Shoulder MRI Capsule Findings Often Mistaken for Frozen Shoulder, Study Warns

Millions of people each year receive a diagnosis of adhesive capsulitis, the painful and disabling condition better known as frozen shoulder. Many of those diagnoses rest, at least in part, on a single imaging finding: an abnormal-looking joint capsule on a magnetic resonance imaging scan. A new study suggests that this practice may be sending a startling number of patients down the wrong diagnostic path. Researchers in South Korea have found that nearly half of all patients undergoing routine shoulder MRI show capsular abnormalities, yet only a small fraction of them actually have clinically diagnosed frozen shoulder. The gap between what the scan shows and what the patient truly has, the authors argue, is wide enough to warrant a fundamental rethink of how radiologists report capsular findings and how clinicians interpret them.

The research, published in BMC Medical Imaging, was led by Kyu-Chong Lee and Kyung-Sik Ahn of Korea University Anam Hospital in Seoul, together with colleagues from several Korean medical centers. The team set out to answer a deceptively simple question: when a shoulder MRI shows abnormalities of the joint capsule, what do those abnormalities actually mean? Adhesive capsulitis has long been associated with characteristic changes in two specific regions of the shoulder joint, the rotator interval and the axillary pouch. The rotator interval is a triangular space between the supraspinatus and subscapularis tendons through which the long head of the biceps tendon passes, while the axillary pouch is the redundant fold of capsule at the lower part of the joint that allows the arm to lift overhead. In classic frozen shoulder, both regions typically show thickening, edema, and inflammation.

To investigate, the researchers conducted a retrospective analysis of 483 adults who underwent shoulder MRI between January and December 2020. Two musculoskeletal radiologists independently reviewed each scan, assessing abnormalities in the rotator interval and the axillary pouch, as well as tears of four key tendons: the supraspinatus, the infraspinatus, the subscapularis, and the long head of the biceps tendon. The team defined radiologic capsular abnormality, abbreviated RCA, as the presence of abnormalities in both the rotator interval and the axillary pouch simultaneously. They then used univariable and multivariable logistic regression analyses to identify which clinical and imaging factors were associated with RCA, and performed subgroup analyses comparing four patient groups: those with no capsular abnormality, those with isolated axillary pouch abnormality, those with isolated rotator interval abnormality, and those with full RCA, adjusting for age where appropriate.

The headline finding was striking. Of the 483 patients studied, 234, or 48.45 percent, had radiologic capsular abnormality. In other words, nearly one in every two shoulders scanned showed the capsular changes that many radiologists and clinicians associate with frozen shoulder. Yet clinically diagnosed adhesive capsulitis was present in only 45 patients, or 9.32 percent of the cohort. Of those 45 patients, 43 did indeed have RCA, confirming that the imaging finding is sensitive for the disease. But the reverse is emphatically not true: the overwhelming majority of patients with RCA, more than four out of five, did not have a clinical diagnosis of adhesive capsulitis. The imaging finding, in isolation, is simply not specific enough to serve as a diagnosis.

The regression analyses revealed a web of associations that complicate the picture further. Older age was independently associated with higher odds of RCA, with each additional year of age raising the odds by roughly five percent, an odds ratio of 1.05 with a 95 percent confidence interval of 1.03 to 1.07 and a p-value below 0.001. This makes biological sense, since the joint capsule, like every other soft tissue in the body, undergoes degenerative change over time. A capsule that looks thickened or edematous in a 65-year-old may simply reflect the accumulated wear of decades rather than an active inflammatory disorder. Any diagnostic framework that treats capsular abnormality as synonymous with frozen shoulder must therefore account for the age of the patient, or risk labeling normal age-related change as pathology.

Tendon tears added another layer of complexity. Tears of the subscapularis tendon, one of the four muscles that make up the rotator cuff, were positively associated with RCA. Patients with Lafosse type I subscapularis tears, the mildest grade, had 1.82 times the odds of RCA compared with those without such tears, while those with type II or higher tears had 2.15 times the odds, both statistically significant results. The long head of the biceps tendon, which runs directly through the rotator interval, also emerged as relevant: in subgroup analyses, isolated rotator interval abnormality was associated with a higher prevalence of both subscapularis and biceps tendon tears than no capsular abnormality, even after adjusting for age. This suggests that inflammation or degeneration in the rotator interval often reflects pathology of the tendons that traverse it, rather than a primary capsular disease.

