With the increasing participation of women in recreational activities and organized sports, differences in the incidence and patterns of athletic injuries between male and female athletes are being studied in greater detail. As risk factors for specific athletic injuries are identified, certain injuries or conditions are found to be sport specific, 1 and the question of whether gender may play a role is being asked.
The association of athletic injuries or conditions with gender may arise from several possible origins. Being female may itself be a risk factor. The hormonal environment unique to the female gender may affect calcium homeostasis and subsequent bone density, predisposing female athletes to certain injuries. Anatomic difference between men and women, such as notch size or anterior cruciate ligament size, may lead to increased injury in some situations. Female behavioral actions or norms, such as footwear choice, may also predispose women to certain conditions. Finally, certain sports may attract more female athletes than male athletes, thus biasing the reported incidence of injuries in female athletes.
Certain athletic injuries and physical conditions have a higher incidence in females than in males. There is a reported 2- to 8-fold higher risk for anterior cruciate ligament injury per unit of sport exposure in women than in men. 2,3,4,5 Other orthopedic problems, such as patellofemoral pain, 6,7 pelvic floor dysfunction, 8 grade 1 ankle sprains, 9 and adhesive capsulitis of the shoulder 10 have also been associated with the female sex. Stress fractures of the femoral neck, pelvis, and metatarsals are more common in women. 11,12,13 Some musculoskeletal conditions, such as spondylolysis and spondylolisthesis, 8 are more prevalent in women, but this may reflect a preponderance of female athletes in certain sports, such as gymnastics. Developmental conditions, such as juvenile idiopathic scoliosis, are more common in women than in men. Other conditions, such as bunions, bunionettes, hammer toes, and neuromas, have all been found to occur 9 times more frequently in females. 14
Generalized ligamentous laxity, and shoulder laxity specifically, have traditionally been associated with the female sex. Recent reviews, however, have failed to conclusively demonstrate a difference in shoulder laxity between men and women. 15 The incidence of shoulder instability and outcome of its treatment in female athletes, however, have not been studied in detail.
This article reviews shoulder instability in the athletic population, focusing on athletes involved in overhead activities. We reviewed studies on the outcome of treatment for shoulder instability, attempting to draw conclusions about the incidence of shoulder instability in the female population and the results of operative interventions for different types of shoulder instability.
The Athletic Shoulder
Shoulder instability in the athlete can occur as the result of a single traumatic event, or from repetitive microtrauma. The most common causes of repetitive microtraumatic shoulder instability are the overhead throwing motion, examples of which are the baseball pitch and the freestyle swimming stroke. An understanding of the biomechanics of these motions is important to understand the forces present across the glenohumeral joint, and how these forces can lead to shoulder instability.
The biomechanics of the baseball pitch have been well studied. The overhead throw consists of five main phases, each of which requires the coordinated function of the rotator cuff, deltoid, and peri-scapular muscles (Fig. 1). The wind-up begins the throwing motion. Little force is found through the shoulder at this stage. The early cocking phase consists of shoulder abduction (deltoid, supraspinatus) and external rotation (infraspinatus, teres minor). The scapular stabilizers function to position and stabilize the glenoid. The rotator cuff muscles rotate and stabilize the glenohumeral joint. The late cocking phase involves maximal external rotation of the shoulder with maximal anterior joint forces exceeding 350 N. 16 The anterior capsular structures experience the greatest strain at this point. 17 There is eccentric loading of the subscapularis and pectoralis muscle to decelerate the arm.
During the next phase, the acceleration phase, the humerus rotates internally, with concentric contraction of the internal rotators and high forces at the glenohumeral joint. The hand accelerates from 0 mph to 90 mph in 50 msec. This is caused by the concentric contraction of the pectoralis major, latissimus, and triceps. The humeral angular velocity can reach 7000°/sec, with rotational torques reaching 14,000 inch/lb. 18,19 The deceleration and follow-through phases involve the dissipation of the kinetic energy, with the external rotators contracting eccentrically and the scapula being stabilized. High forces are generated at the glenohumeral joint during this phase.
