Shoulder

SHOULDER

Chi-Tsai Tang and Scott Simpson

EPIDEMIOLOGY

Shoulder injuries are one of the most common injuries in sports. The incidence of shoulder injury naturally depends on the sport being played. Age of the athlete, level of competition, and gender seem to play a role as well. In epidemiologic studies of high school athletes, significant shoulder injuries accounted for 8.3% to 10.9% of all injuries. Shoulder injuries were more common in boys (11.1% to 14.4%) as compared to girls (1.6% to 3.4%), and this relationship held true even when they participated in the same sport, such as soccer and basketball (1,2). Looking at different sports, the sports with highest injuries to shoulders included boys’ wrestling, boys’ baseball, boys’ football, and girls’ softball (2). Another way to look at injury incidence is to assess injuries per athlete exposure (AE). One study looking at several high school sports found an overall shoulder injury rate of 2.15 per 10,000 AEs. Rates of injury were higher in competition compared to practice. Boys were more likely than girls to sustain their injuries after contact with another person or with the playing surface. Common injuries included shoulder sprain/strain and shoulder dislocation/separation (3). A separate study, one looking at high school baseball and softball only, found an injury rate of 1.72 per 10,000 AEs for baseball and 1.0 per 10,000 AEs for softball (4). It can be assumed that the baseball players were predominantly if not all boys, and softball players were predominantly if not all girls.

Data on whether there is a sex-based bias in shoulder injuries for collegiate and more competitive athletes is a little less clear. In a study comparing seven collegiate sports (including basketball, cross-country running, track, swimming, soccer, tennis, and water polo) from one school, the data overall suggest little difference in the pattern of injury between men and women competing in comparable sports. There was an increased rate of shoulder injuries among female swimmers and water polo players, but the study’s authors suggest this probably resulted from the more rigorous training philosophy of a particular coach at the school (5). One study of elite overhead athletes in sports that included swimming, rowing, wrestling, basketball, volleyball, and handball found males had higher lifetime prevalence of shoulder pain than females (6). However, a separate study of elite male and female badminton players showed no difference in prevalence of shoulder pain (7). In a study of female competitive swimmers of varying ages, it was found that 21.4% of swimmers age 8 to 11, 18.6% of swimmers age 12 to 14, 22.6% of high school swimmers, and 19.4% of masters swimmers had shoulder pain and disability. Greater swimming exposure, higher incidence of previous traumatic injury, patient-rated shoulder instability, and reduced participation in another sport were found to be statistically significantly correlated with shoulder pain and disability (8).

In a population that holds particular interest to physiatrists, those with physical disabilities, the shoulder was the most commonly injured body part in athletes participating in the 2012 Paralympic games, representing 17.7% of all injuries. This should come as no surprise given the stress placed on shoulders from wheelchair use. Rates of shoulder injury were similar in male and female athletes (9).

Looking at the general population of people presenting to primary care, physical therapy, or physical medicine outpatient clinics for shoulder pain, it appears that more patients tend to be female, with percentages ranging from 55% to 74.3% (10–12). The painful shoulder is more likely the dominant shoulder (10). The mean age ranged from 47 to 50 years in these studies (10–12). Moderate intensity pain predominated in males, while severe intensity pain was more frequent in females. Limitations of movements were present mainly in women, likely presenting a female predilection to frozen shoulder. Concurrent cervicalgia was reported in 7.7% of women and 2.9% of men (10). Pain medication consumption was significantly higher among men than women (11). The most common diagnoses included subacromial pain, myalgia, and adhesive capsulitis (12). In a study looking at men and women with neck/shoulder disorders, the highest improvements in pain and disability in both men and women were seen after 3 months. After 5 years, both men and women had significant improvements, but men more than women (13).

