Textbook of Maritime Medicine

Introduction

Musculoskeletal pain causes major diagnostic problems for many physicians. The undergraduate teaching is sparse, and each doctor has variable and mostly limited post-graduate training relating to these diseases. It is perceived by many as difficult to assess the patients and it may take a long time to incorporate the history and examine them. We might even sometimes be in doubt about whether our investment of time and resources in a clinical investigation of these patients really makes a difference. This perception stands in contrast to the major importance of these disorders – not least in an occupational context. Precisely the high prevalence and consequences for the work-ability for our patients and for businesses and communities must commit us to do what we can.

Worldwide, soft tissue and joint diseases including those of a degenerative character are prevalent, in particular in the aging population. These disorders account for the largest proportion of lost “good” years in terms of experiencing functional limitations or suffering from pain in the limbs or the back. The widespread occurrence of locomotor disorders in the maritime setting is no exception. This section does not intend to provide a comprehensive list of musculoskeletal diseases with description of their diagnosis, etiology and management. It is rather the aim to extract specific issues relating to locomotor disorders that may be of particular importance to clinicians dealing with seafarers.

Little research on the epidemiology of musculoskeletal disorders among seafarers has been performed. However, the frequency has been demonstrated previously [1], and it is regarded as likely that the conditions are the same today. In a study of the causes of morbidity among British merchant seamen, disorders in the musculoskeletal system accounted for the largest single reason for reporting sick (28%). Most of these were traumatic and the remaining conditions were dominated by degenerative conditions such as osteoarthritis and low back pain [2]. Surveys have also documented an increased prevalence of musculoskeletal symptoms among commercial fishers in Sweden [3] and in USA [4] where low back symptoms were found to be the most common cause of work impairment followed by pain in the hands and shoulders. Increased hospital contacts for injuries and a number of locomotor diseases among seamen (carpal tunnel syndrome for deck crew) and in particular fishermen (knee osteoarthritis, rotator cuff syndrome, carpal tunnel syndrome) has been found in Denmark [5]. Raynaud’s syndrome was also increased [6]. Previous accidents account for a major part of the morbidity. A high prevalence of rheumatoid arthritis has been demonstrated among fishermen and the influence of heavy outdoor work has been suggested [7].

The statutory medical certification of seafarers regulates this area to a limited extent only. Based on the medical examinations, seafarers with severe locomotor conditions are prevented from employment on a ship, or may be restricted with regard to the duration of employment time or type of services. The focus here is on diseases that impose serious functional limitations relating to work and in particular emergency situations. Examples may be grave cases of, e.g. inflammatory joint disorders such as rheumatoid arthritis, severe osteoarthritis of hip or knee and vertebral spondylosis, or herniated disc with root compression.  Some countries may go further than that by demanding the complete absence of any such condition of even minor severity and also of other mild locomotor ailments, thereby consequently preventing the employment on board a ship of a major part of seafarers, in particular the elderly ones.

Still, in a practical context many seafarers and fishermen in active occupation do suffer from locomotor health consequences of accidents or from age-related degenerative conditions that may restrict them from certain functions on board. Some of these conditions may be work-related, i.e. actually caused by a risk factor on board, examples being chronic low back problems after shoveling ice in a fishing vessel for an extended period of time or an epicondylitis after intensively scraping of rust or painting on board. Other disorders such as lower limb osteoarthritis may be associated with environmental exposure as well but is also related to conditions such as obesity that are prevalent in the maritime population. They may also occur independently from work-exposures or other factors in the maritime setting from which, however, exposures may lead to further progression of the disorder or at least to an aggravation of pain during certain work tasks.

In this situation, the challenge is to accommodate seafarers and fishermen that are able to fulfill their duties on board but still experience degrees of constraints in certain situations. This challenge accounts for the individual seafarer as well as for his supervisor and also for the advising physician. For many such situations it should be recognized that pain aggravation does not necessarily implicate progressive tissue damage. I.e. with uncomplicated low back pain, the best treatment is to pursue normal activity including work-activity while at the same time not misusing the back by for example ongoing bending or extreme heavy lifting or dragging. We are now aware that back exercises benefit the low back patients through strengthening the back and trunk musculature and, in fact, this is the advised treatment for most low back patients. The maintenance of normal use of the back during work should be viewed in this context.

