E4828. Prevalence of the Accessory Infraglenoid Muscle in Patients with and Without Teres Minor Fatty Infiltration on Shoulder MRI
  1. Jennifer Padwal; Stanford University Medical Center
  2. Robert Boutin; Stanford University Medical Center
  3. Kathryn Stevens; Stanford University Medical Center
Teres minor fatty infiltration (TMFI) has important prognostic implications for shoulder surgery. Although quadrilateral space-occupying lesions, such as paralabral cysts or fibrous bands, are typically considered in the evaluation of TMFI, the accessory infraglenoid muscle (AIGM) is an often overlooked etiology that can impinge on the axillary nerve. Previous cadaveric studies have demonstrated a high prevalence of AIGM, but little data exist comparing the prevalence of AIGM in patients with and without TMFI.

Materials and Methods:
A retrospective review of all shoulder MRIs at our institution yielded 100 patients with documented TMFI (76 men, 24 women, mean age 58.6 years). A respective sample of 100 shoulder MRIs in patients without TMFI was obtained for comparison (53 men, 47 women, mean age 56.4 years). All scans were evaluated in consensus by two radiologists (one musculoskeletal attending with 25 years of experience, and a 4th-year radiology resident) for an AIGM with diagnostic confidence, noting muscle origin and insertion. In patients with TMFI, the distance between the AIGM and axillary nerve was measured, and any abnormal axillary nerve signal documented. TMFI was graded using the Goutallier classification, and presence of atrophy and/or edema was noted.

TMFI was grade 1 in 33%, grade 2 in 32%, grade 3 in 13%, and grade 4 in 14%, with intramuscular edema in 33 and atrophy in 21. AIGM was seen in 89/100 patients with TMFI and 30/100 patients without TMFI. In all cases, the AIGM originated from the inferior glenoid neck and inserted onto the humeral neck/proximal humeral diaphysis. The inferior margin of the AIGM in patients with TMFI touched the axillary nerve in 39 (46%), with abnormal signal in 23 (27%).

There was a high prevalence (89%) of AIGM in patients with TMFI compared to those without (30%). In the TMFI group, many AIGM abutted the axillary nerve with concomitant axillary neuropathy. These results stress the importance of looking for AIGM in the setting of TMFI, as TMFI on preoperative MRI has been associated with poor postsurgical outcomes, and the presence of AIGM on shoulder MRI could negate the need for additional imaging to evaluate the cause of TMFI.