2 Common Musculoskeletal Disorders
Overview
For the purpose of this book, the term “musculoskeletal disorders” (MSDs) refers to “conditions involving the experience of pain, discomfort, and/or disability in the human musculoskeletal system, usually as the result of cumulative damage to one or more of the components of this system, including muscles, tendons, ligaments, nerves, cartilage, ligaments, bone, and/or fascia.” Note that this definition excludes musculoskeletal injuries that might occur due to slips, trips, or falls (acute injuries). Instead, MSDs are viewed to be the consequence of exposure to repetitive stress resulting from occupational tasks, athletic pursuits, and other physical activities.
Terms used to describe MSDs over the past several decades include “repetitive stress injuries,” “repetitive strain injuries,” “cumulative trauma disorders,” and “cumulative trauma injuries” (National Research Council – Institute of Medicine, 2001). These labels seem to imply that musculoskeletal injuries/disorders result from (a) exposure of musculoskeletal tissues to stress/strain, (b) the stress/strain application is repetitive in nature, and (c) that the repetitive stress/strain results in the development of cumulative trauma to the tissues. Thus, the very terms used to describe these disorders over the years seem to intuit the presence of a fatigue failure process in the development of MSDs. In fact, the terms above may actually be better descriptors of the actual injury process, as opposed to the all‐encompassing (but vague) term “MSDs.” However, in this book, we will use the latter term due to its standard current usage.
Burden of MSDs
MSDs account for a large societal and economic burden throughout the world. The burden associated with low back pain (LBP) was particularly notable, as its prevalence can be up to 20% in some countries (Fatoye, Gebrye, & Odeyemi, 2019). MSDs, as a whole, accounted for an average of 16% of years lived with disability (YLDs) worldwide in 2017 (Global Burden of Disease 2017 Disease and Injury Incidence and Prevalence Collaborators, 2018). These disorders were seen as a major reason for rising YLD rates per person in this analysis. These increases were attributed to tendencies toward greater age, obesity, and physical inactivity. MSDs were also seen to be an important driver of health care expenditures in middle‐ to high‐income countries (Global Burden of Disease 2017 Disease and Injury Incidence and Prevalence Collaborators, 2018).
In an earlier analysis, a considerable proportion of LBP worldwide was attributed to occupational exposure (Punnett, 2005). On average, 37% of LBP was determined to be workplace related, with considerable variation across different regions of the world. Men had a higher attributable proportion than women, ostensibly due to higher exposure to occupations involving occupational lifting tasks and whole‐body vibration. Approximately 818,000 disability‐adjusted life years were lost annually due to the occupationally related LBP (Punnett, 2005).
The global burden associated with upper extremity MSDs is somewhat less clear due to the lack of a systematic method of defining cases. Studies have shown that the percentage of office workers who suffer from MSDs ranges from 20 to 60% worldwide (Global Burden of Disease 2017 Disease and Injury Incidence and Prevalence Collaborators, 2018). One older study from the United States reported lifetime prevalence of upper extremity (UE) MSDs of 29% in dentists (Stockstill, Harn, Strickland, & Hruska, 1993). Unfortunately, case definitions used by different researchers differed dramatically; thus, global estimates were not possible (Huisstede, Bierma‐Zeinstra, Koes, & Verhaar, 2006). In Europe, MSDs are the leading cause of loss of productivity, sickness absence, and work disability across all European Union (EU) member states. Lost productivity due to MSDs is estimated to represent approximately 2% of the EU gross domestic product (Bevan, 2015).
Clearly, MSDs exact a significant toll on individuals throughout the globe. Not only do these afflictions result in terrible burdens with respect to human disability and suffering, but there are also enormous economic and societal prices to be paid. Any attempt to reduce the effects of these MSD sequelae is clearly more than just a formidable challenge. It should be apparent that the complete control of these disorders, or even a sizable portion of them, is currently well beyond our grasp. Gaining a better understanding of the processes associated with the development of damage in musculoskeletal tissues and physiological mechanisms associated with both damage and repair of these tissues may help our ability to exert some modest degree of control over injury risk (or improved healing) in some circumstances. However, we clearly need to understand our adversary better. To start this process, we will begin by providing a brief review of some of the more common MSDs, providing descriptions and characteristic features of the disorders, prevalence and incidence data, relevant anatomy and pathology, and the risk factors or activities associated with the development of the disorders.
As can be seen below, this review focuses on MSDs affecting the low back, hands/wrists, elbows, and shoulders. While disorders affecting the hips, knees, and ankles are of interest to many musculoskeletal researchers, our emphasis is on the occupational setting, in which the former disorders tend to predominate.
Common Musculoskeletal Disorders
Low Back Pain
Description/characteristic features
LBP is a typically benign condition, though one of the most expensive conditions in industrialized countries (Mayer & Gatchel, 1988). It is associated with substantial loss in the quality of life (Punnett, 2005). LBP is characterized by pain that can range from a dull ache to a sharp, intense, and disabling pain. The focus of this pain is generally in and around the lumbar portion of the spinal column. The lumbar spine is a complex structure, consisting of vertebral bones, intervertebral discs, cartilage endplates, and nerves, and with all of these structures, there are many possible sources of pain.
LBP is often episodic, with most cases being relatively short‐lived. In fact, 40–90% of people obtain complete relief after 6 weeks (Manchikanti, 2000). LBP is often classified by duration as acute (pain lasting less than 6 weeks), sub‐chronic (6–12 weeks), or chronic (more than 12 weeks) (Koes et al., 2010). The condition may be further classified by the underlying cause as mechanical, nonmechanical, or referred pain (Manusov, 2012). While most pain is self‐limited, pain continuing after 3 months of LBP is considered chronic, as this is well beyond the 6‐ to 12‐week time period that normal connective tissues take to heal (Chawla, 2018).
Recent data suggest that approximately 38.5% of work‐related MSDs in the United States in 2016 involved the lower back (Bureau of Labor Statistics, 2018). Many occupational tasks have been associated with a higher prevalence of recurrent LBP, including lifting and/or carrying heavy weight and adoption of awkward postures such as bending, twisting, squatting, and kneeling (Amorim et al., 2019; Hoogendoorn, Poppel, Bongers, Koes, & Bouter, 1999). Exposure to whole‐body vibration in drivers (e.g., tractor drivers, taxi drivers, and helicopter pilots) appears to contribute heavily to LBP risk (Bovenzi, Schust, & Mauro, 2017), as does adoption of awkward postures when driving (Bovenzi, Schust, Menzel, Prodi, & Mauro, 2015). In the professional driving occupations cited earlier, there is also an exposure‐dependent increase in the development of degenerative changes in the spinal column, resulting in spondylolisthesis (Byeon et al., 2013; Chen, Chan, Katz, Chang, & Christian, 2004; Christ & Dupuis, 1966; Froom et al., 1984). The degenerative changes referenced earlier are thought to be due to repetitive mechanical loading and shock to the spinal column that lead to damage accumulation and degeneration (Bovenzi et al., 2017). Individual differences likely impact the effects of loading due to differences in muscle mass and strength as well as due to differences in kinematic and muscle recruitment strategies (Gallagher & Heberger, 2015; Marras et al., 2006).
Prevalence/incidence
LBP is the most common MSD with lifetime prevalence estimates ranging from 65 to 80% (Manchikanti, 2000). Approximately 9–12% of people (632 million) have LBP at any given point in time, and nearly 25% report having it at some point over a 1‐month period (Mayer & Gatchel, 1988; Waddell, 1987). Estimates of the annual prevalence