However, while continued work is generally advocated with most locomotor disorders this does not necessarily mean that the work should always be executed in the same way as usual. Evidently, it is wise to adapt the tasks and the way the work is done so that the seafarer is not bothered more than necessarily.  In many situations one can also learn from such adaptation. The seaman with a locomotor problem can be viewed as a particularly vulnerable person that should be accommodated according to his limitations, but also as a measuring tool that can indicate exposures that may be collectively harmful – not only to the vulnerable individual seafarer but – i.e., if the symptoms reflect a potential collective risk – also to his colleagues. Therefore, the considerations that are taken to the symptomatic seafarer can turn out to be also beneficial to others by leading to a general improvement of the work-methods or the ergonomic circumstances on board.

The overall advice with regard to accommodating seamen and fishers with milder locomotor disorders on board would be to promote the concept that there is room for everyone, and consequently that all crew members can work safely with due respect to their ailments. Possibly, the main constraint here is the psychological character of seafarers and fishermen many of which share the personality of a macho culture with little emphasis on their own well being and more focus on the task. This attitude is likely to expose them unfavorably to excess harmful exposure.

10.9.1   The diagnoses

These diseases do not occur randomly but are rather related to specific stressors and loads in the maritime environment. This, however, is not particularly well documented scientifically, and therefore not recognized by all. Precisely this relationship may provide diagnostic assistance by the history of exposures. Theoretical considerations with regard to the body structures loaded by a given physical exposure may naturally lead to a particularly detailed study of precisely these structures at the physical examination. Similarly, the description of a traumatic event is likely to give rise to hypotheses about the nature and location of the lesion, which may then be studied closely at the physical examination. It is my experience that such considerations are worthwhile.

Another aspect to be emphasized is that the body must be perceived as a coherent structure. A lesion in one place can lead to a changed function elsewhere and therefore to pathology here. Symptoms can spread out and even dominate at an entire third place, as phenomena such as referred pain, radicular pain or the spread of pain in the course of a particular nerve. Contralateral spread is also a well-known phenomenon, which can be explained pathophysiologically. These examples show that the disease is not necessarily located where symptoms dominate. The recognition of this fact requires us to conduct a broader assessment of the patient and a physical examination that extends beyond the symptomatic area(s).

To the experience of the author, the challenges that physicians meet when they diagnose locomotor conditions whether or not of a work-related character may result in a tendency to limit the content and thoroughness of the physical examination of patients. In contrast, the examiner may be inclined to rather rely on paraclinical studies such as imaging or electrophysiological studies of nerves. This is unfortunate and may result in patients being misinterpreted and misdiagnosed and subsequently given wrong or no advice and treatment. It is stressed that plain radiographs are often of little assistance and that even MR imaging and nerve conduction velocity and electromyography may be completely normal even with serious conditions and may also be abnormal in non-symptomatic seafarers.

The diagnostic process aims to identify the tissue with the lesion (e.g., muscle, bone, tendon, insertion, ligament, nerve), more specifically, what structure (e.g. which muscle or tendon), and which pathology (e.g. inflammation, tumor). The history and the physical examination may – provided it is sufficiently thorough – explain the condition in most situations.

Pain is usually the main symptom in musculoskeletal disorders. It is of importance to clarify the nature of pain, whether nociceptive or neuropathic, because the background and treatment differs for these two types of pain. Neuropathic pain may emanate from parts of the nervous system - including nerve roots and peripheral nerves while nociceptive pain is related to other tissues.

Neuropathic pain has a tendency to spread to larger and larger areas (also contralaterally). It is often of a burning or neuralgia like nature and responds poorly or not at all to common analgesics. Typically, it is exacerbated by use and may persist for a long time subsequently. It is frequently accompanied by sensory phenomena such as paresthesia or allodynia, and by subjective weakness. There will be tenderness over the involved nerve segment, while the Tinel's sign (which indicate ongoing nerve-regeneration) should not be regarded as an obligate phenomenon. This pain may often be misinterpreted as nociceptive and involving non-nervous structures or – worse – may be regarded as psychogenic because a standard physical examination does not indicate any pathology.

Nociceptive pain usually remains localized and responds to common analgesics. The pain typically worsens with use and improves/disappears at rest. Sensory phenomena such as hypalgesia or allodynia are absent and there are no paretic muscles. Tenderness is localized over structures such as a joint, a tendon or an insertion and aggravated by compression, by active use or by movement such as passive stretching of a tendon or manipulation of a joint.

