What Part Of The Scapula Articulates With The Humerus

The glenoid cavity, a shallow, pear-shaped depression located on the lateral angle of the scapula, serves as the articular surface for the head of the humerus. This articulation forms the glenohumeral joint, commonly known as the shoulder joint, which is the most mobile joint in the human body.

Anatomy of the Scapula and Humerus

To understand the articulation between the scapula and humerus, it’s essential to first review the anatomy of these bones individually.

Scapula (Shoulder Blade): The scapula is a flat, triangular bone situated in the upper back. It connects the upper limb to the torso and provides attachment points for numerous muscles that control shoulder and arm movement. Key features of the scapula include:

• Body: The main, flat portion of the scapula.
• Spine: A prominent ridge on the posterior surface that divides the scapula into the supraspinous and infraspinous fossae.
• Acromion: A lateral extension of the spine that articulates with the clavicle, forming the acromioclavicular joint.
• Coracoid Process: A hook-like projection on the anterior surface that serves as an attachment point for muscles and ligaments.
• Glenoid Cavity: A shallow, oval-shaped depression located on the lateral angle that articulates with the head of the humerus.
• Supraglenoid Tubercle: A small prominence located just above the glenoid cavity, serving as the attachment site for the long head of the biceps brachii muscle.
• Infraglenoid Tubercle: A small prominence located just below the glenoid cavity, serving as the attachment site for the long head of the triceps brachii muscle.
• Superior, Medial, and Lateral Borders: The edges of the scapula, providing attachment sites for muscles.
• Superior and Inferior Angles: The corners of the scapula.
• Subscapular Fossa: A large, concave depression on the anterior surface of the scapula.
• Supraspinous Fossa: A depression above the spine on the posterior surface.
• Infraspinous Fossa: A depression below the spine on the posterior surface.

Humerus (Upper Arm Bone): The humerus is the longest and largest bone of the upper limb, extending from the shoulder to the elbow. Key features of the humerus include:

• Head: A rounded, proximal end that articulates with the glenoid cavity of the scapula.
• Anatomical Neck: A groove just distal to the head.
• Surgical Neck: A narrowed region distal to the tubercles, which is a common site for fractures.
• Greater Tubercle: A large prominence on the lateral aspect of the proximal humerus, serving as an attachment point for the supraspinatus, infraspinatus, and teres minor muscles.
• Lesser Tubercle: A smaller prominence on the anterior aspect of the proximal humerus, serving as an attachment point for the subscapularis muscle.
• Intertubercular Groove (Bicipital Groove): A groove between the greater and lesser tubercles, housing the tendon of the long head of the biceps brachii muscle.
• Shaft (Diaphysis): The long, cylindrical body of the humerus.
• Deltoid Tuberosity: A roughened area on the lateral aspect of the shaft, serving as the attachment point for the deltoid muscle.
• Lateral and Medial Epicondyles: Bony projections at the distal end of the humerus, providing attachment points for muscles of the forearm.
• Capitulum: A rounded, lateral articular surface that articulates with the head of the radius.
• Trochlea: A spool-shaped, medial articular surface that articulates with the ulna.
• Coronoid Fossa: A depression on the anterior surface that accommodates the coronoid process of the ulna during flexion of the elbow.
• Olecranon Fossa: A depression on the posterior surface that accommodates the olecranon process of the ulna during extension of the elbow.
• Radial Groove (Spiral Groove): A shallow groove that spirals along the posterior aspect of the humeral shaft, lodging the radial nerve and profunda brachii artery.

The Glenohumeral Joint: Articulation Between the Scapula and Humerus

The glenohumeral joint is a ball-and-socket joint, where the "ball" is the head of the humerus and the "socket" is the glenoid cavity of the scapula. This type of joint allows for a wide range of motion, including flexion, extension, abduction, adduction, internal rotation, external rotation, and circumduction.

Glenoid Cavity: The glenoid cavity is a relatively shallow socket, which contributes to the shoulder's remarkable mobility. However, this shallowness also makes the shoulder joint inherently unstable and prone to dislocations. The glenoid cavity is deepened slightly by a fibrocartilaginous rim called the glenoid labrum, which increases the contact area with the humeral head and enhances joint stability. The labrum also serves as an attachment site for ligaments that further stabilize the joint.

Humeral Head: The humeral head is a large, rounded articular surface that faces medially, superiorly, and posteriorly to articulate with the glenoid cavity. The size and shape of the humeral head are significantly larger than the glenoid cavity, which means that only a small portion of the humeral head is in contact with the glenoid cavity at any given time. This limited contact area further contributes to the shoulder's mobility but also its instability.

Factors Contributing to Shoulder Stability

Given the inherent instability of the glenohumeral joint due to the shallow glenoid cavity and the large humeral head, several factors contribute to maintaining shoulder stability:

