Fawad Javed,* Tariq Abduljabbar, Fahim Vohra, Hans Malmstrom,* Irfan Rahman, and Georgios E. Romanos i

Transcription

1 Comparison of Periodontal Parameters and Self-Perceived Oral Symptoms Among Cigarette Smokers, Individuals Vaping Electronic Cigarettes, and Never-Smokers Fawad Javed,* Tariq Abduljabbar, Fahim Vohra, Hans Malmstrom,* Irfan Rahman, and Georgios E. Romanos i Background: To the authors knowledge, there are no studies that have compared periodontal parameters and self-perceived oral symptoms (OSs) among cigarette smokers (CSs) (group 1), individuals exclusively vaping electronic cigarettes (group 2), and never-smokers (NSs) (group 3). The aim of this study is to assess periodontal parameters and self-perceived OSs among vaping individuals, CSs, and NSs. Methods: Ninety-four male participants (groups 1, 2, and 3: 33, 31, and 30 individuals, respectively) were included. Demographic data, self-perceived OSs, and duration and daily frequency of vaping and smoking were gathered using a questionnaire. Full-mouth plaque index (PI), bleeding on probing (BOP), probing depth (PD) 4 mm, and clinical attachment loss (AL) were measured; marginal bone loss (MBL) around all teeth was measured on digital radiographs. Numbers of missing teeth (MT) were also recorded. Odds ratios were calculated for OSs, and periodontal parameters were assessed using analysis of variance and Bonferroni post hoc tests. P <0.05 was considered statistically significant. Results: PI (P <0.01) and PD 4 mm(p <0.01) were significantly higher in groups 1 and 2 than in group 3. BOP was significantly higher in group 3 than in groups 1 (P <0.01) and 2 (P <0.01). There was no difference in number of MT, clinical AL, and MBL among all groups. Gingival pain was more often reported by individuals in group 1 than by individuals in groups 2 (P <0.01) and 3 (P <0.01). Conclusion: Periodontal inflammation and self-perceived OSs were poorer among CSs than among vaping individuals and NSs. J Periodontol 2017;88: KEY WORDS Dental plaque index; electronic cigarettes; periodontal index; periodontal pocket; smoking; tooth loss. * Department of General Dentistry, Eastman Institute for Oral Health, University of Rochester, Rochester, NY. Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University, Riyadh, Saudi Arabia. Department of Environmental Medicine, University of Rochester Medical Center, Rochester, NY. Department of Oral Surgery and Implant Dentistry, Dental School, Johann Wolfgang Goethe, University of Frankfurt, Frankfurt, Germany. i Department of Periodontology, School of Dental Medicine, Stony Brook University, Stony Brook, NY. doi: /jop

2 Periodontal Status, Oral Symptoms, Smoking, and E-Cig Vaping Volume 88 Number 10 Electronic cigarettes (e-cigs) are battery-operated handheld devices in which an e-liquid (containing nicotine and sometimes artificial flavors such as menthol, coffee, candy, butter, and fruits) is heated by a heating element releasing a chemical-filled aerosol for inhalation. 1 In the United States, use of e-cigs is escalating, predominantly in younger adults. 2 Results of a survey conducted in the United States nearly 3 years ago showed that nearly one of 10 never-smokers (NSs) (i.e., individuals who reported to have never used tobacco in any form) had vaped e-cigs at least once. 3 Results also showed that e-cig vaping is more common among younger adults (aged 18 to 24 years) than among older adults (aged 65 years) and is dominant among males rather than females. 3 Quitting combustible smoking is the most common reported reason for vaping e-cigs, 4 whereas some individuals report vaping e-cigs for social entertainment and amusement. 4 Individuals may misconstrue e-cig vaping as less harmful than regular tobacco smoking; recent evidence from in vitro studies 5-7 has shown that e-cig aerosol causes oxidative stress, inflammation, and dysregulation of lung cellular function by impairing myofibroblast differentiation, inducing mitochondrial stress and promoting DNA fragmentation. From an oral health perspective, vaping has been implicated in oral diseases, such as oral submucous fibrosis and periodontal diseases. 