Posted inanatomy

Exploring the Anatomy of the Shoulder: A Complex Marvel of Mobility and Stability

The shoulder is one of the most intricate and versatile joints in the human body. It provides exceptional range of motion while maintaining stability for various activities. I have a love-hate relationship with the shoulder. It is so complex yet works together – when working properly – seamlessly for smooth operation. Understanding its anatomy is essential for diagnosing injuries, designing rehabilitation programs, and appreciating the complexity of human movement. Let’s examine the bones, joints, muscles, ligaments, and nerves in the shoulder. We will highlight their roles in function and movement.

Bones of the Shoulder

Three primary bones form the skeletal framework of the shoulder:

  1. Clavicle (Collarbone):
    • A S-shaped bone that connects the sternum (breastbone) to the scapula (shoulder blade).
    • Acts as a strut to stabilize the shoulder and protect underlying nerves and blood vessels.
  2. Scapula (Shoulder Blade):
    • A flat, triangular bone that serves as the attachment site for multiple muscles.
    • Key features include the glenoid cavity, where the humerus articulates, and the acromion and coracoid process, which provide additional muscle attachment points.
  3. Humerus (Upper Arm Bone):
    • The long bone of the upper arm that fits into the glenoid cavity of the scapula, forming the glenohumeral joint.
    • Its proximal end includes the head, greater tubercle, and lesser tubercle, important for muscle attachments.

Joints of the Shoulder

The shoulders remarkable range of motion is due to the interplay of four joints:

  1. Glenohumeral (GH) Joint:
    • A ball-and-socket joint where the head of the humerus fits into the shallow glenoid cavity.
    • Provides the greatest range of motion but is prone to dislocations due to its shallow depth.
  2. Acromioclavicular (AC) Joint:
    • Formed by the junction of the acromion of the scapula and the clavicle.
    • Important for shoulder elevation and rotation; commonly injured in shoulder separations.
  3. Sternoclavicular (SC) Joint:
    • Connects the clavicle to the sternum, acting as a pivot point for shoulder movements.
    • Provides stability through strong ligaments, allowing for clavicular elevation, depression, and rotation.
  4. Scapulothoracic Joint (Physiological Joint):
    • Not a true anatomical joint but the functional movement between the scapula and the thoracic cage.
    • Essential for full arm elevation, allowing the scapula to glide and rotate along the ribcage.

Muscles of the Shoulder

The shoulder muscles are categorized into two groups: extrinsic and intrinsic muscles.

Extrinsic Muscles

These muscles originate outside the shoulder and contribute to large movements:

The extrinsic muscles of the shoulder connect the upper limb to the axial skeleton. These muscles can be divided into two groups: superficial and deep.

Superficial Extrinsic Muscles:

  1. Trapezius
    • Origin: Occipital bone, ligamentum nuchae, spinous processes of C7-T12
    • Insertion: Lateral third of clavicle, acromion, spine of scapula
    • Action:
      • Upper fibers: Elevate the scapula
      • Middle fibers: Retract the scapula
      • Lower fibers: Depress the scapula
    • Innervation: Accessory nerve (CN XI) and C3-C4 spinal nerves (proprioception)
  2. Latissimus Dorsi
    • Origin: Spinous processes of T7-L5, thoracolumbar fascia, iliac crest, and lower 3-4 ribs
    • Insertion: Intertubercular sulcus (groove) of the humerus
    • Action: Extension, adduction, and medial rotation of the humerus
    • Innervation: Thoracodorsal nerve (C6-C8)

Deep Extrinsic Muscles:

