Shoulder instability patterns have been classified in numerous ways in the orthopaedic literature, including the following:

• Traumatic or atraumatic
• Voluntary or involuntary
• Unidirectional or multidirectional.

In general, surgeons have believed that traumatic, unidirectional instability patterns are more common in males, and atraumatic, multidirectional instability patterns are more common in females.

Sex-associated differences in shoulder stability can be attributed to static restraints (collagen elasticity), dynamic restraints (altered stabilizer muscle firing patterns), and proprioceptive differences. Hormonal differences have also been implicated, in particular due to their influence on collagen. Research in these areas to differentiate patterns between males and females is limited.

Although female sex is associated with benign hypermobility syndrome, it is not clear that generalized ligamentous laxity correlates with shoulder laxity or instability. Most of the hypermobility measures used to measure joint laxity do not include shoulder laxity as a factor to determine generalized hypermobility.

Evidence that the female shoulder is more lax than the male’s is limited; some reports have found no difference between them. Moreover, shoulder laxity may not result in symptoms of instability (Fig. 1). Female sex has been associated with decreased joint proprioceptive ability in the shoulder joint, and proprioceptive deficiencies have also been noted in patients with hypermobility syndrome.

In the United States, shoulder instability rates in athletes have been documented in both male and female athlete populations. In males, the rate is 0.15/1,000 athletic exposures (AE); in females, it is 0.06/1,000 AE. Similarly, in military populations, shoulder instability rates in males are approximately double those in females (1.82/1,000 person years vs 0.9/1,000 person years). These epidemiologic studies, however, do not indicate what percent of male and female patients with a documented shoulder instability event also had shoulder laxity or generalized ligamentous laxity.

Surgical management
Most studies that report results of surgical management for unidirectional instability include a predominance of male patients. When female patients are included, their outcome scores and results are generally not reported separately from results in male patients. In most studies, the small numbers in one or both groups (males and females) do not allow a statistical analysis for comparing the sexes.

Review articles on the topic have identified risk factors for failed stabilization procedures. These include the following:

• Male sex
• Young age
• Number of dislocations prior to treatment
• Generalized ligamentous laxity
• Number of anchors used
• Bony deficiency

Although earlier studies on multidirectional instability had reported on more female patients, more recent studies on surgical treatment for multidirectional instability have had a preponderance of male patients. Results of surgical management in male and in female patients are not independentlyreported.

Posterior instability
Posterior instability has also been studied, and surgical management reports again show a preponderance of male patients. Female sex is not noted to be a risk factor for failed surgical management of shoulder instability in any of these populations.

In comparing the incidence rates for shoulder instability in studies of athletic and military populations to the proportion of male and female patients treated with surgical management in these studies, it would appear that female patients are either offered surgical management at a lesser rate than their male counterparts or choose surgical management less often than their male counterparts. More research is needed to determine whether patient- or surgeon-driven management decisions are responsible for different surgical rates between the sexes, and if so, what the reasoning is behind these decisions.

Nonsurgical management
Early papers on nonsurgical management of shoulder instability suggested that therapy-based treatment outcomes for traumatic instability were poor. Outcomes for atraumatic instability managed nonsurgically were better.

Current results of nonsurgical management may be better than previous studies indicate, as an understanding of shoulder rehabilitation improves. Little research comparing outcomes in male versus female patients addresses specific exercise protocols for shoulder instability. More research is needed to determine whether female patients respond better to therapy-based treatment compared to surgical management, as well as to determine the role of proprioception in neuromuscular shoulder control in male versus female patients.

Sheila M. Algan, MD, is a member of the AAOS Women’s Health Issues Advisory Board and clinical assistant professor for the University of Oklahoma College of Medicine’s orthopaedic surgery department. She can be reached at Sheila-algan@ouhsc.edu

Putting sex in your orthopaedic practice
This quarterly column from the AAOS Women’s Health Issues Advisory Board and the Ruth Jackson Orthopaedic Society provides important information for your practice about issues related to sex (determined by our chromosomes) and gender (how we present ourselves as male or female, which can be influenced by environment, families and peers, and social institutions). It is our mission to promote the philosophy that male and female patients experience and react to musculoskeletal conditions differently; when it comes to patient care, surgeons should not have a one-size-fits-all mentality.