ANATOMY

There are several studies looking at differences in shoulder anatomy between the sexes. Overall, males have larger bony structures compared to females. This includes larger humeral head height and width, larger coracoid process, and larger and rounder glenoid (14–16). For the glenoid, the presence and location of the anterior notch varied between genders. See Figure 2.1 for a depiction of a mean male and female glenoid (15). Differences in glenoid anatomy may have implications in shoulder surgeries, such as planning for the glenoid component in shoulder arthroplasties. The anatomic relationship of the humeral head inclination was not found to be different between the sexes (16). One study showed a difference in humeral head and glenoid version, with males having more retroversion than females, but other studies found no differences in the glenoid version (15,17,18). See Figure 2.2 for a depiction of how to calculate humeral head retroversion and glenoid retroversion. Both the humeral head and glenoid are thought to be retroverted in higher demand shoulders, and the dominant shoulder typically has higher retroversion. Greater humeral head retroversion increases external rotation and decreases internal rotation. Humeral head retroversion is known to be high in the fetus and infant and to become smaller with growth (17).

FIGURE 2.1: Depiction of lateral scapula “Y view” of the right shoulder, illustrating mean differences between male and female glenoids (15)

One of the main differences between male and female bony structure is the size of the scapula (18). Scapular breadth was found to have a high degree of sexual dimorphism (19). In fact, many people have proposed looking at the scapula as a way to differentiate male and female skeletons. The longitudinal scapular length and transverse scapular length were found to be larger in males than females (20). Looking at the morphology of the acromion, there appears to be a predominance of type III (hooked) acromions in men (56.2% versus 43.7%) and type I (flat) in women (56.5% versus 43.4%). Enthesophytes and a rough inferior surface were most frequently found in type III acromions (21).

An interesting difference in bony anatomy between genders that has clinical implications is the shape of the suprascapular notch. Males are approximately three to four times more likely to suffer from suprascapular nerve entrapment than females. A study of the suprascapular notch found that the frequency of type I (maximal depth greater than superior transverse diameter) and type IV (scapulae with bony foramen) was higher in males than females. Type III (superior transverse diameter was greater than the maximal depth) was more common in females than males. Therefore, notches that are “narrow and deep” as opposed to “wide and shallow” and also notches that are a bony foramen may predispose the nerve to injury because of tethering of the nerve within the notch (22).

Looking at differences in soft tissue structures in the shoulder, it is found that the size of the subscapularis insertion onto the humerus is larger in men (23). Looking at ultrasound dimensions of the rotator cuff in young healthy adults, the mean maximum width of the supraspinatus footprint was significantly larger in men than women (14.9 mm versus 13.5 mm). The mean thickness of the supraspinatus tendon (5.6 mm versus 4.9 mm), subscapularis tendon (4.4 mm versus 3.8 mm), and infraspinatus (4.9 mm versus 4.4 mm) tendon were all larger for men, but differences did not reach statistical significance. There was no correlation between height and weight with any tendon measurement (24). Differences in rotator cuff thickness have clinical relevance because increased thickness is one of the main criteria used to diagnose rotator cuff tendinopathy. Other criteria include inhomogeneity and hypoechoic appearance. In a separate study, average thickness of an intact rotator cuff was described to be 4.7 mm and was not found to be related to age, sex, or symptoms (25). The common practice at our institution is to use 5 mm as the cutoff value for normal cuff thickness, though perhaps men should have a slightly higher cutoff. Typically, to make the diagnosis of tendinopathy, other features of tendinopathy need to be seen, and there may oftentimes be a focal area of thickening that is seen. The contralateral shoulder can also be scanned for comparison if it is asymptomatic. This highlights some of the challenges with using ultrasound to diagnosis rotator cuff pathology because there is not always standardized criteria.

FIGURE 2.2: (A) shows how to calculate humeral head retroversion. (a) The humeral head axis (solid line) is the line perpendicular to the cord of the articular surface of the humeral head (dotted line) at the slice with the maximum humeral head diameter. (b) The elbow epicondylar axis is the line drawn between the medial and lateral epicondyles (line) at the slice with the most prominent epicondyles. (c) Humeral head retroversion (alpha) is defined as the angle between the humeral head axis and the elbow epicondylar axis of the humerus. The humeral head is retroverted because the humeral axis is pointing posterior (17). (B) shows how to calculate glenoid retroversion. Glenoid retroversion (beta) is defined as the angle between the glenoid line and a line perpendicular to the scapular axis. The glenoid line is the line connecting the anterior and posterior rims of the glenoid. The scapular axis is the line connecting the tip of the medial pole of the scapula and the center of the glenoid line. The glenoid is considered retroverted because the face of the glenoid is pointing posterior (17).