8,9 In vitro results by Sundar et al. 1 demonstrated that exposure of oral epithelial cells and periodontal ligament (PDL) fibroblasts to e-cig vapor/aerosol with flavorings increased DNA damage and enhanced expression of proinflammatory cytokines such as cycloxygenase (COX))-2 and prostaglandin E2 (PGE2) in these cells. Moreover, it has also been reported that e-cig vaping induces oxidative stress and increases expression of advanced glycation end products (AGEs) and their receptors in oral epithelial cells, including gingival tissues. 1 In this context, vaping c-cigs may be detrimental to oral health. In a recent pilot study, Wadia et al. 9 showed statistically significantly higher gingival inflammation among individuals vaping e-cigs; however, all participants in this study 9 were former cigarette smokers (CSs). Therefore, the exclusive role of e-cig vaping on periodontal status remained unclear. Moreover, to the authors knowledge, there are no clinical studies in indexed literature that have meticulously assessed self-perceived oral symptoms (OSs) and clinical (plaque index [PI], bleeding on probing [BOP], probing depth [PD], and clinical attachment loss [AL]) and radiographic (marginal bone loss [MBL]) parameters of periodontal inflammation among individuals vaping e-cigs. It is hypothesized that: 1) scores of PI, BOP, PD, clinical AL, and mesial and distal MBL are statistically significantly higher in individuals vaping e-cigs and CSs than in NSs; and 2) self-perceived OSs are poorer among individuals vaping e-cigs and CSs than among NSs. The aim of the present pilot study is to compare periodontal parameters and self-perceived OSs among CSs, individuals exclusively vaping e-cigs, and NSs. MATERIALS AND METHODS Ethical Guidelines The study was approved by the Ethics Committee, College of Dentistry, King Saud University, Riyadh, Saudi Arabia. Written informed consent was obtained from all participants before their inclusion in the study. All participants were informed that they reserved the right to leave the research study at any stage of the investigation without penalty. Inclusion and Exclusion Criteria Inclusion criteria were as follows: 1) vaping individuals: individuals without previous history of tobacco use who had been exclusively vaping e-cigs for at least 12 months; 2) CSs: individuals who were smoking at least five cigarettes daily for at least 12 months; and 3) NSs: individuals who reported to have never used any form of tobacco product. Exclusion criteria were as follows: 1) cigar, pipe, and/or waterpipe smoking; 2) dual smoking (cigarette smoking and vaping); 3) smokeless-tobacco use; 4) completely edentulous individuals; 5) individuals with systemic diseases, such as acquired immunodeficiency syndrome, cardiovascular disorders, diabetes mellitus, hepatic diseases (including hepatitis B and C), and renal diseases; 6) individuals with overlapping teeth; 7) third molars; 8) individuals who reported having undergone periodontal therapy within the last 6 months; and 9) individuals who reported having used antibiotics, non-steroidal antiinflammatory drugs, and/or steroids within the last 3 months. Participants Ninety-four male participants (33 CSs, 31 e-cig users, and 30 NSs) with mean ages of (CSs), (e-cig users), and (NSs) years were included. The study was performed between June 2016 and February All study participants were recruited from the outpatient department of the College of Dentistry, King Saud University, Riyadh, Saudi Arabia. Individuals who reported to have been smoking up to five cigarettes daily for at least 12 months were defined as CSs (group 1). 10 E-cig users were defined as individuals who were exclusively e-cig vaping at least once daily (one session) for 12 months (group 2). Participants who reported to have never used tobacco in any form were categorized as NSs (group 3). 11 A standardized 1060

