The aim of this systematic review was to investigate the influence of the presence and position of mandibular third molars in mandibular condyle fractures. An electronic search was conducted in PubMed, Scopus, Web of Science, Cochrane Library, and VHL, through January 2016. The eligibility criteria included observational studies. The search strategy resulted in 704 articles. Following the selection process, 13 studies were included in the systematic review and 11 in the meta-analysis. In terms of the risk of bias analysis, six studies presented ≤6 stars in the Newcastle–Ottawa scale assessment. The presence of a mandibular third molar decreased the probability of condylar fracture (cross-sectional and case–control studies: odds ratio (OR) 0.26, 95% confidence interval (CI) 0.17–0.40, I 2 = 87.8%; case–control studies: OR 0.30, 95% CI 0.16–0.58, I 2 = 91.6%). The third molar positions most favourable to condylar fracture according to the Pell and Gregory classification are class A (OR 1.32, 95% CI 1.09–1.61, I 2 = 0%) and class I (OR 1.37, 95% CI 1.05–1.77, I 2 = 32.8%). Class B (OR 0.69, 95% CI 0.49–0.97, I 2 = 56.0%) and class II (OR 0.71, 95% CI 0.57–0.87, I 2 = 0%) act as protective factors for condylar fracture. The results suggest that the presence of a mandibular third molar decreases the chance of condylar fracture and that the positions of the third molar most favourable for condylar fracture are classes A and I, with classes B and II acting as protective factors.
Mandibular fractures are common injuries and their prevalence is determined by factors such as sex, age, and socio-economic status . Among the mandibular fractures, condylar fractures are the most common, ranging from 29% to 56% . This high incidence of condylar fracture is attributed to the association of biomechanical factors such as bone density and anatomical structures that create weak areas . The join between the ramus of the mandible, which has a high rigidity, and the condyle, which has a lower rigidity, is responsible for indirectly transmitting impact forces and directing them to the condyle .
In addition to these factors, several authors have stated that the presence and position of the third molar are factors influencing the location at which mandibular fractures occur . It has been indicated that the presence of a mandibular third molar creates a weak area in the angle, which favours the incidence of fracture in this region . For this reason, the previously reported systematic review was performed to evaluate the influence of mandibular third molars on angle fractures . The results of the meta-analysis showed that the presence of a third molar increases the chance of fracture in the mandible angle by a factor of 3.27.
As well as their influence on angle fractures, epidemiological studies suggest that the absence of a third molar does not result in brittleness in the angle and would be responsible for transmitting the impact force to the condyle, thereby increasing the incidence of condylar fracture . In a study using finite element methods to assess the influence of the third molar on condylar fractures, it was demonstrated that when a third molar is present, the stress forces increase slightly in the angle region and simultaneously decrease in the condyle . In the model without a third molar, the stress forces were smaller in the angle region and increased in the condyle region. Moreover, it was possible to predict which region would have the largest stress according to the presence or absence of a third molar and the region of impact.
Some studies have even suggested the prophylactic removal of third molars in contact sport athletes, due to their influence in angle fractures . However, several other studies have shown that the incidence of condylar fracture is higher when the third molar is absent. Furthermore, condylar fractures are more difficult to treat because of issues with accessibility and the possibility of complications .
Therefore, the aim of this systematic review was to evaluate the scientific evidence regarding the influence of the presence and position of mandibular third molars in condylar fractures.
Materials and methods
This systematic review and meta-analysis was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The PECO process was used to determine the clinical question (patient, problem, or population; exposure; comparison; outcomes). The population comprised patients with mandibular fractures; the exposure was the presence of the third molar and its different positions; the comparison was with other mandibular fractures; the outcome was a mandibular condyle fracture.
In terms of the study design, the inclusion criteria encompassed cross-sectional, case–control, and prospective or retrospective cohort studies that evaluated the influence of the presence of mandibular third molars on condyle fracture.