There also appear to be differences in bone maturation and bone quality within the shoulder between sexes. This should be no surprise given what we know about sex differences in overall bone metabolism. Females had earlier epiphyseal union of the upper limb and scapular girdle by about 2 years compared to males, reflecting earlier skeletal maturation (26). Looking at bone quality within the greater tuberosity, males tend to have “denser” bone, with higher bone volume to total volume ratio, larger trabecular thickness, greater trabecular number, and greater connectivity density. Furthermore, there was a strong inverse correlation between age and bone volume to total volume ratio, which was more pronounced in females (27).

BIOMECHANICS

There is a difference in absolute shoulder strength between males and females, with males being stronger, which is not surprising. However, this difference is no longer present after normalization for segmental skeletal muscle mass (28). In a study of healthy subjects using surface electromyography (EMG) to look at two portions of the trapezius, deltoid, and infraspinatus through repetitive maximal isokinetic shoulder forward flexions, males were significantly stronger than females and, on average, females produced approximately 60% of the output of the males for peak torque; 76% after normalization for body mass (29). In a separate study testing strength in the shoulder using a dynamometer, no statistically significant differences in agonist/antagonist strength ratios were found between dominant and nondominant sides or between sexes (30). However, a separate study of badminton players showed males were generally stronger than females in all strength measurements except internal rotation on the dominant side. In females only, internal rotation strength of the dominant side was stronger than internal rotation strength of the nondominant side, and a higher internal rotation strength on the dominant side was not balanced by a higher external rotation strength (31). Different positioning of the shoulder for strength testing did uncover some differences in strength between sexes. For women only, the internal rotators demonstrated significantly greater strength in the seated neutral position than in the prone position with the shoulder abducted to the 90° position. Similarly, the external rotators demonstrated greater strength in the prone position with the shoulder abducted to the 90° position, compared to seated positions (32).

There appears to be a sex difference for endurance of shoulder girdle muscles, with women having better endurance than men. In one study, men had significantly lower endurance levels for the trapezius and infraspinatus, but significantly higher activation of the deltoid muscle than women, when performing shoulder flexion (29). A separate study showed that women have better endurance than men in upper body resistance exercises and can perform more repetitions with lower decrement in force output and faster recovery capacity when doing these exercises. Women are thought to have better endurance because of reduced adenosine triphosphate (ATP) depletion, faster ATP recovery, lower blood lactate levels, lower epinephrine levels, a lower respiratory exchange ratio, and lower glycogen breakdown in type I muscle fibers in response to maximal sprint and resistance exercises (33). Also, men and women appear to use different strategies to improve endurance during a repetitive shoulder task. Women increased variability in muscle activation to improve fatigue, whereas men decreased coactivation between muscles to increase endurance (34). There were no significant effects of gender or age on the ability to relax between repetitive contractions (29).

There appears to be some differences in muscle activation patterns between sexes when performing arm elevation in the scapular plane. Females demonstrated higher percent activation levels for three of the four divisions of the trapezius, but had slower onset of timing for activation of the descending trapezius. This just indicates there is a difference in muscle activation between the sexes, but the clinical implication is not clear (35). For isometric exercises with a loading of 50% maximum force, gender-specific differences in functional intermuscular coordination patterns were seen. Women showed less activation of muscles acting in the main force direction, and more activation in those muscles less necessary for the actual force production. This can be interpreted to mean that shoulder activation in men is more precise than in women (36).

Looking at static scapular resting position, there was a significant difference between males and females in nonswimmers but not in swimmers for the distance between the medial spine of the scapula and T3/4. Males had larger values with a mean difference of 11.3 mm (37).