3 Javed, Abduljabbar, Vohra, Malmstrom, Rahman, Romanos oral health related questionnaire was administered to the volunteering participants. The participants were invited to the outpatient department at the College of Dentistry, King Saud University, for clinical and radiographic examinations. Questionnaire Data regarding age, sex, self-perceived OSs (gingival bleeding, pain, and swelling), reason/s for smoking or vaping, and duration and daily frequency of vaping and smoking were gathered using a questionnaire. The questionnaire was administered to participants by a trained investigator (FV). Clinical Periodontal Examination Atrainedandcalibratedexaminer(TA)maskedtothe study groups performed the clinical periodontal examination. Overall k value for intraexaminer reliability was Full-mouth PI, 12 BOP, 12 clinical AL, 12 and PD 4 mm 12 were measured at six sites per tooth (mesio-buccal, mid-buccal, disto-buccal, disto-lingual/ palatal, mid-lingual/palatal, and mesio-lingual/palatal) on all maxillary and mandibular teeth. PD was measured to the nearest millimeter using a graded probe. Number of missing teeth (MT) was also counted. Fractured teeth with embedded root remnants were considered missing. MBL Radiographs were taken by a trained and calibrated investigator (FV) who was masked to the study groups. Overall k for intraexaminer reliability was 0.9. Fullmouth digital intraoral radiographs (four bitewings of posterior teeth, five periapical radiographs in maxillary anterior and posterior teeth, and three periapical radiographs in mandibular anterior and posterior teeth) were taken using a digital radiographic machine. # The long cone paralleling technique was used to standardize angulations of the radiographs. 13,14 All radiographs were viewed on a calibrated computer screen** using a software program. MBL was defined as the vertical distance from 2 mm below the cemento-enamel junction (CEJ) to the most crestally positioned alveolar bone. 15 Tooth surfaces where the CEJ and/or alveolar crest were not visible due to technical reasons (such as dental restorations, interdental caries, overlapping teeth, and/or poor radiographic quality) were excluded. Statistical Analyses Statistical analyses were performed using a software program. One-way analysis of variance (ANOVA) was used to determine whether the dependent variables (PI, BOP, PD 4 mm, clinical AL, and MBL) were related in a statistically significant manner with the independent variables. The Bonferroni adjustment post hoc test was performed for multiple comparisons. Odds ratios (ORs) for self-perceived OSs were also calculated with 95% confidence intervals (CIs). Power analysis was performed using a software package. Sample size estimation was based on the supposition that a mean difference of 0.5 and 1 mm in MBL and PD, respectively, should be detected at a significance level of It was estimated that inclusion of at least 30 individuals per group was required to attain a study power of 90%. P <0.05 was considered statistically significant. RESULTS Characteristics of Study Cohort Mean duration and daily frequency of cigarette smoking and vaping among individuals in groups 1 and 2 were and years and and times, respectively. In groups 1, 2, and 3, 68.5%, 60.6%, and 23.3% individuals, respectively, reported a family history of tobacco smoking (Table 1). Reasons for Smoking and E-Cig Vaping In group 1, 60.6% individuals (n = 20) reported that they smoked cigarettes daily to alleviate psychologic stress. Nine individuals (27.3%) stated that cigarette smoking helped them concentrate on work, whereas 12.1% (n = 4) did not provide any reason for smoking cigarettes. In group 2, all individuals reported that that they vaped e-cigs using flavored e-fluid to enjoy smoking without jeopardizing their health. Clinical Periodontal Parameters and MBL Percentages of sites with dental plaque (P <0.01) and PD 4mm(P <0.01) were statistically significantly higher in group 1 than in groups 2 and 3. BOP was statistically significantly higher in group 3 than in groups 1 (P <0.01) and 2 (P <0.01). There was no statistically significant difference in number of MT, clinical AL, and mesial and distal MBL among individuals in all groups (Table 2). Self-Perceived OSs There was statistically significant difference in OR (95% CI) for self-perceived gingival pain among individuals in group 1 compared with those in groups 2 (21.4 [19.6 to 25.5]) (P <0.01) and 3 (11.5 [9.6 to 13.4]) (P <0.01). There was statistically significant difference in OR (95% CI) for self-perceived gingival swelling among individuals in group 1 compared with individuals in groups 2 (5.2 [4.7 to 5.6]) (P <0.01) and 3 (4.2 [3.8 to 4.4]) (P <0.01). Self-perceived gingival bleeding was more often reported by individuals in group 3 than by those in groups 1 (1.8 [1.4 to 1.9]) (P <0.01) and 2 (3.1 [2.7 to 3.3]) (P <0.01). There was no difference in self-perceived gingival bleeding Hu-Friedy, Chicago, IL. # NOMAD Pro 2 Intraoral X-Ray Systems, Gendex, Hatfield, PA. ** Samsung SyncMaster digital TV monitor, Samsung, Seoul, Korea. VixWin Platinum, Gendex. SPSS version 18, SPSS, Chicago, IL. nquery Advisor 5.0, Statistical Solutions, Saugus, MA. 1061

