Research on Cervical Margin Relocation
The Cervical Margin Relocation (CMR) was proposed more than 15 years ago, and in the last decade became more and more popular among dental practitioners [1-2].
CMR is indicated when the gingival margin of a Class II interproximal cavity cannot be isolated with rubber dam alone, in alternative to perform surgical crown lengthening. CMR consists on placing a base of direct resin composite using a metal interproximal matrix to elevate the interproximal underneath indirect bonded restorations. Consequently, margins can be predictably caught by a conventional impression and/or intraoral optical scanning (IOS) . Different papers described clinical procedures of CMR and evaluated in the lab this technique. [4-10]
Location of a subgingival margin can affect the periodontal health and therefore healthy periodontal tissues, defined by a Probing Pocket Depth (PPD) less/equal than 4mm without Bleeding on Probing (BoP).
Unfortunately, no clinical trial evaluating periodontal tissue response on indirect adhesive restorations placed on posterior teeth with CMR is available till now .
Also, the literature reports about the influence of approximal restorations extension on the development of secondary caries, showing that restorations ending below the CEJ showed significantly increased risk for failure [12-13].
The primary aim of this clinical trial was to evaluate BoP on single adhesive indirect restorations made on posterior teeth with one interproximal margin relocated cervically; and secondary, to analyze the correlation between depth of the interproximal margins and BoP. The null hypothesis tested was that there is no statistically significant difference between margins with or without CMR regarding periodontal tissue inflammation (BoP).
Materials and methods
A consecutive sample of 35 restorations in 35 patients in need of one single partial crown (onlay) on posterior teeth was placed between January and April 2016. A partial restoration was performed from the pool of patients accessing the Department of Prosthodontics and Dental Materials of the University of Siena, Italy. All of them had an old restoration and some carious tissue to be replaced.
Patients written consent to the trial was obtained after having provided a complete explanation of the aim of the study. Ethical approval was achieved beforehand by the University of Siena, Italy.
Caries cleaning of the affected area was performed after placing a first matrix band to retract and simultaneously protect the soft tissue, the curvature of the metal matrix was properly adapted to the curvature of the tooth to achieve the best cervical fit was possible . In one proximal box CMR procedure was performed using G-Premio Bond, simultaneously used to perform hybridization of entire exposed dentin, and universal flow resin composite applied in two or three thin layers depending the depth and size of the cavity (GC Co. Tokyo, Japan). After final cavity’s preparation, an impression was taken (Ex’lance, GC Co., Tokyo, Japan) and sent to the laboratory in order to make the restoration using lithium disilicate (LS2) press material (LiSi Press, GC Co. Tokyo, Japan). A temporary restoration was made with heat-polymerizing polymethylmethacrylate (PMMA) acrylic resin and luted. Patients were instructed to use a 0.2 % chlorhexidine gluconate solution for 7 days until they could perform regular oral hygiene and returned 2 weeks later for the impression procedures, giving enough time for soft tissue adaptation and maturation after teeth preparation. The restorations were made in the laboratory, then tried-in, and margins were examined and carefully verified for fit and extension. Rubber dam was always placed to isolate the abutment. The restorations were luted following manufacturer’s instructions using proprietary’s cement (Link Force, GC Co., Tokyo, Japan) after being sandblasted, etched with fluorhydric acid at 5 % for 60 seconds and a coat of multi primer being applied and left to evaporate for 1 minute.
Cement excess was carefully removed, and occlusion was slightly adjusted when needed. Intra-sulcular margin position was verified, and oral hygiene instructions were given to the patients.
The restorations were placed in the time period between January 2016 and April 2016 and examined for (BoP) at baseline (cementation of the restorations), and after 12 months by two calibrated operators.
At baseline, the restorative margin position in relation to the gingival margin was recorded quantifying by probing in mm , and the linear distance from the bone crest was calculated in mm by intraoral x-ray. In addition, intraoral x-rays were made at the 12-month recall as well.
The CMR was associated with statistically significant increased scores for BoP. Gingival Index (GI) and Plaque Index (PI) were not statistically different between the groups. At the end of the experimental period, 53% and 31.5% of sites (Group 1 and 2 respectively) were positive to BoP: this difference was statistically significant (p=0.10). The linear distance between the bone crest and the restorative margin, assessed by a radiographic analysis, was 2 mm in 13 out of 19 experimental sites of group 1 and 6 out of 11 of group 2.
Included participants completed the 12-month follow-up (Table 2).