A couple of studies looked at shoulder biomechanics in sports. Looking at propelling of the arm in males and females swimming the front crawl at submaximal speeds, the distance covered per stroke is similar before puberty, reaches its maximum at about 20 years of age, and then steadily declines. Shoulder-to-hand distance is significantly larger in males than in females, and this difference tended to offset the difference in distance covered per stroke so that efficiency of the arm stroke is almost the same in male and female swimmers of the same age group and swimming ability (38). Looking at rowing technique, males have greater upper body strength than females, and there is slight difference in rowing techniques adopted by each sex. Male rowers expended more total external energy per stroke and made a larger percentage contribution of angular shoulder energy to their total external energy expenditure. The overall percentage of work done was higher for male rowers, and this difference further increased at higher stroke rates (39).

PATHOLOGY

Frozen Shoulder

Frozen shoulder is a common condition characterized by limited passive shoulder range of motion (ROM) and pain. Typically, shoulder external rotation is particularly restricted because frozen shoulder usually includes scarring of the rotator interval—a triangular area in the anterior superior shoulder where the capsule is reinforced externally by the coracohumeral ligament and internally by the superior glenohumeral (GH) ligament; it is defined at its base by the coracoid process, superiorly by the anterior margin of the supraspinatus tendon, and inferiorly by the superior margin of the subscapularis tendon. Frozen shoulder can occur idiopathically, in which case it is also known as adhesive capsulitis; alternatively, secondary frozen shoulder can occur related to decreased use of the shoulder in the setting of another painful shoulder condition such as rotator cuff tear or tendinopathy.

The etiology of frozen shoulder remains unknown. Histopathological examination of GH capsular tissue in frozen shoulder demonstrates scattered inflammation and mast cell infiltration, suggesting an inflammatory component (40). This is also reinforced by the typically robust improvement of frozen shoulder following GH joint corticosteroid injection.

The incidence of frozen shoulder increases with age and is higher in women than in men. This preponderance of frozen shoulder in females is well known. Data from the United Kingdom published in 2011 demonstrated an incidence of adhesive capsulitis in women of 3.3 per 1,000 person-years versus 2.36 per 1,000 person-years in men. After adjusting for age, a 40% higher incidence was seen in women. The incidence for both men and women increases with age, although the increase plateaus for both sexes somewhat after age 60 (41). That the incidence increases with age may be particularly pertinent for sports medicine providers with the rising numbers of masters athletes (42,43). Data from the United Kingdom show a rising incidence of frozen shoulder among younger women, with no similar trend seen among men (41). The incidence of idiopathic frozen shoulder is higher among diabetics and among women with thyroid disease (44). Results following treatment for idiopathic frozen shoulder may also be somewhat worse among diabetics (45). In the UK study, the increasing incidence of frozen shoulder in younger women persisted despite adjustment for rising rates of diabetes (41).

In the absence of frozen shoulder, shoulder ROM has been demonstrated to remain relatively preserved throughout the aging process, confirming that decreased ROM represents a pathologic state across the age spectrum. Aging women have been demonstrated to have slightly greater range of shoulder flexion than men (46).

Many different treatments for frozen shoulder have been described, ranging from allowing the condition to run its course without intervention, to stretching programs, manipulation of the GH joint under anesthesia, and arthroscopic or open release of joint contracture. Few studies have demonstrated differences in outcomes with adhesive capsulitis between men and women. There are some data to suggest that women are able to achieve greater improvement in shoulder ROM than men with stretching programs for adhesive capsulitis (45), although other studies have found no gender difference in ROM following a stretching program (47). Functional outcomes have not been reliably demonstrated to be different between men and women.

Traumatic/Multidirectional Instability

Instability of the GH joint can be classified based on whether the onset of instability was traumatic or atraumatic and on the direction of instability. Instability is commonly referred to as unidirectional if instability is limited to one plane, or multidirectional if instability includes the anterior, posterior, and inferior directions. The term “instability” implies the presence of symptoms (apprehension or pain) associated with increased motion. “Laxity” refers to abnormal motion (the ability of the shoulder to sublux or dislocate in one or more directions) in the absence of symptoms. Thus, global laxity of the shoulder is not equivalent to multidirectional instability (MDI), although those with laxity are at increased risk for developing symptoms and thus progressing to MDI.