The assessment involves information about the dissemination of pain and provocative factors. Combined with a judgment of the character of pain, one can frequently predict findings that are to be expected at the following physical investigation. In case of a neuropathic pain, it is likely to find patterns of neurological abnormalities at the physical examination. When pain is not assessed as neuropathic, one can rather expect signs suggestive of pathology in tendons or insertions ions (local tenderness, pain provocation by passive drag or active stretch, and possibly swelling, heat or redness), bursa and joints (pain provocation on movement, restricted movement), or ligaments (local tenderness).

Of course, conventional radiography, MRI, ultrasound examination and isotope scintigraphy are important tools, but they can never replace a physical examination. For example, "pathological" MRI changes suggesting, e.g. disc herniation, are frequent in people who have never had back complaints. Thus, there is limited diagnostic specificity. Similarly, the absence of MRI changes does of course not implicate absence of disease. This knowledge may be trivial to the doctor but not to patients and laypersons. Therefore, the physician should communicate his knowledge in order to prevent misinterpretations.

For many diseases, treatment is not just an opportunity for accelerated healing but sometimes a necessity for the patient to get better and to preserve a measure of functionality. This, however, does not always apply for musculoskeletal disorders, many of which often have a chronic course and may involve tissue degeneration for which little or nothing can be done

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Tendinitis, bursitis and other upper limb conditions

The terms tendinitis and tenosynovitis represent painful inflammatory conditions that – similar to affliction of tendinous insertions – may be provoked by straining of a tendon, especially with tasks to which the seafarer is unaccustomed. Active use of the tendon will be painful, and there may be warmth and redness. Especially in the case of inflamed synovial membranes, there will also be swelling and crepitation. In epicondylitis there is mostly not inflammation but rather degeneration at the insertion on the bone (tendinosis). There will be local pain with pressure at the tendon/insertion. The function of the bursae is to fender off the motion of tendons and muscles near to the large joints such as the elbow or knee.  During strong and repetitive strain, the pouches may produce fluid and painless or tender swelling may or may not be accompanied by signs of inflammation. Both conditions require rest and often immobilization. The prevention of relapse it is essential to identify the cause of the disorder.

Example 1: Lateral epicondylitis or "tennis elbow" is characterized by elbow pain provoked by vigorous use of the hand. There is tenderness at the insertion of wrist extensor muscles on the lateral epicondyle where wrist extension provokes pain. Micro-Lesions in the tendinous insertion at the epicondyle may occur during a fierce passive stretch or similar hard impact, such as prolonged or intense use of a heavy hammer. Such pathology, however, would not be expected by simple repetitive work, unless it has been extremely powerful. Since nerve afflictions such as radial tunnel syndrome or pectoralis minor syndrome are important differential diagnosis, the neurological qualities should be examined from the neck and include the whole limb. Lateral epicondylitis and radial tunnel syndrome may co-exist in the same patient. The pain should not be provoked and powerful strain of hand and forearm should be avoided.

Example 2. Rotator cuff disorders may be located in tendons and bursa, and may also involve other structures such as nerves in the vicinity of the shoulder joint. Rotator cuff syndrome often consists of a supraspinatus tendinitis, but other parts of the cuff may also be affected. Since perfusion pressure in the supraspinatus tendon drops to 0 with even minor shoulder abduction, static work with the arm away from the body is a risk factor for degenerative changes here. Likewise, powerful work reduces the perfusion of tendons and contributes to the physiological age-dependent degeneration, which may thus facilitate inflammation with shoulder strain. Therefore rotator cuff tendinitis is rare in younger patients, but frequent among elderly people in particular following long standing heavy physical work. A not unusual exception is when the subacromial space is reduced and the tendon therefore compressed, such as may result from compromised scapulae stability with hanging and/or inwards rotated shoulder. As with other tendon-related pathology there is tenderness over the injured tendon/insertion and pain will be provoked by powerful use of the associated muscle. A “pain arc” is a characteristic feature with supraspinatus tendinitis, i.e. pain, which increases by abduction to the horizontal level and then decreases again on further abduction. With a reduced subacromial space, collision occurs with the impingement maneuver and causes pain. A positive drop-arm test signifies a rotator cuff rupture. A reduced passive movement in the glenohumeral joint can be examined by assessing the outward rotation with the upper arm close to the body. Reduced passive mobility in the glenohumeral joint suggests a periarthrosis.

