Biomechanics/risk management (Working Group 2)

Application of Ultrashort Implants in Posterior Maxilla With Insufficient Bone Height: A Finite Element Analysis

INTRODUCTION AND AIMS

Implantation of the posterior maxilla with insufficient bone height faces challenges. Studies have shown that the use of ultrashort implants can avoid additional damage. This finite element analysis study aimed to evaluate the impacts of different lengths of ultrashort implants and three surgical approaches on stress, strain, and displacement in the posterior maxilla with varying bone heights.

METHODS

Twelve models of different lengths (3.0, 4.0, 5.0, and 6.0 mm) of ultrashort implants combined with unicortical fixation, bicortical fixation, and transalveolar sinus elevation were established, and conventional implants and short implants were considered as control models. Von Mises stresses within the implants and the sinus floor cortical bone, maximum and minimum principal stresses within the alveolar ridge cortical bone, and the maximum principal strain of the cancellous bone were determined using these models. Additionally, the displacement of the implants was analysed.

RESULTS

Stress distribution range and peak values increased as implant length decreased. In the ultrashort implant group, H5L6 exhibited the smallest maximum and minimum principal stresses, 35.77 and 10.66 MPa, respectively. Among groups with different bone heights, 6 mm long implants presented the lowest maximum von Mises stress, whereas 3 mm long implants presented the highest.

CONCLUSION

In the posterior maxilla with bone heights of 3, 4, and 5 mm, the stresses in different lengths of ultrashort implants and surrounding bone tissue were lower than the yield strengths in these areas, and the use of 6 mm-long ultrashort implants combined with osteotome sinus floor elevation can achieve lower stress distribution.

CLINICAL RELEVANCE

This study provides important insights into the biomechanical properties of ultrashort implants combined with three different surgical procedures in severely atrophic maxilla. The 6-mm long implant combined with osteotome sinus floor elevation is most suitable for this area.

Effect of crown-to-implant ratio and crown height space on marginal bone stress: a finite element analysis

BACKGROUND

Crown-to-implant ratio and crown height space, associated with the use of short implants, have been related with marginal bone loss. However, it is unclear which of the two entities would play the most important role on the bone remodelling process. Using a finite element analysis, the present work aims to help clarifying how those two factors contribute for the stress generation at the marginal bone level. A numerical model (reference model), with a crown-to-implant ratio of 4, was double validated and submitted to a numerical calculation. Then, it was modified in two different ways: (a) by decreasing the prosthetic height obtaining crown-to-implant ratios of 3, 2.5 and 2 and (b) by increasing the implants length obtaining a crown-to-implant ratio of 2.08. The new models were also submitted to numerical calculations.

RESULTS

The reference model showed a marginal bone stress of 96.9 MPa. The increase in the implants' length did not show statistically significant differences in the marginal bone stress (p-value = 0.2364). The decrease in the prosthetic height was accompanied with a statistically significant decrease in the marginal bone stresses (p-value = 2.2e- 16).

CONCLUSIONS

The results represent a paradigm change as the crown height space appears to be more responsible for marginal bone stress than the high crown-to-implant ratios or the implants' length. New prosthetic designs should be attempted to decrease the stress generated at the marginal bone level.

The ability to screw-retain single implant-supported restorations in the anterior maxilla: A CBCT analysis

STATEMENT OF PROBLEM

Evidence to validate the routine use of angled screw-channel abutments in the anterior maxilla is sparse. If properly planned, they might provide surgical and prosthetic benefits.

PURPOSE

The purpose of this observational study was to determine the prevalence of digitally placed implants in the anterior maxilla that would allow screw-retained implant-supported restorations with either a straight or an angled screw-channel abutment.

MATERIAL AND METHODS

Two hundred cone beam computed tomography (CBCT) scans met the inclusion criteria for retrospective analysis and digital implant planning. Virtual implants were planned for randomly selected anterior maxillary teeth by using the anatomic crown and root position. Virtual abutments of varying angulation were attached to the implants to determine the ability to screw retain a restoration with either a straight or an angled screw-channel abutment.

RESULTS

One hundred fifty-two (76%) sites required an angled screw-channel abutment to enable screw retention. Forty-eight (24%) sites allowed screw retention with a straight abutment. The percentage of implants requiring angled or straight abutments varied significantly among anterior teeth (P<.005). One hundred nine (71.7%) angled screw-channel abutment sites required a 5-degree abutment, 41 (26.9%) required a 10-degree abutment, and 2 (1.3%) required a 15-degree abutment. Among the anterior teeth, lateral incisors presented a greater need for angled screw-channel abutments. None of the implants in the present study needed cement-retained restorations.

CONCLUSIONS

Angled abutments allowed for screw-retained restorations on digitally planned implants in the anterior maxilla. The required angular correction to a screw-retained restoration was ≤15 degrees. Screw-retained restorations were frequently achievable (76%) with the use of angled screw-channel abutments or with straight abutments (24%), and lateral incisors presented a greater need for angled screw-channel abutments.

