Hand use and patterns of joint involvement in osteoarthritis. A comparison of female dentists and teachers

S. Solovieva, T. Vehmas1, H. Riihimäki, K. Luoma2 and P. Leino-Arjas

Department of Epidemiology and Biostatistics and 1 Department of Occupational Medicine, Finnish Institute of Occupational Health, Helsinki and 2 Department of Radiology, Peijas Hospital, Helsinki University, Central Hospital, Vantaa, Finland.

Correspondence to: S. Solovieva, Department of Epidemiology and Biostatistics, Finnish Institute of Occupational Health, Topeliuksenkatu 41a A, 00250 Helsinki, Finland. E-mail: Svetlana.Solovieva{at}ttl.fi


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Objectives. To investigate the effect of mechanical stress on finger osteoarthritis (OA) by comparing women from two occupations with different hand load but the same socio-economic grade, and to investigate whether hand load may affect the pattern of joint involvement in OA.

Methods. Radiographs of both hands of 295 dentists and 248 teachers were examined. Each interphalangeal (distal, proximal and thumb interphalangeal) and the metacarpophalangeal joints were graded (0 = no OA, 4 = severe OA) separately by using reference images. The co-involvement of different hand joints was analysed by logistic regression.

Results. The distal interphalangeal joints were the most frequently involved joints. The non-dominant hand was more frequently affected by OA of grade 2 or more than the dominant hand. The prevalence of OA of grade 2 or more in any finger joint and also in any distal interphalangeal joint was higher among the teachers compared with the dentists (59 vs 48%, P = 0.020 and 58 vs 47%, P<0.010 respectively). Finger OA showed more clustering in the ring and little fingers and more row clustering and symmetry in the teachers than in the dentists [age-adjusted odds ratio (OR) = 1.57, 95% confidence interval (CI) 1.10–2.23, OR = 1.84, 95% CI 1.28–2.64, and OR = 1.98, 95% CI 1.38–2.86 respectively]. The OR of more severe OA (grade 3 or more) in the right-hand thumb and the index and middle fingers was significantly elevated among the dentists compared with the teachers (OR 2.61, 95% CI 1.03–6.59).

Conclusion. Our findings indicate that finger OA in middle-aged women is highly prevalent and often polyarticular. Hand use may have a protective effect on finger joint OA, whereas continuing joint overload may lead to joint impairment.

KEY WORDS: Hand, Osteoarthritis, Finger joint


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Osteoarthritis (OA) is the most common joint disorder in the adult population, and hand joints are those most often affected by it [1]. The prevalence of OA increases with age with evident sex-specific differences [2]. Before the age of 50 yr, the prevalence of OA in most joints is higher in men than in women, but hand OA is more prevalent among women [2, 3].

Some studies have revealed certain patterns of disease clustering, such as rows (the same joint affected in several fingers) and rays (multiple joints in the same finger), as well as symmetrical associations among the different hand joints [4–7]. Egger et al. [5] provided clear evidence for the polyarticular subset of hand OA in women and pointed out that the three major patterns of polyarticular involvement, in descending order of importance, are symmetry, clustering by row and clustering by ray. Later, these findings were verified in a longitudinal study of the members of the Framingham Study cohort [6]. In a large British cohort study, the pattern of hand joint involvement in clinical OA was found to be almost identical between 53-yr-old men and women [7].

A high prevalence of OA in the right hand compared with the left hand in right-handed persons was found in the New Haven Survey of Joint Diseases [8]. Ever since this result and the ‘wear and tear’ hypothesis proposed by Radin and colleagues [9], joint degeneration has been linked to mechanical loading. According to the hypothesis, wear comes from repeated stresses due to continued loading of the joints. Job tasks that require intensive use (wear) of the same muscles or motions for a long duration increase the likelihood of both localized and general fatigue and in turn the likelihood of injury (tear). Prolonged or repeated overuse of a group of joints has been related to an increased frequency of OA in some studies [10–13] although not all [14–16].

Epidemiological studies provide evidence linking occupational physical activities with osteoarthritis of the lower extremity weight-bearing hip and knee joints [17, 18], and highly repetitive arm motions alone or in combination with other factors (such as awkward posture, force, vibration and duration of the movements) with hand/wrist disorders [19]. Although hand joints are the joints most commonly affected by OA, there is little information on the role of occupation in the aetiology of finger OA and data on women are scarce.