MDI was first described by Neer and Foster in 1980 (48). MDI is defined by the ability to sublux or dislocate the GH joint inferiorly as well as anteriorly and/or posteriorly, as well as the presence of symptoms during the midrange in a given plane of GH motion. Most authors describe inferior instability as a necessary component in the diagnosis of MDI (48–51). The anatomic findings in MDI include an enlarged inferior capsular pouch and a defect in the rotator interval capsule (the anterosuperior portion of the GH joint capsule in the region of the bicipital groove and the attachment of the coracohumeral and superior GH ligaments) (48,49,51).

In general, unidirectional instability is more likely to be related to a particular traumatic injury whereas MDI is more likely to have an atraumatic onset or to follow relatively minor trauma. The initial inciting event for MDI can, however, be more significant trauma (48,51). The repetitive stress from athletics, particularly gymnastics, weightlifting, and swimming the butterfly or backstroke, may be enough to cause the transition from laxity to instability in susceptible individuals (49–52). Female gymnasts are known to be at a particular risk for shoulder instability. In a survey of female NCAA gymnasts, 59% reported shoulder pain, more than 25% reported excess shoulder motion, and 11% met criteria for MDI (50,53).

Whether MDI is more common in females continues to be debated in the medical literature. While generalized laxity is more common in females, generalized laxity does not necessarily correlate with shoulder laxity specifically. Currently, the literature is inconclusive as to whether MDI of the shoulder is more common in females; however, it is our impression that instability is more of a problem in premenopausal females. This may be due to the fact that the sex hormones influence the structure and function of collagen (see Chapter 1 for more information on sex hormones). Studies looking at GH joint excursion in asymptomatic volunteers have demonstrated conflicting results. One study demonstrated more generalized GH hypermobility in women with increased anterior GH translation and decreased anterior GH stiffness compared to men with application of an anteriorly directed force (52), but a subsequent study by the same authors showed no gender difference in translation (54). A retrospective review of pediatric shoulder instability did not find MDI to be more common in girls than boys (53). In a study of shoulder instability at the U.S. Military Academy, MDI events associated with trauma were found to be more common in female cadets (55).

Estimates of the incidence of shoulder instability have focused on traumatic instability, which is more common in males (53,55). A study of shoulder dislocations presenting to the emergency department found an incidence of dislocation in men of 35 per 100,000 person-years and in women of 13 per 100,000 person-years. Dislocations in males occurred predominantly related to trauma from sports or recreation, whereas dislocations in females were more likely than in males to occur at home (56).

The trajectory of laxity during development is different in boys and girls. General joint laxity decreases consistently for boys as they age. In girls, general joint laxity decreases approximately from age 9 to 12 and then increases around the time of puberty (57). A study of young competitive swimmers demonstrated lower general joint laxity in 9-year-old girls but no change at age 12 compared to peer controls. Boys who were competitive swimmers demonstrated higher general joint laxity at both ages compared to peers. The authors postulated that the decrease in laxity in girls was due to increased muscle mass and that the higher laxity in boys was due to repetitive motion toward the extremes of ROM, counteracting the trend of boys generally becoming less lax as they age. They also commented that there is also likely an effect of sex hormone on laxity during and after puberty. They did not, however, find a difference between young swimmers and peer controls with regard to shoulder laxity specifically (58).

Other data on pediatric shoulder instability have shown that girls are more likely to have voluntary shoulder instability than boys and that girls with instability typically are younger at the time of their first instability episode (subluxation or dislocation). In a cohort in which some children with shoulder instability were managed operatively and some nonoperatively, boys demonstrated greater stability in long-term follow-up than girls. Girls also reported more limitation of sports participation due to instability. Boys, however, were more likely to have instability than girls, but the inciting event in boys was usually trauma. The group that did the worst in follow-up were older girls with voluntary instability, regardless of whether they were managed operatively or nonoperatively (53).

There are minimal data regarding outcomes in males versus females for shoulder MDI. In one study, males did slightly worse with nonoperative management than females (59). In studies of operative management, females have a larger increase in GH capsular volume 1 year after capsular shift with Bankart repair. Being an elite athlete was also associated with larger capsular volume following surgery in that study (60). A study of patient satisfaction following surgery for MDI, however, found no sex difference in patient satisfaction following the procedure (61).