It is obviously not good enough to equate shoulder pain and rotator cuff syndrome. It is important to identify the nature of the diseased tissue. Consequently, the physical examination should not only focus on the tendons and insertions, bursa, and insertions but also on bone and nerves. Therefore, besides inspection, assessment of pain provocation at certain movements and local tenderness, the neurological qualities should be carefully studied, including an estimation of the power of representative individual muscles and the sensibility in selected homonymously innervated areas of the skin.

Many patients who are interpreted as suffering from rotator cuff syndrome, do not meet the above requirements for the diagnosis of a rotator cuff tendinitis, and many of these have neurological abnormalities suggesting an involvement of the brachial plexus or other parts of the upper limb nerves. By shoulder asymmetry with a drooping shoulder – for example due to constraints such as a poor agonist-antagonist balance – or with a bony prominence, e.g. caused by an osteophyte in the acromioclavicular joint (osteoarthrosis here is related to physical stress) may compromise the available subacromial space and lead to impingement.

The pain in rotator cuff syndrome should not be provoked, and the arm should be used near the body.

Example 3. Upper limb symptoms related to computer work include pain which may be located anywhere in the upper limb and often with changing dominance of different locations. The pain is of a neuropathic nature and often accompanied by peripheral paresthesia and subjective weakness.

When working with a computer mouse, the dominant arm is stretched forward and the forearm is almost maximally pronated while performing at the same time static-repetitive flexion and extension of fingers and fine static shoulder movements in the horizontal plane of the lateral and medial direction. Therefore certain forearm muscles (m. supinator) are stretched while others are shortened (m. pronator teres). Around the shoulder, the rhomboid and trapezius muscles are stretched while other muscles are contracted and shortened (pectorals). A tense muscle can tighten over one or more nerves with a pressure that exceeds their capillary perfusion. The risk of disease is likely to be increased by intensive work or suboptimal ergonomic conditions. A neurological examination can identify weakness in muscles innervated distal to the brachial plexus and associated characteristic sensory changes. One should not expect absolute paralysis, but only degrees of minor pareses. Similarly, one should not expect analgesia or anesthesia in the investigation of cutaneous sensibility, but only minor changes such as slightly reduced, increased or just altered sensibility. In more severe cases there may be allodynia, which is a condition where a normally non-painful stimuli as touch, cold or vibration leads to pain.

Upper limb activities should be varied, without force, and close to the body. Provocation resulting in pain that is worse after use of the arm should be avoided.

Generally, upper limb disorders are said to represent an inflammation that can be treated by anti-inflammatory medication and by ensuring that the inflamed structure is not provoked so that the inflammation is maintained. Conversely, anti-inflammatory treatment does not help in the absence of inflammation. Neuropathic pain should also not be provoked. Therefore it is important to limit the use of the arm so that the pain subsequent to the activity is not worse than before or during activity. The limb should preferably not use power, or work repetitively or away from the body. Training – e.g. of weak muscles should be done cautiously and only aim to restore muscular balance. Denervated muscles should not be trained. Attempts to do so, e.g. in the gym or by swimming will exacerbate the condition. I perceive painful training as a serious risk factor and consider it important to draw attention to this, because many patients follow doctors' well-intentioned advice about training with weights or swimming, and because many patients often believe that pain "must be worked out." It may be important to stretch tightening structures to reduce tension and promote tissue mobility, including the mobility of nerves, and some physiotherapists are good at it. Ergonomic adjustment aiming to counteract relapse should always be undertaken. 

Osteoarthitis

Osteoarthritis is an extremely common locomotor disease. It may be a “primary” idiopathic condition or it may develop “secondary” to individual constitution or external exposure such as large body weight, heavy work, or injury. The prevalence is rising with age. In actively working people such as e.g. fishermen and deck crew, the most afflicted joints include the acromioclavicular joint, the knee and the hip joint. The disease starts in the cartilage of the joint, but may progress to the adjacent bone and connective tissue. The symptoms include joint pain and stiffness, which is initially present during exercise, and later also on rest. Typically, the pain with osteoarthritis has the character of a “triad” so that pain is present at the start of exercise, e.g. walking, to be reduced or absent after a while and then recurring after some further distance. There may be visible changes such as swellings and disfiguration, and movements may be restricted and painful.

More occupational walking and standing and less sitting are associated with symptomatic knee osteoarthritis, and more bending, twisting and reaching is associated with symptomatic hip osteoarthrosis [8]. Based on metaanalyses, an international study group has concluded that healthy subjects as well as osteoarthritis patients in general can pursue a high level of physical activity, provided that the activity is not painful and does not lead to trauma. In contrast, work activities that produces or maintains pain should be avoided [9].