4 Periodontal Status, Oral Symptoms, Smoking, and E-Cig Vaping Volume 88 Number 10 Table 1. General Characteristics of Study Cohort Parameters Group 1 Group 2 Group 3 n Sex (males) Mean age SD (in years) Duration of habit (in years) Daily frequency of habit Family history of smoking (%) = not applicable. between individuals in groups 1 and 2. There was no difference in gingival pain and swelling between individuals in groups 2 and 3 (Table 3). DISCUSSION It has been reported that vaping individuals are usually either former CSs or dual-smokers. 16,17 Therefore, in the present study, e-cig vaping individuals were strictly defined as individuals who reported to have never used tobacco in any form and were exclusively vaping e-cigs for at least 1 year. Similarly, NSs were also defined as individuals who reported to have never used tobacco in any form. Results of this study are in partial agreement withtheproposedhypothesesasscoresofpiandpd 4 mm were significantly higher among individuals in group 1 than among individuals in groups 2 and 3. Previous studies have also confirmed that CSs exhibit significantly greater number of sites with plaque accumulation, clinical AL, and PD 4mmcompared with NSs. One explanation in this context is that tobacco smoking is associated with increased expression of AGEs and their receptors in gingival tissues, which exacerbate oxidative stress and inflammatory responses. 21 BOP (a classic marker of periodontal disease severity 22 ) was masked among individuals in groups 1 and 2 compared with those in group 3. This may be attributed to the presence in cigarette smoke and e-cig liquid of nicotine, which has a vasoconstrictive effect on gingival blood vessels. 23 Interestingly, there was no statistically significant difference in number of sites with PI, PD 4 mm, clinical AL, and self-perceived gingival pain and swelling among individuals in groups 2 and 3 (Tables 2 and 3). Therefore, it may be argued that e-cig vaping is not as hazardous to periodontal health as cigarette smoking, and vaping individuals may have periodontal status comparable to NSs. However, it is pertinent to mention that a number of factors may have influenced the present results. First, Table 2. Periodontal Parameters Among CSs, Vaping Individuals, and NSs Parameters (mean SD) Group 1 (n = 33) Group 2 (n = 31) Group 3 (n = 30) Number of MT PI (%) * BOP (%) * * PD ( 4 mm) (%) * Clinical AL (in mm) MBL (in mm) Mesial Distal One-way ANOVA and Bonferroni post hoc tests were performed for intergroup comparisons. * Significant difference compared with group 3 (P <0.01). Significant difference compared with group 2 (P <0.01). 1062

5 Javed, Abduljabbar, Vohra, Malmstrom, Rahman, Romanos Table 3. Group Comparisons for Self-Perceived OSs Among CSs, Vaping Individuals, and NSs Yes (n) No (n) OR (95% CI) P Value Gingival pain Group 1 versus group 2 Group 1 (n = 33) (19.6 to 25.5) <0.01 Group 2 (n = 31) 3 28 Group 1 versus group 3 Group 1 (n = 33) (9.6 to 13.4) <0.01 Group 3 (n = 30) 5 25 Group 2 versus group 3 Group 2 (n = 31) (0.3 to 0.8) >0.05 Group 3 (n = 30) 5 25 Gingival bleeding Group 1 versus group 2 Group 1 (n = 33) (0.4 to 0.8) >0.05 Group 2 (n = 31) Group 1 versus group 3 Group 1 (n = 33) (1.4 to 1.9) <0.01 Group 3 (n = 30) 7 23 Group 2 versus group 3 Group 2 (n = 31) (2.7 to 3.3) <0.01 Group 3 (n = 30) 7 23 Gingival swelling Group 1 versus group 2 Group 1 (n = 33) (4.7 to 5.6) <0.01 Group 2 (n = 31) 6 25 Group 1 versus group 3 Group 1 (n = 33) (3.8 to 4.4) <0.01 Group 3 (n = 30) 8 22 Group 2 versus group 3 Group 2 (n = 31) (0.5 to 0.8) >0.05 Group 3 (n = 30) 8 22 the daily frequency of nicotine inhalation was nearly two times higher among individuals in group 1 compared with those in group 2. Next, duration of vaping habit in group 2 was relatively short (2 years) compared with group 1. Furthermore, participants in groups 1, 2, and 3 were relatively young (aged 40 years). It is known that there is a doseand duration-dependent relationship between smoking and periodontal disease. 