Minimal data are available regarding outcomes following surgery for traumatic dislocation that differentiate findings between men and women, but men have been shown to have a higher risk of redislocation following arthroscopic repair after a first anterior dislocation (62).

Rotator Cuff Pathology

Rotator cuff pathology, including partial- and full-thickness rotator cuff tears and rotator cuff tendinopathy, is a common source of shoulder pain and functional impairment. Some articles have also included subacromial impingement in the spectrum of rotator cuff pathology, but whether actual impingement underneath the acromion occurs has been questioned and remains unresolved. Evidence has demonstrated that subacromial spurs are associated with full-thickness rotator cuff tears, but that morphologic variants with a downsloping or hooked acromion (classically referred to as a type II and type III acromion, respectively) are not associated with tears (63). Subacromial impingement as a clinical entity, however, remains controversial. The incidence of rotator cuff tears increases with age (25,64,65). In a study using shoulder ultrasound in patients presenting with shoulder pain, the average age of patients with rotator cuff tears was 58.7 years old for unilateral tears and 67.8 years old for bilateral tears. Patients in whom no rotator cuff tear was demonstrated averaged 48.7 years old (25). Similar to shoulder instability, rotator cuff tears in men are more likely than those in women to be related to trauma or a direct blow to the shoulder (64,66).

There is likely a slightly increased prevalence of females with rotator cuff disease. Looking specifically at the incidence of rotator cuff pathology, it was found to be more common in women (90 per 100,000 person-years) compared to men (83 per 100,000 person-years), with the highest incidence (198 per 100,000 person-years) in those aged 50 to 59 (67). Another study demonstrated a higher prevalence of smaller rotator cuff tears specifically in young women compared with men, but no difference was seen between tear size in older women and men (64). A recent study elucidated that the mechanism behind this may be partly due to an association between a variant of estrogen-related receptor-beta and rotator cuff disease (68). However, not all studies support a gender difference, as there is a study that did not show increased prevalence of rotator cuff tears in young women (69). Focal osteoporosis of the proximal humerus in the setting of rotator cuff tears has been shown using quantitative computed tomography, with greater decrease in focal bone mineral density in women than in men (70).

A study of subjects electing nonoperative management for full-thickness rotator cuff tears demonstrated that women with full-thickness rotator cuff tears were less active with their shoulders compared to men. The authors hypothesized that the difference may be related to differences between men and women with regard to work, sports, and recreational activities (71). Women with full-thickness rotator cuff tears have also been shown to have decreased shoulder function compared to men with full-thickness tears (72,73).

In patients with planned rotator cuff surgery, concerns regarding surgery and postoperative recovery have been shown to be higher in women than in men (74), with concerns including the length of recovery and the postoperative course, as well as postoperative pain. Women have been shown to have different expectations for surgery than men with regard to interacting with and providing care for others, ROM, and return to work after surgery (66). Separately, patients with higher concerns going into surgery have been shown to have worse functional outcomes after rotator cuff repair (75). Women have been shown to report more emotional difficulty related to their shoulder injury preoperatively (64). Women with planned rotator cuff surgery also report a higher level of disability related to activities of daily living and sleep in comparison to men with a similar or lower level of pathology (66). Studies have also demonstrated that women report more interference in social functioning (66), lower activity levels (76), and worse scores for health-related quality of life than men prior to rotator cuff surgery (76).

Following rotator cuff repair, women have been shown to have less shoulder pain relief and more postoperative pain (65,77–79), as well as less recovery of shoulder activity (65,77), ROM (65,77), and strength (65,77,78) as compared to men. Women have also been demonstrated to be slower to return to work than men following rotator cuff repair (80). Younger women demonstrate more postoperative disability than older women after rotator cuff surgery (69); however, there are data that also show greater postoperative satisfaction in younger women than in older women (65). The level of postoperative disability may be more related to the degree of preoperative disability in women than men (69). Decreased outcomes with regard to function, pain, and stability have also been demonstrated in women compared to men following revision rotator cuff surgery (81,82). One study, however, showed greater improvement in pain and function in women than men following arthroscopic rotator cuff repair (83), while another demonstrated no difference in patient satisfaction between men and women following rotator cuff surgery (84).