In a study of knee pathology, seafarers constituted ¼ of patients in an orthopedic clinic. Half of them had a diagnosis of knee osteoarthritis, with a predominance of medial compartment cartilage degeneration. 1/3 of the seafarers had genu varus [10]. The particular vulnerability of the medial compartment (in contrast to the lateral compartment) may be explained by biomechanical factors, and the apparently increased prevalence among seafarers seems to be related to exposures in the maritime setting such as the moving ship and climbing ladders.

Low back problems

The most common conditions include lumbago, which may be triggered by a sudden, uncontrolled strain or by strong muscle effort. I some cases a ruptured intra-vertebral disc may be signaled by an acute attack of lumbago. The symptoms are sudden and strong pain in the lower back, often strongly restricted motion, the back may appear crooked as a result of cramped muscles and pain, and the affected muscles are tender to pressure. The patient should receive a program with strengthening back exercises.

With “sciatica”, neuropathic pain is referred in the sciatic nerve territory. The cause may be found at any level along this nerve as well as near to the roots supplying neurons for the nerve. Pain referred to a limb tends to be mostly regarded as due to a disc herniation, and may of course also afflict roots supplying other peripheral nerves than the sciatic nerve. In most cases, however, irradiating pain represents a phenomenon (referred pain) secondary to a myofascial or ligamenteous problem in the low back and sacral region. Similar pain may be caused by a piriformis syndrome. Root compression causing limb pain reaches usually below the knee. The limb pain is usually worse that the back pain and is aggravated by specific movements, cough and sneeze. With a manifest root compression, severe pain radiates down the leg and is easily provoked, e.g. by elevating the stretched leg. The pattern of muscular weakness, sensory abnormalities and weakened deep reflexes can indicate the level of compression.

A few cases of disc herniation should be viewed as emergencies: The sudden disappearance of pain with persisting paresis, and urine or stool incontinence (cauda equina syndrome). While these conditions should be treated surgically as soon as possible, there is little evidence that other cases of root compression are better treated by surgery than conservatively.

Non-specific chronic low-back pain as well as pelvic pain is extremely common. The latter is frequently associated with pathology around the sacroiliac joints and the pelvic ligaments. It is frequently overlooked and confused with low back pain.

The history and a thorough physical examination can usually rule out "red flags" and therefore also the use of radiological studies, which costs money, and rarely helps the patient. Radiological studies may even harm. Despite the correlation with age rather than with symptoms, “abnormalities” such as degenerative changes are frequently detected and carefully described, and may eventually turn out to become the “diagnosis”.

Degenerative changes in the lumbar region can of course be related to overload, but they may and do as well occur in people who never had physical work. Age equivalent degenerative changes, of course, does not protect the seafarer from further damage by future trauma or cumulative impacts, the consequences of which may not be visualized on x-ray. But nothing suggests that there are fewer symptoms if the patient is relieved from normal everyday use of the back, and despite the absence of differential treatment guidelines based on degenerative radiological changes it is reasonable in most patients to expect reduced pain by continuation of the normal daily activity. Inactivity and sick leave are usually unnecessary and can be harmful. Evidently, major back problems argue against continued exposure to heavy lifting, awkward postures and / or movements. There is no reason to trouble the patient further. By suspending contact with the workplace, however, there is a risk of isolation and undue concern by the patient over the future health of the back. Many fear even with normal use of the back to end up as disabled. Fortunately this is a rare occurrence - and when it happens, it may perhaps relate to well-intentioned medical interventions rather than to than lack thereof.

Most doctors are aware of this newer understanding, which have contributed to a better ability to help the low back patient. Still, however, sick leave remains the common treatment of an uncomplicated back disorder, and low back pain remains one of the dominant reasons for sickness absence. In addition, the patients are typically treated with NSAIDs and referred to physiotherapy and/or an x-ray of the spine. With continuing complaints, referral to a specialist and MRI will be the next step in most situations. None of these are necessarily indicated.

If the doctor lets the patient understand that the pain is caused by a radiologically proven degeneration, the patient perceives that his back is "worn out", often as the result of a heavy work, and that it can go very wrong by further loading of the back, because the radiological change are likely to get only worse unless the back is spared for loading. Such a "diagnosis" that clarifies the situation to the patient is also convenient for the physician. But without careful clinical examination the real explanation may be overlooked, and the easy explanation rarely benefits the patient.