Dentistry is one of the few occupations with an academic background that involve extensive bimanual work. Dentists perform arm movements repeatedly, often rapidly and for extended periods of time. The precision grip is used in the handling of precision tools [20], some of which vibrate. The work postures tend to be particularly awkward for the neck and upper limbs. Dentists frequently assume static postures which require more than 50% of the body's muscles to contract to hold the body motionless while resisting gravity [21]. Static postures are often combined with repetitive movements inherent in the nature of tasks performed. While dentists use the thumb, index and middle fingers heavily in precision gripping with their right hand, the ring and little fingers remain static [22].

Teachers represent an occupational group with a comparable academic background with the dentists but with a distinctively different hand load. Although hand activities such as writing and typing are common in a teacher's work, they are usually of shorter duration, require less force, and frequently alternate with non-hand activities. Furthermore, there is no exposure to vibration.

The aim of the study was to investigate the effect of mechanical stress on finger OA by comparing women from an occupation which places distinctive and continuous demands on the hands (dentistry) with women from a different occupation in the same socio-economic grade (teaching). Assuming that the pattern of hand usage among dentists is characterized by stereotyped repetitive tasks for prolonged periods of time, we expected that the hands of dentists will be significantly more often affected by OA than the hands of teachers. We also investigated whether hand use affects the pattern of joint involvement in OA. Based on the wear and tear theory and on the assumption that the dominant hand is subject to a greater daily stress than the non-dominant hand, we hypothesized that the dominant hand will demonstrate a higher prevalence of finger OA than the non-dominant hand.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Study samples
Potential subjects were identified through the registers of the Finnish Dental Association and the Finnish Teachers’ Trade Union (comprising both the occupationally active and non-active). In Finland 98% of the dentists and 95% of the teachers are trade union members. In 2002, a questionnaire was sent to 436 female dentists and 436 female teachers aged 45–63 yr randomly selected from the registers, using the place of residence (Helsinki or its neighbouring cities) as an inclusion criterion. Of those who received the questionnaire, 295 (67.7%) dentists and 248 (56.9%) teachers participated in a clinical examination between October 2002 and March 2003. Mean age of the dentists was 54 yr (S.D. 6, range 45–63). Corresponding numbers for the teachers were 54 yr (S.D. 4, range 45–61). Mean number of years in occupation was 26 (S.D. 7, range 11–40) for the dentists and 24 (S.D. 7, range 1–37) for the teachers. Ninety-four per cent (n = 277) of the dentists and 98.0% (n = 243) of the teachers were occupationally active at the time the study was conducted.

Participation in the study was voluntary and based on informed consent. The Hospital District of Helsinki and Uusimaa Ethics Committee for Research in Occupational Health and Safety approved the study proposal.

Hand radiography and image analysis
Both hands of the participants were radiographed. Kodak X-ray films were exposed with Siemens X-ray equipment (48 kV, 10 mAs, focus film distance 115 cm). The analogue radiographs were inspected on lighted view boxes and were first evaluated by an experienced radiologist (T.V.) who was blinded to the occupation, age and all health data of the subjects. The ratings were subsequently typed into a computer.

Each distal interphalangeal (DIP), proximal interphalangeal (PIP), thumb interphalangeal (IP) and metacarpophalangeal (MCP) joint of both hands was graded separately, and classified for the presence of OA using the modified Kellgren and Lawrence [23] system. The classification criteria were: grade 0 = no OA (normal finding); grade 1 = doubtful OA (finding possibly slightly abnormal); grade 2 = mild OA (a single radiographic sign indicative of OA, slight to moderate lowering of the joint space, sometimes subluxation, minimal osteophytes, degeneration cysts or slight marginal sclerosis, each of the latter signs without a clear narrowing of joint space but little if any additional pathology) (Fig. 1a); grade 3 = moderate OA (considerable narrowing of joint space with additional degenerative pathology as indicated in grade 2, no destruction of the joint) (Fig. 1b); grade 4 = severe OA (joint space destructed or poorly visible with various advanced degenerative changes) (Fig. 1c).