The exclusion criteria were: (1) case reports, case series, opinion articles, and review articles; (2) studies that reported mandibular fracture during the extraction of third molars; (3) studies that evaluated bad split fractures in orthognathic surgery due to the presence of the third molar; (4) studies including pathological mandibular fractures due to the presence of lesions associated with the presence of the third molar; (5) studies that included patients with diseases of bone metabolism (osteopenia and osteoporosis).
The electronic search was conducted in the PubMed, Scopus, Web of Science, Cochrane Library, and VHL (Virtual Health Library; BIREME (PAHO/WHO)) databases, and included publications through January 2016, without language restriction. A combination of medical subject heading (MeSH) terms was used for the search. The terms used in the databases were: (mandible fracture OR mandibular fracture) AND (third molar OR wisdom tooth OR wisdom teeth).
After searching the databases, the titles and abstracts of articles were read by two authors independently (ACVA and SGMF). Studies that could potentially meet the inclusion criteria for the review were identified at this stage. After independent reading, the authors compared the results of the studies that would pass to the next stage (article read in full), reaching an agreement on their inclusion or not. In the case of disagreement, a third author (ELG) was consulted to obtain a consensus. The studies selected after reading the title and summary were read in full. At this point, it was determined whether or not the study should be included in the systematic review.
The assessment of the quality of the studies was performed using the Newcastle–Ottawa scale (NOS) for case–control studies and a modified NOS for cross-sectional studies . The case–control studies were assessed for the following three components: selection, comparability, and exposure. The cross-sectional studies were assessed for the following three components: selection, comparability, and outcome. The NOS for both study types had a maximum possible score of 9 stars/points for each study.
The data extraction was performed in two stages. In the first stage, the following data were extracted: author, year of publication, study design, country where the research was performed, sample (male, female, and total), type of analysis (patients and fractures), mean age, and main cause of mandibular fracture ( Table 1 ). In the second stage, the following data were extracted: mandibular condyle fracture (present or absent), third molar (present or absent) ( Table 2 ), Pell and Gregory classification when a third molar was present, for the occlusal plane (A, B, C) and for the mandibular ramus (I, II, III), and the Winter classification (vertical V, mesioangular MA, horizontal H, distoangular DA) ( Table 3 ).
|Author||Year||Study design||Country||Sample||Analysis type||Age, years||Main cause of fracture|
|Choi et al.||2011||CS||South Korea||333||52||385||Patients||NR||Violence|
|Mah et al.||2015||CS||South Korea||348||92||440||Patients||NR||Violence|
|Duan and Zhang||2008||CS||China||563||137||700||Patients||NR||Traffic accident|
|Gaddipati et al.||2014||CS||India||95||15||110||Fractures||18–55||Traffic accident|
|Iida et al.||2004||CC||Japan||250||96||346||Patients and fractures||NR||Violence|
|Inaoka et al.||2009||CS||Brazil||38||5||43||Patients||26.35||NR|
|Oh et al.||2006||CC||South Korea||82||23||105||Fractures||35.8 ± 14.4 M||Traffic accident|
|36.9 ± 15.3 F|
|Revanth Kumar et al.||2015||CS||India||56||8||64||Patients and fractures||16–69||Traffic accident|
|Naghipur et al.||2014||CC||Canada||377||69||446||Patients||29.3 ± 11.3||Violence|
|Patil||2012||CS||India||164||26||190||Patients||21–30 (46%)||Traffic accident|
|Zhu et al.||2005||CC||South Korea||359||80||439||Fractures||28.5||Violence|
|Thangavelu et al.||2010||CC||India||345||115||460||Patients||31.2||Traffic accident|
|Lee et al.||2012||CS||South Korea||NR||NR||86||Patients||NR||Violence|
|Author||With condyle fracture||Without condyle fracture||P -value|
|With third molar||Without third molar||With third molar||Without third molar||Total|
|Choi et al.||47||87||145||182||461||0.067|
