High and specialty-related musculoskeletal disorders afflict dental professionals even since early training years

Objective To determine how early musculoskeletal disorders (MSDs) develop in dental professionals and to explore the potential differences among distinct dental specialties. Material and Methods 271 dental postgraduates majoring in five dental specialties were recruited, i.e., orthodontics, prosthodontics, endodontics, periodontics and alveolar surgery. 254 age-matched non-dental postgraduates served as the control. The standardized Nordic questionnaire on MSDs and a self-report questionnaire regarding correlative factors (only for dental postgraduates) were answered through emails. Reliability of responses was assessed applying test-retest method. Results The intraclass correlation coefficient of participants' answers ranged from 0.89 to 0.96. Dental postgraduates had significantly higher prevalence of MSDs than the control group, especially at neck, upper back and lower back. In all dental specialties included, high prevalence of MSDs was reported at neck (47.5%-69.8%), shoulders (50.8%-65.1%), lower back (27.1%-51.2%) and upper back (25.6%-46.5%), with lower prevalence at elbows (5.1%-18.6%), hips (3.4%-16.3%) and ankles (5.1%-11.6%). Periodontics students reported the worst MSDs in most body regions except wrists and knees, which were more prevalent for prosthodontic and alveolar surgery students, respectively. Furthermore, year of clinical work, clinical hours per week and desk hours per week were found as risk factors for MSDs, whereas physical exercise and rest between patients as protective factors. Conclusions High and specialty-related MSDs afflict dental professionals even since very early stage of careers. Prevention aimed at the specialty-related characteristics and the risk/protective factors revealed in this study should be introduced to dental personnel as early as possible.


INTRODUCTION
The nature of dental clinical work could predispose dentists to diverse occupational hazards 8 , among which musculoskeletal disorders (MSDs) have been most frequently reported 2,3,9,17 . In typical clinical settings, dental practitioners frequently adopt unfavorable postures characterized by neck flexion, side bend and rotation, and shoulder abduction for better view and easier access to the oral cavity 20 . This static and awkward posture as well as repetitive movements, high prehension IRUFHV DQG ORQJ ZRUN GXUDWLRQ KDV EHHQ LGHQWL¿HG as the biomechanical risk factor associated with the development of MSDs among dentists 19 , which compromises their health and even causes dropout from dental careers 8 .
While most musculoskeletal symptoms could be relieved by proper preventive strategies, especially before chronic damages have been done 18 , current studies indicate that effective preventive methods are still lacking 4 . This could be caused by the inadequate understanding of MSDs per se. In fact, though numerous reports suggest its high prevalence 8 , few have explored how early MSDs develop in dental careers, which could hinder the application of preventive methods. Cumulative work duration of dental students, who are at the beginning of dental careers, is shorter than dentists. Nevertheless, the same clinical tasks, more uncomfortable postures due to their inexperience, lack of self-protect awareness 1,22 and gradually cumulated exposure to clinical tasks could promote MSDs in the early period of careers. To help prevent MSDs and minimize the negative effect, further studies of the above issue are needed.
Moreover, working postures and duration which are causally related to MSDs vary in distinct specialties. For example, most orthodontic procedures only require a view of buccal surfaces of teeth (with lingual orthodontics as an exception) while the view of pulp cavity is required for endodontists. Another example, most tooth extraction is a short and brisk exertion of force, whereas prosthodontic treatment usually comprises longer and repeated motion. Hence, MSDs could vary among different specialties, which, to the best of our knowledge, has never been reported.
Therefore, we develop this survey to investigate MSDs in dental students of distinct specialties in China. The aim of this study is to evaluate MSDs in the early stage of dental careers and to explore the potential differences of MSDs among distinct specialties.