24 Therefore, it is hypothesized that individuals with a longer history (>5 years) and higher daily frequency of vaping (>15 times daily) are more susceptible to periodontal inflammation compared with individuals with a shorter history (<5 years) and lower frequency of e-cig vaping (<10 times daily). Further well-designed long-term clinical trials/ studies with a larger population (wide range of age groups) are needed to test this hypothesis. The present results showed no statistically significant difference in mesial and distal MBL among all study groups. It has been reported that MBL is significantly higher among older individuals (>65 years) than among younger individuals ( 45 years). 11 As participants in all groups were relatively young, significant difference in MBL between groups could not be established. However, it is emphasized that by no means should e-cig vaping be considered nonhazardous to periodontal health as outcomes of the present study were based on results from a pilot investigation in an exclusive healthy population of e- cig users. Therefore, the reported results should be interpreted with extreme caution. Questionnaires are considered valid tools for assessing oral health. 25,26 Outcomes of the present study support other studies, 25,26 as clinical assessment of periodontal parameters such as BOP was in agreement with self-perceived gingival bleeding. However, the manner in which a questionnaire is administered may influence the response of an individual. In the present study, all vaping individuals perceived that their health was not at risk compared 1063

6 Periodontal Status, Oral Symptoms, Smoking, and E-Cig Vaping Volume 88 Number 10 with smoking cigarettes. This result is in accordance with a recent study 27 in which vaping individuals considered e-cigs as non-hazardous and more beneficial than smoking cigarettes. From a clinical perspective, there are no studies that have shown vaping e-cigs to have detrimental effects on periodontal health status; however, a recent in vitro study 1 showed that exposure of human PDL fibroblast and gingival epithelial cells to e-cig vapor with flavorings increased oxidative stress and DNA damage and enhanced release of proinflammatory cytokines (PGE2 and COX- 2). In this regard, it is premature to label vaping as safe. There are a number of limitations associated with the present study. First, as reported earlier, the present results were based on a pilot investigation. A large number of participants could not be recruited for the present study as it was exigent to identify individuals who were exclusively vaping e-cigs and had never consumed tobacco in any form in the past. Moreover, individuals in group 2 had a relatively short history of vaping (2 years). Further research in a larger cohort with different sexes and a longer history of vaping (at least5years)isneededtoassesstheroleofchronic vaping and its flavorings on oral and systemic health in healthy individuals and current and former smokers. It is also emphasized that community health awareness programs should routinely be conducted to educate the public about the detrimental effects of habitual tobacco use on overall health (including periodontal status). CONCLUSION Within the limitations of the present study, it is concluded that periodontal inflammation and selfperceived OSs are exacerbated in CSs compared with vaping individuals and NSs. ACKNOWLEDGMENTS The authors extend their appreciation to the Deanship of Scientific Research at King Saud University, Riyadh, Saudi Arabia, for funding this work through Research Group Grant RG The authors report no conflicts of interest related to this study. REFERENCES 1. Sundar IK, Javed F, Romanos GE, Rahman I. E-cigarettes and flavorings induce inflammatory and pro-senescence responses in oral epithelial cells and periodontal fibroblasts. Oncotarget 2016;7: Delnevo CD, Giovenco DP, Steinberg MB, et al. Patterns of electronic cigarette use among adults in the United States. Nicotine Tob Res 2016;18: Schoenborn CA, Gindi RM. Electronic cigarette use among adults: United States, NCHS Data Brief 2015;(217): Ayers JW, Leas EC, Allem JP, et al. Why do people use electronic nicotine delivery systems (electronic cigarettes)? A content analysis of Twitter, PLoS One 2017;12:e Lei W, Lerner C, Sundar IK, Rahman I. Myofibroblast differentiation and its functional properties are inhibited by nicotine