Shoulder is the region of the body that connects the upper limb with the trunk. The shoulder girdle (composed of the articulation between the scapula and clavicle) and pelvic girdle are the two points in the body where the axial and the appendicular skeletons meet.

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diagram showing the acromioclavicular jointamnd the ligaments reinforcing it and other joints of the shoulder

Image: “Diagram showing the acromioclavicular jointamnd the ligaments reinforcing it and other joints of the shoulder.” by Henry Vandyke Carter, Henry Gray (1918) Anatomy of the Human Body. Bartleby.com: Gray’s Anatomy, Plate 326. License: Public Domain


Contituents of the Shoulder

  • Bones: clavicle, scapula, humerus
  • Joints: shoulder joint composed of the glenohumeral and acromioclavicular joints
  • Cartilages: hyaline cartilage, glenoid labrum
  • Muscles: anterior axioappendicular muscles, posterior axioappendicular muscles, scapulohumeral muscles

Bones

Clavicle

Diagram identifying the two clavicles between the sternum and the upper limbs

Image: “Diagram identifying the two clavicles between the sternum and the upper limbs.” by Anatomography – en:Anatomography. License: CC BY-SA 2.1 JP

The clavicle is the only horizontal long bone in the body. It is also the only bone connecting the upper limb with the trunk. The clavicle, along with the scapula, contributes to the formation of the shoulder girdle. Due to its location, it serves as a major supporting mechanism for the scapula and the upper limb.

The upper limbs are suspended away from the thorax due to their attachment to the clavicle. Additionally, the clavicle also functions to protect the cervicoaxillary canal that encompasses the long thoracic nerve and other neurovascular structures. Clavicle also transmits physical shock from the upper limb to the axial skeleton.

In the shoulder joint, the clavicle articulates with the clavicular notch of the manubrium to form the sternoclavicular joint as well as its articulation with the acromion of the scapula to form the acromioclavicular joint.

Clavicle also serves as attachment surface of numerous important muscles of the shoulder region.

Scapula

Image identifying the posterior surface of the scapula

Image: “Image identifying the posterior surface of the scapula.” by Anatomography – en:Anatomography. License: CC BY-SA 2.1 JP

Scapula is a triangular shaped bone with lateral, superior and inferior angles, superior, lateral and medial borders and posterior and costal surfaces.

Glenoid cavity is found on the lateral aspect of the scapula, articulates with the head of the humerus to form the ‘ball and socket’ glenohumeral joint of the shoulder.

The scapula also consists of three processes:

  • Acromion: projects from the anterolateral aspect of the spine and articulates with the clavicle
  • Spine: divides the posterior surface of the clavicle into supraspinus and infraspinus processes.
  • Coracoid process: hook-like projection from the anterolateral aspect of the scapula right under the clavicle. Functions to stabilize the shoulder joint by providing attachment to various shoulder muscles and ligaments. It is also known as the “surgeon’s lighthouse” because of being a palpable hallmark of where important neurovascular structures lie in the upper limb.

Scapula functions to bring about important movements on the shoulder joint by providing attachment sides to various shoulder muscles. These movements include elevation, depression, abduction (protraction), adduction (retraction), upward rotation (laterally), downward rotation (medially), anterior and posterior tipping.

Proximal part of the humerus

A few structures in the proximal part of the humerus play a vital role in the formation of the shoulder joint. These structures are as follows:

  • Head of the humerus: it is spherical in shape and articulates the glenoid cavity in the scapula.
  • Greater and lesser tubercles: projections found on the proximal part of the humerus serve as attachment points for the four rotator cuff muscles. The greater tubercle is located on the lateral side and has the superior, middle and inferior facet that serve as attachment sites for supraspinatus, infraspinatus and teres minor muscle, respectively. The lesser tubercle, located anteriorly, provides attachment for the subscapularis muscle.
  • Bicipital groove: lies between the greater and lesser tubercles and extends inferiorly towards the shaft of the humerus. It provides attachment for the long head of the Biceps brachii muscle.

Deltoid tuberosity is found on the anterolateral part of mid-shaft of the humerus and serves as an attachment site for the deltoid muscle.

