Shoulder instability is broadly defined as abnormal motion of the humeral head relative to the glenoid that is symptomatic during the patient’s active range of motion (ROM).

Female athletes are commonly thought to have higher rates of shoulder instability, as well as poorer outcomes after treatment. However, few studies have examined sex-based differences in the epidemiology, treatment, and outcomes of shoulder instability—even after passage of the National Institutes of Health (NIH) Revitalization Act of 1993, which required that all NIH-supported research on human subjects include and analyze the impact on women and underrepresented minorities.

Recognition of sex-based differences is vital to further enhance the care of orthopaedic patients, especially female athletes. AAOS has been a leader on the topic, hosting symposia, including the AAOS/Clinical Orthopaedics and Related Research/Orthopaedic Research Society/Center for Musculoskeletal Health, University of California, Davis/Society for Women’s Health Research Musculoskeletal Sex Differences Throughout the Lifespan Research Symposium in 2014 and the symposium on Sex-based Considerations in Caring for Common Sports Injuries held during the AAOS 2016 Annual Meeting.

What evidence supports sex-based differences in shoulder (in)stability?

Shoulder instability (especially multidirectional) is thought to be more common in females than in males due to overall joint laxity. Although clinical studies examining sex differences in the glenohumeral joint are rare and females typically comprise less than 50 percent of the general study populations, we know the following:

1. Females demonstrate lower patient-reported outcomes (PROs) after arthroscopic stabilization.
2. Female hormonal cycles vary more than those of males. Although hormones, such as estrogen, are known to affect the anterior cruciate ligament, there is insufficient evidence to support any hormone-related differences in the ligaments that stabilize the shoulder.
3. Female glenoids are smaller in height and width and have higher inclination angles (a morphology favoring instability) compared to male glenoids.
4. Females have higher rates of generalized joint laxity that persists longer past puberty than their age-matched male counterparts.

Barnes et al., demonstrated that females have greater ROM in all planes of motion. The greatest differences in planes of motion between men and women were abduction and external rotation, increasing the risk of false-positive examination-under-anesthesia (EUA) results.

What evidence disproves a sex-based difference in shoulder (in)stability?

EUA findings may overestimate shoulders at risk: Females have greater passive ROM in all planes of motion compared to males. The greatest difference is observed with the shoulder abducted and externally rotated (load-shift position)—thus, EUA results may not be as specific for females.

Men have higher rates of dislocation: The overall incidence of glenohumeral dislocations is relatively low, with 0.08 to 0.24 dislocations per 1,000 person-years, and it is highest in young, athletic males. The incidence is higher in the military population (1.69/1,000 person-years) and highest in military academy cadets (4.65/1,000 person-years). Traumatic dislocations occur twice as often in males than in females (1.82 versus 0.9, respectively), with a direct relationship to activity level and an inverse relationship to age.

Multivariate analysis supports that men are more likely to have recurrent instability: Univariate analysis has demonstrated that age, sex, generalized ligamentous laxity (Brighton score ≥ 4), participation in sports, intensity of activity, and return to contact sports were all contributing factors to recurrent instability. However, after multivariate analysis, only male sex and younger age were independently predictive of recurrent instability.

Previous research methodology may not have been as accurate as once thought: Patients are a valuable and arguably most authoritative source of information on outcomes such as health-related quality of life, functional status, symptom burden, and health behaviors. Unfortunately, patient ratings of symptoms, function, or perceived disability are affected by multiple factors, such as psychological status, social environment, and activity. Additionally, patient expectations of outcomes after orthopaedic surgical procedures have been shown to vary by diagnosis, age, sex, education, level of function, and overall health status. Although both shoulder pain and disability generally improve after well-indicated surgical intervention, there remains a lack of reliably identifiable risk factors for failure in either domain.

Our outcome assessment tools may not be calibrated according to sex: Outcome research requires overall generic health assessments, as well as disease- and site-specific outcome measures and patients’ own perceptions of their health status.

However, studies have established that outcomes may vary due to factors other than shoulder pain, impairment, or disability, thus creating intrinsic instrument bias. The often-used Constant-Murley, University of California at Los Angeles Shoulder, American Shoulder and Elbow Surgeons, Shoulder Pain and Disability Index, and Oxford Shoulder scoring instruments have been demonstrated to record higher total normalized scores for males than for females. For example, males consistently score higher on the Constant-Murley scale (strength) as well as the Oxford Shoulder scale (pain). As higher scores are interpreted as better outcomes, scores may be most appropriate for measuring intrasex differences rather than intersex differences.

Previous research did not account for patients’ pre- and postoperative activity levels: A patient’s level of activity can affect the biological process of healing, with or without surgical treatment, as well as influence the patient’s perception of outcome. Low activity levels can falsely improve scores on many outcome measures—particularly for assessment of sports-related shoulder injuries. High activity levels, on the other hand, have been correlated with decreased long-term outcomes. Thus, at least theoretically, there is an ideal level of baseline, preoperative, and postoperative activity for most orthopaedic conditions and treatments that must be taken into consideration in relation to outcomes.

However, few of the more than 30 shoulder outcome measures that have been described measure activity level (intensity, duration, frequency, and type) to any degree of specificity.

Conclusions

Although females may be anatomically and physiologically susceptible to higher rates of shoulder instability based on glenohumeral bony morphology favoring decreased stability, increased rates of generalized ligamentous laxity, and higher baseline ROM—which are all accepted risk factors for instability and lower success rates for instability procedures—females appear to have lower rates of traumatic instability and recurrent instability based on epidemiological studies.

The sex/instrument bias of commonly used outcome measures, which may contribute to confirmation bias, calls into question the findings of inferior outcomes of shoulder stabilization surgery in females. Thus, there is sufficient evidence to challenge the commonly held notion that females have higher rates of shoulder instability and inferior outcomes after shoulder surgery.

This discrepancy may be attributable to the fact that few studies are methodologically based or sufficiently powered to look for sex-based differences in shoulder instability. Methodologically sound, adequately powered studies utilizing sex-neutral outcome measures, especially studies that account for shoulder activity level, are necessary to further advance the current body of literature.

Tierney A. Shannon, MD, and David J. Tennent, MD, are with the Department of Orthopaedic Surgery at the San Antonio Military Medical Center, JBSA-Fort Sam Houston in Texas. Both are Nth Dimensions Scholars.

William N. Levine, MD, is with the Department of Orthopaedic Surgery at Columbia University Medical Center in New York City.

Anthony E. Johnson, MD, is with the Department of Surgery and Perioperative Care at the University of Texas at Austin Dell Medical School in Austin, Texas.

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