Research has demonstrated baseline differences between men and women on the scores used to evaluate shoulder symptoms and function. The Constant Score, which measures pain, ROM, activity level, and strength (85), has in particular been shown to highlight baseline sex differences across all age groups in subjects without shoulder complaints, with higher scores seen in men (86,87). Some of the sex differences demonstrated in prior studies, particularly those using the Constant Score as an outcome measure, may thus overestimate sex differences in the setting of shoulder pathology. Due to the known baseline differences in shoulder functional scores, many studies now evaluate male and female subjects separately with no comparison between sexes.

Fractures

Fractures of the humerus are more common in women than in men when including all humeral fractures together, with an incidence nearly 1.5 times higher in women than in men (88). The incidence of proximal humeral fractures increases with age in both men and women, with a higher incidence of proximal humeral fractures in women. The incidence of proximal humeral fractures increases for both males and females in the 10- to 19-year-old age group, decreases in the following decade, and then slowly rises until age 50 with similar rates in both sexes. Following age 50, the incidence rises for both sexes but for women much more steeply than for men, with data from Rochester, MN, demonstrating a peak incidence of 439 fractures per 100,000 person-years in women aged 80 and over with a female-to-male ratio of proximal humeral fractures of approximately 4:1 after age 80. The proportion of proximal humeral fractures due to mild or moderate trauma as opposed to severe trauma also increases with age and is higher in women than men (88–90).

The complexity of proximal humeral fractures also increases with age. A study of patients hospitalized for humeral fractures in Germany found the majority of complicated, four-part proximal humeral fractures to be in women over age 60, with fractures in older women typically related to low-energy trauma such as ground level falls. Males under age 60 were more likely to have three- or four-part fractures than women in the same age group, with more than half of the fractures in men under age 60 from high-energy trauma (89). Isolated fractures of the greater tuberosity in particular are more common in men and are typically seen in younger patients than other proximal humeral fractures (91,92).

Although the scapula is uncommonly fractured (accounting for 1% to 3% of all fractures, including patients with polytrauma), data from motor vehicle collisions show a more than threefold higher rate of scapular fracture in men than women (93). Similarly, data from the military have shown the incidence of clavicular fractures to be more than twice as high in men than women (94).

Labral Tears

The glenoid labrum is a fibrous structure around the rim of the glenoid that deepens the cavity and adds support. The labrum can be torn from a single trauma or repeated microtrauma, and labral tears are oftentimes associated with shoulder instability. Tears of the glenoid labrum typically present as shoulder pain with mechanical symptoms that can include popping, locking, catching, or clicking.

Tears of the labrum have been described to occur at various locations. The most common location is the superior labral tear with extension from anterior to posterior (SLAP lesion). Repetitive overhead activities have been hypothesized as a mechanism for causing SLAP lesions. One theory of how this happens is that high eccentric activity of the biceps muscle creates tension on the long head of the biceps and its attachment onto the labrum during the deceleration and follow-through phase of throwing. Another theory is termed the “peel back” mechanism—when the shoulder is placed in a position of abduction and maximal external rotation, the rotation causes a torsional force at the base of the biceps and its attachment onto the labrum. SLAP lesions have also been associated with shoulder instability (108). Mechanical symptoms are present in approximately half of all patients with superior labral tears (95). In a single-institution retrospective review of all arthroscopic shoulder surgeries over an 8-year period, superior labral injuries were seen in 6% of cases with an average patient age of 38. In that study, 91% of superior labral injuries seen were in men with 9% of tears found in women (95). Management of glenoid labral tears, particularly SLAP tears, remains controversial. A retrospective study utilizing a national database to examine trends in labral repair surgery demonstrated that repair of SLAP tears was performed three times more often in men than women in the period from 2004 to 2009. One reason for the higher incidence and rate of repair in men may be the link between overhead throwing sports and SLAP tears. Additionally, orthopedic surgeons may be more likely to repair SLAP tears seen arthroscopically in men than women (96).