|Mah et al.||84||57||149||30||320||<0.001|
|Duan and Zhang||133||167||237||163||700||<0.001|
|Gaddipati et al.||6||41||60||11||118||<0.001|
|Iida et al.||78||97||111||60||346||NR|
|Inaoka et al.||39||52||NR||NR||91||<0.001|
|Oh et al.||42||32||234||103||411||<0.05|
|Revanth Kumar et al.||36||4||NR||NR||40||NS|
|Naghipur et al.||51||79||182||134||446||<0.001|
|Zhu et al.||38||108||136||34||316||<0.001|
|Thangavelu et al.||89||172||171||28||460||<0.001|
|Lee et al.||12||11||51||12||86||0.008|
|Author||Pell and Gregory classification||P -value||Pell and Gregory classification||P -value||Winter classification||P -value|
|With condyle fracture||Without condyle fracture||With condyle fracture||Without condyle fracture||With condyle fracture||Without condyle fracture|
|Mah et al.||40||29||15||60||61||28||<0.005||25||43||16||23||98||28||<0.005||33||28||7||1||42||64||23||7||<0.05|
|Duan and Zhang||194||79||31||462||269||63||0.003||164||75||65||361||247||186||0.003||NR||NR||NR||NR||NR||NR||NR||NR||NR|
|Iida et al.||76||133||NR||248||235||NR||0.0003||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR||NR|
|Inaoka et al.||65.4%||30.8%||NR||NR||NR||NR||NR||73.1%||NR||NR||NR||NR||NR||NR||75%||21.1%||NR||NR||NR||NR||NR||NR||NR|
|Naghipur et al.||38||25||6||234||162||74||0.301||32||27||10||188||206||76||0.603||20||42||6||1||119||311||34||6||0.859|
|Thangavelu et al.||51||23||27||167||174||78||<0.0001||50||33||27||184||155||71||<0.0001||75||83||29||64||76||167||6||11||<0.0002|
|Lee et al.||9||3||0||NR||NR||NR||0.042||7||5||0||NR||NR||NR||0.949||NR||NR||NR||NR||NR||NR||NR||NR||NR|
The meta-analysis was performed using R software, with ‘meta’ and ‘metafor’ packages (R Foundation for Statistical Computing). Statistical heterogeneity was tested by I 2 statistic. This statistic expresses the percentage of the variation across studies. Heterogeneity of I 2 < 25% was considered low, of I 2 = 50% was considered moderate, and of I 2 > 75% was considered high. When I 2 was equal to 0, a fixed-effects model was used; when I 2 was >0, a random-effects model was used. The dependent variable was fracture of the mandibular condyle. Analyses were performed for the following categorical variables: presence of mandibular condyle fracture (‘yes’ or ‘no’) and presence of mandibular third molar (‘present’ or ‘absent’). When a third molar was present, the variables extracted were: (1) angulation of the third molar according to the Winter classification (V, MA, H, DA), which was assessed to determine the risk of mandibular condyle fracture at each position compared with the sum of the other positions; (2) position of the third molar according to the Pell and Gregory classification (A, B, C and I, II, III), which was assessed to determine the risk of mandibular condyle fracture for each classification compared with the sum of the other positions (e.g., A compared with B + C). Forest plots were generated for each meta-analysis. Publication bias was calculated using Egger’s test when there were at least 10 studies to be included in the model . Funnel plots to assess the possibility of publication bias were analysed visually. When asymmetry is observed in a funnel plot, publication bias may be present.
The search strategy resulted in 704 articles (Cochrane seven articles, PubMed 253 articles, Scopus 217 articles, VHL 33 articles, and Web of Science 194 articles). Three hundred and thirty-six of these articles were duplicates and were removed, leaving 368 articles for analysis ( Fig. 1 ). The titles and abstracts of these articles were read by two independent authors in order to determine whether they fulfilled the inclusion criteria for this review. At this stage, 322 articles were excluded for not fulfilling the inclusion criteria. Therefore, the full texts of 46 articles were read. Twenty-two were then excluded because they assessed the influence of third molars on angle fractures, and a further 11 were excluded for being outside the scope of this research, resulting in 13 articles being included in this review : five were case–control studies and eight were cross-sectional studies . Eleven of these 13 articles were included in the meta-analysis ( Fig. 1 ).
