Osseointegration was defined as the direct structural and functional connection between ordered living bone, and the surface of a load-carrying implant. Osseointegrated dental implants were designed by Brånemark et al1 in the early 1960s. Today, treatment with prostheses that are supported by different kinds of implants may improve the physical, psychological, and social well-being of edentulous patients.Hence, dental implants have been popularly used to replace the missing teeth in the last 2 decades.
Despite the high success and survival rates of dental implants, failures may occur. Many studies2,3 have been done to determine the success criteria of dental implants. Several success criteria are reported including hard and soft tissue responses. Especially, marginal bone loss (MBL) around the implant is the most important criteria to obtain success and it is evaluated by means of radiography, and is directly associated with the long-term success of implant treatments. Albrektsson et al2 suggested that radiographic vertical bone loss must be less than 0.2 mm during the first year and ≤0.2 mm during each successive year. In addition, the authors stated that absence of pathological signs such as persistent pain, infection, neuropathies and paresthesia, radiolucency in radiography, and absence of implant mobility determine implant success.
The main theories explaining MBL are controversial and include infection or overloading the implants. The overloading theory stated that MBL could occur as a result of altered occlusion, and excessive occlusal forces could cause further bone resorption around implants. According to the infection theory, implants behave like natural teeth and they are susceptible to similar types of diseases as natural teeth. Another theory explains MBL with 3 combined factors that include surgical, prosthodontic, and patient disorders. During the first year after implantation, initial MBL may be influenced by a number of parameters such as surgical trauma, occlusal overload, peri-implantitis, microgap, smoking, location, biologic width and implant crest module, and flapless or flapped procedures.4
The aim of the current study is to analyze the presented evidence behind suggested reasons for long-term MBL around 600 endosseous titanium dental implants according to the radiological findings up to 60 months.
Materials and Methods
The data of 151 patients and 600 implants were received from the prosthodontic department of a university clinic and have been analyzed in a 6-year period in this retrospective study.
Patients undergoing radiotherapy in the maxillofacial area, who received chemotherapy and bisphosphonate therapy, and had excessive alcohol and tobacco use, uncontrolled diabetes, HIV (+), rheumatoid arthritis, serious psychiatric and mental disorders, and bruxism were excluded from the study. All patients were informed about the surgical and prosthetic procedure and written consent on an institutionally approved form was taken. This study was designed according to Declaration of Helsinki's medical protocol and ethics. Ethical approval was obtained from the Istanbul University Ethical Committee (April 22, 2011, Permit Number: 07).
All patients were placed under local anesthesia by a single experienced surgeon (G.A.). In all patients, full-thickness mucoperiosteal flap was raised after mid-crestal incision. Subsequently, implants were placed according to implant system recommendations. Both one-stage and 2-stage surgical techniques were used to prepare the surgical sites. Resorbable collagen membrane and synthetic bone graft materials were used if necessary.
After more than 3 months of healing period, second-stage surgery was performed to nongrafted sites and 6 months after at grafted sites. Prosthetic treatment was made by a single prosthodontist (G.O.O.). After the completion of the planned prosthetic procedure, periodic recall check for 6 months was performed by a trained prosthodontist (S.D.) including extra- and intra-oral clinical examinations and assessment of periodontal health. For evaluation of the MBL, periapical radiographs were taken using a long-cone paralleling technique. A conventional radiograph Rinn holder (XCP Evolution 2000 Instrument Kits; Dentsply Rinn, Elgin, IL) was used. Vertical distance in millimeters from the implant shoulder to the most apical initial point of first visible bone contact was measured by first author (G.O.O.) and it was calibrated with a blinded assessor. The images were digitized for measurement using specialized software (ImageTool, version 1.28; University of Texas Health Science Center at San Antonio, San Antonio, TX). A computer-assisted calibration was performed for each radiograph by evaluation of the previous known values (eg, fixture length). Unclear radiographs or implant sites were excluded.
The evaluated information belonged to the patient pool of 5 experienced prosthodontists:
- Sociodemographic data: patient name, gender, and age.
- Dental implants and dimensions: Length, diameter, and location of the implants were recorded.
- Seven brands of dental implant systems were installed: Straumann (Institute Straumann AG, Waldenburg, Switzerland), Astra tech (Astratech AB, Mölndal, Sweden), Biohorizons (Biohorizons, Birmingham, AL), Implant Direct (Paragon Implant Co., Encino, CA), Zimmer Tapered Screw Vent (Zimmer Inc., Carlsbad, CA), Zimmer SwissPlus (Zimmer Inc., Carlsbad, CA), and CamLog (Henry Schein Co., Carlsbad, CA).