The freestyle swimming stroke is also a complex athletic movement, which serves as a model for the biomechanics of the overhand motion. Pink et al. 20 have characterized the kinematics of the freestyle swimming stroke (Fig. 2). During hand entry, the muscles that position the scapula (trapezii, rhomboids supraspinatus, middle deltoid, and anterior deltoid) are firing. The serratus anterior and subscapularis muscles are continuously active. During pullthrough, the pectoralis major, latissimus dorsi, and posterior deltoid provide most of the force. Pink et al. 21 performed stroke analysis and electromyographic analysis comparing pain-free to painful shoulders during the freestyle swimming stroke. They demonstrated that the painful shoulders had an asynchronous firing pattern. 21 Competitive distance swimmers may make up to 16,000 shoulder revolutions per week or 2500 revolutions per day, and shoulder problems are reported in 66% of these swimmers. 20
SHOULDER INSTABILITY IN THE ATHLETE
The shoulder is a highly mobile joint that trades a high functional range of motion for a lack of inherent stability. The bony geometry of the shoulder does not offer much stability. The glenoid labrum increases the area of contact but is relatively smaller and shallower than the large humeral head. In contrast to the bony constraint of joints such as the hip, the soft tissues that span the joint are what provide stability in the shoulder joint. The glenohumeral ligaments and glenoid labrum provide static restraint to excessive glenohumeral motion. The rotator cuff, deltoid, and peri-scapular muscles provide dynamic control of the humerus and scapula. This maintains the net humeral joint reaction force centered into the glenoid, thus keeping the joint located. 22 These tissues also provide a sealed space that can provide stability to the joint by creating a negative intraarticular pressure through a suction cup-like situation. Repetitive overhead motions, such as throwing or swimming, can upset this delicate balance of forces and increase the instability of the humeral head within the glenoid.
Shoulder laxity has been defined as “the physiological motion of the glenohumeral joint that allows a normal range of motion.”15 The amount of laxity in the shoulder varies from person to person and is, by definition, asymptomatic. 23,24 Carter and Wilkinson 25 defined patients as having generalized joint laxity when the results of four or more laxity tests were positive. Beighton et al. modified the criteria of Carter and Wilkinson 26 to define a mobility score (range, 0–9 points), with each extremity given a separate point. Nicholas 27 defined the shoulder of an individual as loose with one or more positive result(s) of laxity tests. He found a statistically significant association between looseness and knee ligament injuries in male professional football players. Women were not studied. Grana and Moretz 28 evaluated secondary school athletes, and were unable to confirm Nicholas' association between looseness and ligament injury.
There is controversy about whether there is a difference in the prevalence of generalized joint laxity between men and women. Although Carter and Wilkinson 25 found an overall prevalence of generalized joint laxity in 6.7% (19 of 285) of children, they found no difference between boys (6.2%, nine of 145) and girls (7.1%, 10 of 140). Using the criteria of Nicholas, 27 Grana and Moretz 28 reported that the shoulders of female high school basketball players were looser than the shoulders in males (P < 0.05). Beighton et al. 26 found that girls and women were more mobile than boys and men at any age, and that mobility decreased with age in both genders. They also found a correlation between mobility scores and musculoskeletal symptoms for men (R = 0.80) and for women (R = 0.96). The Beighton et al. 26 study focused on subjects from an African population.
When motion that occurs around the glenohumeral joint becomes symptomatic, a distinction is made between shoulder laxity and shoulder instability. 29 Instability is defined as abnormal symptomatic motion of the glenohumeral joint that results in painful subluxation or dislocation of the shoulder. 30 Shoulder instability, as a diagnosis, is varied, and can be classified according to timing, degree, direction, etiology, and volition (Table 1). 31 The hypermobility syndrome describes the situation in which generalized joint laxity is associated with musculoskeletal symptoms, and seems to be more common in women. 31,32,33 Kirk et al. 33 used the criteria of Carter and Wilkinson 25 to define the hypermobility syndrome. This syndrome has been reviewed by several authors, 29,31,32,33,34 and shoulder symptoms were often reported to be uncommon (4.2% to 6.7%). 29,33,34 Treatment of the unstable shoulder in the athlete will vary depending on the type of instability present, and therefore, correct classification of the instability is vital.
In the athlete, anterior instability can result from an acute dislocation (macrotrauma) or subluxation of the shoulder due to repetitive microtrauma. The pathology found with these two types of anterior instability is quite different. The athlete with a traumatic dislocation usually has a Bankart lesion with detachment of the labrum and capsule. This type of instability has been labeled TUBS (Traumatic, Unidirectional, Bankart lesion, Surgical repair). The athlete with repetitive microtrauma usually has an intact labrum, but has excessive capsular laxity.
