Original Research Article

The prevalence of Temporomandibular disorders among a group of Libyan dental students.


Bugaighis I.1, Elgehani R.2, Orafi M.2, Elatrash A.2.

1 Department of Orthodontics, Faculty of Dentistry, University of Benghazi, Benghazi, Libya.

2 Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, University of Benghazi, Benghazi, Libya.


* Corresponding author: Iman Bugaighis, P.O. Box 595 Benghazi-Libya. E.mail: isbugaighis@yahoo.com



Background: It has been reported that Tempro-Mandibular-disorders (TMD) is the most common orofacial pain source of non-dental origin. The aim of this cross-sectional observational study is to explore the prevalence of signs and symptoms of TMD among a group of dental students in Benghazi-Libya.


Methods:   The examined group comprised 100 students [17 males and 83 females; mean age: 24.8±2.23 years. The assessment was undertaken following Helkimo’s measures. The prevalence and severity of TMD was determined using a self-reported anamnestic questionnaire and clinical examination. Descriptive statistics was employed to explore the frequencies of the examined variables. Linear correlation coefficient was conducted between the reported symptoms and recorded signs and between the reported symptoms and the clinical dysfunction scores (CDS). The level of significance was set at P<0.05.


Results: A total of 37% of the students reported some level of TMD symptoms [32% mild (Ai1); 5% severe (AiII)]. On the other hand, majority of the students (93%) demonstrated a range of TMD signs [76% mild (DiI); 17% moderate (DiII)]. Although linear correlation coefficient between the reported symptoms (Ai) and the recorded signs (Di) (r=0.32) and between the reported symptoms (Ai) and the CDS (r=0.37) were low, they were statistically significant (P<0.05).


Conclusion: Mild to moderate prevalence of TMD appears to exist among Libyan dental students.


Keywords: Libya, Benghazi, Dental Students, Tempromandibular joint, Tempro-Mandibular-disorder.


      Tempro-Mandibular disorder (TMD) is a term used to describe a heterogeneous group of correlated anomalies involving the Tempro-Mandibular Joint (TMJ), cranio cervico facial muscles and occlusion, all of which have shared symptoms of orofacial pain, articular clicking as well as impaired jaw movement [1-3].

      It is generally accepted that the aetiology of TMDs is multifactorial, including craniofacial causes (such as: changes in the TMJ, dysfunction of the masticatory muscles, occlusal interferences, bruxism and tooth loss) [2,46], generalized factors (such as immune-mediated systematic diseases and neoplastic tumors) [7], Psychological stress  [8]  or a combination of the reported features [2,46]. Oral prosthesis, orthodontic management, orthognathic surgery as well as mandibular fracture have been linked to TMJ alterations and worsening of present TMD [9]

      It has been reported that TMD is the most common orofacial pain source of non-dental origin [10], and the second most prevalent musculoskeletal condition, subsequent to chronic lower back pain, that might lead to pain and disability [10]. The estimated prevalence of TMD at a population level varies between 12% and 50% [4,1114]. This wide prevalence discrepancy might be attributed to variations in the examined races, different study designs and lack of homogeneity in the diagnostic criteria [5], as well as to the environment in which subjects are examined [8]. Furthermore, TMD appears to be relatively common, occurring in young and middle-aged adults (20-40 years of age), rather than in children and the elderly [15].

      The diagnostic assessment of the TMD presents a challenge to the researchers and clinicians because of the broad range of musculo-skeletal, inflammatory, and dental disorders that can be manifested in the TMJ [8,16]. Furthermore, a simple appraisal remains difficult because of the absence of uniform assessment criteria.

      A number of TMD evaluation tools have been proposed in the literature. One of the most widely used is Helkimo’s index [17] which combines anamnestic and clinical dysfunction indices. However, a universal assessment measures have yet to be recognized [5,8]. 

      Whilst the prevalence of TMD has been extensively reported in western countries, similar studies in the Arab world are less frequent [3,1821]. The literature search on key words such as TMJ, TMD, Libyans, students revealed that there were no published studies on TMD for Libyan subjects [September 2016]. Therefore, the aim of the present study was to examine the prevalence of signs and symptoms of TMDs among a group of Libyan dental students in Benghazi, using questionnaires and clinical examination.