  1. Levator Scapulae
    • Origin: Transverse processes of C1-C4
    • Insertion: Medial border of the scapula, superior to the spine
    • Action: Elevates the scapula and tilts the glenoid cavity inferiorly by rotating the scapula
    • Innervation: Dorsal scapular nerve (C5) and cervical nerves (C3-C4)
  2. Rhomboid Major
    • Origin: Spinous processes of T2-T5
    • Insertion: Medial border of the scapula, below the spine
    • Action: Retracts the scapula, rotates it to depress the glenoid cavity, and fixes the scapula to the thoracic wall
    • Innervation: Dorsal scapular nerve (C5)
  3. Rhomboid Minor
    • Origin: Spinous processes of C7-T1 and ligamentum nuchae
    • Insertion: Medial border of the scapula, at the level of the spine
    • Action: Retracts the scapula, rotates it to depress the glenoid cavity, and fixes the scapula to the thoracic wall
    • Innervation: Dorsal scapular nerve (C5)
  4. Serratus Anterior
    • Origin: External surfaces of ribs 1-8 or 1-9
    • Insertion: Anterior surface of the medial border of the scapula
    • Action: Protracts the scapula, holds it against the thoracic wall, and rotates the scapula upwards
    • Innervation: Long thoracic nerve (C5-C7)

      Intrinsic Muscles

      The intrinsic shoulder muscles originate from the scapula and/or clavicle and insert onto the humerus. They primarily move the glenohumeral (shoulder) joint.

      Intrinsic Shoulder Muscles:

      1. Deltoid
        • Origin: Lateral third of the clavicle (anterior fibers), acromion (middle fibers), and spine of the scapula (posterior fibers)
        • Insertion: Deltoid tuberosity of the humerus
        • Action:
          • Anterior fibers: Flexion and medial rotation of the arm
          • Middle fibers: Abduction of the arm (after the initial 15°)
          • Posterior fibers: Extension and lateral rotation of the arm
        • Innervation: Axillary nerve (C5-C6)
      1. Teres Major
        • Origin: Posterior surface of the inferior angle of the scapula
        • Insertion: Medial lip of the intertubercular sulcus (groove) of the humerus
        • Action: Adduction, medial rotation, and extension of the arm
        • Innervation: Lower subscapular nerve (C5-C6)

      Rotator Cuff Muscles:

      These four muscles stabilize the humeral head within the glenoid cavity.

      1. Supraspinatus
        • Origin: Supraspinous fossa of the scapula
        • Insertion: Superior facet of the greater tubercle of the humerus
        • Action: Initiates abduction of the arm (first 15°) and stabilizes the humeral head
        • Innervation: Suprascapular nerve (C5-C6)
      2. Infraspinatus
        • Origin: Infraspinous fossa of the scapula
        • Insertion: Middle facet of the greater tubercle of the humerus
        • Action: Lateral rotation of the arm and stabilization of the humeral head
        • Innervation: Suprascapular nerve (C5-C6)
      3. Teres Minor
        • Origin: Middle part of the lateral border of the scapula
        • Insertion: Inferior facet of the greater tubercle of the humerus
        • Action: Lateral rotation of the arm and stabilization of the humeral head
        • Innervation: Axillary nerve (C5-C6)
      4. Subscapularis
        • Origin: Subscapular fossa of the scapula
        • Insertion: Lesser tubercle of the humerus
        • Action: Medial rotation of the arm and stabilization of the humeral head
        • Innervation: Upper and lower subscapular nerves (C5-C7)

      These muscles contribute to a wide range of shoulder movements and provide critical stability to the shoulder joint during dynamic activities.

      Ligaments and Bursae

      Ligaments of the Shoulder

      These ligaments provide stability by reinforcing the joints:

