Subscapularis tendon tears hidden by the medial biceps sling can be missed on arthroscopic examination.

INTRODUCTION: To investigate (1) the prevalence of "hidden lesions" and "non-hidden lesions" of subscapularis tendon tears requiring repair during arthroscopic examination that would be missed by a 30° arthroscope, but could be identified by a 70° arthroscope, from the standard posterior portal and (2) the correlation of preoperative internal rotation weakness and findings of magnetic resonance imaging (MRI) indicating hidden lesions. MATERIALS AND METHODS: We retrospectively examined 430 patients who underwent arthroscopic subscapularis repair between was initially nonvisible with a 30° arthroscope but became visible only with a 70° arthroscope from the standard posterior portal. The preoperative and intraoperative findings of the hidden lesion group (n = 82) were compared with those of the non-hidden lesion group (n = 348). 2016 and 2020. A hidden lesion was defined as a subscapularis tendon tear requiring repair that preoperative internal rotation weakness was assessed using the modified belly-press test. Preoperative MR images were reviewed using a systemic approach. RESULTS: The prevalence of hidden lesions was 19.1% (82/430). No significant difference was found in preoperative internal rotation weakness between the groups. Preoperative MRI showed a significantly lower detection rate in the hidden lesion group than in the non-hidden group (69.5% vs. 84.8%; P = 0.001). The hidden lesions were at a significantly earlier stage of subscapularis tendon tears than the non-hidden lesions, as revealed by the arthroscopic findings (Lafosse classification, degree of retraction; P = 0.003 for both) and MR findings (muscle atrophy, fatty infiltration; P = 0.001, P = 0.005, respectively). CONCLUSIONS: Among the subscapularis tears requiring repair, 19.1% could be identified by a 70° arthroscope, but not by a 30° arthroscope, through the posterior portal. The hidden lesions showed a significantly lower detection rate on preoperative MRI than the non-hidden lesions. Thus, for subscapularis tears suspected on preoperative physical examination, the 70° arthroscope would be helpful to avoid a misdiagnosis.