Statement of problem
Osseointegrated implants can be prone to occlusal overloading because of the absence of the periodontal ligament and limited tactile sensitivity. However, current scientific evidence of the occlusion variation of implant-supported fixed prostheses is lacking.
The purpose of this clinical study was to analyze changes in occlusal force distribution and occlusal contact in single posterior partial fixed implant–supported prostheses over time.
Material and methods
Partially edentulous patients who had received implant-supported single crowns in the posterior region between December 2012 and December 2013 were enrolled. The participants underwent occlusal examinations by using the T-Scan III system at 0.5, 3, 6, 12, 24, and 36 months after implant prosthesis delivery. The relative occlusal forces (ROFs) of implant prostheses, mesial adjacent teeth, and control natural teeth were recorded, and implant prosthesis occlusion time ratios were calculated. The paired t test was used to compare the implant prosthesis occlusion time ratios and ROFs of implant prostheses at 2 different times as a self-control. The differences in ROFs between implant prostheses and control teeth in the same participant at the same time were also analyzed by using a paired t test. The Pearson correlation coefficient was used to analyze the statistical correlation between implant prosthesis occlusal force and the implant prosthesis occlusion time ratio (α=.05).
Thirty-seven posterior partial fixed implant–supported prostheses in 33 participants (18 women and 15 men aged 23.9 to 70 years) were followed up for 3 to 36 months (mean: 31.4 months). The ROFs of implant prostheses increased significantly ( P <.05) from 2 weeks (7.46 ±4.21%) to 3 months (9.87 ±6.79%), whereas those of control natural teeth decreased significantly ( P <.05) from 13.78 ±6.00% to 11.43 ±5.47%. The ROFs of implant prostheses continued to increase from 6 to 12 months and from 12 to 24 months, with significant differences ( P <.05). However, they were statistically similar to those of control natural teeth at 6, 12, 24, and 36 months after restoration. Implant prosthesis occlusion time ratios also increased significantly between 2 weeks and 3 months and between 3 and 6 months ( P <.05). No significant differences were found between the other time points ( P >.05).
The occlusal force and occlusal contact time of implant prostheses changed significantly with time.
The occlusal force and occlusal contact time of single posterior implant–supported fixed prostheses can change with time. The occlusion of posterior implant prostheses should be carefully monitored during follow-up examinations, and occlusal adjustment should be considered when necessary.
Implant-supported fixed prostheses are widely used to replace missing teeth in partially edentulous patients. Osseointegrated implants react biomechanically to occlusal force in a manner distinct from natural teeth because of the absence of the periodontal ligament and limited tactile sensitivity. Consequently, dental implants can be prone to occlusal overloading, which is considered a cause of mechanical complications such as screw loosening and fracture, prosthesis fracture, and implant fracture, eventually compromising implant longevity.
The control and maintenance of implant occlusion may reduce mechanical and biologic complications, thus increasing the longevity of prostheses. To minimize the excessive force, light contact during forceful occlusion and no contact in the maximum intercuspal position (MIP) are considered as reasonable approaches. The occlusal schemes of implant prostheses can result in a quantifiable time delay such that the natural teeth occlude in advance of the implant prostheses by fractions of seconds. However, in the natural dentition, several factors can cause changes in occlusion force distribution and occlusal contacts, such as occlusal abrasion, periodontal disease, temporomandibular diseases, orthodontic treatment, mesial tooth movement, and continuous eruption. Even in adulthood, 3D positioning of natural teeth in the dental arches is constantly changing as a consequence of continued slow tooth eruption and mesial tooth movement of about 0.1 to 0.2 mm annually. Dental implants cannot mimic the positional changes of the natural teeth because of the absence of the periodontal membrane. Therefore, the occlusion of partial fixed implant–supported prostheses may change.
The T-Scan III system can accurately identify occlusal force distribution and the sequence of occlusal time in natural teeth and implant prostheses. The evaluation of precision of this method has provided positive results in previous studies. Other studies that focused on the accuracy and reliability of the T-scan system have demonstrated the system’s high degree of reliability for the analysis and evaluation of occlusal contact distribution in maximum intercuspation.
Current scientific evidence of implant occlusion is lacking and is mainly limited to in vitro, animal, retrospective, and case studies. Whether the occlusion of implant prostheses remains light with use is unknown. Therefore, the purpose of this clinical study was to describe and analyze the longitudinal variation of occlusal force distribution and occlusal contact, including changes in occlusion time (OT), in single posterior fixed implant–supported prostheses by using the T-Scan III system. The research hypothesis was that the occlusion of partial posterior implant–supported fixed dental prostheses changes with time.
Material and methods
The study was registered with the Chinese Clinical Trial Registry and with the World Health Organization (ChiCTR-ROC-17012240). Ethics committee approval was obtained from the Peking University School of Stomatology Biomedical Institutional Review Board (no.: PKUSSIRB-201310062). Participants were consecutively recruited from a group of partially edentulous patients who received implant-supported single crowns in the posterior region. Written informed consent was obtained from all participants. All surgical and restorative phases were performed by dentists at the Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, China, between December 2012 and December 2013.