When sick leave is a reality, it is hard to get going again as long as the pain continues. And if the doctor find indication for sick leave in the primary stage - why not later when the pain remains? But there are fortunately alternatives to sick leave. The patient and the physician may agree on the required considerations for accommodating the patient at the workplace, which is mostly willing to comply with such advice – particularly when the likely alternative could be a lengthy sick leave.

The patients must be able to tell his story, to accept his pain and to be thoroughly physically investigated. Obviously, paraclinical investigations as MRI scanning may in certain situations provide an assistance to reach a diagnosis, but these should never replace a thorough physical examination. After a detailed physical examination most patients can be insured that the state is peaceful and that neither of X-ray, MRI, prescription medication, nor sick leave is required. Patients must understand that the back should be used but not abused and that activity is better than inactivity for most of them. Back exercises are also accepted by patients as the right treatment - even despite the pain. But the removal of unhealthy work-exposures would tend to positively influence the course and also tend to benefit work colleagues in which unnecessary work-related pain may also be avoided. Pain at work without special heavy loads must be accepted as it is by back exercises.

The treatment with pelvic dysfunction differs from the exercises in uncomplicated lumbago, and should also include training of proprioception by coordination and balance exercises, e.g. with great balls or rocker boards.

Raynauds phenomenon

Attacks of white fingers involving one or several phalanges of one or several fingers are frequent. This condition is mostly idiopathic but may also arise secondary to an autoimmune condition or to prolonged occupational exposure to vibrating tools (so-called hand-arm-vibration syndrome, or traumatic vasospastic syndrome).  It is aggravated and triggered by smoking, exposure to local vibration and climatic exposures such as cold and humidity. Therefore patients should be advised to limit their use of vibrating tools, to quit smoking, and to protect their hands with gloves when required by climatic conditions. The majority of persons with hand-arm vibration syndrome have abnormal cold sensitivity and the same applies following amputations and nerve injuries.

10.9.2   Return-to-work

It is a medically important contribution to explain that the resuming of work after sick leave should be gradual with regard to time and/or intensity. This is likely to prevent relapses of sick leave. Within sports medicine we have learned that a controlled process with gradually increasing loads in terms of intensity and extent over time - where the sick or injured worker or sportsman is actively involved - gives the most viable course and outcome.

One should also caution with regard to pain reduction by cover of analgesic medication with the aim of being able to continue a harmful work. Recent research suggests that non-steroidal anti-inflammatory medication, which is widely applied for many locomotor ailments including tendinopathy may actually hamper the healing of tissue such as tendons. For security reasons, opiates would be contraindicated during work and therefore its use aboard during extended periods of time is discouraged.

References

1. Chmielewski, J., B. Jaremin, and P. Gandurski, The problem of rheumatic disease among seamen and fishermen. Biul.Inst.Med.Morsk.Gdansk., 1973. 24(3): p. 291-303.

2. Howard, J.K., Causes of morbidity among British merchant seamen. IMS.Ind.Med.Surg., 1973. 42(6): p. 13-17.

3. Törner, M., et al., Musculo-skeletal symptoms as related to working conditons among Swedish professional fishermen. Applied Ergonomics, 1988. 19(3): p. 191-201.

4.  Lipscomb, H.J., et al., Musculoskeletal symptoms among commercial fishers in North Carolina. Appl.Ergon., 2004. 35(5): p. 417-426.

5.  Kaerlev, L., et al., Hospital contacts for injuries and musculoskeletal diseases among seamen and fishermen: a population-based cohort study. BMC.Musculoskelet.Disord., 2008. 9: p. 8.

6.  Kaerlev, L., et al., Hospital contacts for chronic diseases among danish seafarers and fishermen: A population-based cohort study. Scand.J Public Health, 2007. 35(5): p. 481-489.

7.  Hellgren, L., The prevalence of rheumatoid arthritis in occupational groups. Acta Rheumatol.Scand., 1970. 16(2): p. 106-113.

8.  Allen, K.D., et al., Associations of occupational tasks with knee and hip osteoarthritis: the Johnston County Osteoarthritis Project. J.Rheumatol., 2010. 37(4): p. 842-850.

9.  Vignon, E., et al., Osteoarthritis of the knee and hip and activity: a systematic international review and synthesis (OASIS). Joint Bone Spine, 2006. 73(4): p. 442-455.

10. Pearce, M.S., Y.E. Buttery, and R.N. Brueton, Knee pathology among seafarers: A Review of 299 patients. Occupational Medicine, 1996. 46(2): p. 137-140.