Joints

shoulder joint

Image: “Shoulder joint.” by Blausen.com staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. – Own work. License: CC BY 3.0

Sternoclavicular joint: saddle shaped joint that occurs between the proximal part of the clavicle and the clavicular notch of the manubrium (of sternum). It is a synovial double plane surrounded by a joint capsule. It brings out movement of clavicle on the verticle and anteroposterior plane.

Acromioclavicular joint: a plane synovial joint that occurs between the medial surface of the acromion (of scapula) and the acromial end of the clavicle. Movement on vertical and anteroposterior planes as well as axial rotation. This joint allows the movement of the arms above the head.

Glenohumeral joint: articulation between the head of the humerus and the glenoid cavity, which is reinforced by the rotator cuff muscles as well as the long head of the Biceps brachii muscle. It is a synovial ball in socket and moves for flexion, extension, medial and lateral rotation, abduction and adduction as well as circumduction.

Ligaments

  • Costoclavicular ligament: connects the proximal half of the clavicle with the 1st rib and its costal cartilage.
  • Interclavicular ligament: connects both of the clavicles to each other and the manubrium in between
  • Posterior sternoclavicular ligament: situated posterior to the joint
  • Anterior sternoclavicular ligament: situated anterior to the joint
  • Acriomioclavicular ligament: situated on top of the joint

  • Coracoclavicular ligament: divided into two additional ligaments: trapezoid ligament and conoid ligament. Extends from the coracoid process to the acromial end of claivlce (inferior surface)

  • Glenoid labrum is a cartilaginous fiber ring attached to the circumference of the glenoid cavity; it helps deepen the cavity for articulation with the head of humerus. Tendon of the long head of the Biceps brachii muscle continues from the top of the glenoid labrum and is attached to the supraglenoid tubercle. It passes through the articular cavity. Synovial membrane stretches over all articulating surfaces.

Bursae are fluid-filled spaces that occur between tendons of the muscles. They work to reduce friction between tendons and joint capsule. Following are the bursae found in the shoulder joint:

  1. Subtendinous bursa of the subscapularis located between the subscapularis muscle and fibrous membrane
  2. Subacromial-subdeltoid bursa between the joint capsule and deltoid muscle
  3. Subcoracoid bursa between the coracoid process and the joint capsule.

Ligaments work to thicken the fibrous membrane of the joint capsule. Ligaments reinforcing the shoulder joint are as follows:

  1. Superior glenohumeral ligament, middle glenohumeral ligament and the inferior glenohumeral ligament stretch from the superomedial aspect of the glenoid cavity to the lesser tubercle found on the proximal part of the humerus.

  2. Coracohumral ligament stretches from the inferior aspect of the coracoid process to the greater tubercle found on the proximal part of the humerus.

  3. Transverse humeral ligament works to stabilize the tendon of the long head of the Biceps brachii muscle between the greater and lesser tubercle.

Muscles

Anterior axioappendicular muscles

There are four anterior axioappendicular muscles and they function to allow the movement of the pectoral girdle. These muscles, along with their origins, insertions, functions and innervations are as follows:

Pectoralis major originates as two different heads: clavicular head – anterior border of medial clavicle, and sternocostal head – superior surface of the first six costal cartilages as well as the anterior aspect of the sternum. It inserts the bicipital groove in the anteromedial aspect of the proximal humerus.

The clavicular head works to flex the humerus, whereas the sternocostal head works to extend the humerus. Together they work to adduct and medially rotate the arm, while pulling the scapula anteroinferiorly. Pectoralis major is innervated by the lateral and medial pectoral nerves.

location of pectoralis minor

Image: “Location of pectoralis minor.” Original by sv:Användare:Chrizz, 1 June 2005. License: CC BY-SA 3.0

Pectoralis minor originates from the area near the costal cartilages of 3rd – 5th ribs and inserts the coracoid process superior surface medially. Its function is to pull the scapula anteriorly and inferiorly as well as stabilizing it. It also helps raising the ribs during breathing. Pectoralis minor is innervated by the medial pectoral nerve.

