Study Finds Link Between Posterior Acromial Morphology and Posterior Shoulder Instability

Researchers have recently found that posterior acromial morphology is correlated with the direction of glenohumeral instability. This study was conducted to see whether acromial morphology correlates to anteroposterior shoulder stability. An article detailing the work was published last month in the Journal of Bone & Joint Surgery.

In their work, the researchers hypothesized that a horizontal and higher position of the acromion in the sagittal plane would be tied to posterior instability.

“In shoulders with posterior instability, the acromial is situated higher and is oriented more horizontally in the sagittal plane than in normal shoulders and those with anterior instability; this acromial position may provide less osseous restraint against posterior humeral head translation,” the authors wrote.

Background of the Study

The research team conducted a retrospective study of 41 patients posterior shoulder instability and 41 with anterior instability. Those with instability were compared to 53 control participants with no symptoms of instability or degenerative changes to the shoulder joint. Metrics evaluated in the participants through radiographs included posterior acromial tilt, anterior acromial coverage, posterior acromial coverage, critical shoulder angle, and posterior acromial height.

Using a logistic regression model, the researchers found that posterior acromial height was most significantly correlated with posterior instability. The posterior instability group displayed significantly greater posterior acromial heights than those with anterior instability, measure at 30.9 and 19.5 millimeters, respectively.

The odds ratio for posterior instability was 39 when the researchers used a posterior acromial height cutoff of 23 mm. Also, the shoulders with posterior and anterior instability differed significantly in their posterior acromial coverages. When analyzing shoulders with anterior instability, the team noted there was no significant difference between them and the normal shoulders other than anterior acromial coverage.

“A steep “Swiss chalet roof-type” acromion virtually excluded recurrent posterior instability in an albeit relatively small cohort of patients. Additional investigation is needed to determine the relevance of these findings for future treatment,” the authors concluded.

Author Affiliations

Authors of this Journal of Bone & Joint Surgery are Dominik C. Meyer, MD, Lukas Ernstbrunner, MD, Glenn Boyce, MD, Mohamed A. Imam, MD, Rany El Nashar, MD, and Christian Gerber, MD.