Methods and Materials

      This was a cross-sectional observational study. The examined cohort comprised 100 students attending the Faculty of Dentistry in Benghazi-Libya; 17 males and 83 females with an age range from 21 to 30 years.

      The examined students were of Libyan decedent for at least two generations with no history of chronic illness, craniofacial abnormality, systematic disease or hypodontia. None had a history of either tooth extraction or orthodontic treatment and none was on medication.

      A functional assessment was undertaken using the Helkimo clinical dysfunction (Di) and anamnestic (Ai) indices [17]. The anamnestic questionnaire composed of ten questions. The questions explore the possibility of the presence of pain in the TMJ, pain in the head and neck while chewing, parafunctional habits, movement limitations, joint clicking, Subsequently the findings were categorized according to the anamnestic dysfunction index to (Ai) as 0, I, or II. While Ai0 comprised individuals with subjectively symptom-free, AiI and AiII characterize those presented with mild and severe symptoms of TMD correspondingly. 

      The clinical examination included the assessment of the following; 1- Maximum opening of the mandible taking into consideration the measurement of the overbite (or openbite) while examining any deviation of the mandible during opening movement 2- Protrusive movement of the mandible in view of the measurement of over jet. 3- Right and left lateral mandibular excursions. All measurements were computed in millimeters (mm) and were obtained by a digital caliper with a sensitivity of 0.01mm. Each movement was repeated three times in order to compare the values and record the average score. The TMJs were assessed for sound and pain. The masticatory muscles (temporalis, masseter, medial pterygoid) were palpated for tenderness and the lateral pterygoid muscle was assessed against forced contraction. Subsequently, the results were categorized following the clinical dysfunction index; Di of point zero (Di0) for subjects with clinically sign-free; DiI (1-4 points) and DiII [5-9] respectively for subjects with moderate and severe signs of TMD correspondingly. The occlusal components were beyond the scope of this study.

      The clinical examination was undertaken by one consultant (M.M) who has significant experience in TMD clinical assessment. The use of one examiner ensured the consistency of the procedure and uniformity of the interpretation of the responses provided by the subjects.

Statistical analysis

      The collected data were coded and entered into an Excel 2010 spread sheet. Statistical analysis was undertaken using SPSS, Chicago, Illinois version 17. Descriptive statistics, Frequency and percentage of each variable were computed. The positive answers in each score were added and the percentage means were computed. Correlation coefficients between both scores were calculated. Statistical significance was set at the level P<0.05.

      An intra-examiner reliability study was carried out to confirm the consistency of the results. Twenty students were randomly selected for repeated clinical assessment on the same day of the first examination to minimize the possibility of symptoms variability. The average intraclass Correlation Coefficient (ICC) of all variables was found to be greater than 0.90, indicating an excellent level of agreement between the two examinations.


      The examined subjects were 100 students (83 males and 17 females; (mean age= 24.8, SD=2.23). Unpaired Student t-test showed that there was a significant statistical male/female discrepancy (P>0.05). Thus further statistical analysis to evaluate sex differences in the prevalence of TMD could not be undertaken and the data were pooled together for further analysis.

      Figure 1 shows that whilst 63% of the participants had no TMD symptoms (Ai0), 32% reported mild symptoms (AiI). On the other hand, just 5% described the existence of severe TMD symptoms (AiII).

Figure 1: Pie chart displaying the percentage (%) of recorded symptoms of the tempromandibular disorders

      Figure 2 illustrates that out of the ten explored symptoms pain was the most featured at 55%; minor neck aches and shoulder pain (24%), tenderness and or muscular pain (17%) and slight pain in or around the ear (14%). Mild recurrent headache was the second most reported symptom in 34%. The least described symptom was slight difficulty in opening the mouth (7%) and minor difficulty in moving or using the jaw (8%).

Figure 2: Bar graph displaying the percentage (%) of the most recorded symptoms of the Tempromandibular disorders  

      Assessment of the signs of the TMD among the examined participants revealed that impaired range of movement was the most frequently recorded (20%) followed by TMJ sounds during movement (17% out of which 9% were bilateral). Furthermore, only 15% of the participants had joint pain during movement (out of which only 3% suffered from bilateral TMJ pain). Masticatory muscle pain during palpation was the least observed sign (12%) (Figure 3).

