Lecture 8: Biomechanics of Shoulder Joint PDF

# Lecture 8: Biomechanics of Shoulder Joint ## Introduction * **Arthrology** is the study of joints * **Kinematics** is the study of movement. * **Kinetics** is the study of the forces associated with motion. ## Structure of Shoulder Joint * The shoulder joint consists of **5 joints**, **3** are...

# Lecture 8: Biomechanics of Shoulder Joint ## Introduction * **Arthrology** is the study of joints * **Kinematics** is the study of movement. * **Kinetics** is the study of the forces associated with motion. ## Structure of Shoulder Joint * The shoulder joint consists of **5 joints**, **3** are **true joints** and **2** are **false joints**. * The shoulder joint is a **girdle** (complex joint), not a single joint. * There are **4** interdependent joints: * **Gleno-humeral joint (GH)** - "true joint" * **Sterno-clavicular joint (SC)** - "true joint" (synovial joint with 3 degrees of motion) * **Acromio-clavicular joint (AC)** - "true joint" * **Scapulo-thoracic joint (ST)** - "false joint". * The 5 joints of the shoulder are divided into **2 groups**. ## 1st Group: * **Gleno-humeral joint (GH)** - "true joint" - This joint is a **ball and socket** with **3 degrees** of motion. - It is the articulation between the **humeral head** and **glenoid cavity**. - The **humeral hea**d is **one third** (1/3) of a sphere, with a diameter of **37 to 55mm**. - The humeral head is oriented **superiorly**, **medially** and **posteriorly**. - The **glenoid fossa** is a small bony depression that measures **41mm** longitudinally and **25mm** in transverse direction. - The surface of the **glenoid fossa** is **1/3** to **1/4** that of the humeral head. - The **longitudinal diameter** of the glenoid fossa represents **75%** of the humeral head and its **transverse diameter** represents **60%** of the humeral head. - The glenoid fossa is tilted about **7-5° posteriorly**. * **Subdeltoid joint** (subdeltoid bursa) - "false joint" - It consists of **2 surfaces sliding with respect to each other**. - It is also called **sub-aromial bursa** or **sub-deltoid bursa**. - It separates **supraspinatus tendon** & **humerus** from the **acromion**, **coracoid**, **coracoacromial ligament** & **deltoid muscles**. ## 2nd Group: * **Scapulo-thoracic joint (ST)** - "false joint" - This joint provides range for arm elevation and a stable base for the rolling and sliding of the humeral head. - This joint consists of: - The space between the **scapula** and **serratus anterior**. - The space between the **thoracic wall** and **intercostal muscle**. * **Sterno-clavicular joint (SC)** - "true joint" - This joint is a **synovial joint** with **3 degrees of motion.** - This joint forms the articulation between the **sternal head of the clavicle** (medial) and **manubrium of the sternum** and **1st costal cartilage**. - It is the only structural attachment of the **scapula** to the rest of the body. - This joint possesses a **joint disc**(meniscus) which serves an important **stability**, increasing the congruence of the joint and absorbing forces that are transmitted along the clavicle. * **Acromio-clavicular joint (AC)** - "true joint" - This joint forms the articulation between the **acromion** (concave) and the **lateral end of the clavicle** *(convex)*. - This joint offers limited **motion of the scapula** *(*rotation*, *winging*, and *tipping*)*. ### Stability of the **Sterno-clavicular joint** * **Anterior** and **posterior sternoclavicular ligaments**. - These ligaments prevent anterior and posterior movement of the head of the clavicle. * **Costo-clavicular ligaments**. - This site of elevation, depression, retraction, and protraction limits the elevation of the clavicle. ### Stability of the **Acromio-clavicular joint** * **Superior** and **inferior acromio-clavicular ligaments**. - The *superior ligament* is stronger as it is reinforced by fibers of the **trapezius** and **deltoid muscles**. * **Coraco-clavicular ligaments** (*conoid* and *trapezoid ligaments*). - This joint prevents superior dislocation of the clavicle and transmits compressive force from the scapula to the clavicle. ## Scapulo-thoracic Joint (ST) * This joint is a false joint. * It performs **mobility** and **stability** functions for the shoulder joint. * The scapula performs the following movements: * **Elevation** and **depression** - "**10-12 cm**" * **Abduction** and **adduction** - "**15 cm**" * **Upward** and **downward rotation** - "**60 degrees**" * The movement of the scapula on the thorax is associated with the motion of the **sterno-clavicular** and **acromio-clavicular joints**. ## Coraco-acromial Arch * The **coracoacromial arch** also called the **supra-humeral arch** is formed by: * **Coracoid process** - "anteriorly" * **Acromion process** - "posteriorly" * **Coracoacromial ligament** - "superiorly" * This a**rch protects** the top of the humeral head, sensitive muscles, tendons, and bursa from direct trauma. * The arch also prevents *superior dislocation* of the humerus. * When the humeral head impacts into the arch, it can cause **painful impingement** of the structures lying between the humeral head and arch. ## Gleno-humeral Joint (GH) * This joint is a **ball and socket** joint with **3 degrees** of motion. * The **articular cartilage** of the **glenoid fossa** is **thicker** on the periphery. * The **glenoid labrum** increases the depth the glenoid cavity by **50%**, increasing **stability** (tangential) by **20%**. ### Gleno-humeral Capsule * This joint is surrounded by a **large, loose capsule**. * This capsule is **taut superiorly**, and **slack anteriorly** and **inferiorly**. * The capsule is **twice** the size of the humeral head. * When the capsule is slack, it allows more than **1 inch** of distraction of the humeral head from the glenoid fossa. ### Gleno-humeral Ligaments * These ligaments including **superior**, **middle**, and **inferior** ligaments form a "Z" shape on the anterior capsule. * They all become **taut** in the **lateral rotation** of the shoulder and in any movement that causes an anterior glide of the humeral head. * The **coraco-humeral ligament** originates from the **coracoid process**, blends with the **superior capsule** and **supraspinatus tendon** to insert on the **greater tubricle**. ## Retroversion Angle of Scapula * The retroversion angle of the scapula is **30 degrees**. * This is the angle between the **horizontal plane** *(cronal plane)* and the **scapula**. ## **Angle of Retroversion & Inclination of Humeral Head** * **Retroversion angle of the humeral head**. - This angle, also known as **torsion**, is the angle between the **coronal plane** *(frontal or horizontal)* and the **humeral head**. - This angle is equal to **30 degrees**. * **Inclination angle of the humeral head**. - This is the angle between the **longitudinal** *(vertical)* axis of the humerus shaft and the **humeral head**. - The angle ranges from **130 to 150 degrees**. ### Angle of Retroversion & Inclination of Glenoid Fossa * **Inclination angle** - **5 degrees** * **Retroversion angle** - **7 degrees** ## Rotator Cuff Muscles * The **rotator cuff muscles** pull the **humeral head** inside the **glenoid cavity**. * These muscles are crucial for creating **dynamic stability** in the shoulder joint. **The four rotator cuff muscles:** * **Supraspinatus** - this muscle elevates the shoulder joint out to the side * **Infraspinatus** - this muscle externally rotates the shoulder joint. * **Teres Minor** - this muscle externallly rotates the shoulder joint * **Subscapularis** - this muscle helps to move the humerus freely during elevation of the arm.