Anatomy Lab Ch 11
General Functions and Categorization of Articulations
Articulations, or joints, are the points where bones connect. With very rare exceptions, every bone in the human body forms a joint with at least one other bone. These structures serve two primary purposes:
Bone Cohesion: They hold the bones together to maintain the structural integrity of the skeleton.
Flexibility: They provide the rigid skeletal system with flexibility, allowing for gross body movements to occur.
Functional and Structural Classification of Joints
Joints are categorized using two distinct systems: functional (based on movement) and structural (based on anatomical composition).
Functional Classification
This system focuses on the amount of movement the joint permits:
Synarthroses: Immovable joints.
Amphiarthroses: Slightly movable joints.
Diarthroses: Freely movable joints (predominantly found in the appendicular skeleton).
Structural Classification
This system is based on the type of connective tissue between the bones and whether a joint cavity is present:
Fibrous Joints: Adjoining bones are connected by dense fibrous connective tissue; no joint cavity is present. Most are synarthrotic.
Suture: Short fibers connect irregular edges of bone (e.g., squamous suture between parietal and temporal bones). These are synarthroses.
Syndesmosis: Joints connected by longer fibers (ligaments). Examples include the joint between the distal tibia and fibula. These can be amphiarthrotic or synarthrotic.
Gomphosis: A "peg-in-socket" fibrous joint involving a periodontal ligament (e.g., a tooth in its bony alveolar socket). These are synarthroses.
Cartilaginous Joints: Adjoining bones are united by cartilage; no joint cavity is present.
Synchondrosis: Bones united by hyaline cartilage (e.g., the epiphyseal plate in growing long bones or the joint between the costal cartilage of rib and the sternum). These are synarthroses.
Symphysis: Bones united by a broad, flat disc of fibrocartilage (e.g., intervertebral discs and the pubic symphysis). These are amphiarthroses.
Synovial Joints: Adjoining bones are covered in articular cartilage, separated by a joint cavity, and enclosed in an articular capsule lined with a synovial membrane. All are diarthroses.
Anatomy and Components of Synovial Joints
Synovial joints are the most common joints in the body and possess several distinguishing structural characteristics:
Joint (Articular) Cavity: A physical space between articulating bones containing a small volume of synovial fluid.
Articular Cartilage: A layer of hyaline cartilage covering the bone surfaces within the joint to provide a smooth surface.
Articular Capsule: A two-layered envelope enclosing the cavity. The outer fibrous layer is dense irregular connective tissue, while the inner synovial membrane is loose connective tissue.
Synovial Fluid: A viscous lubricant with the consistency of egg whites. It reduces friction during movement.
Reinforcing Ligaments: Thickenings of the capsule (capsular), or distinct structures outside (extracapsular) or inside (intracapsular) the capsule.
Nerves and Blood Vessels: Sensory nerves detect pain and stretch; blood vessels primarily supply the synovial membrane.
Articular Discs (Menisci): Fibrocartilage pads that improve the fit between bone ends and minimize wear.
Bursa and Tendon Sheath: Friction-reducing sacs filled with synovial fluid. A bursa is a small fluid-filled sac; a tendon sheath is an elongated bursa that wraps around a tendon.
Classification of Synovial Joints by Movement
Synovial joints are further classified based on the shape of their articular surfaces and the planes of movement allowed:
Plane Joint: Flat articulating surfaces allowing gliding movements. Nonaxial (e.g., intercarpal and intertarsal joints).
Hinge Joint: A rounded/cylindrical bone fits into a concave surface. Uniaxial allowing flexion and extension (e.g., elbow, interphalangeal joints).
Pivot Joint: A rounded bone fits into a sleeve composed of bone and/or ligaments. Uniaxial allowing rotation (e.g., proximal radioulnar, atlantoaxial joint).
Condylar Joint: An oval condyle fits into an oval depression. Biaxial allowing flexion, extension, adduction, and abduction (e.g., metacarpophalangeal/knuckle joints).