Among the 13 articles included, 3814 subjects were evaluated. The primary outcome assessed by the articles was the presence of condylar fracture. However, the articles assessed the outcome in different ways. Eight articles evaluated condyle fracture by the number of individuals with the fracture , three studies evaluated condylar fracture by the number of fractures, considering bilateral fractures , and two studies evaluated the two forms, assessing the number of fractures and the number of individuals with condylar fracture . Among the 13 studies included, 12 reported the cause of the fractures; the main causes were found to be traffic accidents and violence.
The 13 articles evaluated the presence or absence of condylar fracture in the presence or absence of a third molar. Eight articles also evaluated the influence of the different third molar positions (positions A, B, C and I, II, III according to Pell and Gregory, and V, H, MA, DA according to Winter) on the frequency of condylar fracture. Most of the articles found evidence indicating the influence of the presence of a third molar in decreasing the frequency of condylar fractures; only one article did not find statistically significant results .
Figure 2 shows the meta-analysis of the case–control and cross-sectional studies that assessed the chance of condyle fracture in the presence of third molars. When the third molar was present, there was a lower chance of mandibular condyle fracture (events of experimental group) than when the third molar was absent (odds ratio (OR) 0.26, 95% confidence interval (CI) 0.17–0.40, I 2 = 87.8%). Figure 3 shows the meta-analysis of case–control studies that assessed the chance of condyle fracture (experimental group) compared to other types of mandibular fracture, without condyle fracture (control group), in the presence of third molars. When the third molar was present, there was also a lower chance of mandibular condyle fracture (events of experimental group) than other mandibular fractures without condyle fracture (OR 0.30, 95% CI 0.16–0.58, I 2 = 91.6%). Thus, the third molar acts as a protective factor.
An assessment of the influence of the third molar position (Pell and Gregory and Winter classifications) on condylar fracture was performed. Four studies were included in the meta-analysis to evaluate the influence of third molar position on the occlusal plane − Pell and Gregory classes A, B, and C. The presence of a class A third molar resulted in a higher probability of condylar fracture (events of experimental group) when compared to the other classes (B + C) by analysis using the model (OR 1.32, 95% CI 1.09–1.61, I 2 = 0%) ( Fig. 4 ). The presence of a class B third molar (events of experimental group) acted as a protective factor compared to the other classes (A + C); there was a lower probability of the occurrence of condylar fracture using the random-effects model (OR 0.69, 95% CI 0.49–0.97, I 2 = 56.0%) ( Fig. 5 ). No significant difference was found for class C third molars when compared to the other classes (A + B) (OR 1.12, 95% CI 0.74–1.68, I 2 = 46.5%).
The four studies were also used in a meta-analysis to assess the position of the third molar in relation to the mandibular ramus − Pell and Gregory classes I, II, and III . Using the random-effects model, there was a higher chance of fracture of the condyle in the presence of a third molar of class I (events of experimental group) when compared to the other classes (II + III) (OR 1.37, 95% CI 1.05–1.77, I 2 = 32.8%) ( Fig. 6 ). Using the fixed-effects model, a third molar of class II (events of experimental group) acted as a protective factor when compared to the other classes (I + III) (OR 0.71, 95% CI 0.57–0.87, I 2 = 0%) ( Fig. 7 ). No significant difference was found for class III third molars when compared to the other classes (I + II) (OR 1.04, 95% CI 0.79–1.36, I 2 = 0%).
Three studies were included in the meta-analysis for the assessment of third molar position for the Winter classification (V, H, MA, and DA); no significant difference was found for each position in relation to the other positions.
