The influence of crown-to-implant ratio on marginal bone loss: a narrative review
Review Article

The influence of crown-to-implant ratio on marginal bone loss: a narrative review

Adolfo Di Fiore, Francesco Maniero, Edoardo Stellini

Department of Neurosciences, School of Dentistry, Section of Prosthodontics and Digital Dentistry, University of Padova, Padova, Italy

Contributions: (I) Conception and design: All authors; (II) Administrative support: All authors; (III) Provision of study materials or patients: All authors; (IV) Collection and assembly of data: All authors; (V) Data analysis and interpretation: All authors; (VI) Manuscript writing: All authors; (VII) Final approval of manuscript: All authors.

Correspondence to: Adolfo Di Fiore. Department of Neurosciences, School of Dentistry, Section of Prosthodontics and Digital Dentistry, University of Padova, Padova, Italy. Email: adolfo.difiore@unipd.it.

Abstract: Marginal bone loss (MBL) is considered a fundamental criterion for implant success. Many factors may influence MBL, but it is still not clear how MBL is influenced by crown-to-implant ratio (C/I-R). In addition, the introduction of short implants, to avoid bone augmentation procedure, lead to ratios higher and higher. The aim of this study is to elucidate about the role of high C/I-R on MBL. The articles were identified through MEDLINE database (via PubMed) and checking references. After the selection, 20 articles were included in the review. The highest follow-up among the selected studies was 16 years for the retrospective studies and 5 years for the perspective studies. The highest ratio reported was 2.53. In conclusion, is possible to state that C/I-R not exceed 2.2 did not influence the manifestation of biological complications and lead to a not significant MBL.

Keywords: Crown-to-implant ratio (C/I-R); marginal bone loss (MBL); short implants


Received: 01 September 2020; Accepted: 15 October 2020; Published: 20 November 2020.

doi: 10.21037/fomm-20-57


Introduction

Dental implants are considered nowadays a reliable solution to replace teeth. High implants and prosthesis survival rates (above 98% after 5 years) were achieved (1,2). To consider successful an implant therapy the criteria at implant level are: absence of pain, bone loss at 1st year <1.5 mm, annual bone loss <0.2 mm thereafter, no radiolucency, no mobility, no infection (3). Therefore, marginal bone loss (MBL) represents a favourable factor for long-term implant stability (4). Unfavourable crown-to-implant ratio (C/I-R), occlusal overload, occlusal table width, off-axis loading and cantilevers are factors that play a role in the failure of prosthetic implant therapy (5-7). The influence of cantilever (8), occlusal overload (9), and off-axis loading in prosthetic complications are largely demonstrated.

However, the role of C/I-R on MBL is still unclear. C/I-R is the application in implantology of the prosthodontics parameter of crown-to-root ratio. Ideally, the ratio between the crown and the root should be 1:2, and a minimum of 1:1 for a tooth abutment is recommended. To avoid unfavourable C/I-R, these prosthetic concepts have been used in implant dentistry. C/I-R is calculated as the ratio between the crown and implant lengths. We can distinguish two different C/I-R:

❖ Anatomical ratio is the ratio between the distance from the apex to the shoulder of the implant, and the distance from the shoulder of the implant to the end of the crown;

❖ Clinical ratio is the ratio between the distance from the apex to the bone level, and the distance from the bone level to the end of the crown.

In literature there are evidences that attest both the presence (10) and the absence (11,12) of a relationship between MBL and C/I-R, and one studies even deducted that high CI might provide a protective effect on bone loss (13). In addition, the introduction of short implant, to avoid bone augmentation procedure, lead to C/I-R higher and higher. A security threshold should be investigated to avoid biological complication and to guarantee the success of the therapy. The aim of this study is to elucidate about the role of C/I-R on MBL. We present the following article in accordance with the Narrative Review reporting checklist (available as http://dx.doi.org/10.21037/fomm-20-57).


Methods

The articles were identified through MEDLINE database (via PubMed) combined the key words: “crown-implant ratio”, “marginal bone loss”, “alveolar bone loss”, “short implants”. The search was supplemented checking references of the relevant review articles. The studies had to meet the following requirement:

❖ At least 1 year follow-up;

❖ Mean C-I/R should be reported;

❖ Outcome measures should include at least bone changes;

❖ Publication must be reporting in the English.

The selection path of the articles is shown in Figure 1.

Figure 1 Flowchart of study selection.