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FIG. 1. (a) Mild OA (grade 2). See text for further definition. (b) Moderate OA. Considerable narrowing of joint space with additional degenerative pathology. (c) Severe OA. Joint space destroyed/poorly visible. Various advanced degenerative changes.

 
The reliability of the readings was estimated by measuring intraobserver and interobserver agreement, using the weighted Cohen's {kappa} coefficient with quadratic weights [24], for 46 randomly chosen subjects. A second reading was independently performed by T.V. and another experienced radiologist (K.L.). The {kappa} values can be interpreted as follows: less than 0.20, poor; 0.21–0.40, fair; 0.41–0.60; moderate; 0.61–0.80; good; 0.81–1.0, very good agreement. The interobserver agreement for OA ranged from 0.67 to 0.85 for DIP joints, from 0.39 to 0.61 for PIP joints and from 0.18 to 0.69 for MCP joints. The intraobserver agreement for OA ranged from 0.73 to 0.88 for DIP joints, from 0.67 to 0.92 for PIP joints and from 0.59 to 1.0 for MCP joints. The matrix of weighted {kappa} coefficients is shown in Appendix 1 (available as supplementary data at Rheumatology Online). The readings of T.V. only were used in the subsequent statistical analyses.

If the subject had at least one finger joint with radiographic OA, she was classified as having finger OA. Two cut-offs for radiographic finger OA were used: OA of grade 2–4 (mild OA) and OA of grade 3–4 (moderately severe OA). A particular joint group (i.e. DIP, PIP, thumb IP, MCP) was defined as having OA if at least one joint of that joint group had radiographic OA. A particular finger group (i.e. thumb, finger and middle fingers, little and ring fingers) was defined as having OA if at least one joint of that finger group had radiographic OA. Symmetrical OA was defined as a subcategory of OA: OA in at least one symmetrical pair of the joints—if the joint of one hand is affected, the same joint of the opposite hand is also affected.

Statistical analyses
The prevalence of OA was calculated for each joint separately. The differences in the prevalence of finger OA between dentists and teachers were compared using Fisher's exact probability test. To assess the relationship between handedness and finger OA, the prevalence ratio (prevalence of OA in the dominant hand/prevalence of OA in the non-dominant hand) and 95% confidence interval was calculated by the Mantel-Haenszel method.

To test whether OA is likely to affect multiple finger joints in some individuals, the subjects were divided into three age groups: 45–49, 50–54, 55–63 yr, and the age-specific prevalence of OA was obtained separately for each joint. The number of subjects expected to have 0, 1, 2, 3 or 4+ joints with OA, assuming that the presence of the disease in different hand joints in a subject is independent of the presence in other joints, was calculated using the Poisson distribution. The distribution parameter used to generate the expected number of subjects was the average number of joints per individual. The observed frequencies were compared with the expected frequencies using the {chi}2 test.

The interrelation of OA occurrence in different finger joints was analysed by logistic regression. First, for each pair of joints, the relationship was described by the conditional odds ratio (OR: the relative odds of having OA in one joint if the other joint is also affected by OA). Then, a single summary OR was obtained for associations within the groups of joints using the Mantel-Haenszel OR estimator. Estimates of OR were obtained for the clustering of joint involvement by row (the same joint of several fingers) and ray (several joints of the same finger involved) in the same hand and symmetrical (same joint, same-finger, opposite hands) involvement of the two hands, and were adjusted for age (year of birth).

To evaluate whether occupational hand load was associated with the location of OA at a particular site (left vs right hand, in the thumb, index and middle fingers, or in the little or ring fingers) a further analysis was performed among women with OA in any finger joint: the odds of having a particular pattern of multiple joint involvement among dentists was estimated by logistic regression analysis, using the teachers as a reference group. ORs and their 95% confidence intervals (CIs) were adjusted for age (year of birth).

Analyses were performed with the SAS statistical software version 8.2 (SAS, Institute, Cary, NC, USA).