- Four groups were formed based on the location of the implant: anterior maxilla, posterior maxilla, anterior mandible, and posterior mandible.
- Two groups were formed with regard to implant length: <10.5 mm (short implants) and >10.5 mm (standard implants).
- Two groups were formed based on the diameter of the implants: implants with a diameter of <3.75 mm (narrow) and >3.75 (standard).
- Four groups were formed based on crown/implant (C/I) ratio (0–0.49, 0.5–0.99, 1–1.49, and 1.5–2).
- Five groups were formed based on occlusal table width (Occlusal table width)/implant diameter (OT/I) ratio (1–1.49, 1.5–1.99, 2–2.49, 2.5–2.99, and >3).
- The patients were grouped as nonsmokers, mild smokers (up to 10 cigarettes per day), and heavy smokers (more than 10 cigarettes per day).
Evaluation of Vertical Bone Loss
Full particulars of related factors were collected and put in order during the study. Vertical bone level was measured indirectly by peri-implant radiographic findings at the end of the retrospective study.
The periapical radiographs were taken with the paralleling technique and digital x-rays (XCP Evolution 2000 Instrument Kits; Dentsply Rinn) were used. Photoshop 7.0 software was used to measure the amount of mesial and distal vertical bone loss. These values were analyzed according to restoration types which were crowns or bridges.
Nonparametric Kruskal-Wallis analyses of variance and post hoc Mann-Whitney U test were used to assess the effects between the 2 groups. Logistic regression models for correlated data were employed to relate implant brand, bone type and quality, implant diameter, and length, smoking, crown/implant and occlusal table width/implant diameter ratios, and cantilever to MBL rates. Chi-square tests were used to analyze qualitative variable. P < 0.05 was considered statistically significant.
In this study, 600 implants were placed from the year 2006 to 2011 in 151 patients (95 female, 56 male, 51 ± 14.3 years ranging from 20 to 80).The longest follow-up time was 6 years and cumulative survival rate was 95.7%. Twenty-six implants failed after occlusal loading (late failure). Early failed implants were excluded. The distribution of implants according to demographic features of patients are shown in Table 1. The distribution of MBL according to implant brand is shown in Table 2. MBL is significantly higher in Zimmer SwissPlus implant system than Astra Tech, Implant Direct, Zimmer Tapered Screw Vent, Straumann, and CamLog implant systems. The distribution of MBL was correlated according to location (Table 3).The bone loss in posterior region was higher than anterior region for maxilla. There was no significant difference in mandible. Table 4 shows that implant diameter and length does not affect MBL. C/I ratio was analyzed and Kruskal-Wallis test showed that MBL was significantly higher when the C/I ratio was 1.5/2 (P < 0.05) (Table 5). OT/I ratio was analyzed and it showed that MBL was significantly higher when the ratio was 2.5 to 2.99 and higher than 3 (P < 0.05) (Table 6). Table 7 shows association between MBL and cantilever. Mann-Whitney U test revealed that cantilever does not affect MBL. The distribution of MBL according to tobacco usage is shown in Table 8. No significant difference was found between groups. Multivariate analysis showed that Zimmer SwissPlus implant brand, maxilla posterior region, 1.5 to 2 C/I ratio, and OT/I ratio 2.5 to 2.99 and more than 3 are the major risk factors for MBL (Table 9).
Osseointegration is the most important requirement for implant survey. Several authors3–5 suggest that the rate of MBL is the most important criteria for implant survey. In addition, occlusal loading of implant supported restorations affects the peri-implant bone loss.6–10 For instance, overloading may cause bone microfractures and may lead to mechanical failure of implant or implant prosthesis.4–6
Measurements of MBL changes over time in radiographs have been reported to be important parameters. Canullo et al11 suggested that MBL could be measured from a standard landmark at the collar of the implant. Thus, MBL should be detected by periapical radiographs and in the current study parallel radiographic technique was used to measure bone level. However, 3-dimensional imaging including volumetric tomography would be of value to use in future studies to gain insight into the time-dependent changes in buccal and lingual/palatal walls around implants, because esthetic results depends also on the maintenance of the buccal wall.
The relationship between MBL and implant brand is controversial. Ravald et al12 treated 66 patients with full-arch restoration with Astra Tech and Brånemark implant systems and no significant differences were found between the groups for 12 to 15 years in function. Bilhan et al13 reported similar MBL level around ITI and AstraTech implants at 2 years. Ozkan et al14 reported similar positive treatment outcomes in ITI, Camlog, and Frialit implants. In the current study, MBL is significantly higher in Zimmer SwissPlus implant system. No significant difference among other groups was found. We believe that more MBL seen in Zimmer SwissPlus implant system is caused by excessive implantation of polished surface of these implants into the bone due to the esthetic reasons.