The stability of the glenohumeral joint is a function of both dynamic (rotator cuff and scapular stabilizers) and static (capsule, ligaments, and labrum) entities. The rotator cuff functions dynamically to keep the humeral head centered in glenoid fossa. The anterior glenohumeral ligaments function as primary static restraints to excessive translation of the humeral head in various arm positions (Table 2). If these anterior structures become loose due to repetitive strain, increased humeral translation results. This can lead to rotator cuff overload, subsequent tendinopathy, and, ultimately, further instability. These patients tend to present with symptoms of pain more than instability. 35 However, the primary pathology is instability. Anterior instability patterns generally present with “dead arm” or coming-apart symptoms during the late cocking phase or early acceleration phase of the overhand motion. 36
The overlap between pain and instability in the throwing or overhead athlete can be confusing. Table 3 lists possible differential diagnoses for an unstable shoulder. Jobe et al. 37 suggested a classification system to aid in reaching a more definite diagnosis (Table 4).
The standard of care for symptomatic patients with recurrent dislocations is either an open or an arthroscopic capsulolabral reattachment. This has been variably combined with capsular procedures such as capsular shift, suture plication, and thermal capsulorrhaphy. Table 5 summarizes publications reporting the results of operative treatment of anterior shoulder instability. Where the numbers of men and women were included, of 2083 patients, 1619 (77.7%) were male and 464 (22.3%) were female.
Remarkably, few authors have analyzed the success of surgical stabilization based on gender. All of the above-mentioned studies have shown a preponderance of men. Manta et al. 38 performed a retrospective review of 30 shoulders in 29 patients who underwent an arthroscopic transglenoid suture repair technique for traumatic or recurrent anterior instability. Three of the patients' shoulders demonstrated increased laxity with a positive sulcus sign and increased posterior laxity. These authors reported an overall success rate of 40%, with success defined as no episodes of instability at 5 years. In male subjects, they reported a 45% success rate, and in female subjects, a 30% success rate. A second, open procedure was required in 20% of the male subjects and 40% of the female subjects. They felt that this showed a trend toward higher failure rates in women. Pagnani et al. 39 reported on arthroscopic shoulder stabilization using transglenoid sutures for recurrent anterior instability in 37 shoulders. There were 33 male patients and 4 female patients. Seven (19%) patients, all male, had recurrent episodes of instability after surgical stabilization. Youssef et al. 40 performed arthroscopic transglenoid Bankart suture repair for recurrent anterior instability in 30 patients (23 men, 7 women). They evaluated outcome using the Rowe functional grading system. They found no correlation between outcome and gender. Landseidl 41 reported on 65 cases of recurrent anterior instability treated with arthroscopic capsular repair. There were 46 men and 19 women. They reported 9 redislocations, 8 in male patients and 1 in a female patient. Hayashida et al. 42 performed a retrospective review on 82 arthroscopic Bankart transglenoid suture repairs for traumatic anterior instability. There were 63 men and 19 women, with overall 67% excellent results based on the Rowe scale. On multivariate analysis, gender was not statistically related to a poor result.
In general, in the under-30 age group, there has been no correlation shown for gender as a prognostic variable of anterior dislocation. Hoelen 43 and Hovelius 44 found no difference between men and women, Simonet 45 found no difference for age-matched men and women, and Rowe 46 found a recurrence of 60% for men and 30% for women (these were not evaluated by age).
Multidirectional instability (MDI) is a clinical diagnosis defined as symptomatic glenohumeral instability or translation in more than one direction (anterior, inferior, and posterior) on physical examination. 30 The wide variation in normal shoulder laxity demands that a diagnosis of MDI be made only if the laxity is in more than one direction and that it is symptomatic for the patient.
Brown et al. 15 modified the classification of Pagnani and Warren. 47 Type I multidirectional instability, asdescribed by Pagnani and Warren, would be called multidirectional laxity with global instability and subluxation, or dislocation occurring anteriorly, posteriorly, and inferiorly. There may be generalized joint laxity within the family, with or without underlying connective tissue disorders, such as in individuals with Marfan syndrome or Ehlers-Danlos syndrome. Patients with Type II multidirectional instability, or multidirectional laxity with anterior and inferior instability, typically sustain one traumatic event with the presence of underlying asymptomatic multidirectional laxity. These patients frequently will have a Bankart lesion and therefore are best treated surgically with a Bankart repair and a capsular shift procedure. Patients with Type III multidirectional instability, or multidirectional laxity with posterior and inferior instability, usually have progressive capsular laxity due to repetitive microtrauma, from activities such as swimming or throwing, or after a relatively minor traumatic event. Finally, patients with Type IV multidirectional instability, or multidirectional laxity with anterior and posterior instability without inferior instability, have been reported to exist, but have not been seen in the authors' clinical practice.