    Results
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Prevalence of finger OA
Only 2.7% of the dentists and 4.8% of the teachers were left-handed. The prevalence of OA in each joint group and each finger is presented in Table 1. OA of grade 2 or more occurred most frequently in the DIP joints and the joints of the little finger. The MCP joints and the thumb joints were the least frequently affected by OA. A similar tendency was observed for OA of grade 3 or more. The prevalence of OA of grade 2 or more was higher in the non-dominant hand compared with the dominant (Table 1). The difference between non-dominant and dominant hand involvement reached statistical significance for the joints of the ring and middle fingers [prevalence ratio (PR) = 0.76, 95% CI 0.61–0.95 and PR = 0.75, 95% CI 0.57–0.98 respectively]. The prevalence of more severe OA (grade 3 or more) was slightly higher, although not statistically significantly so, in the joints of the dominant hand compared with the non-dominant hand.


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TABLE 1. Association between handedness (dominant or non-dominant) and prevalence of finger OA among female dentists and teachers (N = 543)

 
Prevalence of finger OA and occupation
The prevalence of OA for each joint separately by hand laterality and occupation is shown in Appendix 2 (available as supplementary data at Rheumatology Online). The prevalence of OA increased with age in both occupations (Fig. 2). The prevalence of OA of grade 2 or more in any finger joint as well as OA in any DIP joint was significantly higher among teachers than dentists (59 vs 48%, P = 0.020 and 58 vs 47%, P = 0.010, respectively; Fisher's exact probability test). There was no difference in the prevalence of more severe OA (grade 3 or more) in any finger joint between the dentists and teachers (13 and 17% respectively, P = 0.19; Fisher's exact probability test).



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FIG. 2. Prevalence of OA of grade 2 or more in any finger joint (Any), in any distal interphalangeal (DIP) and proximal interphalangeal (PIP) joints among dentists and teachers by age categories. The numbers of females (dentist/teachers) were 93/47 in the 45–49 age group, 61/68 in the 50–54 age group, 74/108 in the 55–59 age group and 67/25 in the 60–63 age group.

 
Effect of hand use on the patterns of joint involvement
Table 2 shows the observed frequencies of the number of joints involved per person and the corresponding expected frequencies assuming that there was no clustering within the individual. The clustering of hand joint involvement was statistically highly significant ({chi}2 test, P = 0.001).


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TABLE 2. Observed and expected numbers of hand joints affected by OA in 543 women (295 dentists and 248 teachers)

 
There were distinct patterns of multiple joint involvement in the hands (Table 3). In both occupations, the strongest association was between the ring finger's joint involvement and the little finger's joint involvement of the same hand (OR = 16.90, 95% CI 10.24–27.80 for the dentists and OR = 47.33, 95% CI 21.52–104.09 for the teachers). Furthermore, if one DIP or PIP joint was involved the OR of any other DIP or PIP joint in that hand being affected increased about 3-fold among the dentists and about 6-fold among the teachers. The clustering of finger joint involvement in OA was symmetrical. Clustering within the ring and little fingers, clustering by row, and symmetrical joint involvement were clearly more apparent among the teachers than the dentists (age-adjusted OR = 1.57, 95% CI 1.10–2.23, OR = 1.84, 95% CI 1.28–2.64, and OR = 1.98, 95% CI 1.38–2.86, respectively).


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TABLE 3. Presence of OA of grade 2 or more in a particular finger joint according the OA status of other finger joints

 
The patterns of joint involvement for moderately severe (grade 3 or more) OA were different between dentists and teachers. Among the dentists, OA (grade 3 or more) was more frequently clustered within the thumb, index and middle fingers (OR = 1.80, 95% CI = 1.16–2.80) than within the ring and little fingers (OR = 1.47, 95% CI = 0.95–2.27). The pattern among the teachers was quite the opposite, with moderately severe OA more frequently clustered within the ring and little fingers (OR = 9.75, 95% CI = 4.78–19.88) than in the thumb, index and middle fingers (OR = 1.50, 95% CI = 0.96–2.23). No symmetrical occurrence of moderately severe OA was observed in either occupation (OR = 0.83, 95% CI = 0.54–1.26 in dentists and OR = 0.90, 95% CI = 0.58–1.40 in teachers).

The age-adjusted OR of having a particular pattern of OA among the dentists compared with the teachers was calculated (Table 4). Among the dentists with OA of grade 2 or more in any finger joint, the OR of having it in the ring and little fingers of the left hand was significantly lower than the corresponding OR among the teachers with OA (OR 0.16, 95% CI 0.05–0.47). However, the OR of more severe OA (grade 3 or more) in the thumb, index and middle fingers of the right hand was significantly elevated among the dentists with severe OA compared with the teachers (OR 2.61, 95% CI 1.03–6.59).