MATERIAL AND METHODS
Two groups, i.e., the dental and non-dental postgraduates were recruited from Sichuan University, China. The dental postgraduates (n=327) were volunteers recruited from 5 dental specialties, including orthodontics, prosthodontics, endodontics, periodontics and alveolar surgery. They had received general dental training in the last undergraduate year, and had sticked to their specialties as postgraduates for at least 1 year. The non-dental postgraduates (n=350) were volunteers proportionally recruited from 26 non-dental and non-medical specialties, including economics, literature, chemistry, and so on. All the subjects had no idea about the purpose of the investigation.
The standardized Nordic questionnaire 11 was used to investigate the prevalence, frequency and impact of musculoskeletal symptoms in different DQDWRPLFDO UHJLRQV %ULHÀ\ WKH IUHTXHQFLHV RI MSDs were categorized as "never" (negative MSDs); or "everyday" (ED), at least once in the "last week" (LW), "last month" (LM) or "last year" (LY). Numbers of ED, LW, LM and LY were summed as positive MSDs, based on which the prevalence of MSDs was calculated. The impacts of MSDs were categorized as "no interference"(N), "work activities reduced"(W), "leisure activities reduced" (L), or "treatment sought" (T). An accompanying self-report questionnaire was designed to collect personal information of the dental postgraduates including specialty, gender, age, year of clinical work, etc. A small-scale pilot study has previously been done to evaluate the effectiveness and reliability of the accompanying questionnaire (data not shown). An email was sent to the students, asking them to ¿OO LQ WKH LQIRUPHG FRQVHQW IRUP DQG DQVZHU WKH standardized Nordic questionnaire, and the selfreport questionnaire (for the dental postgraduates only). Three weeks later, the same questionnaires were emailed to 40 randomly selected subjects (20 dental and 20 non-dental postgraduates) to assess the reliability of the responses. Throughout the study, an official email account from the postgraduate institute has been used for contact, so that the identities of the researchers have been kept in dark to the students 15 .
The chi-square test was utilized to examine the differences of MSDs prevalence between the dental and control group, and the differences among distinct dental specialties. The Wilcoxon rank-sum test was employed to compare the frequency and impact of MSDs among dental specialties. The logistic regression was used to reveal the association between prevalence of MSDs and exposure variables among dental postgraduates. All analyses were conducted using SPSS 13.0.
The study protocol has been approved by West China Hospital of Stomatology Institute Review Board in Jul. 2012. The Approval ID is WCHSIRB-D-2012-033.

RESULTS
A total of 288 dental postgraduates and 279 other postgraduates responded respectively. The researchers excluded 17 responses of dental students and 25 responses from the control group due to previous injuries/diseases which could induce musculoskeletal symptoms, inadequate clinical hours per week (<4 hours), information loss or misunderstanding of questions. Altogether, 254 questionnaires of the control group and 271 questionnaires of the dental group (2.78±1.57 years of clinical experience) including 58 from students in orthodontics, 59 in prosthodontics, 68 in endodontics, 43 in periodontics and 43 in alveolar surgery, were subjected to statistical analysis. In the reliability test, responses of participants showed a high consistency. The intraclass correlation FRHI¿FLHQW ,&& UDQJHG IURP WR

MSDs of dental personnel and other professionals
In general, 85.6% dental postgraduates experienced positive MSDs in at least one anatomic UHJLRQ ZKLFK ZDV VLJQL¿FDQWO\ KLJKHU WKDQ RWKHU SURIHVVLRQDOV :LWK UHJDUGLQJ WR VSHFL¿F regions, dental postgraduates experienced more MSDs in neck, upper back and lower back, while less LQ DQNOHV ZLWK VLJQL¿FDQFH P<0.05). No differences were observed in other regions ( Figure 1).
However, differences were observed in wrists and knees. These two regions were thus further analyzed to detect where the difference existed DW DQ DGMXVWHG VLJQL¿FDQFH OHYHO Į ¶ RI DFFRUGLQJ WR WKH IRUPXOD P NN Į ¶ N WKDW UHSUHVHQWHG ¿YH VSHFLDOWLHV 14 .

Frequency and impact of musculoskeletal symptoms
The four regions reported with high rate of MSDs (neck, shoulders, upper back and lower back) and the two regions detected with differences among specialties were further analyzed. The suffering of MSDs among dental specialties was summarized in Figure 3. The frequencies were categorized as "everyday" (ED), at least once in the "last week" (LW), "last month" (LM) or "last year" (LY). The LQÀXHQFHV ZHUH FDWHJRUL]HG DV ³QR LQWHUIHUHQFH´ (N), "work activities reduced" (W), "leisure activities reduced" (L), or "treatment sought" (T). Consistent ZLWK RXU ¿QGLQJV RI WKH SUHYDOHQFH SHULRGRQWLF students reported more severe symptoms in neck, upper back and lower back. In terms of wrists and knees, more severe symptoms were reported by prosthodontic students and alveolar students, respectively.

Figure 3-Frequency and impact of Musculoskeletal disorders (MSDs) in different dental specialties
The frequencies were categorized as "everyday" (ED), at least once in the "last week" (LW), "last month" (LM) or "last year" (LY). The impacts were categorized as "no interference" (N), "work activities reduced" (W), "leisure activities reduced" (L), or "treatment sought" (T) .a, b, c and d, compared to orthodontists, prosthodontics, endodontics and alveolar surgery respectively (P