• Glenoid Labrum: As mentioned earlier, the glenoid labrum deepens the glenoid cavity and increases the contact area with the humeral head, thereby enhancing joint stability. Damage to the labrum, such as a tear, can lead to shoulder instability and dislocations.
• Capsule and Ligaments: The glenohumeral joint is surrounded by a fibrous capsule that encloses the joint and helps to hold the bones together. The capsule is reinforced by several ligaments, including the superior, middle, and inferior glenohumeral ligaments, as well as the coracohumeral ligament. These ligaments provide static stability to the shoulder joint and resist excessive translation of the humeral head.
• Rotator Cuff Muscles: The rotator cuff muscles are a group of four muscles that surround the shoulder joint and provide dynamic stability. These muscles include the supraspinatus, infraspinatus, teres minor, and subscapularis. The tendons of these muscles blend with the joint capsule and help to compress the humeral head into the glenoid cavity, stabilizing the joint during movement.
• Scapulothoracic Joint: Although not a true joint in the anatomical sense, the scapulothoracic articulation is the interface between the anterior surface of the scapula and the posterior rib cage. This articulation allows the scapula to glide along the rib cage, which is essential for normal shoulder function. Coordinated movement between the scapula and humerus, known as scapulohumeral rhythm, is crucial for achieving full range of motion at the shoulder joint.
• Other Muscles: Other muscles that cross the shoulder joint, such as the deltoid, pectoralis major, latissimus dorsi, and teres major, also contribute to shoulder stability by controlling movement and resisting forces that could lead to dislocation.
• Neuromuscular Control: Proprioceptors in the muscles, tendons, and joint capsule provide feedback to the nervous system about joint position and movement. This feedback allows for coordinated muscle activation and adjustments to maintain joint stability during dynamic activities.

Clinical Significance

The glenohumeral joint is a common site for injuries due to its inherent instability and wide range of motion. Some common clinical conditions involving the shoulder joint include:

• Shoulder Dislocations: Shoulder dislocations occur when the humeral head separates from the glenoid cavity. This can happen due to trauma, such as a fall or direct blow to the shoulder, or due to chronic instability. Anterior dislocations are the most common type of shoulder dislocation, accounting for over 95% of cases.
• Rotator Cuff Tears: Rotator cuff tears are a common cause of shoulder pain and dysfunction, especially in older adults. These tears can occur due to acute trauma, such as a fall, or due to chronic overuse and degeneration. The supraspinatus tendon is the most commonly affected rotator cuff tendon.
• Glenoid Labrum Tears: Tears of the glenoid labrum can occur due to acute trauma, such as a shoulder dislocation, or due to repetitive overhead activities, such as throwing. Labral tears can cause pain, clicking, and instability in the shoulder.
• Shoulder Impingement: Shoulder impingement occurs when the rotator cuff tendons are compressed or irritated as they pass through the subacromial space, the space between the acromion and the humeral head. This can lead to pain, inflammation, and limited range of motion.
• Adhesive Capsulitis (Frozen Shoulder): Adhesive capsulitis is a condition characterized by stiffness and pain in the shoulder joint. It is caused by inflammation and thickening of the joint capsule, which restricts movement.
• Osteoarthritis: Osteoarthritis can affect the glenohumeral joint, leading to pain, stiffness, and decreased range of motion. This condition is more common in older adults and can be caused by age-related degeneration of the cartilage.
• Shoulder Instability: Chronic shoulder instability can result from previous dislocations or subluxations (partial dislocations) of the shoulder. This can lead to recurrent episodes of pain and giving way of the shoulder.

Treatment Options

Treatment options for shoulder conditions vary depending on the specific diagnosis and severity of the condition. Some common treatment options include:

• Conservative Treatment: Conservative treatment options include rest, ice, compression, and elevation (RICE), as well as pain medication, anti-inflammatory drugs, and physical therapy. Physical therapy may involve exercises to strengthen the rotator cuff muscles, improve range of motion, and restore normal shoulder mechanics.
• Injections: Corticosteroid injections can be used to reduce pain and inflammation in the shoulder joint. Hyaluronic acid injections may also be used to lubricate the joint and reduce pain.
• Surgery: Surgery may be necessary for more severe shoulder conditions, such as rotator cuff tears, labral tears, and recurrent shoulder dislocations. Surgical options include arthroscopic procedures, which are minimally invasive and involve the use of a small camera and instruments to repair the damaged tissues. Open surgery may be necessary for more complex cases.
• Shoulder Replacement: In cases of severe osteoarthritis or other conditions that cause significant damage to the shoulder joint, shoulder replacement surgery may be an option. This involves replacing the damaged joint with an artificial joint.

Exercise and Rehabilitation

Rehabilitation plays a crucial role in recovery from shoulder injuries and surgery. The goals of rehabilitation are to reduce pain and inflammation, restore range of motion, strengthen the rotator cuff muscles, and improve shoulder stability. Some common exercises used in shoulder rehabilitation include:

• Pendulum Exercises: These exercises involve gently swinging the arm in a circular motion to improve range of motion and reduce pain.
• Isometric Exercises: These exercises involve contracting the muscles without moving the joint. They can be used to strengthen the rotator cuff muscles and improve stability.
• Theraband Exercises: Therabands are elastic bands that provide resistance during exercise. They can be used to strengthen the rotator cuff muscles and improve range of motion.
• Scapular Squeezes: These exercises involve squeezing the shoulder blades together to strengthen the muscles that control scapular movement.
• Wall Slides: These exercises involve sliding the arms up a wall to improve range of motion and strengthen the shoulder muscles.
• Progressive Weight Training: As the shoulder heals, progressive weight training can be used to gradually increase the strength of the shoulder muscles.

Conclusion

The glenoid cavity of the scapula articulates with the head of the humerus to form the glenohumeral joint, or shoulder joint. This articulation allows for a wide range of motion, making the shoulder the most mobile joint in the human body. However, the shallowness of the glenoid cavity also makes the shoulder inherently unstable and prone to injuries. Understanding the anatomy of the scapula and humerus, as well as the factors that contribute to shoulder stability, is essential for preventing and treating shoulder conditions. A combination of conservative treatment, surgery, and rehabilitation can help to restore function and relieve pain in individuals with shoulder problems.