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TABLE 4. ORs of finger OA patterns as a function of occupation among those with OA in any (≥1) finger joint

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
This study investigated the association of extensive hand use with finger OA and characterized the patterns of hand joint involvement among middle-aged women from two occupations with different hand loads. Our findings provide evidence for a high prevalence of radiographically defined finger OA in women aged 45–63 yr and for that of a polyarticular subset of finger OA. The joints most commonly affected were the same in both occupations, i.e. the DIP joints, while the MCP joints were less often involved. Clustering within the little and ring fingers, row clustering and symmetry were the most pronounced features in the patterns of joint involvement. Clustering within the thumb, index and middle fingers was more commonly observed in the dentists.

Effect of hand load on the pattern of joint involvement
We examined the pattern of joint involvement by using mild and moderate/severe cut-offs for radiographic finger OA. The clustering of affected joint groups within a hand was similar to that observed in previous studies [5, 6], with a stronger relationship between multiple involvement of the same joints in several fingers than with involvement of joints of any given ray. A high degree of symmetrical hand joint involvement was also confirmed. Furthermore, clustering within the little and ring fingers was revealed.

The remarkably high rate of OA co-occurrence in the joints of the little and ring fingers can not be easily explained by the impact of joint loading only. Possible explanations for this result include an inherited predisposition to OA. The tendency towards symmetrical joint involvement also suggests the importance of genetic [25] and systemic factors [26] in the aetiology of OA. It is possible that there are differences between the two occupations in the presence of risk factors or protective factors. The socio-economic status of the dentists and the teachers was similar, although the average income of dentists tends to be higher.

A number of differences in the pattern of joint involvement between the two occupations were discovered. In the teachers, there was a tendency to develop multiple joint involvement in the little and ring fingers, in the same row of fingers (same joint but different fingers), and symmetrically. The dentists were more likely to develop severe OA in the joints of the right hand's thumb and the index and middle fingers compared with the teachers. These fingers are the ones used by dentists in holding instruments accompanied by precise and repetitive movements. These fingers seem to be subject to overload rather than limited mechanical stress.

Hand use and finger OA
Our data did not support the hypothesis of increased radiographic OA in the dominant hand compared with the non-dominant hand. This is in conflict with the findings of the New Haven Survey of Joint Diseases, in which a statistically significant increase in the prevalence and severity of OA in the dominant hand was observed [8]. No radiographic or clinical differences were found between the right and left hands in a study of 134 consecutive community subjects with a detailed description of the hand use history [14]. In agreement with the Mini-Finland study [16], the prevalence of OA was higher in the joints of the non-dominant hand than in the dominant hand. However, Cvijetíc et al. [27, 28] and Egger et al. [5] reported the reverse association.

In a Virginia textile mill, female workers whose jobs required a repeated pinch grip had a much higher rate of OA in DIP joints than other female workers [10]. The results of the present study showed that the prevalence of OA in any finger joint or in any DIP joint of the dentists was lower compared with those of the teachers. Lehto et al. [14] did not find any differences in the prevalence of OA between female dentists and female controls randomly selected from the general population in a study of radiographic arthritis of the hands in 136 dentists.

The current findings regarding higher prevalence of mild degenerative changes in the little finger compared with other fingers, in the non-dominant compared with the dominant hand and in the teachers compared with the dentists indicate that wear alone may be insufficient to explain the prevalence and location of OA. This raises the question whether the non-dominant hand and the little finger, which are engaged in somewhat more static activities compared with the dominant hand and other fingers, could be subject to some specific stress. It seems more plausible, however, that persistent and intensive hand use increases the strength of muscles and ligaments and through this or other mechanisms protects against wear and tear injury. The results of the study by Rogers et al. [29] showed that moderate/high joint stress was associated with a reduced risk of hip/knee OA among women.

