Musculoskeletal Disorders. Sean Gallagher
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Although it was thought that the pronator teres and flexor carpi radialis were originally thought to be most commonly affected, more recent literature suggests all muscles may be equally affected, with the possible exception of the palmaris longus (Kiel & Kaiser, 2019). As the tendon undergoes repetitive microtears, there is remodeling of the collagen fibers and increased mucoid ground substance. Focal necrosis or calcification can occur. Subsequently, collagen strength decreases leading to increased fragility, scar tissue formation, and thickening of the tendon. Although less common, acute trauma can also cause medial epicondylitis from a sudden violent contraction of the muscles (Kiel & Kaiser, 2019).
Risk factors/activities associated with medial epicondylitis
Medial epicondylitis is sometimes referred to as “Golfers Elbow”; however, numerous other repetitive athletic activities such as overhead throwing, bowling, and weightlifting have also been associated with the development of medial epicondylitis. However, 90% of cases appear not to be associated with athletic activities (Kiel & Kaiser, 2019). Studies have demonstrated that occupational risk factors include activities repeatedly stressing the flexor tendons including wrist bending, rotational motions of the forearm, and forceful gripping are associated with medial epicondylitis.
Shoulder tendinopathy (rotator cuff)
Characteristics/description
Shoulder disorders are a common cause of occupational pain and disability in working populations. Rotator cuff disorders (RC) emerge as the dominant source of shoulder pain, accounting for an estimated 65–70% of all shoulder pain (Hales & Bernard, 1996). Symptoms of RC tendinopathy commonly include pain situated at the top and front of the shoulder or outer portion of the upper arm. Shoulder pain can progress from the anterolateral shoulder margin and lateral surface of the arm down to the elbow (Gumina et al., 2014). Pain can be exacerbated if the afflicted individual raises the arm or reaches back behind the body. These disorders are most commonly observed in middle‐aged to older patients. Injuries to the shoulder are associated with longer than average recovery times compared to other occupational MSDs with a median of 26 days of lost work. Night pain can occur in 83% patients, and 41% may experience muscle weakness (van Kampen et al., 2014).
Epidemiology
Several studies have been performed to assess the overall prevalence of shoulder pain in the general population. In general, prevalence rates tend to suggest overall prevalence rates of approximately 7–20% (Breivik, Collett, Ventafridda, & R., C., & Gallacher, D., 2005; Hasvold & Johnsen, 1993; Parsons et al., 2007; Pope, Croft, Pritchard, & Silman, 1997). In the workforce, MSDs involving the shoulder accounted for 14.9% of all work‐related MSDs in the United States in 2016, with heavy tractor‐trailer truck drivers and laborers/material movers having a greater proportion of injuries affecting the shoulder than other occupations (Bureau of Labor Statistics, 2018). However, certain personal characteristics have been shown to be influential in the experience of shoulder pain, particularly sex and age. Breakdown by sex show a consistently higher prevalence of shoulder pain in females than in males. Females typically have an approximately 10% higher prevalence than that reported in males (Breivik et al., 2005; Picavet & Schouten, 2003; Treaster & Burr, 2004). Age is another significant factor, with an increased general prevalence in older individuals and a notable increase of shoulder pain prevalence in the 45–64 age‐group (Pribicevik, 2012). In addition, adolescents aged 12–18 years appear to have a greater than average shoulder pain prevalence. In 2014, 88,980 nonfatal shoulder injuries and illnesses occurred that involved days away from work (Bureau of Labor Statistics, 2015).
A systematic review examining physical occupational risk factors for shoulder pain disclosed that jobs involving high force demands, highly repetitive work activities, adoption of non‐neutral shoulder postures, and exposure to vibration and duration of employment were observed to be common physical occupational risk factors (Van der Windt, Thomas, & Pope, 2000). This review also examined psychosocial factors (e.g., job dissatisfaction, lack of control at work, poor social support, and/or psychological demands); however, while certain psychophysical factors were found to be significant, these factors were assessed to be inconsistent across the studies examined. Moderate to high levels of physical demand have commonly been associated with the development of shoulder pain (Ariens et al., 2000; Bergenudd, 1987; Devereux, Vlachonikolis, & Buckle, 2002; Malchaire, Cock, & Vergracht, 2001; Miranda, Punnett, Viikari‐Juntura, Heliövaara, & Knekt, 2008). Exposure to vibration has also been implicated in the development of shoulder pain (Ariens et al., 2000; Miranda et al., 2008; Stenlund, Goldie, & Hagberg, 1993; van der Windt et al., 2000). Continuous low‐intensity muscle contractions also increase the prevalence of neck‐shoulder complaints and syndromes, including acromioclavicular syndrome (Balogh et al., 2019; Huysmans, Blatter, & Beek, 2012; Visser & van Dieen, 2006). Finally, the adoption of non‐neutral shoulder postures has been associated with shoulder outcomes in a number of studies (Larsson, Sogaard, & Rosendal, 2007; Miranda et al., 2008; Pope et al., 1997; van der Windt et al., 2000). Many studies have failed to examine potential interactions between these physical risk factors; however, Frost and Andersen (1999) provide data suggestive of an interaction between force and repetition and shoulder tendinitis.
Anatomy/pathology
Rotator cuff (RC) injuries are generally considered to be the result of a degenerative disorder of the RC tendons, which begins with acute tendinitis (inflammation) that leads to tendinosis (degeneration), ultimately leading to tendon tears (partial or full), as shown in Figure 2.6 (Neer, 1983). There is mounting evidence of some level of inflammation in all tendon injuries (Abraham, Shah, & Thomopoulos, 2017). Animal models of RC pathology in which the tissue are collected long before a surgical repair endpoint show clear evidence of inflammation in rotator cuff tendon proper, epitendon, or surrounding capsule (Abraham et al., 2017; Kietrys, Barr, & Barbe, 2011; Thomas et al., 2014). Yet, histological studies of the affected RC tendons tend to show a minimal amount of inflammatory cells at the time of surgery (Fukuda, Hamada, & Yamanaka, 1990), and serum biomarker studies of patients with long‐term RC injuries show increased levels of circulating markers of enzymes that contribute to tissue degradation (matrix metalloproteinases), angiogenesis (increased production of vascular endothelial growth factor, VEGF), and axonal sprouting that may be related to the enhanced pain associated with RC injuries (summarized in Gold et al., 2016). These inflammation and degeneration changes are typically intrinsic factors that can enhance RC injuries (Seitz, McClure, Finucane, Boardman, & Michener, 2011). Extrinsic factors can also enhance RC injuries and impingement of the RC tendons. Some injuries are also thought to be the result of a combination of extrinsic and intrinsic factors.
Risk factors/activities associated with shoulder tendinopathy
The development of RC disorders is associated with a number of factors, including personal characteristics (such as age and gender), occupational exposures, and certain sports‐related activities. Among personal characteristics, factors such as gender and age tend to be most highly implicated in RC injuries (details). In terms of physical exposures, a history of occupations involving heavy lifting and/or highly repetitive tasks has been linked to the increased risk of RC disorders, which often result in greater than average lost time from work compared to other MSDs. In terms of physical exposures, a history of occupations involving heavy lifting and/or highly repetitive tasks have been linked to increased risk of RC disorders, which often result in greater than average lost time from work compared to other MSDs. In 2014, 88,980 nonfatal shoulder injuries and illnesses occurred that involved days away from work (Bureau of Labor Statistics, 2015).