      1. Glenohumeral Ligaments (Superior, Middle, Inferior): The glenohumeral ligaments (GHLs) are key stabilizers of the glenohumeral (shoulder) joint, which is the articulation between the humeral head and the glenoid cavity of the scapula. These ligaments reinforce the joint capsule and provide passive stability by preventing excessive movement.
        • There are three main glenohumeral ligaments, named based on their position:
          • Superior Glenohumeral Ligament (SGHL)
            • Origin: Upper part of the glenoid cavity
            • Insertion: Lesser tubercle of the humerus
            • Function:
              • Limits inferior translation of the humeral head when the arm is at the side.
              • Provides stability during external rotation in the adducted position.
          • Middle Glenohumeral Ligament (MGHL)
            • Origin: Anterior surface of the glenoid cavity
            • Insertion: Anterior part of the humeral neck
            • Function:
              • Limits anterior translation of the humeral head.
              • Provides restraint during external rotation when the arm is abducted to 45°.
          • Inferior Glenohumeral Ligament (IGHL)
            • Origin: Lower part of the glenoid cavity
            • Insertion: Inferior part of the humeral neck
            • Function:
              • Primary stabilizer when the arm is abducted to 90°.
              • Consists of an anterior and posterior band with an axillary pouch in between.
              • Prevents anterior dislocation during abduction and external rotation (anterior band) and posterior dislocation during internal rotation (posterior band).
        • Static Stability: Help maintain the humeral head’s position within the shallow glenoid cavity, especially when the joint is under stress.
        • Prevent Dislocation: Protect against anterior, inferior, and posterior dislocations of the humeral head.
        • Reinforce the Capsule: Strengthen the joint capsule, particularly anteriorly, where the shoulder is more vulnerable.
      2. Coracoclavicular Ligament: The coracoclavicular ligament is a key stabilizing structure of the shoulder, connecting the coracoid process of the scapula to the clavicle. It plays a critical role in maintaining the stability of the acromioclavicular (AC) joint.
        • Components:The coracoclavicular ligament is composed of two distinct parts:
          • Conoid Ligament:
            • Located medially
            • Runs from the coracoid process to the conoid tubercle of the clavicle
            • Provides vertical stability, preventing the clavicle from being displaced superiorly.
          • Trapezoid Ligament:
            • Located laterally
            • Runs from the coracoid process to the trapezoid line on the clavicle
            • Provides horizontal stability, resisting shearing forces that could separate the clavicle from the scapula.
        • Functions:
          • Stabilization of the AC Joint:
            • Prevents excessive upward displacement of the clavicle relative to the scapula.
          • Force Transmission:
            • Transfers forces from the upper limb to the axial skeleton through the clavicle.
          • Prevention of Dislocation:
      3. The coracoacromial ligament (CAL): a strong fibrous band that forms part of the roof of the shoulder joint. It connects two parts of the scapula—the coracoid process and the acromion.Forms an arch over the humeral head, preventing superior dislocation.
        • Origin: Coracoid process of the scapula
        • Insertion: Acromion of the scapula
        • Forms the Coracoacromial Arch:
          • Along with the acromion and coracoid process, the ligament forms the coracoacromial arch, which acts as a protective roof over the glenohumeral joint.
          • This arch protects the structures beneath it, including the supraspinatus tendon, subacromial bursa, and the long head of the biceps tendon, from direct trauma.
        • Functions:
          • Prevents Superior Displacement of the Humeral Head:
            • Acts as a barrier to upward dislocation of the humeral head, especially in the absence of rotator cuff integrity.
          • Maintains Shoulder Stability:
            • Contributes to passive stability by preventing excessive upward movement of the humeral head, particularly during shoulder abduction and flexion.
        • Clinical Significance:
          • The coracoacromial ligament is implicated in subacromial impingement syndrome. When the space beneath the coracoacromial arch is narrowed (due to thickening of the ligament, bony spurs, or inflammation), it can compress the supraspinatus tendon or subacromial bursa, leading to pain and restricted movement.
      4. Transverse Humeral Ligament: The transverse humeral ligament (THL) is a small but important ligament that plays a key role in stabilizing the long head of the biceps brachii tendon.
        • Origin and Insertion: The transverse humeral ligament spans the intertubercular groove (bicipital groove) of the humerus, extending between the greater tubercle and lesser tubercle of the humerus.
        • Functions:
          • Stabilizes the Biceps Brachii Tendon:
            • It holds the tendon of the long head of the biceps brachii in the intertubercular groove, preventing it from dislocating or slipping out of the groove during shoulder and elbow movements.
          • Protects the Synovial Sheath:
            • The ligament protects the synovial sheath that surrounds the biceps tendon, ensuring smooth gliding within the groove.
          • Maintains Integrity During Shoulder Motion:
            • The ligament helps maintain the positioning of the biceps tendon during movements like flexion, extension, and rotation of the shoulder joint.
        • Clinical Significance:
          • Biceps Tendon Subluxation or Dislocation:
            • If the transverse humeral ligament is torn or weakened (often due to trauma or rotator cuff injuries), the biceps tendon can dislocate or subluxate, leading to pain and shoulder dysfunction.
          • Bicipital Tendinitis:
            • Inflammation of the long head of the biceps tendon, often associated with repetitive overhead activities, may be exacerbated by issues with the transverse humeral ligament.