The prevalence rates of radiographically-defined osteoarthritis in this study were higher than those reported in several epidemiological studies [4, 6, 30, 31] and similar to those found in the Zoetermeer survey [32] and the Mini-Finland study [16]. Our findings on the prevalence of OA in different joint groups are consistent with the results from other studies [5, 16, 31]. However, some studies found a higher prevalence of OA in MCP than in PIP joints [4, 32]. The forces generated by diverse hand activities produce different loads on the finger joints. The MCP joints are known to be especially affected by heavy manual labour [33]. Schmid et al. [34] found that the distribution of OA in the hands with overproportional involvement of the MCP joints was associated with lifelong farming. Using biomechanical analysis of loads in the finger joints, Radin et al. [9] found that forces were greatest across the DIP joints and that fine pinch grip resulted in greater mechanical stress than power grip. Therefore, the preferential involvement of the DIP rather than the MCP joints in hand OA among dentists and teachers in the present study was expected.

Methodological considerations
Some of the discrepancies of our findings with the previously published reports can be due to differences in the classification criteria. Most studies to date have used the system of grading radiographic severity (0–4) developed by Kellgren and Lawrence [23], though there has been some variation in the description of these grades [35]. The Kellgren and Lawrence criteria perhaps put too much emphasis on osteophytes, assuming that joint space narrowing occurs after osteophyte formation [36]. A number of alternative grading systems have been developed to overcome these deficiencies [37–39]. A recent study by Neame et al. [12] showed that osteophytes were more marked on the right side for all DIP and PIP joints, whereas there were no right–left differences in joint space width. The classification system used in the present study gave less weight to the osteophytes, especially in the definition of mild OA (of grade 2 or more).

The reliability of the OA classification criteria is critical for the credibility of results. While there was substantial variation (especially for the less frequently affected joint groups) in the readings between the two radiologists, intraobserver agreement was generally good. The lower level of interobserver agreement was not surprising. Despite training and the use of reference images, each reader graded the radiographs according to his or her own inherent standard about what constituted a positive finding. The high intraobserver agreement suggests that the classification criteria applied here are highly reproducible. Because all radiographs were evaluated by one observer who was blind to the subjects’ occupational status, the high intraobserver repeatability implies that the comparison between teachers and dentists was unbiased.

The assumption made in the last set of the logistic regression analyses led to reduction in the number of subjects to be analysed. Thus, the estimation of the odds of OA location at a particular site, especially in the thumb, index and middle fingers, may not be precise.

Representativeness of the sample
There was some difference in the study participation rate between the two occupational groups, especially among the participants aged 45–49 and 60–63 yr. No teachers belonging to the age group of 62–63 yr took part in the study. Although the official age for old-age pension in Finland is 65 yr, the actual average retirement age is 59 yr [40]. Workplace stress is a significant reason for reduced hours or part-time retirement, particularly for workers in their 60s [41]. Perhaps it is the heavy workload of the teachers, characterized by long hours devoted to teaching-related activities, and relatively low reward through monetary compensation that tend to push the teachers towards early retirement. The registers of the Finnish Dental Association and the Finnish Teachers’ Trade Union, through which the potential subjects were identified, include both occupationally active and non-active persons. Thus, the largest difference in participation rate, observed among the older age group, may be due to a difference in willingness to participate in a study on workload and health. Among the younger age group, where the difference was smaller (63% for the dentists and 58% for the teachers), those women who were occupationally active, had experienced symptoms and/or had a family history of arthritis were possibly more willing to take part in the study. However, the most noticeable difference in the prevalence of OA between the occupations was observed among those aged 50–54 yr, in whom the participation rates were similar (62% for the dentists and 60% for the teachers). Thus, the difference in participation rate is not likely to account for the difference in the prevalence of OA between the occupations.


    Conclusions
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
The present findings suggest that moderate hand use does not have a detrimental effect on finger OA but may even be protective, whereas continuous joint overload may lead to joint impairment. Furthermore, the excessive prevalence of OA found among the teachers, and in the non-dominant hand joints of both occupational groups, as well as the noticeable symmetry of joint involvement seem to suggest the importance of non-mechanical aetiological factors.


    Acknowledgments
 
This work was financially supported by the Finnish Work Environment Fund.

There is no conflict of interest.

Supplementary data

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Supplementary data are available at Rheumatology Online.


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 

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Submitted 9 November 2004; revised version accepted 1 December 2004.



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