Figure 3: Pie chart showing the percentage (%) of subjects with recorded signs of Tempromandibular disorders

      The clinical dysfunction scores revealed that 7% of the examined cohort had no signs of TMD (Di 0), 76% demonstrated mild dysfunction symptoms (DiI), while only 17% showed signs of moderate clinical TMD (DiII). None of the participants revealed severe TMD clinical signs.

      The linear correlation coefficient (r) between the reported symptoms (Ai) and the recorded signs (Di) was 0.32, and between the reported symptoms (Ai) and the clinical dysfunction score (CDS) was 0.37. Although these values were low, they were statistically significant (P<0.05)

      The outcome of the assessed three dimensional mandibular movements (Table 1) showed that just over half of the examined subjects (51%) demonstrated maximum mouth opening capacity of 40 mms or more while only 2 patients (2%) had restricted vertical movement. Furthermore, over 40% of the participants had the potential of eccentric mandibular movement of 7mm or more (right side 48%; left side 44%).  Moreover, 13% had restricted eccentric lateral movement towards the right side and 10% towards the left side. Almost one third of the participants (30%) demonstrated the potential of protrusive mandibular movement of 7mm or more, while only 16% of the sample had limited protrusive movement of zero to 3mm.

Table 1: Displaying the percentage (%) of participants maximum mandibular opening, (mm) maximum eccentric mandibular movements (right and left) and maximum protrusive capacity of the sample.


Percentage %

Maximum vertical opening







Maximum right lateral movement







Maximum left lateral movement







Maximum protrusion









      This cross-sectional study firstly examined a sample of Libyan dental students to derive a preliminary data on the prevalence of signs and symptoms of TMD using anamnestic questionnaires and clinical examination. Most of the approached students declined to participate due to the fragile security status when the study was conducted. Thus, the present group was considered a convenient sample and the number was accepted as a pilot study with a recommendation for further study with greater sample size.

      The present findings might offer an insight to the health care practitioner to detect the presence of TMD in their patients. Furthermore, the current findings would provide a reference for comparison with other similar studies and for future national research with larger sample size and representable male/female numbers.

      This questionnaire was designed to evaluate the anamnestic and clinical dysfunction indices according to Helkimo’s epidemiological index [17]. Helkimo’s index [17] includes five frequently observed physical signs and symptoms to score. This index remains the most widely used scoring system in TMD research, but it is not without serious flaws related to its validity. This limits its wide-ranging applicability. Van Der Weele and Dibbets [22] explained that it seems that a random set of cut-off standards for the varied groups within the subcategories have been selected, yet not weighed accordingly. Furthermore, it’s analysis is limited only to one dimension [22]. They concluded that although Helkimo’s index [17] might be a seminal tool in the evaluation of TMD, more work is required to enhance its efficiency.

      It had been noticed that the peak of prevalence of TMD symptoms is between 20-40 years of age, with a lower prevalence in younger and older cohorts [5,23]. Therefore, this study was planned to examine subjects within this age group. Furthermore, students are voluntarily available and easier to recruit. However, their dental background could influence the results of the study [24].

      The recruited females in the present study (83) were significantly more than males [17]. This discrepancy limited the possibility of undertaking statistical analysis for each sex separately. Manfredini et al. [5] confirmed in their systemic review and a number of other studies [25,26] reported as well that treatment-seeking individuals with TMD were mainly females.

      Whilst 63% of the present subjects had no symptoms of TMD (Ai0], 32% presented with mild symptoms (AiI) and only 5% suffered from severe symptoms (AiII). A similar trend was observed in subjects with comparable age groups from Jordan [18] at 63%, 21.5% and 15,5% respectively and from Niger [24] at 70%, 26.3% and 2.9% correspondingly. All the reported groups were assessed by the same method (the anamnestic questionnaire). Other studies noticed lower prevalence of TMD in their countries; 12% in Finish subjects [27], 57% in Egyptians [28] and 53% in Brazilians [26].     