and e-cigarette via mitochondrial OXPHOS complex III. Sci Rep 2017;7: Lerner CA, Rutagarama P, Ahmad T, Sundar IK, Elder A, Rahman I. Electronic cigarette aerosols and copper nanoparticles induce mitochondrial stress and promote DNA fragmentation in lung fibroblasts. Biochem Biophys Res Commun 2016;477: Lerner CA, Sundar IK, Watson RM, et al. Environmental health hazards of e-cigarettes and their components: Oxidants and copper in e-cigarette aerosols. Environ Pollut 2015;198: Javed F, Kellesarian SV, Sundar IK, Romanos GE, Rahman I. Recent updates on electronic cigarette aerosol and inhaled nicotine effects on periodontal and pulmonary tissues [published online ahead of print February 6, 2017]. Oral Dis. doi: /odi Wadia R, Booth V, Yap HF, Moyes DL. A pilot study of the gingival response when smokers switch from smoking to vaping. Br Dent J 2016;221: Bjartveit K, Tverdal A. Health consequences of smoking 1-4 cigarettes per day. Tob Control 2005;14: Javed F, Näsström K, Benchimol D, Altamash M, Klinge B, Engström PE. Comparison of periodontal and socioeconomic status between subjects with type 2 diabetes mellitus and non-diabetic controls. J Periodontol 2007;78: Javed F, Al-Kheraif AA, Rahman I, et al. Comparison of clinical and radiographic periodontal status between habitual water-pipe smokers and cigarette smokers. J Periodontol 2016;87: Khocht A, Janal M, Harasty L, Chang KM. Comparison of direct digital and conventional intraoral radiographs in detecting alveolar bone loss. J Am Dent Assoc 2003; 134: Updegrave WJ. The paralleling extension-cone technique in intraoral dental radiography. Oral Surg Oral Med Oral Pathol 1951;4: Javed F, Al-Kheraif AA, Al Amri MD, et al. Periodontal parameters and whole salivary cytokine profiles among habitual gutka chewers and non-chewers. J Periodontol 2015;86: McCabe SE, Veliz P, McCabe VV, Boyd CJ. Smoking behaviors and intentions among current e-cigarette users, cigarette smokers, and dual users: A national survey of U.S. high school seniors. Prev Med 2017;99: Farsalinos KE, Romagna G, Tsiapras D, Kyrzopoulos S, Spyrou A, Voudris V. Impact of flavour variability on electronic cigarette use experience: An internet survey. Int J Environ Res Public Health 2013;10: Gupta N, Gupta ND, Goyal L, et al. The influence of smoking on the levels of matrix metalloproteinase-8 and periodontal parameters in smoker and nonsmoker patients with chronic periodontitis: A clinicobiochemical study. J Oral Biol Craniofac Res 2016;6(Suppl. 1): S39-S Azizi A, Sarlati F, Bidi M, Mansouri L, Azaminejad SM, Rakhshan V. Effects of smoking severity and moderate and severe periodontitis on serum C-reactive protein levels: An age- and gender-matched retrospective cohort study. Biomarkers 2015;20: Javed F, Al-Kheraif AA, Salazar-Lazo K, et al. Periodontal inflammatory conditions among smokers and 1064

7 Javed, Abduljabbar, Vohra, Malmstrom, Rahman, Romanos never-smokers with and without type 2 diabetes mellitus. J Periodontol 2015;86: Biswas SK, Mudi SR, Mollah FH, Bierhaus A, Arslan MI. Serum soluble receptor for advanced glycation end products (srage) is independently associated with cigarette smoking in non-diabetic healthy subjects. Diab Vasc Dis Res 2013;10: Gutweniger CA. Gingival bleeding as an indication of periodontal disease. J Oreg Dent Assoc 1983;52: Clarke NG, Shephard BC. The effects of epinephrine and nicotine on gingival blood flow in the rabbit. Arch Oral Biol 1984;29: Torrungruang K, Nisapakultorn K, Sutdhibhisal S, et al. The effect of cigarette smoking on the severity of periodontal disease among older Thai adults. J Periodontol 2005;76: Heaton B, Gordon NB, Garcia RI, et al. A clinical validation of self-reported periodontitis among participants in the black women s health study. J Periodontol 2017;88: doi: /jop : Buhlin K, Gustafsson A, Andersson K, Håkansson J, Klinge B. Validity and limitations of self-reported periodontal health. Community Dent Oral Epidemiol 2002; 30: Hershberger AR, Karyadi KA, VanderVeen JD, Cyders MA. Beliefs about the direct comparison of e-cigarettes and cigarettes. Subst Use Misuse 2017;52: Correspondence: Dr. Fawad Javed, Department of General Dentistry, Eastman Institute for Oral Health, University of Rochester, Rochester, NY fawad_javed@urmc. rochester.edu. Submitted March 22, 2017; accepted for publication May 17,