      Bursae

      Bursae are fluid-filled sacs that reduce friction between tissues:

      1. Subacromial Bursa: The subacromial bursa is a fluid-filled sac located in the shoulder that plays a crucial role in reducing friction between structures during shoulder movement.
        • Anatomy:
          • Location: Positioned between the acromion and coracoacromial ligament above, and the supraspinatus tendon and superior part of the shoulder joint capsule below.
          • Often continuous with the subdeltoid bursa, so it is sometimes referred to collectively as the subacromial-subdeltoid bursa.
        • Functions:
          • Reduces Friction:
            • Acts as a cushion to reduce friction between the acromion, coracoacromial ligament, and the supraspinatus tendon during shoulder movements, particularly in abduction and flexion.
          • Facilitates Smooth Motion:
            • Allows smooth gliding of the supraspinatus tendon under the acromion during arm elevation, enhancing the efficiency of shoulder motion.
          • Protects Tendons and Muscles:
            • Shields the supraspinatus tendon and rotator cuff from mechanical stress and potential damage caused by contact with the overlying structures like the acromion.
        • Clinical Significance:
          • Subacromial Bursitis:
            • Inflammation of the subacromial bursa can result from repetitive overhead activities, trauma, or rotator cuff injuries.
            • Symptoms include shoulder pain, especially during abduction and flexion, with tenderness over the acromion.
          • Impingement Syndrome:
            • The subacromial bursa is often involved in subacromial impingement syndrome, where it is compressed between the acromion and the supraspinatus tendon, leading to pain and restricted movement.
          • Rotator Cuff Tears:
            • Chronic inflammation of the bursa can contribute to or result from rotator cuff tears, complicating shoulder function further.
      2. Subdeltoid Bursa: The subdeltoid bursa is a fluid-filled sac in the shoulder region that helps reduce friction during movement. It often works with the subacromial bursa, and in many cases, the two are continuous, forming the subacromial-subdeltoid bursa.
        • Anatomy:
          • Location: Lies between the deltoid muscle and the shoulder joint capsule, particularly over the supraspinatus tendon and greater tubercle of the humerus.
        • Functions:
          • Reduces Friction:
            • Acts as a cushion between the deltoid muscle and the rotator cuff, preventing friction and wear during arm elevation, abduction, and rotation.
          • Facilitates Smooth Movement:
            • Ensures the deltoid muscle can glide smoothly over the underlying rotator cuff tendons and joint structures, enhancing efficient shoulder motion.
          • Protects Rotator Cuff:
            • Shields the supraspinatus tendon from mechanical stress as the deltoid contracts during shoulder abduction and flexion.
        • Clinical Significance:
          • Subdeltoid Bursitis:
            • Inflammation of the subdeltoid bursa, often occurring with or independently of subacromial bursitis, can result from overuse, trauma, or rotator cuff injuries.
            • Symptoms include pain over the lateral shoulder, tenderness, and difficulty in lifting the arm, especially in abduction.
          • Shoulder Impingement Syndrome:
            • The subdeltoid bursa can become compressed between the humeral head and the acromion, contributing to impingement syndrome, which causes pain during overhead activities.
          • Rotator Cuff Pathology:
            • Chronic bursitis can lead to or coexist with rotator cuff tendinopathy or tears, exacerbating shoulder dysfunction and pain.