      Interestingly, great discrepancies in the frequency of TMD clinical signs were noticed in the literature. While Finish [27], Jordanians [18] and Nigerians [24] showed prevalence of 61%, 62.8% and 68.8% respectively, lower values were observed in Tanzanian (40%) [29] and Iranian (37%) [20] samples. The higher values of TMD clinical signs in Libyan subjects revealed in the present study (95%) might be attributed to the observed high percentage of impaired range of mandibular movements (61% limited protrusion, 56% restricted lateral movement and 49% impaired vertical movement). Although, there is no clear reason for such high prevalence of clinical signs of TMD in Libyan students, the level of stress they are exposed to and their psychological wellbeing for living in an unstable city (Benghazi) might be the main reason for the high TMD. Bonjardim et al. [2] reported a significant association between their examined group of university students and psychological status. Conducting another study with greater sample size, and when the country is more stable might be more informative.

      It is generally accepted that pain is a significant symptom of TMD and the main cause for seeking treatment [30]. In this study pain was the most reported symptom (27%), followed by headache (20%) and then TMJ sounds (17%). The percentage of pain symptoms is considered the highest in the literature. North Americans and European studies showed that clinically detectable pain was found in 3% and 18% respectively of their examined subjects of similar age group to our students [17,31-33]. Furthermore, 13.1% of the examined Jordanian subjects had TMJ pain [18], as well as 5.5% of Nigerian students [24]. This high incidence of pain in the Libyan subjects might be a result of their increased stress due to the unstable security status and their worries of how this might negatively affect their study and future. Furthermore, Isong et al. [34] reported that pain threshold of TMD varies considerably by race, sex, age and socioeconomic status.

      Up to date, the normal range of the three dimensional mandibular movement in Libyan subjects have not been investigated. Thus, the outcome of this analysis has to be interpreted with caution and the significant but weak correlation coefficient between the noted signs and the described symptoms, in addition to the clinical dysfunction score do not principally designate that a significant percentage of the cohort suffer from TMD. This makes our results open to various interpretations.

      Although our study provides some information about the prevalence of TMD among Libyan dental students, long term clinical studies with greater sample size should be considered in this domain. Early diagnosis and prevention of future complications associated with TMDs is the key for successful TMD treatment.


      In this study, mild to moderate prevalence of TMD appears to exist among this examined cohort of Libyan university dental students in Benghazi. Pain was the most common reported symptom, while impaired range of movement was the most frequently recorded sign. Special attention and follow up should be paid to those subjects who had received diagnoses of TMD.

     Conflict of Interest

      The authors declare that they have no conflict of interest.



[1]           Dimitroulis G. Temporomandibular disorders: a clinical update. British Medical Journal. 18;317(7152):190-94


[2]           Bonjardim LR, Lopes-Filho RJ, Amado G, Albuquerque RLC, Goncalves SRJ. Association between symptoms of temporomandibular disorders and gender, morphological occlusion, and psychological factors in a group of university students. Indian Journal of Dental Research. 2009;20(2):190–94.


[3]           Habib SR, Qasim M, Rifaiy A, Habib Awan K, Alsaif A, Alshalan A, et al. Prevalence and severity of temporomandibular disorders among university students in Riyadh Production and hosting by Elsevier The Saudi Dental Journal. 2015;27:125–30.


[4]           de Santis TO, Motta LJ, Biasotto-Gonzalez DA, Mesquita-Ferrari RA, Fernandes KP, de Godoy CH, Alfaya TA, Bussadori SK. Accuracy study of the main screening tools for temporomandibular disorder in children and adolescents. Journal of bodywork and movement therapies. 2014;18(1):87-91.


[5]           Manfredini D, Guarda-Nardini L, Winocur E, Piccotti F, Ahlberg J, Lobbezoo F. Research diagnostic criteria for temporomandibular disorders: a systematic review of axis I epidemiologic findings. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology. 2011;112(4):453-62.


[6]           Manfredini D, Lobbezoo F. Relationship between bruxism and temporomandibular disorders: a systematic review of literature from 1998 to 2008. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology. 2010;109(6):e26-50.


[7]           KOPP S, HARALDSON T. Normal variation in skin surface temperature over the temporomandibular joint and masseter muscle. European Journal of Oral Sciences. 1983;91(4):308-11.


[8]           Okeson JP. TMJ disorders and occlusion. Mosby, editor. 2003.


[9]           Goldstein BH. Temporomandibular disorders: a review of current understanding. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology. 1999;88(4):379-85.


