Acromioclavicular joint (AC) arthritis (A) is a common incidental finding on shoulder imaging. An improved knowledge of the age- and sex-specific prevalence (or base rate) of incidental ACA can inform diagnosis and treatment strategies for shoulder pain.

There is no relationship between age or gender and the presence or severity of MRI findings consistent with osteoarthritis of the acromioclavicular joint.

We rated ACA on 475 MRIs from a database of patients who had MRIs for non-AC indications. The cohort consisted of 51% men, 49% women and had an average age of 55. Bivariate analyses were used for analysis of age and sex-specific prevalence.

The prevalence of findings consistent with ACA on MRI for non-AC indications was 83%. The prevalence increased from 75% between ages 40 and 50 to 100% after age 70. Logistic regression demonstrated an association between age and ACA (Odds Ratio 2.89; 95% confidence interval [CI]=2.30 to 3.63; p=0.001). No difference was seen by sex (Chi-Square, 0.16; p=0.67). There was a positive correlation between age and ACA severity (Spearman's rho=0.43; p=0.000010).

The observation that MRI evidence of ACA is the norm (75%) after age 40 and is universal with human aging (100% after age 70), makes it very difficult to discern ACA as a cause of shoulder symptoms. Given the near universal prevalence of radiological ACA, imaging cannot be used as a reference standard for diagnosis of symptomatic ACA based on symptoms and signs.

: Not applicable.