and 2-implant–supported noncantilevered TUFPP with sinus augmentation.

Abbreviations

ANOVA:

analysis of variance

ISFPP:

implant-supported fixed partial prosthesis

TUFPP:

3-unit fixed partial prosthesis

The Statistical Analysis of the study was performed by Dr Muzaffer Bilgin, who is an independent statistician at Eskişehir Osmangazi University, Faculty of Medicine, Department of Biostatistics. The authors thank him for his contribution. The authors also thank Dr Ekim Onur Orhan for his help in designing the figures.

The authors report no conflicts of interest.