[10]         LeResche L. Epidemiology of temporomandibular disorders: implications for the investigation of etiologic factors. Critical Reviews in Oral Biology & Medicine. 1997;8(3):291-305.


[11]        National Institute of Dental and Craniofacial Research [Internet]. Facial pain. [cited 2017 Jan 24]. Available from: https://www.nidcr.nih.gov/DataStatistics/FindDataByTopic/FacialPain/.


[12]        Lee JY, Kim YK, Kim SG, Yun PY. Evaluation of Korean teenagers with temporomandibular joint disorders. Journal of the Korean Association of Oral and Maxillofacial Surgeons. 2013;39(5):231-7.


[13]         Farsi NM. Symptoms and signs of temporomandibular disorders and oral parafunctions among Saudi children. Journal of oral rehabilitation. 2003;30(12):1200-8.


[14]         Ebrahimi M, Dashti H, Mehrabkhani M, Arghavani M, Daneshvar-Mozafari A. Temporomandibular disorders and related factors in a group of Iranian adolescents: a cross-sectional survey. Journal of dental research, dental clinics, dental prospects. 2012;5(4):123-7.

[15]         Carlsson GE. Epidemiological studies of signs and symptoms of temporomandibular joint-pain-dysfunction. A literature review. Australian Prosthodontic Society Bulletin. 1984;14:7-12.


[16]         Benson BW, Otis LL. Disorders of the temporomandibular joint. Dental Clinics of North America. 1994;38(1):167-85.


[17]        Helkimo M. Studies on function and dysfunction of the masticatory system. 3. Analyses of anamnestic and clinical recordings of dysfunction with the aid of indices. Svensk tandläkare tidskrift. Swedish dental journal. 1974;67(3):165-81.


[18]        Dweiri A, Alomari F, Habahabeh R, Alshdafat N. The Prevalence of Symptoms and Signs of Temporomandibular Disorders among a Group of Young Adult Jordanian Population. Journal of the Royal Medical Services. 2013;20(3):44-50.


[19]        Abu-Ghazaleh SB, Rajab LD, Sonbol HN. Psychological stress among dental students at the University of Jordan. Journal of dental education. 2011;75(8):1107-14.


[20]        Nourallah H, Johansson A. Prevalence of signs and symptoms of temporomandibular disorders in a young male Saudi population. Journal of oral rehabilitation. 1995;22(5):343-7.


[21]        Larheim TA, Katzberg RW, Westesson PL, Tallents RH, Moss ME. MR evidence of temporomandibular joint fluid and condyle marrow alterations: occurrence in asymptomatic volunteers and symptomatic patients. International journal of oral and maxillofacial surgery. 2001;30(2):113-7.


[22]        WEELE LT, Dibbets JM. Helkimo's index: a scale or just a set of symptoms?. Journal of oral rehabilitation. 1987;14(3):229-37.


[23]        Clark GT, Mulligan R. A review of the prevalence of temporomandibular dysfunction. Gerodontology. 1984;3(4):231-6.  


[24]        Otuyemi OD, Owotade FJ, Ugboko VI, Ndukwe KC, Olusile OA. Prevalence of signs and symptoms of temporomandibular disorders in young Nigerian adults. Journal of Orthodontics. 2000;27(1):61-6.


[25]        Loster JE, Osiewicz MA, Groch M, Ryniewicz W, Wieczorek A. The Prevalence of TMD in Polish Young Adults. Journal of Prosthodontics. 2015 Dec 1.


[26]        Nomura K, Vitti M, Oliveira AS, Chaves TC, Semprini M, Siéssere S, Hallak JE, Regalo SC. Use of the Fonseca's questionnaire to assess the prevalence and severity of temporomandibular disorders in Brazilian dental undergraduates. Brazilian dental journal. 2007;18(2):163-7.


[27]        Helkimo MI, Bailey JO, Ash MM. Correlations of electromyographic silent period duration and the Helkimo dysfunction index. Acta odontologica scandinavica. 1979 Jan 1;37(1):51-6.


[28]        Abdel-Hakim AM. Stomatognathic dysfunction in the western desert of Egypt: an epidemiological survey. Journal of oral rehabilitation. 1983;10(6):461-8.