Subclavius muscle originates from the 1st rib and its cartilage and inserts the subclavian groove, found inferiorly, in the middle third of the clavicle. It elevates the 1st rib while depressing the clavicle and is innervated by the subclavian nerve.

Serratus anterior originates from the 8th-9th ribs and inserts the costal surface of the scapula, medially. It moves the scapula anteriorly (protraction) while stabilizing it. It also moves it upwards. Serratus anterior is innervated by the long thoracic nerve.

Posterior axioappendicular muscles

Also known as the “extrinsic group” the posterior axioappendicular muscles are divided into two further groups: superficial and deep.

Superficial muscles, along with their origins, insertions, functions and innervations are as follows:

  1. Trapezius, which originates from the occipital bone and the spinous processes of vertebrae C7-T12 and inserts the acromion process, lateral third of clavicle posteriorly, external occipital protuberance and nuchal ligament. It works to rotate, elevate and depress the scapula along with retracting it (moving posteriorly). It works antagonistically to the serratus anterior muscle, which causes protraction of the scapula. Trapezius is innervated by the accessory nerve along with cervical spinal nerves.
  2. Latissimus dorsi muscle originates from the spinous processes of vertebrae T7-T12 and inserts the intertubercular groove of humerus. Its functions are the adduction and internal rotation of the arm, while working antagonistically to deltoid and trapezius muscle. It is innervated by the thoracodorsal nerve.

Deep muscles, along with their origins, insertions, functions and innervations are as follows:

levator scapulae

Image: “Location of the levator scapulae muscle.” by modified by Uwe Gille – Gray Image:Gray409.png. License: Public Domain

  1. Levator scapulae muscle originating from the transverse processes of C1-C4 and their posterior tubercles and inserting the medial border of the scapula superiorly. Its function is to elevate the scapula. It is innervated by the cervical and dorsal scapular nerve.
  2. Rhomboid minor muscle originating from the spinous processes of C7-T1 as well as the nuchal ligaments and inserting the medial border of the scapula superiorly. It retracts the scapula, hence working antagonist to serratus anterior muscle. It also attaches the scapula to the thoracic wall, as well as helping in rotation. This muscle is innervated by the dorsal scapular nerve.
  3. Rhomboid major muscle originating from the spinous processes of vertebrae T2-T5 and inserting the medial border of scapula. It retracts the scapula, depresses the glenoid cavity and works antagonist to the serratus anterior muscle. It is innervated by the dorsal scapular muscle.

Scapulohumerus muscles are a group of 7 muscles that assist in stabilizing the glenohumeral joint by connecting the humerus to the scapula.

Rotator cuff muscles are a group of 4 muscles that work to stabilize the shoulder joint during its movements. For example, during abduction of the arm, it is the rotator cuff muscles that prevent detachment of the head of humerus from the glenoid cavity as the deltoid muscle works to elevate the arm. This mechanism is known as “concavity compression”.

Similarly, rotator cuff muscles stabilize the glenohumeral joint during external and internal rotation of the arm.

  • Supraspinatus and infraspinatus muscles are both innervated by suprascapular nerve and originate from the supraspinous and infraspinous fossae, respectively. They both insert on the greater tubercle of the humerus. Supraspinatus works to abduct the humerus, while the infrapsinatus muscle works to externally rotate the humerus.
  • Teres minor muscle originates from the lateral border of the scapula and inserts the posterior facet of the greater tubercle of the humerus. It is innervated by the axillary nerve and rotates the humerus externally.
  • Subscapularis muscle originates from the subscapularis fossa, inserts the lesser tubercle of the humerus, and is innervated by the subscapular nerve. It rotates the humerus internally.
  • Deltoid muscle is responsible for giving the shoulders their shape. It originates from the anterior borders of the clavicle, acromion and the spine of the scapula and inserts the deltoid tuberosity of the humerus. Deltoid muscle is innervated by the axillary nerve and flexes, extends, and abducts the shoulders.
  • Teres major muscle originates from the inferior angle of scapula, inserts the intertubercular sulcus of humerus, and adducts the humerus as well as rotates it internally. It is innervated by the subscapular nerve (lower).
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