title: MRI of the axillary arch muscle: prevalence, anatomic relations, and potential consequences.
abstract: OBJECTIVE
  The purpose of this study was to use MRI of the shoulder to analyze the axillary arch muscle and its anatomic relations to lymph nodes and the brachial plexus.
  
  
  METHODS
  In this retrospective study at a single clinic, five observers blinded to the patient's condition assessed images from 1,109 consecutive initial shoulder MRI examinations for the presence and anatomic relations of the axillary arch. MRI interpretation reports were reviewed for documentation of previous injuries and upper extremity radicular pain or numbness for possible correlations between presence of the arch and symptoms of nerve entrapment. Results were reported as prevalence percentage or mean ± SD with 95% CI. Groups were compared by use of Student's t test or chi-square test as indicated (p < 0.05).
  
  
  RESULTS
  An arch muscle was found in 71 of 1,109 (6%) examinations, and variability was found in arch insertion and visualization. A statistically significant 65 of 71 (92%) arches had a course superficial to the lymph nodes. The insertion of 50 of 71 (70%) arches was within 5 mm of the brachial neurovascular bundle. Excluding documented injuries, significantly more patients with an arch had upper extremity neurologic abnormalities than did patients without an arch (p = 0.02).
  
  
  CONCLUSIONS
  The axillary arch muscle is situated in such a way that it can conceal lymph nodes and impinge on the brachial plexus, causing symptoms of upper extremity nerve entrapment. Radiologists' familiarity with the arch can improve their recognition of this muscular variant so that they can communicate appropriate clinical correlations to referring physicians.
authors.affiliation: A. T. Still University of Health Sciences, 800 W Jefferson St., Kirksville, MO 63501, USA. mguy@atsu.edu
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Guy
authors.forename: Sandra K
authors.initials: SK
authors.lastname: Sandhu
authors.forename: John M
authors.initials: JM
authors.lastname: Gowdy
authors.forename: Cameron C
authors.initials: CC
authors.lastname: Cartier
authors.forename: James H
authors.initials: JH
authors.lastname: Adams
mesh_terms: D000293:Adolescent; D000328:Adult; D001365:Axilla / Q000033:anatomy & histology*; D016009:Chi-Square Distribution; D005260:Female; D006801:Humans; D007592:Joint Diseases / Q000175:diagnosis*; D008198:Lymph Nodes / Q000033:anatomy & histology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000033:anatomy & histology*; D012189:Retrospective Studies; D012785:Shoulder Joint / Q000033:anatomy & histology*