[29]        Mazengo MC, Kirveskari P. Prevalence of craniomandibular disorders in adults of Ilala District, Dar‐es‐Salaam, Tanzania. Journal of oral rehabilitation. 1991;18(6):569-74.


[30]        Moss RA, Garrett J, Chiodo JF. Temporomandibular joint dysfunction and myofascial pain dysfunction syndromes: parameters, etiology, and treatment. Psychological Bulletin. 1982;92(2):331.


[31]        Von Korff M, Dworkin SF, Le Resche L, Kruger A. An epidemiologic comparison of pain complaints. Pain. 1988;32(2):173-83.


[32]        Goulet JP, Lavigne GJ, Lund JP. Jaw pain prevalence among French-speaking Canadians in Quebec and related symptoms of temporomandibular disorders. Journal of dental research. 1995;74(11):1738-44.


[33]        Mohlin B. Prevalence of mandibular dysfunction and relation between malocclusion and mandibular dysfunction in a group of women in Sweden. The European Journal of Orthodontics. 1983;5(2):115-23.


[34]        Isong U, Gansky SA, Plesh O. Temporomandibular joint and muscle disorder-type pain in US adults: the National Health Interview Survey. Journal of orofacial pain. 2008;22(4):317.

ملخص باللغة العربية


انتشار الاضطرابات الصدغية الفكية بين مجموعة من طلاب طب الأسنان الليبيين.



إيمان بوقعيقيص 1& *، رافع إلجهاني 2 ، مرعي العرفي 2 ، أيمن الأطرش 2


1 قسم تقويم الأسنان، كلية طب و جراحة الفم و الأسنان، جامعة بنغازي، بنغازي، ليبيا.

2  قسم جراحة الفم والوجه والفكين، كلية طب و جراحة الفم و الأسنان، جامعة بنغازي، بنغازي، ليبيا.

* المؤلف المسؤول: د. إيمان أبوقعيكيص ، صندوق بريد 595 بنغازي-ليبيا. البريد الإلكتروني  isbugaighis@yahoo.com



تعتبر اضطرابات الصدغية الفكية مصدراً للألم الأكثر شيوعا من أصل اضطرابات الأسنان. الهدف من هذه الدراسة استكشاف انتشار علامات وأعراض اضطرابات الصدغية الفكية بين مجموعة من طلاب طب الأسنان في بنغازي ليبيا.



تضمنت المجموعة التي تم فحصها 100 طالب [17 ذكور و 83 إناث[. كان متوسط العمر: 24.8 ± 2.23 سنة. وقد أجري التقييم باستخدام مقاييس هلكيمو Helkimo’s. تم تحديد انتشار وشدة اضطرابات الصدغية الفكية باستخدام استبيان أنامنيستيك المبلغ عنها  ذاتياً من قبل المريض وكذلك عبر الفحص السريري. وقد استخدمت إحصائيات وصفية لاستكشاف ترددات المتغيرات التي تم فحصها. كما تم حساب معامل الارتباط ما بين الأعراض والعلامات المسجلة، وكذلك ما بين الأعراض التي تم الإبلاغ عنها وعجز الوظائف السريرية. تم تحديد مستوى الدلالة عند P <0.05.



 أبلغ ما مجموعه 37٪ من الطلاب عن مستوى معين من أعراض اضطرابات الصدغية الفكية (32٪ خفيفة 5٪ شديدة).  من ناحية أخرى، أظهرت الفحوص الإكلينيكية أن غالبية الطلاب (93٪) يعانون من علامات اضطرابات الصدغية الفكية (76٪ خفيفة و17٪ معتدلة). على الرغم من أن معامل الارتباط الخطي بين الأعراض المسجلة والعلامات المسجلة (r = 0.32)  وما بين الأعراض المبلغ عنها و عجز الوظائف السريرية (r = 0.37)  كانت منخفضة، غير أنها كانت ذات دلالة إحصائية  مقبولة (P < 0.05).



 هناك انتشار من بسيط إلى معتدل لأعراض اضطرابات الصدغية الفكية بين طلاب الأسنان في ليبيا.


الكلمات المفتاحية:                                      

ليبيا، بنغازي، طلاب طب الأسنان، مفصل الفك السفلي، اضطرابات الصدغية الفكية.



  • There are currently no refbacks.