Discussion/summary

The primary selection identified 87 studies for the MEDLINE search. Checking of relevant reviews revealed five additional articles that met the inclusion criteria. After having removed duplicates and analysed titles and abstracts, 58 studies were excluded because they did not meet the inclusion criteria. The remaining 29 papers were evaluated by full-text analysis. Nine articles were finally excluded after full-text reading due to lack of information as marginal bone changes and C/I-R measurement. Characteristics of the selected studies are shown in Tables 1 and 2. Anitua et al. (14) in a retrospective study analysed 45 implants with a mean C/I-R of 2.4. The mean MBL reported was 1.01 mm for mesial bone, 0.89 mm for distal bone. Birdi et al. (15) studied 309 implants in a retrospective study. The mean C/I-R reported was 2.0 and the MBL was 0.2 mm. Blanes et al. (16) in a perspective study analyzed 109 implants. The mean C/I-R was 1.77. MBL was calculated for three different groups: group A (C/I-R 0–0.99): 0.35 mm, group B (C/I-R 1–1.99): 0.03 mm, group C (C/I-R >2): 0.02 mm. Di Fiore et al. (17) studied 108 implants retrospectively with a mean C/I-R of 2.21 mm. The mean MBL reported was 1.42 mm. Guljé et al. (18) analyzed 47 implants in a perspective study. The mean C/I-R was 2.14 with a MBL reported of 0.13 mm. Hadzik et al. (19) studied perspectively 30 implants with an anatomical ratio of 1.69. MBL reported was 0.34 mm. Hingsammer et al. (20) in a perspective study analysed 76 implants. The mean C/I-R was 1.70 with a MBL of 0.71 mm. Lee et al. (21) studied retrospectively 175 implants with a mean C/I-R of 1.06, MBL reported was 0.93 mm. Malchiodi et al. (22) analyzed 280 implants in a perspective study. In the study was reported both the anatomical and the clinical ratio, that was respectively 1.84 and 2.08. The MBL calculated was 0.48 mm. Mangano et al. (23) studied 68 implants perspectively. The mean C/I-R was 1.70. After 5 years of loading, a mean MBL of 0.38 and 0.48 was reported in the C/I <2 and C/I >2 groups, respectively. Naenni et al. (24) analyzed 96 implants in a perspective cohort study. The 6-mm and the 10-mm had a mean C/I-R of 1.75 and 1.04, respectively. The mean MBL calculated was 0.29 mm for the 6-mm group, and 0.15 mm for the 10-mm group. Nunes et al. (25) studied 118 implants retrospectively. The mean C/I-R was 2.53 and the MBL calculated was 0.67 mm. Pieri et al. (26) in a perspective study analyzed 61 implants with a mean C/I-R of 1.94 mm. The MBL calculated at 2 years was 0.60 mm. Ramaglia et al. (27) studied perspectively 78 implants with a mean C/I-R of 1.08. MBL at 5-year was calculated separately for the C/I-R <1 and C/I-R >1 group and was 0.653 and 0.287 mm respectively. Rossi et al. (28) in a perspective study confronted two groups of 30 implants each. The mean C/I-R for test and control group was 1.55 and 0.97 respectively. After 5-year loading MBL calculated was 0.14 mm for test group and 0.18 mm for control group. Schneider et al. (29) in a retrospective study analyzed 100 implants. In the study was reported both the anatomical and the clinical C/I-R of 1.04 and 1.48 respectively. The MBL calculated was 0.008 mm. Sharmann et al. (30) analyzed perspectively two different groups of 47 implants each. The mean C/I-R reported was 0.86 for the control group and 1.48 for the test group. The mean MBL calculated after 3 years were 0.19 and 0.33 for the test and control groups respectively. Urdaneta et al. (31) in a retrospective study analyzed 326 implants with a mean C/I-R of 1.6. The mean MBL found was 0.33 mm. Villarinho et al. (32) analyzed 46 implants perspectively. The mean C/I-R reported was 1.6 with a MBL of 0.3 mm. Zadeh et al. (33) analyzed perspectively for 3 years 209 implants divided in two groups. The C/I-R was 1.78 for the test group and 0.93 for the control group. The analysis of MBL revealed that the test group gained 0.04 mm while the control group losed 0.02 mm. Overall, the highest follow-up reported was 16 years for the retrospective studies and 5 years for the prospective studies. The highest C/I-R reported was 2.5.

Table 1
Table 1 Studies included in the review
Full table
Table 2
Table 2 Studies included in the review—part 2
Full table

Biomechanically, unfavourable ratio may influence marginal bone through the lever mechanism. When subjected to lateral forces, implant restorations with long lever arm generate greater stress at the bone crest. It has been demonstrated in a cantilever model that crown height increased from 10 to 20 mm, lead to a proportional moment increment (34). C/I-R is calculated as the ratio between the crown and implant lengths. We can distinguish two different C/I-R:

❖ Anatomical ratio is the ratio between the distance from the apex to the shoulder of the implant, and the distance from the shoulder of the implant to the end of the crown;

❖ Clinical ratio is the ratio between the distance from the apex to the bone level, and the distance from the bone level to the end of the crown.

