Skeletal System: Articulations Study Guide

The Sternoclavicular Joint

• Joint Classification: This is a saddle joint formed at the intersection of the manubrium of the sternum and the sternal end of the clavicle.

• Internal Anatomy: The joint contains a fibrocartilaginous articular disc. This disc serves to partition the joint into two distinct parts, creating two separate synovial cavities.

• Mobility and Range of Motion: The structure allows for specific movements of the clavicle, including elevation, depression, and circumduction.

• Structural Stability: Stability is primarily provided by various fibers and ligaments. These reinforcements make the sternoclavicular joint exceptionally stable and difficult to dislocate.

The Acromioclavicular Joint

• Joint Classification: Classified as a plane joint, it exists between the acromion of the scapula and the lateral end of the clavicle.

• Supporting Structures:

  • It contains a fibrocartilaginous articular disc within the joint cavity.

  • Acromioclavicular Ligament: Strengthens the joint capsule on the superior side.

  • Coracoclavicular Ligament: Binds the clavicle to the coracoid process.

• Clinical Condition: Shoulder Separation: This injury occurs when these ligaments are torn, resulting in the misalignment of the acromion and the clavicle.

The Glenohumeral (Shoulder) Joint

• Anatomic Overview: A ball-and-socket joint between the head of the humerus and the glenoid cavity of the scapula.

• Functional Characteristics:

  • Permits the greatest range of motion of any joint in the human body.

  • Conversely, it is the most unstable joint and the one most frequently dislocated.

• Internal Features:

  • Glenoid Labrum: A fibrocartilaginous ring that encircles and deepens the socket.

  • Bursae: Abundant bursae are present to reduce friction in areas where tendons and muscles cross the articular capsule.

• Ligamentous Support: Includes the coracoacromial, coracohumeral, and glenohumeral ligaments.

• Muscular Support (The Rotator Cuff): Much of the joint's stability is provided by the rotator cuff muscles. These four muscles work as a collective to secure the humeral head within the glenoid cavity:

  1. Subscapularis

  2. Supraspinatus

  3. Infraspinatus

  4. Teres minor

• Tendon Interaction: The tendons of these muscles encircle the joint and fuse directly with the articular capsule. Additionally, the tendon of the long head of the biceps brachii contributes to joint integrity.

Clinical Perspectives on the Shoulder

• Dislocation Vulnerability: High mobility correlates with high instability.

• Shoulder Separation: Specific to the dislocation of the acromioclavicular joint. Symptoms include pain when the arm is abducted beyond $90^{\circ}$ and a prominent appearance of the acromion.

• Glenohumeral Dislocation: Typically occurs when a fully abducted humerus undergoes a hard strike. This causes the shoulder to look flattened or "squared-off," with the humeral head shifting anteriorly and inferiorly relative to the capsule.

The Elbow Joint

• Structural Composition: The elbow is a hinge joint featuring two distinct articulations enclosed in a single capsule:

  1. Humeroulnar Joint: The trochlea of the humerus articulates with the trochlear notch of the ulna.

  2. Humeroradial Joint: The capitulum of the humerus articulates with the head of the radius.

• Stability Mechanisms:

  • Articular Capsule: Characterized by its thickness.

  • Bony Interlock: The humerus and ulna interlock very closely.

  • Reinforcement Ligaments:

    • Radial Collateral Ligament: Extends around the radial head and stabilizes the lateral surface.

    • Ulnar Collateral Ligament: Extends from the medial epicondyle to the coronoid and olecranon processes, stabilizing the medial side.

    • Annular Ligament: Surrounds the neck of the radius and binds the radial head to the ulna.

• Clinical Condition: Subluxation of the Radial Head: An incomplete dislocation often called "nursemaids elbow." The radial head is pulled out of the annular ligament. This is seen almost exclusively in children under the age of $5$ because their annular ligament is thin and the radial head is not fully formed.

The Hip Joint

• Anatomic Overview: A ball-and-socket articulation between the head of the femur and the acetabulum of the hip bone.

• Mobility vs. Stability: Compared to the glenohumeral joint, the hip joint is significantly more stable but less mobile.

• Structures:

  • Acetabular Labrum: A fibrocartilaginous ring that deepens the acetabular socket.

  • Articular Capsule: Extends from the acetabulum to the trochanters, enclosing the femoral head and neck.

  • Retinacular Fibers: Ligamentous fibers of the capsule that reflect around the femoral neck. These fibers house the retinacular arteries, which provide the primary blood supply to the head and neck of the femur.

• Intracapsular Ligaments:

  1. Iliofemoral Ligament: Supports the anterior capsule.

  2. Ischiofemoral Ligament: Located posteriorly.

  3. Pubofemoral Ligament: A triangular thickening of the inferior capsule.

  4. Ligament of the Head of the Femur (Ligamentum Teres): Extends from the acetabulum to the fovea of the femoral head. It provides little stability but contains a small artery for the femoral head.

Clinical Perspectives on the Hip and Femur

• Fractures of the Femoral Neck: Often mislabeled as "hip fractures."

• Intertrochanteric Fractures: These are extracapsular and generally occur in younger or middle-aged individuals.

• Subcapital Fractures: Occur within the hip's articular capsule. Most common in older patients with osteoporosis. These can lead to the tearing of retinacular arteries, potentially resulting in avascular necrosis.

