Boundaries and External Landmarks
Topographic anatomists consider the infraclavicular region as part of the shoulder girdle. It has clear osteomuscular boundaries:
- Superior: clavicle (clavicula).
- Inferior: horizontal line passing at the level of the 3rd intercostal space.
- Medial: lateral edge of the sternum.
- Lateral: anterior edge of the deltoid muscle (m. deltoideus).
A key landmark is the infraclavicular fossa (fossa infraclavicularis), also known as Mohrenheim's fossa. It is located below the clavicle, at the border of its lateral and middle thirds. The distal continuation of this fossa is the deltopectoral groove (sulcus deltopectoralis). Deep within this groove, approximately 1.5–2 cm below the clavicle, the coracoid process of the scapula (processus coracoideus) can be palpated.
Layer-by-layer structure and fasciae
The tissues of the infraclavicular region are arranged in layers, forming important fascial spaces:
- Skin and subcutaneous tissue: the skin is thin and mobile. Supraclavicular nerves (nn. supraclaviculares) from the cervical plexus run within the adipose tissue.
- Superficial fascia: in the upper third, it forms a sheath for the platysma (platysma). At the level of ribs II–III, it thickens, forming the suspensory ligaments of the breast (Cooper's ligaments).
- Deep pectoral fascia (fascia pectoralis): consists of superficial and deep layers that form a closed sheath for the pectoralis major muscle (m. pectoralis major). Fascial septa containing lymphatic vessels pass between the muscle bundles. Purulent processes and metastases in breast cancer can spread deeply along these pathways.
- Subpectoral space (spatium subpectorale): the cellular space posterior to the pectoralis major muscle.
- Clavipectoral fascia (fascia clavipectoralis): a deep fascial layer forming osteofibrous sheaths for the pectoralis minor and subclavius muscles.
Thoracic triangles
For the description of the topography of the axillary neurovascular bundle, the infraclavicular region is divided into three triangles. The main landmark is the pectoralis minor muscle (m. pectoralis minor):
- Clavipectoral triangle (trigonum clavipectorale) — located between the clavicle and the superior border of the pectoralis minor muscle. The first part of the axillary artery is projected here.
- Pectoral triangle (trigonum pectorale) — corresponds to the contours of the pectoralis minor muscle itself. It contains the second part of the axillary artery, which is surrounded on three sides (laterally, medially, and posteriorly) by the cords of the brachial plexus.
- Subpectoral triangle (trigonum subpectorale) — lies between the inferior borders of the pectoralis minor and pectoralis major muscles. It corresponds to the third part of the artery, where major nerve trunks begin to diverge toward the upper limb.
Clinical anatomy of vessels and cellular spaces
The projection of the axillary bundle runs from the medial half of the middle third of the clavicle downwards and laterally to the border of the lower and middle thirds of the deltopectoral groove. Meanwhile, the axillary vein (v. axillaris) occupies the most medial position.
The cellular tissue of the infraclavicular region is not isolated. Through fascial defects and along the vessels, purulent processes can freely spread into the lateral cervical triangle, the axillary fossa, and its inferior regions.
The region of the clavicle is of particular clinical significance: here, the clavipectoral fascia is tightly fused with the adventitia of the subclavian vein. With sharp arm abduction, the vein can be compressed between the fascia, the clavicle, and the 1st rib, leading to the development of so-called «effort thrombosis».
Briefly: answers to topical questions
Which branches arise from the axillary artery in the clavipectoral triangle?
The following branches arise from the axillary artery in the clavipectoral triangle:
- Superior thoracic artery (a. thoracica superior) — branches out in the first and second intercostal spaces.
- Thoracoacromial artery (a. thoracoacromialis) — almost immediately divides into deltoid, pectoral, and acromial branches.
To memorize the topography of the neurovascular bundle in the clavipectoral triangle (from anterior to posterior and medial to lateral), use the mnemonic VAPlex: Vein, Artery, Plexus (brachial plexus).
Class and exam questions
What is Mohrenheim's fossa?
This is a synonym for the infraclavicular fossa (fossa infraclavicularis). It is located below the clavicle, between its lateral and middle thirds, and is bounded by the clavicular part of the pectoralis major muscle and the deltoid muscle.
How do breast cancer metastases spread in this region?
They often follow the lymphatic vessels located along the connective tissue septa of the deep pectoral fascia, spreading to the deep surface of the pectoralis major muscle.
Which muscles are enclosed in the sheaths of the clavipectoral fascia?
This fascia (fascia clavipectoralis) forms isolated osteofibrous sheaths for two muscles: the pectoralis minor (m. pectoralis minor) and subclavius (m. subclavius).
What else to study on this topic
- Branches of the axillary artery in each of the three triangles
- Syntopy of the brachial plexus cords in the pectoral triangle
- Topography of the cephalic vein of the upper limb (v. cephalica)
- Boundaries and contents of the subpectoral space
- Quadrangular space (Foramen quadrilaterum) of the axillary region
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