For the case–control studies, the results of the quality assessment using the NOS varied from 5 to 8 points from a total possible score of 9. One study presented 8 stars in the evaluation of quality . Three studies had a total of 6 stars and one study presented 5 stars . The domain that was least contemplated in the evaluation of the case–control articles was ‘selection’ (definition of controls), followed by ‘comparability’ ( Table 4 ). Cross-sectional studies presented a risk of bias ranging between 4 and 8 points from a total possible score of 9. Among the studies evaluated, only two obtained 8 stars in the quality assessment . Four had a total of seven stars , one had 5 stars , and one had 4 stars . The domain least contemplated in the quality assessment of the cross-sectional studies was the ‘comparability’ domain ( Table 5 ).
|Case definition||Representativeness of the cases||Selection of controls||Definition of controls||Comparability of cases and controls||Ascertainment of exposure||Same method of ascertainment for cases and controls||Non-response rate|
|Iida et al.||★||0||★||0||★||★||★||★||6/9|
|Oh et al.||★||0||★||0||0||★||★||★||5/9|
|Naghipur et al.||★||★||★||0||★★||★||★||★||8/9|
|Thangavelu et al.||★||0||★||0||★||★||★||★||6/9|
|Zhu et al.||★||★||★||0||0||★||★||★||6/9|
a Selection: (1) (a) yes, with independent validation (1 star), (b) yes, e.g. record linkage or based on self-report, (c) no description; (2) (a) consecutive or obviously representative series of cases (1 star), (b) potential for selection biases or not stated; (3) (a) community controls (1 star), (b) hospital controls, (c) no description; (4) (a) no history of disease (endpoint) (1 star); (b) no description of source. Comparability: (1) (a) study controlled for the most important factor (position of third molar) (1 star), (b) study controlled for any additional factor (1 star). Exposure: (1) (a) secure records (e.g. surgical records) or (b) structured interview where blind to case/control status (1 star), (c) interview not blinded to case/control status, (d) written self-report or medical record only, (e) no description; (2) (a) yes (1 star), (b) no; (3) (a) same rate for both groups (1 star), (b) non-respondents described, (c) rate different and no designation.
|Studies||Selection (Maximum 4 stars)||Comparability (Maximum 2 stars)||Outcome (Maximum 3 stars)||Total|
|Representativeness of the sample||Sample size||Ascertainment of the exposure||The subjects in different outcome groups are comparable, based on the study design or analysis. Confounding factors are controlled.||Assessment of the outcome||Statistical test|
|Choi et al.||★||★||★★||★||★★||★||8/9|
|Mah et al.||★||★||★★||★||★★||★||8/9|
|Duan and Zhang||★||★||★★||0||★★||★||7/9|
|Gaddipati et al.||★||★||★★||0||★★||★||7/9|
|Inaoka et al.||★||★||★★||0||★★||★||7/9|
|Revanth Kumar et al.||0||0||★★||0||★★||0||4/9|
|Lee et al.||0||0||★★||0||★★||★||5/9|
a Selection: (1) (a) truly representative of the average in the target population or (b) somewhat representative of the average in the target population (1 star), (c) selected group of users, (d) no description; (2) (a) justified and satisfactory (1 star), (b) not justified; (3) (a) validated measurement tool (2 stars) or (b) non-validated measurement tool, but the tool is available or described (1 star), (c) no description of the measurement tool. Comparability: (1) (a) the study controlled for the most important factor (position of third molar) (1 star), (b) the study controlled for any additional factor (1 star). Outcome: (1) (a) independent blind assessment or (b) record linkage (2 stars), (c) no description; (2) (a) the statistical test used to analyze the data is clearly described and appropriate, and the measurement of the association is present (1 star), (b) the statistical test is not appropriate, not described, or incomplete.
When the influence of the presence of the third molar in condyle fractures was analysed for the case–control and cross-sectional studies, the funnel plot was asymmetric ( Fig. 8 ). However, Egger’s test suggested that the differences between the original and the adjusted meta-analyses was not significant ( P = 0.089). As the other analyses included fewer than 10 articles, neither a funnel plot nor Egger’s test was performed.