The Knee Joint

• Joint Characteristics: The largest and most complex diarthrosis. It functions primarily as a hinge joint but allows for slight rotation and lateral gliding when flexed.

• Key Articulations:

  1. Tibiofemoral Joint: Between the femoral condyles and tibial condyles.

  2. Patellofemoral Joint: Between the patella and the patellar surface of the femur.

• Capsule and Ligaments:

  • The articular capsule encloses the medial, lateral, and posterior regions only.

  • Quadriceps Femoris Tendon: Passes over the anterior surface and surrounds the patella.

  • Patellar Ligament: Extends from the patella to the tibial tuberosity.

  • Oblique Popliteal Ligament: Protects against hyperextension.

  • Collateral Ligaments: The Fibular (lateral) collateral ligament prevents hyperadduction; the Tibial (medial) collateral ligament prevents hyperabduction.

• Internal Cartilage: Medial and lateral menisci are C-shaped fibrocartilage pads that cushion the joint and stabilize it.

• Cruciate Ligaments:

  • Anterior Cruciate Ligament (ACL): Prevents hyperextension and anterior displacement of the tibia.

  • Posterior Cruciate Ligament (PCL): Prevents hyperflexion and posterior displacement of the tibia.

Clinical Perspectives on Knee Injuries

• Vulnerability: The knee is prone to injury during forcible abduction (Tibial collateral), medial strikes (Fibular collateral), hyperextension (ACL), or hyperflexion (PCL).

• The Unhappy Triad: A severe injury resulting from a lateral blow to the knee involving three structures: the tibial collateral ligament, the medial meniscus, and the ACL.

• Knee Replacement Surgery: Chronic ligamentous injuries can lead to osteoarthritis. If severe, arthritic bone ends are replaced with metal or plastic components.

The Talocrural (Ankle) Joint

• Classification: A highly modified hinge joint with two articulations in one capsule (Tibia-Talus and Fibula-Talus).

• Movements: Permits dorsiflexion and plantar flexion.

• Anatomy and Ligaments:

  • Malleoli: The medial and lateral malleoli prevent the talus from sliding laterally or medially.

  • Deltoid Ligament: Binds the medial side; prevents overeversion.

  • Lateral Ligament: Binds the lateral side; prevents overinversion and is prone to sprains.

• Clinical Conditions:

  • Ankle Sprain: Tearing/stretching of ligaments (usually lateral) caused by overinversion.

  • Pott Fracture: Caused by overeversion; involves an avulsion fracture of the medial malleolus and a subsequent fracture of the fibula.

Joint Development and Aging

• Early Development: Joints begin forming by the $6^{\text{th}}$ week of development from mesenchyme.

  • Fibrous: Mesenchyme becomes dense regular connective tissue.

  • Cartilaginous: Mesenchyme becomes fibrocartilage or hyaline cartilage.

  • Synovial: Mesenchyme forms the capsule, ligaments, synovial membrane, and internal discs/menisci.

• Aging and Exercise: Exercise increases synovial fluid flow and strengthens supporting muscles. However, excessive stress can worsen osteoarthritis.

Comparative Clinical Views: Arthritis

• General Definition: Inflammatory or degenerative diseases involving joint swelling, pain, and stiffness.

• Gouty Arthritis: Caused by increased uric acid levels; seen mostly in older males.

• Osteoarthritis: A degenerative "wear and tear" condition common with age, impacting articular cartilage in joints like the hips, knees, and shoulders.

• Rheumatoid Arthritis: An autoimmune disorder affecting the synovial membrane. It can lead to ankylosis (bone fusion) and is treated with NSAIDs, corticosteroids, and DMARDs (disease-modifying antirheumatic drugs), including biologics produced in living cells.

Questions & Discussion

• Q: What movements are allowed at the temporomandibular joint?

  • A: This joint allows for elevation, depression, protraction, retraction, and side-to-side lateral excursion.

• Q: Why is the shoulder joint considered the most mobile and at the same time the most unstable joint in the human body?

  • A: It is a ball-and-socket joint with a shallow glenoid cavity and a relatively loose articular capsule, which allows for extreme range of motion (mobility) but lacks the deep bony socket and tight ligaments required for high stability.

• Q: What is the function of the annular ligament in the elbow joint, and what injury may occur to this ligament and joint in young children?

  • A: The annular ligament binds the head of the radius to the ulna. In young children under $5$, the radial head can be pulled out of this ligament, a condition known as subluxation (nursemaids elbow).

• Q: How do the glenohumeral and hip joints compare with respect to their mobility and stability?

  • A: Both are ball-and-socket joints, but the hip joint is significantly more stable and less mobile due to its deeper socket (acetabulum) and stronger ligaments/muscles.

• Q: What are the functions of each of the intracapsular ligaments of the knee joint?

  • A: The ACL prevents hyperextension and anterior displacement of the tibia; the PCL prevents hyperflexion and posterior displacement of the tibia.

• Q: What bones articulate at the talocrural joint, and what movements are permitted at this joint?

  • A: The distal ends of the tibia and fibula articulate with the talus. It permits dorsiflexion and plantar flexion.

• Q: What are some ways that joints change as a person ages?

  • A: Aging often leads to the degradation of articular cartilage (osteoarthritis), decreased synovial fluid production, and loss of ligament flexibility.