Perhaps most intriguingly, tears of the supraspinatus and infraspinatus tendons, the two most commonly torn rotator cuff tendons, showed the opposite relationship: they were negatively associated with RCA. Patients with these tears were less likely to show capsular abnormalities than patients without them. The authors interpret this finding as consistent with the traditional concept of adhesive capsulitis as a diagnosis of exclusion, a condition diagnosed only after other shoulder pathologies have been ruled out. In practical terms, a shoulder dominated by a large supraspinatus or infraspinatus tear appears less likely to harbor the capsular changes of frozen shoulder, perhaps because the two conditions represent distinct biological processes that do not readily coexist, or because the mechanical picture of a cuff tear overshadows and excludes the capsular one.

The clinical implications of the study are considerable. If nearly half of shoulder MRI scans show capsular abnormalities but only about nine percent of patients have frozen shoulder, then any radiology report that flags capsular abnormality without careful qualification invites overdiagnosis. A patient told that their MRI shows findings consistent with adhesive capsulitis may undergo months of stretching programs, corticosteroid injections, or even capsular distension procedures that address the wrong problem, while the true culprit, perhaps a subscapularis tear or biceps tendon pathology, goes untreated. The authors argue that the term radiologic capsular abnormality itself should be used in reports, precisely because it describes an imaging observation without asserting a clinical diagnosis, and that particular caution is warranted in older patients and in those with subscapularis or biceps tendon pathology.

The study does have limitations inherent to its design. It was retrospective, drawing on patients who had already been referred for shoulder MRI at a single Korean institution, which may limit generalizability to other populations. The clinical diagnosis of adhesive capsulitis was extracted from existing records rather than standardized prospectively, and the researchers note that the article was published initially as an early citable version subject to further edits. Still, the size of the cohort, the independent double reading by two musculoskeletal radiologists, the multivariable statistical framework, and the age-adjusted subgroup analyses give the findings substantial weight. The work was supported by a grant from the National Research Foundation of Korea, and the authors declared no competing relationships with companies whose products might relate to the subject matter.

For patients and clinicians alike, the message is ultimately one of humility about what imaging can and cannot tell us. MRI is a magnificent tool for revealing the anatomy of the shoulder in exquisite detail, but anatomy is not destiny, and a thickened capsule on a scan is a finding, not a verdict. Frozen shoulder remains a clinical diagnosis, rooted in a patient’s history of progressive pain and profound restriction of both active and passive motion, and it should be confirmed by examination rather than inferred from a radiology report. As this study makes clear, the capsule that lights up on MRI may be telling a story about age, about a torn subscapularis tendon, or about an inflamed biceps pulley, and only careful clinical correlation can determine which story is true. In an era when imaging findings increasingly drive diagnostic decisions, the humble conclusion that a scan must never replace the physical examination has rarely been more vividly illustrated.

Subject of Research: Radiologic capsular abnormalities on shoulder MRI and their association with rotator cuff and biceps tendon tears in the diagnosis of adhesive capsulitis

Article Title: Radiologic capsular abnormalities on shoulder MRI: associations with rotator cuff and biceps tendon tears and implications for the diagnosis of adhesive capsulitis

Article References: Radiologic capsular abnormalities on shoulder MRI: associations with rotator cuff and biceps tendon tears and implications for the diagnosis of adhesive capsulitis. (n.d.). https://doi.org/10.1186/s12880-026-02895-y

Image Credits: AI Generated

DOI: 10.1186/s12880-026-02895-y

Keywords: shoulder MRI, adhesive capsulitis, frozen shoulder, rotator cuff tear, subscapularis tendon, biceps tendon, rotator interval, axillary pouch, radiologic capsular abnormality, musculoskeletal radiology, overdiagnosis, BMC Medical Imaging