Anatomical Borders of the Breast
The breast tissue forms a pentagon shape with five specific anatomical borders: superiorly at the clavicle, inferiorly at the inframammary fold (5th-6th rib), medially at the lateral edge of the sternum, laterally at the midaxillary line, and returning superolaterally back to the midaxilla. 1
The Pentagon-Shaped Perimeter
The traditional perception of the breast as a conical structure is anatomically incorrect. 1 Understanding the true pentagon shape is essential for complete clinical examination and surgical planning.
Five Specific Landmarks Define the Borders:
Superior border: The clavicle marks the upper extent of breast tissue 1, 2
Inferior border: The inframammary fold, located at the level of the 5th-6th rib 1, 2
Medial border: The lateral edge of the sternum (not the midline of the sternum itself) 1, 2
Superolateral return: From the midaxillary line, the border returns back to the midaxilla, completing the pentagon 1
Clinical Significance of Border Recognition
The upper outer quadrant and the area under the areola/nipple are the two most common sites for breast cancer to arise, making thorough examination of these regions particularly critical. 1, 3
Mobility of Borders:
The upper and lateral breast borders are relatively mobile, while the inferior and medial breast borders are relatively fixed. 4 This anatomical reality affects surgical planning and reconstruction outcomes.
Examination Implications
When performing clinical breast examination, all tissue within this pentagon must be systematically examined using a vertical strip pattern to ensure complete coverage. 1, 3 The breast is a skin structure held in place by skin/fascial zones of adherence, and the breast itself is mobile over the pectoralis fascia. 4
Common Pitfall to Avoid:
Incomplete coverage of breast tissue by not following the pentagon-shaped perimeter is a frequent examination error that can result in missed lesions. 3 Examiners must resist the temptation to examine only the visible breast mound and must extend palpation to all five borders of the pentagon.