The standard of treatment for patients with MDI is a course of rehabilitation. 48 If this is unsuccessful, open surgical stabilization in the form of an inferior capsular shift has been shown to be effective. 49 Other techniques of arthroscopic stabilization, such as capsular plication and thermal capsulorrhaphy, are also currently being investigated.
Table 6 summarizes publications reporting the results of operative treatment of MDI. Of those where gender was included, a total of 465 patients with MDI were treated. There were 289 (62.2%) men and 166 (35.7%) women. Although the ratio of men to women is lower than in the anterior instability population, 11 out of 14 papers indicated a male preponderance. Gartsman et al. 50 reported on the results of 43 men and 11 women with bidirectional instability (inferior, with either anterior or posterior), based on history, physical exam, radiographic tests, and arthroscopy, excluding patients with unidirectional instability. Operative treatment consisted of labral repair, if required, followed by suture imbrication ± thermal capsulorrhaphy of the glenohumeral ligaments and rotator interval closure. Evaluation included the American Shoulder and Elbow Surgeon's Shoulder Index, the Constant, Rowe, and UCLA Scores. They found statistically significant differences in the Rowe score according to gender (P = .039). The mean score for men was 90.5, and for women, 98.1.
Posterior shoulder instability is relatively uncommon and has a wide range of presentations. Table 7 reviews the classification of posterior instability of the shoulder. Recurrent posterior subluxation is the most common posterior shoulder instability found in the overhead athletic population. During the follow-through phase of throwing, the backhand stroke in tennis, and initiation of the pull-through phase in swimming, strain is placed on the posterior capsule as the humeral head is forced posterior. With the arm at 45° of abduction, the posterior band of the IGHL is the primary restraint to posterior translation. 51 Patients with recurrent posterior instability may have pain or feelings of instability with the shoulder in this flexed, internally rotated, and adducted position. Therefore, the throwing athlete with posterior subluxation or instability will primarily complain of pain during the follow-through phase. 36
The results of treatment of recurrent posterior subluxation have been more unpredictable than for anterior instability. 52,53 Surgical treatment for posterior subluxation includes soft tissue capsular tightening procedures, such as a posterior capsulorrhaphy or a posterior-inferior capsular shift. Bony procedures, such as glenoid osteotomies, have also been used to treat this condition.
Table 8 summarizes publications reporting the results of operative treatment of posterior shoulder instability. Of those in which gender was included, there was a total of 247 patients; 180 (72.9%) were men and 67 (27.1%) were women. All studies reported a male preponderance. None of the studies included here reported outcome according to gender.
Shoulder instability is common in athletes. As girls and women continue to become more active, it will be more commonly seen in female athletes. Advances in sport have challenged surgeons to work harder to find improved methods for treatment of this often difficult problem. Combining and comparing the results of treatment is difficult because of a lack of standardized, valid pathologic categories, consistent clear surgical techniques, and reliable valid outcome tools. The role of gender in the presentation and outcome of treatment of shoulder instability has yet to be fully defined. This article reviews papers reporting on the different types of shoulder instability and their treatment, to try to draw some conclusions on the incidence of shoulder instability in the male and female population. In the literature reviewed, of a total of 2083 patients with unidirectional anterior instability, women were 22.3%. Of 465 patients with multidirectional instability, 35.7% were female. Of 247 patients with posterior instability, 27.1% were female.
Conclusions about the incidence of shoulder instability in female athletes must be drawn with caution. First, not all of these patients were athletes, but part of the general patient population. Second, many articles were retrospective reviews. It was also not clear in many of the articles whether the patients were consecutive. There was some heterogeneity in the classification of the different types of instability, and therefore, some patients may also not have been consistently classified between the different studies. Finally, patients with instability that did not require operative intervention were not included in this review.
In analyses of the outcome of surgical intervention, patients were rarely subclassified according to gender. Even when data were analyzed according to gender, no consistent conclusions could be reached. Certainly, the differences in outcome between male and female risk needs more study. At this time, the most that can be said is that women are less likely than men to undergo surgery for stabilization of an unstable shoulder of any category, and that there is no strong evidence to suggest that the outcome of surgery is significantly different than that of men. The higher incidence of shoulder instability in the male population may simply reflect the fact that men are still more likely than women to participate in high-risk activities and overhead sports. Data on the rate of shoulder instability per unit of exposure to high activities is not available at this time.
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Nicola Maffulli, M.D., Guest Editor
Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
Female; Athletes; Shoulder instability; Review; Surgery