Patients were eligible for inclusion if they were aged over 20 years; had good periodontal health; had single-implant crowns; and if their implant prostheses exhibited less occlusal contact area and lighter contact marks than the adjacent teeth. Occlusal contacts were evaluated by using a 30-μm articulating paper during forceful occlusion, and no contact was evaluated by using an 8-μm articulating film, which could be pulled out with no resistance during light occlusion.
Patients were excluded if they were experiencing pain in the temporomandibular joint; had a history of periodontitis; had undergone therapy involving occlusal adjustments, composite resin restorations, crown restorations, orthodontic procedures, or tooth extraction after implant prosthesis delivery; had failed to exhibit a stable occlusal relationship (premature contact and/or occlusal interference); or had parafunctional habits (clenching and/or bruxism) detected by inquiry and examination.
The prostheses were screw- or cement-retained and included ceramic crowns, metal-ceramic crowns, metal-resin crowns, and cast metal crowns delivered 4 to 5 months after implant placement. No occlusal contact was detected on the composite resin used to seal the screw access hole of screw-retained prostheses. Each implant prosthesis corresponded to 1 control tooth, most of which were the corresponding tooth on the contralateral side of the arch. If the corresponding tooth on the contralateral side of the arch was missing, an adjacent tooth was chosen. The occlusion of the mesial adjacent teeth, as a part of partial occlusion variation, was also evaluated as another control group.
At 0.5, 3, 6, 12, 24, and 36 months after prosthesis delivery, the occlusion was examined by using the T-Scan III system (Tekscan, Inc), which expressed the relative occlusal force (ROF) of each tooth as the percentage of the overall ROF. Before the examination, the participants were taught to clench their teeth in the MIP. When the participants could perform the clenching movement correctly, they were asked to sit in a relaxed upright position in the dental chair. The participants were then instructed to clench firmly on the sensor 3 times, and a video was recorded by the computer for analysis. The position and number of points of occlusal contact were verified by using 30-μm articulating paper ( Fig. 1 ). The same operator (Q.L.) performed all recordings.
The following occlusion parameters were evaluated in this study: ROFs (expressed as a percentage) of implant prostheses, mesial adjacent teeth, and control teeth at the MIP; OT, which was defined as the time from the first occlusal contact to the MIP and automatically calculated by using the digital occlusion analysis system, as measured from the first tooth contact until the last tooth contact was attained; the implant prosthesis occlusion time (IOT), which was defined as the time from the first occlusal contact of implant prostheses to the MIP and calculated according to the recorded system movie ( Fig. 2 ); and the IOT-to-OT ratio, which showed the relative occlusal time of implant prostheses. The MIP was determined by using the digital occlusion analysis system at the frame in which maximum intercuspation occurred, that is, the largest area of tooth contact. All ROFs and OT values were calculated as the mean values of the 3 repeated recordings.
All data were collected, and a statistical analysis was performed by using a statistical software program (PASW Statistics v18.0; SPSS Inc). The assumption of normality was justified because the values did not tend to have extreme outliers and homogeneity of variance was evident. Descriptive statistical methods were used for the evaluations. All values were expressed as means ±standard deviations. The paired t test was used to compare the IOT-to-OT ratios and ROFs of implant prostheses at 2 different times as a self-control. The differences in ROFs between the implant prostheses and control teeth of the same patient at the same time were also analyzed by using a paired t test. The Pearson correlation coefficient was used to analyze the statistical correlation between the implant prosthesis occlusal force and the IOT-to-OT ratio (α=.05).
In total, 33 participants (18 women and 15 men) with 37 single posterior partial fixed implant–supported prostheses, including 22 metal-ceramic crowns, 12 metal-resin implant crowns, 2 cast metal crowns, and 1 ceramic crown, were enrolled. There were 3 first premolars, 4 second premolars, 5 second molars, and 25 first molars. The 37 control teeth included 5 first premolars, 5 second premolars, 4 second molars, and 23 first molars.
The age of the participants ranged from 23.9 to 70 years at the first examination (mean: 42.8 years). The follow-up period ranged from 3 to 36 months and exceeded 12 months in 85% of the participants (mean: 31.4 months). During the follow-up period, all participants were free of occlusion-related discomfort. Five participants (6 implant prostheses) were lost to follow-up or withdrew early because they had either lost contact (n=3), were not compliant (n=1), or had moved from the area (n=1). Two participants (2 implant prostheses) had crown restorations of natural teeth and stopped participating in follow-up examinations at 24 months. Six participants (6 implant prostheses) fractured the prosthesis veneer and stopped participating in follow-up examinations early, yielding a complication rate of 16.2%. Four veneers fractured after 12 months, and the remaining 2 occurred after 3 months. One implant prosthesis was lost because of screw loosening, and the patient received a new prosthesis. Therefore, 22 implant prostheses underwent occlusal examination at 36 months. The data of the 15 implant prostheses not included in the 36-month follow-up were included in the follow-up data of other review times.
The longitudinal changes in the ROFs of implant prostheses, mesial teeth, and control teeth with time are shown in Figure 3 . The ROFs of the implant prostheses increased significantly in the first 3 months ( P <.05), whereas those of the control teeth decreased significantly ( P <.05). Between 3 and 6 months, the ROFs of the implant prostheses remained relatively stable. However, the ROFs of the implant prostheses increased significantly between 6 and 12 months and between 12 and 24 months ( P <.05). No significant difference was found between 24 and 36 months ( P >.05).