      Nerves of the Shoulder

      The shoulder is innervated by branches of the brachial plexus, a network of nerves originating from the cervical spine (C5–T1). These nerves provide motor and sensory innervation to the muscles, skin, and joints of the shoulder region.

      • Major Nerves of the Shoulder:
        • Axillary Nerve (C5-C6) Origin: Posterior cord of the brachial plexus
          • Course: Passes through the quadrangular space along with the posterior circumflex humeral artery
          • Innervates:
            • Motor: Deltoid and teres minor muscles
            • Sensory: Skin over the deltoid region (lateral shoulder via the superior lateral cutaneous nerve)
          • Clinical Significance: Injury can occur during shoulder dislocation or humeral surgical neck fractures, leading to deltoid weakness and loss of sensation over the lateral shoulder.
        • Suprascapular Nerve (C5-C6) Origin: Upper trunk of the brachial plexus
          • Course: Passes through the suprascapular notch under the transverse scapular ligament
          • Innervates:
            • Motor: Supraspinatus and infraspinatus muscles
            • Sensory: Glenohumeral and acromioclavicular joints (no cutaneous sensory distribution)
          • Clinical Significance: Compression at the suprascapular notch can cause supraspinatus/infraspinatus weakness, affecting shoulder abduction and external rotation.
        • Dorsal Scapular Nerve (C5) Origin: Directly from the C5 root of the brachial plexus
          • Course: Runs along the medial border of the scapula
          • Innervates:
            • Motor: Rhomboid major, rhomboid minor, and levator scapulae
          • Clinical Significance: Injury can cause weakness in scapular retraction and elevation.
        • Long Thoracic Nerve (C5-C7) Origin: Roots of C5-C7 of the brachial plexus
          • Course: Runs along the lateral thoracic wall, superficial to the serratus anterior
          • Innervates:
            • Motor: Serratus anterior muscle
          • Clinical Significance: Injury (e.g., from trauma or surgery) can lead to winging of the scapula and difficulty with overhead movements.
        • Subscapular Nerves (Upper and Lower) (C5-C6) Origin: Posterior cord of the brachial plexus
          • Course: Pass directly to subscapularis and teres major
          • Innervates:
            • Upper Subscapular Nerve: Subscapularis
            • Lower Subscapular Nerve: Subscapularis and teres major
          • Clinical Significance: Rarely injured, but weakness can affect internal rotation and adduction.
        • Thoracodorsal Nerve (C6-C8) Origin: Posterior cord of the brachial plexus
          • Course: Runs along the posterior axillary wall
          • Innervates:
            • Motor: Latissimus dorsi
          • Clinical Significance: Injury can impair shoulder adduction and internal rotation.

        Blood Supply of the Shoulder

        The shoulder receives its blood supply from branches of the subclavian artery and the axillary artery, which are responsible for delivering oxygenated blood to the muscles, bones, joints, and other soft tissues of the shoulder region.

