Home » Conditions and Syndromes » Thoracic Outlet Syndrome » Thoracic Outlet – Functional Anatomy

Thoracic Outlet – Functional Anatomy

Then anatomy of thoracic outlet structures is complex in several ways. Many bones are both origin and insertion for muscles in this complex. Secondly, several of the muscles have statistically significant variations in cadaver studies. This post starts with a brief overview, then, a video that walks through the structures from front to back. After that, it discusses the structures in more detail.

Structures of Thoracic Outlet Syndrome

Overview

Thoracic Outlet Syndrome (TOS) is a group of related of symptoms that indicate a restriction in the neurovascular bundle that
supplies the upper extremity. Like other syndromes, such as Chronic Fatigue Syndrome or Piriformis Syndrome, it is not a clear
diagnosis. It’s a condition that may be resolved by addressing one or many of the dysfunctions indicated by the symptoms.

The name “thoracic outlet syndrome” implies that the dysfunction has occurred in the area where the nerves and vascular structures pass through the shoulder to the upper extremity. Restrictions in myofascial structures of the neck are usually included in
this syndrome. This includes restrictions in the brachial plexus, subclavian artery, subclavian vein, and the subclavian lymph trunk.

These structures may become restricted in one or more places between the cervical vertebrae and the axilla. The first section of this document reviews the boney structures that are most directly involved. The second section reviews the soft tissues, building from posterior to anterior.

Casually Speaking

Casually, people use “Thoracic Outlet Syndrome” when talking about pain and dysfunction down the upper extremity. Here, we focus on the structures that create pain and dysfunction with a neurovascular restriction in the thoracic outlet. Interestingly, the muscles discussed here have trigger point patterns that refer down the arm as well.

Several other muscles, not in the thoracic outlet structure, refer from the torso into the arm. Look at the post on “Torso into Arm” for those muscles and their trigger point patterns.

Video of Thoracic Outlet Structures

This is a short video walking through the structures from posterior to anterior followed by a more detailed document of the same structures.

This video uses the convention throughout the site of bones of origin in blue, bones of insertion in green, and bones that are trapped in tan. That makes for an interesting perspective on how the bones and structures interact.


Details of Thoracic Outlet Structures

Bones

C2-C7 (shown in yellow) Cervical vertebrae are particularly important in the treatment of TOS. Trigger point activity in the scalenes is strongly governed by joint displacement in the cervical vertebrae. Although radiculopathy is technically not a part of TOS, these vertebrae can compress the brachial plexus’ nerve roots and should be addressed before treatment of the muscles.

C1 (atlas) does not directly attach any of the soft tissues that press on the brachial plexus. However, it is more difficult to resolve displacements in the other cervicals, with lasting results, without addressing displacements in the articulations of C1.

T1-T2 (shown in green) These two thoracic vertebrae surround the nerve roots of the lower branch of the brachial plexus. It isn’t easy to resolve FORWARD HEAD POSTURE, perpetuating TOS, without addressing shearing in the upper thoracis. Displacements of the costovertebral joints of these vertebrae govern trigger points in the scalenes.

Clavicle (not shown on the left) When depressed, the clavicle can apply pressure on the neurovascular bundle. Padget-Schroder’s syndrome is a variation of TOS where the subclavian vein deviates medially and gets compressed between the clavicle and the first rib.

First Rib (shown in tan) An elevated fist rib can press into the brachial plexus. Also, an elevated first rib governs trigger point activity in the scalenes. Ribs 2-5 (shown in tan) serve as attachments for posterior scalene, and pectoralis minor – muscles that strongly contribute to TOS.

Scapula (shown in hot pink) – The coracoid process may press into the neurovascular bundle when the scapula has an extreme forward tilt, e.g., the pec minor is very short, and the lower trapezius is overstretched.

Scalenus posticus and medius

Scalenus Medius and Posticus

The posterior scalene muscle attaches to the second rib and the posterior tubercles of C4-C6.

  • Elevation of the first two ribs proprioceptively perpetuate trigger points in the scalene muscles.

The middle scalene muscle attaches to the first rib and all the posterior tubercle of cervical vertebrae from C2-C7.

  • It creates a veil of muscle just posterior to where the brachial plexus exits.
  • Tension on this muscle creates pressure on the brachial plexus and subclavian artery from the back.
  • It elevates the first rib, pulling it up against the clavicle.
Brachial Plexus

Brachial Plexus

This plexus is composed of nerve roots from C5-T1. Close the vertebrae, they form two main bundles. C5, C6, and C7 combine into the upper trunk. C8 and T1 combine into the lower trunk. These trunks are formed before the plexus passes through the scalenes.

Scalenus Minimus

Scalenus Minimus

Scalenus minimus is a small muscle that extends from the C7 and/or C6, to the fascia of covering the chest cavity. It appears to reinforce the thick fascia that connects the anterior tubercle of C7 to the first rib. Travell found scalenus minimus on at least one side of about half the subjects studied. She remarks that it can be a strong, thick muscle. Contributions to neurovascular pressure:

  • It elevates the first rib, pulling it into the clavicle
  • It presses the brachial plexus into the medial scalene
  • It presses the subclavian artery into the anterior scalene
Subclavian Artery

Subclavian Artery

The subclavian artery exits the thoracic cage near the mid-line and loops over the first rib. It is mostly medial and, at times, anterior to
the brachial plexus as it extends toward the axilla.

Scalenus anticus

Scalenus Anticus

The anterior scalene usually attaches to the anterior tubercles of the 3rd, 4th, and 5th cervical vertebrae; just anterior to the where the nerve roots exit. Contributions to neurovascular pressure:

  • It can press the subclavian artery and brachial plexus against scalenus medius.
  • When its attachment along the 1st rib extends more posteriorly than normal, it is particularly prone to cinch on the plexus and artery.
  • It elevates the first rib, pulling it up to compress the neurovascular bundle against the clavicle.
  • It can press the subclavian artery into the scalenus minimus.
Subclavian Vein

Subclavian Vein

The subclavian vein exits the thoracic cage anterior to scalenus anticus. It travels through a groove in the first rib and then
joins the neurovascular bundle before the bundle passes into the axilla.

Subclavian Lymphatic Trunk (not shown)

The subclavian lymphatic trunk inserts into the subclavian vein medial to the insertion of the muscles on the ribs. Some of the lymph vessels that extend off of the main trunk follow the subclavian vein and can be entrapped to restrict lymphatic flow.

Clavicle

Clavicle

When depressed, the clavicle can apply pressure on the neurovascular bundle.

Padget-Schroder’s syndrome is a variation of TOS where the subclavian vein deviates medially and gets compressed between the clavicle and the first rib.

Subclavius

Subclavius

Subclavius is a cylindrical muscle that originates on the first rib’s costal-cartilage and inserts on the underside of the medial third of the clavicle. Contributions to neurovascular pressure:

  • It draws the clavicle down, trapping the neurovascular bundle between the first rib and upper belly of the serratus anterior.
  • When over-developed, its belly presses into the neurovascular bundle.
Pectoralis minor

Pectoralis Minor

Pectoralis minor originates on the 3rd, 4th and
5th rib and inserts on the medial portion of the coracoid process. More common anomalies include origins on the 1st, 2nd and 6th rib as well insertions that extend to the greater tubercle of the humerus. Contributions to neurovascular pressure:

  • Protraction of the scapula, to the point of pulling the coracoid process onto the ribs.
  • It can apply pressure on the costocoracoid membrane, which covers and protects the neurovascular bundle as it passes under the clavicle to the axilla
Thoracic Outlet Structures

Costocoracoid Membrane and Ligament

The costocoracoid membrane is a thick fascial membrane that invests pectoralis minor, invests subclavius, and occupies the area in between them.

It attaches to the clavicle, with a layer on either side of the subclavius. Those layers blend over the upper ribs and then split again as it invests the pectoralis minor. It continues laterally past the pectoralis minor to become the axillary fascia and fuse with the facia of the biceps’ short head. This, of course, varies with the more common anomalies of pectoralis minor. It fuses with the fascia over the first two ribs, extending medially past the attachment of the subclavius.

The costocoracoid ligament is a thickened portion of this membrane extending from the first rib to the coracoid process. It blends with the coracoclavicular ligament that secures the coracoid process to the clavicle.

Origin, Insertion, Trapped

From a perspective of elevating ribs, this complex structure originates from the upper vertebrae and scapula. Watch the video to see the changes in origin and insertion as each muscle is added. The scapula is, of course, suspended by the trapezius, levator scapula, and other extrinsic back muscles. The ribs are directly elevated by the scalenes and pectoralis minor. Other bony structures are caught in the middle. Even the manubrium is in limbo. It has been colored as an origin bone for anchoring the subclavius. But, in reality, it is elevated with the upper ribs by the scalenes.

It Gets a Bit More Complicated

The thoracic outlet structures are complex. To make it more complicated, muscles like scalenes, pec minor, and levator scapula have statistically significant variations. Scalenes are quite variable in size, attachments and, even, number of muscles. Pectoralis minor is variable in its attachment on ribs. Levator scapula sometimes attaches to a rib. Also, posture, activity, and weight all add to how this area works.

A detailed assessment is important for effective treatment.


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Tony Preston has a practice in Atlanta, Georgia where he sees clients. He has written and taught about anatomy, trigger points, and cranial therapies since the mid-90s.

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