Labral tears can also occur antero-inferiorly and postero-inferiorly, and these are mainly due to shoulder instability. Postero-superior lesions have been described in overhead athletes, and this entity was first described as Walch’s internal impingement, where repeated contact between the deep surface of the rotator cuff and the posterior superior labrum causes undersurface rotator cuff tears and degenerative labral tears (109). Internal impingement is associated with GH internal rotation deficit (GIRD) and scapular malposition, inferior medial scapular winging, coracoid tenderness, and scapular dyskinesis (SICK scapula syndrome). It has been suggested that internal impingement is most likely caused by fatigue of the muscles of the shoulder girdle resulting from lack of conditioning. As the shoulder girdle muscles become fatigued, the humerus drifts out of the scapular plane, which can lead to tensile stressing of the anterior aspect of the shoulder capsule. This can compromise the obligatory posterior rollback of the humeral head, leading to anterior translation and therefore causing the undersurface of the rotator cuff to abut the margin of the glenoid and labrum (110).

Myofascial Pain and Delayed Onset Muscle Soreness

Myofascial pain is characterized by pain related to sensitive trigger points in muscles. Myofascial trigger points can be either latent, where pain is present only with pressure over the trigger point, or active, with pain present constantly, including in the absence of overlying pressure. Trigger points prevent full lengthening and relaxation of the muscle (97) and produce a local muscle twitch response when stimulated. Trigger points can refer pain locally and even distally into an extremity (97).

Myofascial trigger points are commonly present in the shoulder musculature. The prevalence of myofascial trigger points in the shoulder region may be higher in women than in men (97). Although the prevalence of myofascial pain is unknown, a study of patients with chronic, atraumatic, unilateral shoulder pain presenting to a physical therapy clinic demonstrated active trigger points in shoulder girdle musculature in all 72 examined patients with a median of six involved muscles. The most common locations were the infraspinatus and upper trapezius. Myofascial trigger points were also commonly found in the middle trapezius, anterior deltoid, middle deltoid, and teres minor (97).

In asymptomatic subjects, women have been shown to have a lower pain threshold than men with pressure applied over the trapezius (98). A higher pain intensity has also been reported in women versus men following nociceptive stimulation with a noxious agent (hypertonic saline or glutamate) into the upper trapezius in subjects without pain at baseline (99–101). With repeated injections, a decrease in pain is seen in men but not in women, suggesting a gender difference in pain modulation (101). Additionally, motor control strategies during experimentally generated pain in the trapezius are different in women and men. Without experimentally induced pain, both genders show differential activation of different portions of the trapezius with sustained contraction (potentially to prevent overload of a specific muscle region during sustained contraction). That pattern has been shown to be maintained in men but not in women during experimentally induced pain in the trapezius, with women demonstrating no change in the recruitment pattern of the trapezius during experimentally induced pain (99). Men have also been shown to recruit more motor units of the trapezius during sustained contraction in the setting of experimentally induced pain with no similar increase in women (100).

Delayed onset muscle soreness following exercise has not been shown to be different between women and men when comparing pain pressure thresholds in shoulder girdle muscles up to 48 hours after resistance exercise (102) or when comparing pain with muscle contraction during delayed onset muscle soreness (103).

Osteoarthritis

Arthritis in general is more common in women than in men (46,104,105). GH osteoarthritis is associated with aging and is more common in elderly women than in elderly men (12). The prevalence of acromioclavicular osteoarthritis is more similar between the sexes (12), but may be more common in men than in women in the subset of patients with rotator cuff tears (66). A cross-sectional study of men and women living independently after age 65 showed decreased ROM with shoulder abduction in both men and women associated with aging, but a greater decrease was seen in women than in men. The prevalence of disability related to the shoulder was also higher in elderly women than men (105). In middle-aged and elderly individuals without shoulder pain, however, sex has been shown not to be predictive of degenerative changes on shoulder x-rays (106).

Management of severe GH osteoarthritis frequently includes total shoulder arthroplasty. There are minimal data to suggest postoperative differences between men and women; however, loosening of the glenoid component is more common in women than in men (107).

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