        • Main Arteries Involved:
          • Subclavian Artery
            • Origin: The subclavian artery arises from the brachiocephalic trunk on the right side and directly from the aortic arch on the left side.
            • It supplies the upper limb and parts of the neck and chest. Some of its branches contribute to the blood supply of the shoulder.
            • Key Branches Relevant to the Shoulder:
              • Vertebral Artery: Supplies the cervical spine and brain, indirectly supporting shoulder stability.
              • Thyrocervical Trunk: Gives off important branches that contribute to shoulder blood flow.
                • Suprascapular Artery: Supplies the supraspinatus and infraspinatus muscles, as well as the shoulder joint and acromioclavicular (AC) joint.
                • Transverse Cervical Artery: Supplies the trapezius muscle and the levator scapulae, and also branches to the rhomboids.
                • Inferior Thyroid Artery: Supplies the thyroid gland and parts of the shoulder region.
          • Axillary Artery
            • Origin: The subclavian artery becomes the axillary artery as it passes the first rib and enters the axilla (armpit).
            • The axillary artery is divided into three parts. Each part gives rise to branches that supply various muscles, joints, and structures of the shoulder and upper arm.
              • Part 1 (Proximal to the Pectoralis Minor):
                • Superior Thoracic Artery: Supplies the first and second intercostal spaces, as well as the serratus anterior muscle.
              • Part 2 (Posterior to the Pectoralis Minor):
                • Thoracoacromial Artery: A short trunk that gives off multiple branches that supply the shoulder region:
                  • Clavicular Branch: Supplies the subclavius muscle.
                  • Acromial Branch: Supplies the deltoid and acromion.
                  • Deltoid Branch: Supplies the deltoid muscle.
                  • Pectoral Branches: Supply the pectoralis major and pectoralis minor muscles.
                • Lateral Thoracic Artery: Supplies the serratus anterior, pectoralis muscles, and lateral chest wall.
              • Part 3 (Distal to the Pectoralis Minor):
                • Subscapular Artery: The largest branch of the axillary artery. It supplies the subscapularis, latissimus dorsi, and teres major muscles, and gives rise to the circumflex scapular artery and thoracodorsal artery.
                  • Circumflex Scapular Artery: Passes through the triangular space and supplies the muscles around the scapula, including the infraspinatus, teres minor, and supraspinatus.
                  • Thoracodorsal Artery: Supplies the latissimus dorsi muscle.
                • Posterior Circumflex Humeral Artery: Passes through the quadrangular space and supplies the deltoid and teres minor muscles, and also provides blood to the shoulder joint.
                • Anterior Circumflex Humeral Artery: Provides blood to the shoulder joint, particularly the glenohumeral joint and the biceps brachii.
        • Key Venous Drainage:
          • Subclavian Vein: Collects blood from the arm, shoulder, and head and neck regions, draining into the brachiocephalic vein.
          • Axillary Vein: Formed by the union of the brachial vein (from the arm) and basilic vein (from the forearm), and it drains into the subclavian vein.
          • Cephalic Vein: A superficial vein that drains the lateral side of the upper limb and enters the axillary vein.

        Conclusion

        Wow. That was a lot. But good information to learn! The shoulder’s complex anatomy allows for an incredible range of motion while maintaining stability. Its interplay of bones, joints, muscles, ligaments, and nerves is crucial for daily activities, athletic performance, and fine motor control. Understanding the anatomy of the shoulder provides valuable insights for healthcare professionals in diagnosing injuries, developing rehabilitation programs, and improving overall musculoskeletal health. I love Trail Guide to the Body by Andrew Biel for studying the anatomy. It gives straight to the point information and the drawings are very helpful! If you are looking for more in depth learning, or are a hand therapist studying for the CHT, I highly recommend Rehabilitation of the Hand and Upper Extremity. It is a 2 volume set and is exactly what you need for the full explanation of the anatomy. I used both of these when studying for the CHT exam.

        Whether you’re a clinician, athlete, or someone fascinated by human anatomy, appreciating the shoulder’s structure can deepen your understanding of how the body achieves such remarkable movement and function.

        Thanks for learning with me!

        -Logan

        Citations

        Biel, A. (2019). Trail Guide to the body: A hands-on guide to locating muscles, bones, and more. Books of Discovery.

        Osterman, A. L., Skirven, T. M., Fedorczyk, J. M., Amadio, P. C., Feldscher, S. B., & Shin, E. K. (2021). Rehabilitation of the hand and upper extremity. Elsevier Mosby.

        Tags: