Scapulothoracic ‘joint’

The scapulothoracic ‘joint’ is not a true synovial joint but is instead describing the movements of the scapula on the posterior thoracic cage. Many muscles that have their distal attachments on the scapula affect this ‘joint,’ such as trapezius, serratus anterior, and pectoralis minor mm. This joint affects movement of the scapula, such as elevation/depression, protraction/retraction, and upward/downward rotation of glenoid fossa.

Acromioclavicular (AC) joint

The acromioclavicular (AC) joint is the articulation between the acromial (lateral) end of the clavicle and the acromion of the scapula. It is an example of a plane, synovial joint, which only allows slight movements - gliding/sliding of the scapula on the clavicle.

While there are other ligaments closely associated with the fibrous joint capsule, the integrity and strength of this joint is actually greatly reinforced by an extrinsic ligament, the coracoclavicular ligament, which is composed of two bands that connect the coracoid process of the scapula and the clavicle.

The term ‘shoulder separation’ describes acromioclavicular joint injuries, particularly dislocations. Depending on the severity, the acromion becomes more prominent than usual.

Glenohumeral (shoulder) joint

The glenohumeral joint is the true shoulder joint. It is the most mobile joint of the body, but with increased mobility, there is noted decreased stability. Again to reach the full range of motions that can occur at the shoulder joint, the joints mentioned above also need to move, so if an injury or pathology affects those joints, this will often present with reduced mobility in the glenohumeral joint as well.

This joint is a synovial ball and socket joint, which is multiaxial allowing the actions:

• Flexion: movement of the shoulder in an anterior direction in the sagittal plane

• Extension: movement of the shoulder in a posterior direction (or back to anatomical position and beyond) in the sagittal plane

• Abduction: movement of the shoulder in a lateral direction in the coronal/frontal plane (think moving away from the body; median plane)

• Adduction: movement of the shoulder in a medial direction in the coronal/frontal plane (think adding back towards the body)

• Circumduction: this is a complex, circular movement that involves the sequential movements of flexion, abduction, extension, and adduction (or opposite order)

• Medial (Internal) rotation: rotates anterior surface of humerus (and all superficial parts) towards the median plane

• Lateral (External) rotation: rotates anterior surface of humerus (and all superficial parts) away from the median plane

The glenohumeral joint is formed by the relatively large head of the humerus and relatively shallow glenoid fossa. The arrangement and shape of these bones play an important role in increased mobility of this joint.

There are numerous accessory structures associated with this complex joint (ligaments, tendons, bursae, etc.), but we will focus on a few of the most clinically relevant areas.

The rotator cuff tendons are the main source of stability for the glenohumeral joint, forming an almost complete musculotendinous ‘cuff’ around the joint. There is a notable and clinically important inferior deficiency to the ‘cuff.’ Dislocations of the glenohumeral joint are almost always oriented inferiorly, but are typically clinically described as anterior (most frequent) or posterior.

The coraco-acromial arch is formed by the coracoid process and acromion of the scapula with the coraco-acromial ligament. This region of the shoulder is important in preventing superior dislocations of the glenohumeral joint. This region also allows passage of the supraspinatus m. tendon (a rotator cuff muscle). Repetitive actions, such as full flexion or abduction, can cause increased friction in this area and can cause tendonitis or other injury to the supraspinatus tendon.

There is a bursa located between the arch and glenohumeral joint to help facilitate these movements and reduce friction, the subacromial (subdeltoid) bursa. 

The glenoid labrum is a fibrocartilaginous lip circumscribing the glenoid fossa, which helps to provide a better fit of the humeral head.

2. Describe the location, innervation, and actions of the deltoid muscle.

The deltoid muscle is a very powerful and coarsely-textured muscle of the shoulder region. It is this muscle, which is located superficial to and covering the glenohumeral (shoulder) joint, that gives the shoulder region its distinct rounded contours.

Proximal attachments of the deltoid include the clavicle and scapula, with a distal attachment on the shaft of the humerus. Thus, the deltoid crosses and directly affects the actions of the glenohumeral joint. 

The major/prime action of the deltoid muscle is abduction of the arm at the shoulder, but it does rely on the supraspinatus muscle to initiate the first 15° of abduction. Since this muscle is so complex and is really composed of 3 parts that can act independently of each other, its different components may play antagonistic (opposing) roles in specific actions at the glenohumeral joint: 

• flexion/extension, and

• medial (internal) rotation/lateral (external) rotation.

The deltoid muscle is innervated by the axillary nerve, and may be paralyzed and atrophy with injury to the nerve. As this is a common site for intramuscular injections, care must be taken to avoid the deep axillary nerve.

3. Describe the location, major actions, and role in stability with the glenohumeral joint of the rotator cuff muscles. Describe the basic location of teres major m., and the boundaries and contents of the quadrangular space.

Rotator cuff muscles

There are four muscles that make up the rotator cuff. Three are located on the posterior surface of the scapula, from superior to inferior: supraspinatus, infraspinatus, and teres minor. These muscles distally attach to the greater tubercle of the humerus.

One rotator cuff muscle (subscapularis m.) is located on the anterior surface of the scapula. and distally attaches to the lesser tubercle of the humerus.

These attachments on the tubercles of the humerus allow the rotator cuff muscles to form a nearly complete musculotendinous ring around the glenohumeral (shoulder joint), which serves as the main source of stability of this joint.

The most frequently injured/damaged rotator cuff muscle is the supraspinatus m., due to its location between the coraco-acromial arch and glenohumeral joint.

All rotator cuff muscles, except supraspinatus m. play a role in lateral & medial rotation of the shoulder joint, dependent on muscle. The supraspinatus m. initiates the first 15° abduction of the shoulder joint, after which the deltoid muscle is the primary abductor.

Teres major muscle

The teres major muscle is not a rotator cuff muscle - it does not play a role in the musculotendinous ring around the glenohumeral joint. However, it is located in close proximity to posterior rotator cuff muscles, and just inferior to the teres minor m. The distal attachment is on the proximal humerus, and thus, teres major m. directly affects the actions of the shoulder joint, in particular adduction and medial rotation.

This is a comparatively thick and robust muscle to the rotator cuff mm. and can be visible and palpated as the inferolateral portion of the posterior axilla.

The quadrangular space is important in understanding where the axillary n. and posterior circumflex humeral artery are located deep to the deltoid muscle. It is bounded by:

• Teres minor m. (superiorly)

• Teres major m. (inferiorly)

• Triceps brachii m., long head (medially)

• Humeral shaft (laterally)

4. Describe the collateral blood flow to the shoulder region.

The blood supply to the shoulder consists of several anastomosing arteries. High mobility joints typically have extensive collateral supply. The arteries that supply the shoulder come from the subclavian a. and its continuation, the axillary a.

The two most clinically relevant areas of arterial anastomoses in the shoulder region include:

• Posterior scapular region includes the anastomosis of:

  • Dorsal scapular a. (medial scapula; br. of subclavian a.)

  • Suprascapular a. (superior scapula; br. of subclavian a.)

  • Circumflex scapular a. (lateral scapula; br. of axillary a.)

  • Clinical consideration: With ligation,injury, or atherosclerosis of the axillary a., there is typically still sufficient arterial supply to region (muscles, etc.) with the anastomoses with subclavian artery branches, as these come off proximal to the axillary a.

• Surgical neck of the humerus includes the anastomosis of branches of the axillary artery:

  • Anterior circumflex humeral a.

  • Posterior circumflex humeral a. (typically more robust)

  • Clinical consideration: The surgical neck is a relatively weak region of the humerus and is an area of increased incidence of fracture. 

    • If only one circumflex humeral artery is affected by a fracture, the other should provide collateral circulation to surrounding tissues.

    • The axillary nerve travels in close approximation with the posterior circumflex humeral a. through the quadrangular space and around the surgical neck of the humerus.

5. Diagram the brachial plexus beginning proximally with nerve roots and ending with terminal branches. List the major muscle groups innervated by the terminal branches.

Brachial plexus components (proximal to distal):

• ROOTS

  • Ventral primary rami (VPR) of spinal nerves C5-T1

• TRUNKS

  • Superior (C5-C6)

  • Middle (C7)

  • Inferior (C8-T1)

• DIVISIONS

  • Anterior divisions (innervate anterior/flexor compartments)

  • Posterior divisions (innervate posterior/extensor compartments)

• CORDS (names based on location relative to axillary a.)

  • Medial (anterior division of inferior trunk)

  • Posterior (all 3 posterior divisions)

  • Lateral (anterior divisions of superior & middle trunks)

• Terminal BRANCHES

  • Musculocutaneous n. - a terminal br. of lateral cord

    • Innervates muscles of anterior compartment of arm

  • Axillary n. - a br. of posterior cord

    • Innervates the deltoid & teres minor mm.

  • Radial n. - terminal br. of posterior cord

    • Innervates posterior compartments of arm & forearm

      • Radial n. = posterior!

  • Median n. - contributions from lateral & medial cords

    • Innervates majority of mm. of anterior compartment of forearm; 5 intrinsic hand mm.

  • Ulnar n. - continuation of medial cord

    • Innervates a small component of the anterior compartment of forearm; most intrinsic hand mm.

6. Identify the major muscles, innervation, actions, and important anatomical/clinical relationships of the anterior compartment of the arm.

The anterior compartment of the arm is located anterior to the humerus and is separated from the posterior compartment by thick, deep fasciae referred to as intermuscular septa.

MUSCLES

• Biceps brachii m. - most superficial

  • Often referred to as a ‘three-joint muscle’ as its tendons cross 3 joints: 

    • shoulder, 

    • elbow, and 

    • proximal radio-ulnar

  • 2 proximal tendons (‘heads’; short & long) - both of which are originate on the scapula

    • The long head of biceps brachii actually traverses within the joint cavity of the glenohumeral joint

      • Tendon travels distally in the intertubercular sulcus, which is converted into more of a canal by a ligament

        • This is a common area of inflammation of this tendon (biceps tendinitis) due to wear and tear, or even more severe issues such as dislocation or rupture

  • 1 distal tendon that attaches to the radial tuberosity of the radius

• Brachialis m. - deep to biceps brachii m.

  • Extends from mid-humerus to ulna

• Coracobrachialis m. - deep to biceps brachii m.; superomedial arm

  • Scapula to humerus

  • The musculocutaneous n. pierces this muscle

INNERVATION

• The muscles of the anterior compartment of the arm are primarily innervated by the musculocutaneous n.

ACTIONS

• Flexion of the elbow joint - primary action of this compartment

  • Brachialis m. is typically prime mover

  • Biceps brachii m.’s involvement is contingent on position of elbow and forearm

• Supination of radio-ulnar joints

  • Biceps brachii m.

• Synergistic for some actions at shoulder joint (e.g. flexion, adduction)

  • Coracobrachialis m.; long head of biceps brachii m.

7. Identify the major muscles, innervation, and actions of the posterior compartment of the arm.

The posterior compartment of the arm is located posterior to the humerus and is separated from the anterior compartment by thick, deep fasciae referred to as intermuscular septa.

MUSCLES

• The major muscle of the posterior compartment of the arm is the triceps brachii m.

  • As the name triceps suggests, there are three proximal tendons (or heads): long, medial, and lateral heads

    • The long head originates on the scapula (thus crossing both the shoulder and elbow joint), whereas the other two originate on the humerus (crossing only the elbow joint)

  • Distal attachment: olecranon of ulna

• Anconeus: small muscle on posterolateral elbow region - often difficult to differentiate from distal triceps

INNERVATION

• The posterior compartment of the arm is innervated by the radial n.

  • Remember radial n. = posterior!

ACTIONS

• Primary action is extension of elbow joint

• Long head of triceps plays small role in extension & adduction of shoulder joint

8. Identify the major joints and associated ligaments in the elbow region. Explain what types of movement occur at these joints and other pertinent clinical information.

There are 3 articulations associated with a shared articular capsule in the elbow region: humero-ulnar (elbow), humeroradial, & proximal radio-ulnar joints.

Humero-ulnar (elbow) joint

• Hinge joint (works with humeroradial joint)

• Articulation of trochlea of humerus and trochlear notch of ulna

• Movement:

  • Flexion and extension of elbow joint

• Ligaments: collateral ligaments (ulnar & radial) on sides of joint to help guide correct movements

• Bursae: olecranon bursae - typically 3 sets

  • Bursitis of elbow (olecranon bursitis): inflammation of bursa(e) associated with elbow joint

Proximal radio-ulnar joint

• Pivot joint

• Articulation of head of radius and radial notch of ulna

• Movement:

  • Pronation and supination of proximal radio-ulnar joint (forearm)

• Ligament: anular ligament of radius

  • Circumferential ligament surrounding the head of the radius

• Clinical Consideration: subluxation or dislocation of the radial head (‘pulled elbow’): common injury in juveniles after sudden pulling or lifting event

  • Injury to anular ligament of radius and displacement of radial head (typically distally)

9. Differentiate between a dermatome and peripheral cutaneous nerve map.

There are two types of cutaneous innervation maps: dermatome & cutaneous peripheral maps.

Dermatome map

Dermatome map will depict different areas of skin afferently innervated by a single cranial or spinal nerve. There are numerous dermatome maps available that may show slight variations. While the maps show very distinct territories, there is a considerable amount of overlap between territories.

To understand the dermatome map in the upper limb, understanding the development of limbs is helpful. The limbs begin as lateral protrusions from the trunk, and the 1st digit (thumb) will be directed superiorly (or on the cranial side). This means that the lateral surface of the upper limb will have more cranially directed spinal nerves innervating skin.

Cutaneous peripheral map

Cutaneous peripheral map will depict the different areas of skin afferently innervated by a specific multisegmented peripheral nerve (or its branches). A cutaneous nerve map fills in the granular details of a dermatome map. 

Paresthesia over a specific area can indicate damage or injury to a cranial or spinal nerve and/OR a peripheral nerve. Knowledge of both is important in differentiating potential issues.

You do not have to memorize (will not be assessed on in Anatomy) any dermatome or peripheral nerve maps for the Anatomy portion of the curriculum, but you can use some of these resources for reference.