8.5 Work-related upper limb disorders
8.5.2 Over-use injuries to process workers
1992, 1994, Cohen et al. 1992, Gibson et al. 1991, Helme et al. 1992). Via various feedback loops in the CNS, disturbances of motor control and bloodflow may also ensue. The entrapment or irritation of peripheral nerves (which physiotherapists call
‘adverse neural tension’) may also be part of the picture, although how this relates to the other mechanisms is again not clear.
Psychological factors may well play a role in this process, but they are certainly not the sole causative factors involved—and there is good evidence that in many cases psychological symptoms that RSI victims report (anxiety, depression) are secondary or tertiary developments from the initial physical disorder (see below). In other words they are consequences, not causes. Sleep disturbance may be an intervening causative link.
The classification of the conditions falling into the overall RSI/WRULD category into Types I and II is a useful one—but it is something of an oversimplification. Some of the possibilities are summarized in Table 8.5 and Figure 8.15 (see also Pheasant 1994).
although job rotation is always in theory to be encouraged, it may be of minimal benefit if the jobs between which the person rotates all have similar biomechanical profiles—that is, if they impose similar patterns of loading on the muscles and soft tissues.
The magnitude of the forces that it is necessary for the person to exert in the task in question is also relevant. In an elegant study of workers from a number of industrial plants, Silverstein et al. (1986) compared the prevalence of WRULDs in those whose jobs entailed high and low levels of force and repetitiveness respectively. Both force and repetition were statistically significant risk factors on their own—and the combination of the two carried a particularly high level of risk.
This basic pattern of association was confirmed in a subsequent study of the specific disorder carpal tunnel syndrome, which was reported by the same authors, although in this latter study, force on its own was not a significant risk factor (Silverstein et al. 1987). The data of both studies suggest that when force and repetition are combined, their effects are at least multiplicative. Overall, these findings are just about what we should expect, on the assumption that the pathologies of these conditions are the consequence of ‘abnormal wear and tear’
which results from hand-intensive work.
It is generally recognized that the jobs that carry the highest levels of risk are ones in which forceful gripping actions are combined with turning actions and/or are made with a deviated wrist. In practice, turning actions will generally involve wrist deviation; but you can have a deviated wrist without making a turning action (e.g., when using certain badly designed hand tools).
Anatomical and biomechanical considerations would lead us to predict that there will be certain relatively consistent associations between particular disorders and particular patterns of movement. For example, we should expect ‘clothes wringing’
types of action (in which flexion and ulnar deviation are combined with supination of the forearm) to result in tenosynovitis or peritendinitis affecting the tendons of the extensors (particularly those that act on the thumb); and likewise that repetitive flexion and extension of the wrist would lead to carpal tunnel syndrome (either directly due to mechanical irritation of nerve, or secondary to a flexor tenosynovitis).
(For an explanation of these anatomical terms, see section 5.2.)
Experience suggests that these predictable associations do indeed occur reasonably consistently—and such little epidemiology as there is would tend to confirm this (Pheasant 1991a). But they do not by any means occur infallibly—and in practice, just about any of the disorders in question may be associated with any of the motion patterns. In one way this is not particularly surprising. The functional anatomy of the hand is complex. All of the forearm muscles, whose tendons cross the wrist to insert on the bones of the hand, have multiple functions—as prime movers, synergists or muscles of stabilization. It would seem probable, furthermore, that the various anatomical structures of the ‘muscle tendon unit’ (i.e. the muscle itself and its soft-tissue attachments at either end) may be subject to over-use injury both when they repeatedly contract and when they are repeatedly stretched by their antagonists. So the number of potentially injurious permutations and combinations is considerable.
The particular type of gripping action that the task entails is also a factor. Pinch grips and claw-like grips (i.e. ‘precision grips’) both entail a higher internal bio- mechanical loading for a given externally applied force (and thus a higher level of risk) than full grasping actions (i.e. ‘power grips’; see section 5.3). The ‘over- spreading’ of the hand is also a risk factor.
There is epidemiological evidence that carpal tunnel syndrome may be caused by the use of vibrating tools (Cannon et al. 1981). It may also be caused by repeated impact as in using a hammer. Tenosynovitis of the finger flexors may likewise be caused by repeated blunt trauma—for example when using badly designed hand tools that have pressure ‘hot-spots’ on their handles.
It is widely recognized that people become accustomed to repetitive (and otherwise hand-intensive) work over a period of time. This process of adaptation is sometimes known as ‘work hardening’. It probably has a number of physiological components. A simple muscle training effect is part of the story, as in all probability is the greater ‘economy of movement’ that comes with increasing skill. But the physiology may well be more complicated than this, and the process of adaptation may well be confounded with ‘survivor effects’ (i.e. those people who are unable to adapt leave the job).
Whatever the underlying physiology, however, it stands to reason that, given that such processes of adaptation do indeed occur, then newcomers to a particular job will be at an elevated level of risk as compared with ‘old hands’. This has particularly been shown to be so for peritendinitis crepitans (Thompson et al.
1951). By the same token, we should expect any change in working practices that entails an increase in task demands to result in an increase in injury rate. Again this is borne out in practice. It also seems that people lose some of their physiological adaptation during holidays, periods of sickness absence and other lay-offs. (This is sometimes called de-adaptation.) So when they return to work they are at risk (Thompson et al. 1951).
It must be stressed, however, that these conditions are by no means to be regarded solely as ‘training injuries’, in that although unaccustomed work is clearly an important risk factor, the process of injury may also occur insidiously, leading to the onset of the condition in a seasoned worker and without any change in working practices being involved. It may be that when the condition arises in this way, it is because the person’s capacity to tolerate hand-intensive work is diminishing with time (due to the cumulative effects of the work itself or to normal ageing) until the point is reached when the demands of the task come to exceed that capacity.
We are now in a position to summarize the principal ergonomic risk factors that are associated with over-use injuries to the hand, wrist and forearm in process workers. These are set out in Table 8.6.
Workers on assembly lines are also prone to over-use injuries to the muscles and soft tissues of the neck and shoulder region. These are most commonly caused by working for lengthy periods with the arms in a raised position, and/or from making frequent or repeated reaching actions (particularly overhead reaching or reaching behind the body).
Of the various WRULDs that affect the process worker, tenosynovitis has traditionally been regarded as the most important in the UK, whereas in the US carpal tunnel syndrome has the greater prominence, and in the Scandinavian countries ergonomic and epidemiological studies tend to focus on conditions affecting the neck and shoulder region (such as ‘tension neck’ or trapezius myalgia). It is difficult to account for these differences in emphasis. It seems wholly unlikely that they would reflect underlying differences in the true prevalence of these conditions. Nor does it seem likely that the conditions (at least as they are normally defined) can be mistaken for each other by anyone with more than a rudimentary knowledge of such matters.