It was reported that posterior sites especially in maxilla had negative effect on success and survival rates of the implants.15,16 Bryant et al8 researched 72 studies through literature review and they declared that the location of restoration is important than the restoration type in complete edentulous arch. Goodacre et al17 presented that the location of restoration had more important effect than prosthesis type. Also they reported little difference of success rate in mandible according to restoration type. Neart et al10 suggested that the estimated MBL for the first 6 months is higher in the maxilla than the mandible. In this study, we found no significant difference in mandible; however, MBL was higher at posterior region than anterior region in maxilla. It should be related with distribution of type IV bone in the posterior maxilla.
Longer and wider dental implants have always been considered more reliable due to an improved C/I ratio and a greater surface area available for osseointegration, which dissipates the imposed occlusal forces. Many studies demonstrated18,19 that the influence of implant diameter on crestal bone strains dominates over the effect of the implant's length. The majority of studies18–21 have implicated occlusal overload as the primary factor for MBL around endosseous implants, whereas very little stress is transferred to the apical portion, and the increase of implant length from 7 to 10 mm did not significantly improve its anchorage. Therefore, implant length may not be a primary factor in distributing prosthetic loads to the bone-implant interface. Bone-implant interface area could be reduced by the effect of occlusal forces. In the current study it was found that each of the implant diameter and length does not affect MBL.
Excessive C/I ratio is altering the biomechanics of the implant. Rossi et al22 investigated the clinical outcomes of single crowns supported by short implants (6 mm long), with a follow-up period of 2 years. They reported moderate marginal bone loss and increased clinical C/I ratio with time from 1.5 at the delivery of the prosthesis to 1.8 at the end of the follow-up period. Misch23 revealed that a C/I ratio of 0.5 to 1.0 reduces stress on the MBL, thereby preventing bone loss. Lee et al19 revealed that implants with a C/I ratio below 1 exhibited greater peri-implant MBL than implants with a C/I ratio more than 1. Contrarily, Tawil et al24 reported that C/I ratio did not prove to be a major biomechanical risk factor, as long as the occlusion is properly adjusted and occlusal contacts are placed as closely as possible to the emerging axis of the implant. The current study demonstrated that MBL was significantly higher when the C/I ratio was 1.5/2.
Occlusal table width/implant diameter ratio increment could result as a cantilever effect and increase mechanical forces on implant. Reducing the buccolingual width of a restoration is not a new concept in dentistry. Schuyler25 advocated reducing the contacting surfaces as a means of adjusting occlusal disharmony, which could result in occlusal trauma. Tawil et al24 showed that occlusal table width/implant diameter ratio did not seem to be a major risk factor in cases of favorable loading at implants shorter than 10 mm, and the authors recommended reducing the width of the occlusal table to favor axial load on the implant in nonesthetic regions. The current study found that MBL was significantly higher when the occlusal table width/implant length ratio was 2.5 to 2.99 and higher than 3.
Romeo et al20 followed the patients treated with implant supported prostheses and found them to have 34 mesial and 14 distal cantilevers for 7 years. They measured an MBL of 0.82 ± 0.62 mm near the cantilever and 0.69 ± 55 mm distant from the cantilever. Semper et al26 reported that the length of cantilever extensions had no influence on MBL. In this study, we measured these values as 0.82 ± 1.05 mm near the cantilever and −0.81 ± 1.00 mm distant from the cantilever. This study demonstrates that cantilever does not affect MBL.
Several studies27,28 have suggested different results for smoking. Bain et al27showed that smoking increased implant failure rate. Similarly, Lindquist et al29 reported that smokers had greater bone resorption than nonsmokers. In contrary, Kan et al28 reported no significant relationship between smoking and failure rates. The reason for the difference between the studies is unknown and it could be said that the negative effects of smoking on MBL depend on local or systemic factors. In our study, results showed no significant relationship between smoking and MBL.
In conclusion, our findings revealed that the marginal bone loss in posterior region is higher than anterior region for maxilla, although there is no difference for mandible. In addition, marginal bone loss is not affected by implant diameter and length or existence of cantilever; whereas excessive crown/implant and occlusal table width/implant diameter ratios increase marginal bone loss. However, evaluation of implant success rate and determining implant brand-marginal bone loss relationship is complex and difficult to standardize because of so many related factors.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
This study was designed according to Declaration of Helsinki's medical protocol and ethics. Ethical approval was obtained from the Istanbul University Ethical Committee (April 22, 2011, Permit Number: 07).
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