Saddle Joint: Articulating surfaces are both concave and convex (saddle-shaped). Biaxial allowing flexion, extension, adduction, and abduction (e.g., carpometacarpal joint of the thumb).
Ball-and-Socket Joint: A ball-shaped head fits into a cuplike depression. Multiaxial allowing flexion, extension, adduction, abduction, and rotation (e.g., shoulder and hip joints).
Body Movements and Muscle Attachments
Movement occurs when skeletal muscles contract across diarthrotic joints. Every muscle has two attachment points:
Origin: The stationary or less movable attachment point.
Insertion: The more movable attachment point. During contraction, the insertion moves toward the origin.
Primary Movement Types
Flexion: Decreases the joint angle and distance between bones in the sagittal plane (e.g., bending the knee).
Extension: Increases the joint angle/distance; the opposite of flexion. Hyperextension occurs when the movement goes beyond anatomical position.
Abduction: Movement of a limb away from the midline in the frontal plane (or fanning of fingers/toes).
Adduction: Movement of a limb toward the midline.
Rotation: Movement of a bone around its longitudinal axis (e.g., rotation of the atlas around the dens of the axis).
Circumduction: A combination of flexion, extension, abduction, and adduction. The limb describes a cone in space.
Pronation: Rotating the forearm so the palm faces posteriorly (radius crosses over the ulna to form an ).
Supination: Rotating the forearm so the palm faces anteriorly (radius and ulna are parallel).
Dorsiflexion: Lifting the foot so the superior surface moves toward the shin.
Plantar Flexion: Flexing the foot downward (pointing the toes).
Inversion: Turning the sole of the foot medially.
Eversion: Turning the sole of the foot laterally.
Anatomy of Specific Body Joints
The Hip Joint
A high-stability ball-and-socket joint. Its stability is attributed to:
Deep Acetabulum: The socket is deepened by a fibrocartilage rim called the acetabular labrum.
Ligaments: The ligament of the head of the femur (ligamentum teres) connects the fovea capitis to the acetabulum. The iliofemoral, pubofemoral, and ischiofemoral ligaments "screw" the femur into the socket during standing.
The Knee Joint
The largest and most complex joint, functionally a hinge joint that allows some rotation when flexed. It consists of three joints in one: the tibiofemoral (bicondyloid) and femoropatellar joints.
Menisci: C-shaped fibrocartilage pads (medial and lateral) that secure the femur.
Extracapsular Ligaments: Fibular and tibial collateral ligaments prevent rotation during extension.
Intracapsular Ligaments: The Anterior Cruciate Ligament (ACL) and Posterior Cruciate Ligament (PCL) prevent anterior-posterior displacement.
Locking Mechanism: The knee must be "unlocked" by the popliteus muscle before flexion can occur.
The Shoulder (Glenohumeral) Joint
The most mobile joint in the body. Its mobility comes at the expense of stability:
Shallow Glenoid Cavity: Deepened slightly by the glenoid labrum.
Ligaments: Coracohumeral and three weak glenohumeral ligaments.
Rotator Cuff: Tendons from four muscles provide the majority of the joint's stability.
The Temporomandibular Joint (TMJ)
A modified hinge joint where the condylar process of the mandible meets the mandibular fossa and articular tubercle of the temporal bone.
Articular Disc: Divides the joint cavity into superior and inferior compartments.
Movement: Allows elevation/depression (hinge) and anterior gliding (to prevent damage during hard biting).
Joint Pathology and Disorders
Bursitis: Inflammation of a bursa (e.g., "water on the knee" from damage to the patellar bursa).
Sprains: Damage to reinforcing ligaments via overstretching or tearing. These heal slowly due to poor blood supply in dense regular connective tissue.
Dislocations: Bones forced out of alignment, typically requiring reduction (returning the bone to its proper position) by a physician.
Aging: Years of wear can lead to adhesions (fibrous bands between bones) or spurs (extraneous bone tissue) that cause joint stiffness.