DISCUSSION
This study, to the best of our knowledge, is WKH ¿UVW WKDW UHSRUWV GLIIHUHQW GLVHDVH SDWWHUQV RI MSDs suffered by dental professionals of distinct specialties, particularly with a focused scrutiny on the early development of MSDs.
There are 85.6% dental postgraduates complaining at least one anatomic region with PXVFXORVNHOHWDO V\PSWRPV VLJQL¿FDQWO\ KLJKHU WKDQ the control group (70.4%). Together with higher prevalence in neck, upper back and lower back reported by dental postgraduates, it is suggested that, compared to the general population, higher 06'V DIÀLFW GHQWDO SHUVRQQHO 7KLV FRXOG PDLQO\ be due to the more uncomfortable postures they assume to undertake during work.
The prevalence of MSDs in dental postgraduates in our study is similar to the results of other findings regarding dentists 2,3,8 . However, their clinical experience (2.78±1.57 years) is much shorter, suggesting that the progression and deterioration of MSDs spark from the very beginning of the practitioners' career life. Ergonomic studies demonstrate that protection and treatment should begin early before musculoskeletal injuries become persistent 18 . Thus, more concerns should be put on the prevention in the early stages of dental careers.
The comparison regarding MSDs among different dental specialties provides more fresh and in-depth understandings to this issue. It is interesting that prosthodontic students experience more suffering of wrist symptoms while fewer in almost all other body regions (Table 1). This discrepancy may be due to the nature of prosthodontic work. Compared with other specialties, prosthodontic students may assign relatively less time for intraoral operation, as they have more off-chair work to do, e.g., making the temporary crown and choosing the color for porcelain. This advantage may more or less relieve them of the burden from the long-term awkward posture in chair-side operation. On the other hand, more frequent use of vibrating instruments, as indicated to be a risk factor for wrist injuries in previous studies 12 , probably subjects them to higher risk of wrist disorders.
Despite the generally low prevalence of knee disorders in other studies 13 , alveolar surgery students have significantly higher prevalence in knees compared to other specialties in our investigation. It is observed that when performing dentoalveolar surgeries, like tooth extraction, students usually stand to facilitate their operations. 0RUHRYHU WKH\ PD\ ZHOO WDNH PRUH WLPH WR ¿QLVK an extraction due to inexperience, which puts them long in an unfavorable standing posture. Knee joints would be more vulnerable to chronic injuries when keeping stance is combined with large cumulative loads 16 . This could account for the vulnerability of knees among alveolar surgery students. However, experienced alveolar surgeons tend to sit whenever possible, which is also suggested to be a more protective posture 5 . It indicates that harmful postures are not always inevitable, thus further highlights the importance of corresponding training for the prevention of MSDs.
Periodontic students suffer the most serious MSDs in neck, shoulders, upper back and lower back. Long work duration and repetitive movements of periodontal treatments could be the main reasons that predisposes them to higher risk of MSDs. Interestingly, though using ultrasonic instruments for tooth cleaning frequently, periodontic students do not experience severe wrist symptoms as prosthodontic students do. Prehension force, which serves as a risk factor for MSDs 6 , may explain this difference, as ultrasonic periodontal instruments do QRW UHTXLUH FRQVLVWHQW ¿UP JUDVS ZKLOH KLJKVSHHG handpieces do, which are more frequently used by prosthodontists.
Only a few variables of potential correlative factors are found significantly associated with MSDs. The differences among distinct specialties exist, but are not obvious. Clinical time and desk time positively correlate to musculoskeletal disorders, whereas physical exercise and rest between patients play a protective role, which is in accordance with previous studies concerning dentists 9,10 . Female students are shown to present higher risk of MSDs than male, especially those who specialize in alveolar surgery. This could be due to the fact that surgeries like tooth extraction demand high and continuous prehension forces and, therefore, female students have to struggle harder to exert the required strength. Moreover, young women have less physical exercise than young PHQ 1R VLJQL¿FDQW DVVRFLDWLRQ LV GHWHFWHG EHWZHHQ MSDs and some suspicious risk factors, such as poor illumination satisfaction %ULHÀ\ GHYHORSPHQW of MSDs in dental students is related to multiple factors, mainly consistent with those of dentists. It indicates that, even though the accumulation of exposure to these factors is low in the early WUDLQLQJ \HDUV WKHLU LQÀXHQFHV DUH VWLOO VLJQL¿FDQW Thus, prevention aimed at these factors should be explored.
Nevertheless, there are some limitations in this study. Firstly, though 271 dental postgraduates were recruited, each specialty only had a sample size of 43-68. Secondly, as most data are collected by self-report questionnaires, memorial mistakes could hardly be avoided. Therefore, prospective studies with larger sample sizes are necessary in the future.

CONCLUSION
+LJK 06'V DIÀLFW GHQWDO SURIHVVLRQDOV VLQFH HDUO\ WUDLQLQJ \HDUV ZKLFK DUH VSHFLDOW\ VSHFL¿F 7KXV protection against MSDs of dental practitioners should be taken at the early stage of careers, precisely, while still as dental students. Moreover, the preventive methods should match the nature of clinical task in distinct specialties.