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Which is worse—shoulder subluxations or dislocations?

By Annie Hayashi

Award-winning study of more than 4,000 West Point cadets shows that shoulder subluxations may be as serious as dislocations

In a major study of more than 4,000 West Point cadets, researchers found that the incidence of shoulder subluxations was more than five times higher than that of dislocations over the course of a year. In addition, subluxations were frequently accompanied by other injuries such as Bankart lesions or capsulolabral avulsions, which are normally associated with dislocations. The findings are significant for surgeons who treat shoulder injuries, according to MAJ Brett D. Owens, MD, because “the morbidity of a traumatic subluxation event is underappreciated.”

Dr. Owens presented the results of the National Collegiate Athletic Association award-winning study at the 2007 annual meeting of the American Orthopaedic Society for Sports Medicine.

“We saw a unique opportunity to look at some of the epidemiological variables and take a hard scientific look. What are the risk factors, if any, for developing instability? Is there anything from a demographic standpoint, for example, that makes someone more predisposed to instability? In terms of causation, why is one prone to an instability event?” added coauthor COL Thomas M. DeBerardino, MD.

West Point cadet (in red) misses a punch in boxing match. (Picture courtesy of U.S. Military Academy)

Shoulder instability events defined
Dr. Owens and his colleagues analyzed a closed cohort of 4,141 young, active, male (3,509) and female (632) cadets with a mean age of 20 years for any shoulder instability events. “We were very concerned about how we defined the study parameters,” he said. “We classified instability events in the following three ways: direction—either anterior or posterior; occurrence—first time or recurrent; and type—dislocation or subluxation. In our study, dislocations were those events that required a manual reduction by a healthcare provider. Subluxations were short of that.”

Patients were examined by an orthopaedic surgeon for shoulder instability. Within the time frame of the study, a “new event” was defined as the first time a patient was seen for a shoulder injury. The event was considered “recurrent” if the cadet had a history of either dislocation or subluxation in that shoulder prior to or during the study. The report tracked only new shoulder instability events. The patient’s demographics, participation in sport(s), and mechanism of injury were also tracked.

An ideal study cohort
All cadets at West Point are required to participate in organized sports and physical activities including boxing, football, rugby, gymnastics, an obstacle course, and military training. Boxing is a mandatory physical education class for males. Females are required to do “close-quarters” combat training that requires intensive, upper-extremity activity.

“Our goal was to capture every event right after it happened. All orthopaedic surgeons, athletic trainers, and physical therapists made a concerted effort to refer any cadet who sustained any type of shoulder injury. We captured all of the shoulder instability events within 72 hours of the injury. After an examination by an orthopaedic surgeon, as well as an X-ray or magnetic resonance image (MRI), we were able to exclude those with an acromioclavicular (AC) sprain or a contusion. We included those who experienced suspected instability events,” explains Dr. Owens.

He admits that the student population has a “captured nature” from a healthcare standpoint—making it ideal for this type of study. Cadets don’t go outside the West Point complex for any medical evaluations. This enabled the researchers to collect data in a uniform manner and follow the outcome of injuries.

High incidence of subluxations reported
From the total student population, 117 cadets sustained a new, traumatic, shoulder instability event during the course of the academic year. Of that number, 101 (86.3 percent) were men and 16 (13.7 percent) were women, which is in proportion to the actual student body. The majority (80.3 percent) of the instability events were anterior, while 10.3 percent were posterior, and 11 percent were multi-directional. (Some injuries fit more than one category.)

Subluxations accounted for 99 out of the 117 events—84.6 percent; the number of dislocations (18) was substantially lower. Of the subluxations, 45 percent were primary events, and 54.5 percent were recurrent. Of the 18 dislocations, 66.7 percent were primary events and 33.3 percent were recurrent.

All but one of the injuries occurred during athletic or physical activities. The sports that caused the most injuries involved punching, falling, or tackling. Boxing caused the greatest number of injuries (16.2 percent), followed by football (15.4 percent). The most common mechanism of injury was a fall (15.4 percent), followed by a collision (14.5 percent). Boxing caused the most noncontact injuries (14.5 percent); in all cases, missed punches caused an anterior shoulder subluxation.

Football falls or tackles accounted for 15 percent of the injuries. Injuries also occurred during a mandatory physical education class that included both gymnastics and a rigorous obstacle course.

MAJ Brett D. Owens, MD

Long-term consequences
“One unexpected finding in our study was the large proportion of subluxations. They have received little attention in the literature. However, in our population, subluxation comprised the majority of the instability events and was associated with MRI findings of labral tears in 49.4 percent of cases,” according to Dr. Owens. Forty-six percent of subluxations had humeral head osteochondral lesions.

Drs. Owens and DeBerardino see these subluxation events as a serious problem that may be as serious as a dislocation in the long term. They found that many patients with these subluxation events sustained Bankart lesions or capsulolabral avulsions, which have been thought to be caused only during a dislocation event. “The pathoanatomy, the actual biology, the structure that is getting injured is exactly the same,” according to Dr. DeBerardino. “Subluxation is underrepresented in the literature and the morbidity of a traumatic subluxation event is underappreciated.”

For more information, refer to: Owens BD, Duffey ML, Nelson BJ, DeBerardino TM, Taylor DC, Mountcastle SB: The incidence and characteristics of shoulder instability at the United States Military Academy. Am J Sports Med 2007;35:1168-1173.

Annie Hayashi is the senior science writer for AAOS Now. She can be reached at hayashi@aaos.org.

September 2007 AAOS Now
http://www.aaos.org/news/bulletin/sep07/clinical1.asp