Narrative review regarding the applicability, accuracy, and clinical outcome of flapless implant surgery with or without computer guidance

Background

The advent of computer‐guided surgery removed the need for complex surgical interventions such as extensive flap elevations, second stage implant exposure, and complications usually associated with conventional protocols.

Purpose

(a) Analyze available literature reporting on applicability, accuracy, clinical outcome of flapless surgery with or without computer guidance. (b) Evaluate quality of studies, in terms of scientific level of evidence and ethical committee approval.

Materials and methods

A PUBMED search was performed in July 2018. A first search was based on a general search string limited to “Dental Implants” and “flapless surgery.” A second search focused on accuracy of computer‐guided surgery using search string “Surgery, Computer‐Assisted” or “guided surgery,” and “Dental implants.” The following inclusion criteria were applied: (a) studies in English; (b) human studies (excluding cadaver); (c) systematic reviews; (d) systematic reviews with meta‐analysis. Reviews not mentioning accuracy were excluded in search 2.

Results

Nine reviews included in total. Implant survival ranged between 89% and 100%. Early surgical and prosthetic complications reported in 9.1% to 36.4% of reviewed papers. Tooth‐supported guides show more accuracy than bone or mucosa‐supported guides. Fully guided surgery yields higher accuracy, with lower values for horizontal coronal, horizontal apical and angular deviation (1.00, 1.23, and 3.13°mm, respectively) than those placed with half guided surgery (1.44, 1.91, and 4.30 mm, respectively). Thirty‐four of 71 human studies included in nine reviews, mentioned ethical committee approval or compliance with Declaration of Helsinki.

Conclusions

Guided flapless surgery is comparable to free‐hand surgery in terms of implant survival, marginal bone remodeling, and peri‐implant variables. Clinicians advised to take care in all steps of the protocol, and include safety margins around virtually planned implants. Regarding compliance with research ethics, we should question whether scientific reports of clinical trials performed without an ethical umbrella are trustworthy. Compliance of ethics standards is imperative for submitted research papers.

A Finite Element Analysis to Compare Stress Distribution on Extra-Short Implants with Two Different Internal Connections

Background: The goal of this study was to analyze the stress distribution on two types of extra-short dental implants with 5 mm of length: An internal hexagon (IH) and morse taper connection (MT). Methods: The three-dimensional model was composed of trabecular and cortical bone, a crown, an extra-short dental implant and their components. An axial load of 150 N was applied and another inclined 30° with the same magnitude. Results: Stress concentrations on the IH implant are observed in the region of the first threads for the screw. However, in the MT implant the highest stress occurs at the edges of the upper implant platform. Conclusions: In view of the results obtained in this study the two types of prosthetic fittings present a good stress distribution. The Morse taper connections presented better behavior than the internal in both loading configurations.

Technical and biological complications related to crown to implant ratio: a systematic review

AIM

To review the occurrence of prosthetic failure and biological complications with respect to the crown to implant (C/I) ratio.

METHODS

Accurate search was made on the subject C/I ratio with the following criteria: (1) studies on humans with data on prosthetic failure and/or biological complications related to C/I ratio; (2) partial edentulous patients; (3) randomized clinical trials, prospective, longitudinal, retrospective, and multicenter studies with a minimum of 48 months mean follow-up; (4) language: English; (5) radiographic measurements by peri-apical x-ray; (6) implant material: titanium; and (7) no implant type selection was applied.

RESULTS

Six articles were considered eligible for full-text analysis. Unfavorable C/I ratio can be considered a potential risk factor for single crown and abutment loosening (C/I ratio ≥ 1.46) and abutment fractures in posterior areas (C/I ratio ≥ 2.01).

CONCLUSIONS

Despite the limited data, high C/I ratio may be related to some prosthetic failures. Unfavorable C/I ratio does not affect biological complications and implant failure.

Bisphosphonates, vitamin D, parathyroid hormone, and osteonecrosis of the jaw. Could there be a missing link?

It is estimated that over 190 million bisphosphonates have been prescribed worldwide. But this drug can produce adverse effects, of which osteonecrosis of the jaw and severe hypocalcemia are the most serious. It is evident that bisphosphonate administration affects multiple and diverse biochemical mediators related to bone metabolism. This review of literature investigates four basic parameters in patients treated with bisphosphonates - parathyroid hormone (PTH), bisphosphonates, vitamin D, calcium, and jaw osteonecrosis - which are fundamental for assessing bone metabolism and so the efficacy and correct use of the drug. The imbalances generated by vitamin D and calcium deficiencies, together with their multiple systemic repercussions, have been widely researched but the outcomes of these imbalances in relation to bisphosphonate administration are not well known, and some research has indicated that they may be associated with osteonecrosis of the jaw (ONJ). The present review set out to explain the functioning of bone metabolism, the importance of different chemical mediators, the imbalances produced by incorrect use of this drug, in order to forewarn against the possible relation of these parameters with ONJ, whose physiopathology remains unknown. Medical and dental clinics should keep detailed anamneses of the use of vitamin D and calcium supplements, as it is of vital importance to maintain their correct levels in blood, given that these are related to ONJ as well as other adverse effects; this procedure is also necessary in order to ensure the correct use of the drug.