Clinical ratio is considered a more accurate since the implant-bone interface is less rigid than the connection at the implant shoulder, due to bone viscoelasticity (16). Among the studies included in this review, many concluded that the relationship between C/I-R and MBL is not significant. Urdaneta et al. (31) concluded that larger CI ratio led to an increase in prosthetic complications but had no significant influence on MBL. In this study 16% of the sample had CI ratio >2. Nunes et al. (25) found a weak inverse correlation between CI ratio and MBL. Blanes et al. (16) stated that there were no statistical relationship between CI ratio and MBL. Birdi et al. (15) found no significant relationship between CI ratio and first bone-to-implant contact levels. Schneider et al. (29) concluded that technical and biological CI ratio had no significant correlation on the MBL, technical and biological complications and implant survival. Mangano et al. (23) found no correlation between MBL and CI ratio along time, with a 0.023 mm increase in 1-year bone resorption for every 0.1 increase in CI ratio. However, other studies found a stronger correlation between C/I-R and MBL. Di Fiore et al. (17) showed that, at multivariable analysis, C/I-R >2 was correlated with higher MBL. It was estimated that C/I-R >2 led to an increase of 0.28 mm in MBL. However, the authors underlined that this increment could be considered clinically irrelevant. Hingsammer et al. (20) concluded that a MBL of 0.71 mm is considered to be satisfactory and a C/I-R of 1.7 can be considered as a threshold to avoid early marginal bone changes. Malchiodi et al. (22) showed that implants with C/I ratio >2 had a MBL of 0.72 mm. Statistical analysis revealed that both anatomical and clinical CI ratio had a correlation with MBL. The authors concluded that, from a biomechanical point of view, to avoid excessive bone loss, anatomical and clinical C/I ratio should not exceed respectively 3.1 and 3.4. It was seen that C/I ratio resulted the main factor correlated to implant success and crestal bone loss.

The difference between all this findings can be found in various aspects. Many studies mixed together implants of different lengths; moreover, the definition of short implant is not standardized. Tawil and Younan considered short an implant <10 mm (35). Nisand and Renouard defined “short” implants with a length of <8 mm and extra-short the one <5 mm (36). Since the length of the implant seems not to be correlated to the lever mechanism, the portion that may be significantly correlated is the crown. In many of the previous mentioned studies, the crown-height-space was not considered in relation to CI ratio. Anitua et al. (14) analysed 45 extra-short implants with a mean CI ratio of 2.4 and a mean CHS of 17.05. Results showed that implants with MBL <2 mm had a mean CHS of 17 mm, while implants with MBL >2 had a mean CHS of 21 mm. Statistical analyses revealed that CHS had a positive correlation with bone loss. Nissan et al. (37) in an in vitro study showed that an increased crown height from 6 to 12 mm, in case of off-axis loading of 30 degrees determined a proportional increase stress distribution (17.72 vs. 30.09 MPa). CHS higher than 15 mm is considered as biomechanically unfavourable, resulting in increased stress at bone level. Failure were noted at CHS >15 mm and CI ratio at 1.75 when force application was at 30 degrees. The authors concluded that CHS is more significant than CI ratio in the evaluation of the adverse effects related to biomechanics, and studies about the effects of C/I ratio should mention both implant length and CHS in their results.

In this review was considered both splinted and unsplinted implants. Many studies underline that splinting crowns together could better distribute non-axial forces, minimizing their load to the restoration and bone, and increasing the load area (7). Moreover, splinting is recommended in case of poor quality bone to be reduced the marginal bone stress under horizontal load (38). Therefore, splint implants could alter the real impact of C/I-R on MBL resulting in less cervical stress for a better forces distribution. However, in an in vitro study Nissan et al. (37) proved the splinting implants can result in greater crestal bone loss.

In conclusion, according to a recent consensus of EAO (39), is possible to state that crown-to-implant not exceed 2.2 did not influence the possibility of biological complications and lead to a not significant MBL. Further research should investigate the complications of higher ratios, the implications of splinted implants on forces distribution and the role of crown height on MBL.


Acknowledgments

Funding: None


Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at http://dx.doi.org/10.21037/fomm-20-57

Conflicts of Interest: All the authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/fomm-20-57). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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doi: 10.21037/fomm-20-57
Cite this article as: Di Fiore A, Maniero F, Stellini E. The influence of crown-to-implant ratio on marginal bone loss: a narrative review. Front Oral Maxillofac Med 2020;2:29.