At 12 months follow-up, changes from baseline were observed in GI, PI, and BoP: 20 % of the sites of Group 1 (CMR) and 8.5 % of Group 2 (shoulder preparation) presented dental plaque (PI), while at baseline dental plaque was not present. Teeth at baseline did not show any degree of gingival inflammation (GI) or BoP, while at 12 months 31.5 % of Group 1 and 18.5 % of Group 2, the GI scoring ranged from 1 to 3, and BoP was presented in 53 % of Group 1 and 31.5 % of Group 2, respectively.
Statistically significant differences existed for PPD at mesial and distal sites at baseline (P = .001) (Table 2). Considering the two different groups, differences were identified for PPD from baseline to 12 months (P = .340).
PI and GI at 12 months were similar in both groups (P = .250 and P = .465), respectively. Significantly more sites in Group 1 had BoP (53 %) compared with group 2 (31.5 %) (P = .010) (Table 3).
Evaluating Group 1 cases with positive BoP at one year recall, the recorded margin-bone crest distance was mainly 2 mm (13 of 19 margins) and similarly in 6 cases of 11 in Group 2.
Because the CMR technique is usually made on posterior teeth that had an interproximal decay and/or an existing restoration to be replaced and in need to receive an adhesive partial restoration the present clinical trial focused on BoP of subgingival margins in the interproximal area.
Scientific publications available on CMR are mainly based on ‘marginal quality’ [14-17]; however, neither leakage tests under laboratory conditions nor clinical investigations, such as randomized controlled trials, evaluating CMR are available yet.
However, the evaluation of margins’ quality under scanning electron microscopy (SEM) when conducted at low magnification can not clarify if the margins sealed efficiently: leakage to be present does not require an evident gap visible at low magnifications, and it can be clearly detected only using micro computerized tomographic analysis and/or cutting the samples after being processed for marginal leakage. Recently, no correspondence between SEM quality margin assessment and presence of nano-leakage was found .
From a clinical point of view, the effectiveness and the “bio-integration” of CMR of posterior indirect restoratives should be related to both BoP, as a measure of periodontal tissue stability, and to a radiographic examination, able to assess the marginal bone stability.
Recent articles confirm how the presence of a deep subgingival margin is otherwise associated with an increase of bleeding after probing [19-23].
Lang et al. clearly described the periodontal inflammation mechanism that occur when overhanging margins are found interproximally . However in the same preclinical model, Lang and co. demonstrated how periodontal inflammation is a reversible process, and restitutio ad integrum can be again established if proper margin is present . The periodontal inflammation experimentally provoked  can be similar to that took placed in the interproximal areas where CMR was applied in this study.
The CMR clinical procedure is advocated to get a better control of margins of the indirect restoration at the time of preparation, impression and luting (1,2), but cannot improve quality of bonding to cementum-dentin substrates [24-25], and the progressive degradation of the hybrid layer at the bonding interface can not avoided [25-26]. The seal of the cervical margins below the CEJ remains an important unsolved issue.
While no differences were present between the groups at baseline, at the 12-month follow-up 53 % of sites in Group 1 was positive to BoP versus 31.5 % in Group 2 (P = .010) (Table 2).
When the margin-bone crest distance was considered, it was noticed that in Group 1 samples, 13 margins of 19 were located at a distance of 2 mm from the bone crest and in Group 2 in 6 of 11 cases.
If it is considered that the recorded distance between the restorative margins and the bone crest of all cases with positive BoP (in both Groups 1 and 2) was between 2-3 mm, it can be speculated that one of the reason of the bleeding might be related to an invasion of the biological width [27-28].
At the authors’ best knowledge, the present investigation offers for the first time short-term clinical results about the periodontal tissues response to the CMR procedure.
BoP was evident in the majority of relocated margins. Among the clinical reasons that can justify the BoP positive sites in group 1, it’ s worth to mention the difficulties to keep clean deep margins by the patients, roughness of the cervical resin margins, incomplete control of adhesive and resin composite flow in between the interproximal margin and the metal matrix in an amount that can not be visible in the x-ray.
The results of this study and limited information of medium-long term clinical behavior of CMR procedure suggest a prudent selection of clinical cases in which CMR can be made, and a periodic recall of patients in order to keep under control all periodontal parameters.
Within the limitations of this study, higher incidence of BoP can be expected around CMR margins and in coincidence with margins. CMR of margins is a sensitive-technique, especially when deep subgingival margin is selected and bonding restorative procedures are performed below the cementum enamel margins.
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