Key words:Bisphosphonate-related osteonecrosis of the jaw, vitamin D, parathyroid hor

Clinical advantages of computer-guided implant placement: a systematic review

OBJECTIVES

To systematically scrutinize the current scientific literature regarding the clinical advantages of computer guidance of implant placement.

MATERIALS AND METHODS

Four electronic databases were searched using specified indexing terms. The reference lists of publications were also searched manually. For inclusion, publications had to meet pre-established criteria.

RESULTS

The searches yielded 1028 titles and abstracts. After data extraction and interpretation, 28 publications and 2 systematic reviews remained for inclusion. Fifteen studies were prospective observational and four were retrospective observational. Nine studies included a control group (controlled clinical trials) of which seven were prospective and two retrospective. Only three of the prospective studies were randomized (RCT's). A total of 852 patients were treated with 4032 implants using computer-guided implant surgery. The number of patients included in each study ranged from 6 to 206. The age ranged from 16 to 92 years and the follow-up period varied between 1 and 49 months.

CONCLUSIONS

The limited scientific evidence available suggests that guided placement has at least as good implant survival as conventional protocols. However, several unexpected procedure-linked adverse events during guided implant placement indicate that the clinical demands on the surgeon were no less than those during conventional placement. A clinical advantage with flapless guided surgery is that the technique is likely to decrease pain and discomfort in the immediate postoperative period.

The effects of loading on the preload and dimensions of the abutment screw for a 3-unit cantilever-fixed prosthesis design

OBJECTIVE

The purpose of this study was to use an in vitro model system to compare the effects on the screw torque and screw dimensions within 2 commercially available implant systems from occlusal loading on a cantilevered-fixed partial denture.

MATERIALS AND METHODS

Cantilevered implant-supported 3-unit prostheses with 2 premolar abutments and 1 premolar pontic (7.3 mm in length) were made on resin casts containing 2 implant analogs for 2 implant systems: BioLok Silhouette Tapered Implant System (Birmingham, AL) and Zimmer Tapered Screw-Vent Implant System (Carlsbad, CA) with 10 samples in each group. Each sample was loaded with either of 2 protocols: (1) a load of 50 N on the cantilevered pontic unit and (2) a loading of 150 N on all 3 units. The outcome measures were (1) changes in residual torque of the abutment screws and (2) changes in screw dimension.

RESULTS

The BioLok Silhouette Tapered Implant group demonstrated slight but statistically significant torque loss 18.8% to 28.5% in both abutment screws for both protocols, P ≤ 0.05, without any changes in screw dimension. In the Zimmer Tapered Screw-Vent Implant group, there was a significant elongation of the abutment screws and a markedly significant 44.4%, (P ≤ 0.01) loss in torque in the mesial screw and a 28.5%, (P ≤ 0.05) loss in torque in the distal screw when the cantilever alone was loaded.

CONCLUSIONS

Differences in screw design influence the maintenance of preload and distortion of the shank. The influence of the interface design, namely an internal hex of 1 mm versus an external hex did not influence the preload. Cantilevered prostheses can cause loss of torque and dimensional changes in abutment screws.

Virtual implant planning in the edentulous maxilla: criteria for decision making of prosthesis design

OBJECTIVES

To evaluate prosthetic parameters in the edentulous anterior maxilla for decision making between fixed and removable implant prosthesis using virtual planning software.

MATERIAL AND METHODS

CT- or DVT-scans of 43 patients (mean age 62 ± 8 years) with an edentulous maxilla were analyzed with the NobelGuide software. Implants (≥3.5 mm diameter, ≥10 mm length) were virtually placed in the optimal three-dimensional prosthetic position of all maxillary front teeth. Anatomical and prosthetic landmarks, including the cervical crown point (C-Point), the acrylic flange border (F-Point), and the implant-platform buccal-end (I-Point) were defined in each middle section to determine four measuring parameters: (1) acrylic flange height (FLHeight), (2) mucosal coverage (MucCov), (3) crown-Implant distance (CID) and (4) buccal prosthesis profile (ProsthProfile). Based on these parameters, all patients were assigned to one of three classes: (A) MucCov ≤ 0 mm and ProsthProfile≥45(0) allowing for fixed prosthesis, (B) MucCov = 0-5 mm and/or ProsthProfile = 30(0) -45(0) probably allowing for fixed prosthesis, and (C) MucCov ≥ 5 mm and/or ProsthProfile ≤ 30(0) where removable prosthesis is favorable. Statistical analyses included descriptive methods and non-parametric tests.

RESULTS

Mean values were for FLHeight 10.0 mm, MucCov 5.6 mm, CID 7.4 mm, and ProsthProfile 39.1(0) . Seventy percent of patients fulfilled class C criteria (removable), 21% class B (probably fixed), and 2% class A (fixed), while in 7% (three patients) bone volume was insufficient for implant planning.

CONCLUSIONS

The proposed classification and virtual planning procedure simplify the decision-making process regarding type of prosthesis and increase predictability of esthetic treatment outcomes. It was demonstrated that in the majority of cases, the space between the prosthetic crown and implant platform had to be filled with prosthetic materials.