BIO 171 - Articulations/Joints (Chapter 9) Notes

Articulations/Joints (Chapter 9)

• Articulation (joint): A site where two or more bones meet.

A. Joint Classification

1. Functional Joint Classification:

• Based on the degree of movement the joint allows.

  • a. Synarthroses: Immovable joints (e.g., skull sutures).

  • b. Amphiarthroses: Slightly moveable joints (e.g., pubic symphysis, intervertebral discs - IVDs).

  • c. Diarthroses: Freely moveable joints (include all synovial joints).

• Synarthrotic and amphiarthrotic joints predominate in the axial skeleton, while diarthrotic joints predominate in the extremities. This distribution relates to the different functional requirements of these regions.

2. Structural Joint Classification:

• Based on joint structure - what material binds bones together and if a joint cavity is present.

  • a. Fibrous Joints: Bones connected by fibrous tissue, no joint cavity, not very mobile.

    • 1) Suture: A tight seam between bones formed by short connective tissue (CT) fibers (e.g., skull sutures).

    • 2) Syndesmosis: A sheet of dense CT (a ligament) connects the bones (e.g., the interosseous membrane connecting the radius and ulna).

    • 3) Gomphosis: Peg in socket; the only example is the articulation of a tooth in its alveolar socket - periodontal ligament connects tooth to bone.

  • b. Cartilaginous Joints: Cartilage connects to bone, no joint cavity, not very mobile.

    • 1) Synchondrosis: A plate of hyaline cartilage connects the bones (e.g., epiphyseal growth plate, costal cartilages between ribs and sternum).

    • 2) Symphysis: A fibrocartilage pad connects the bones; allows more movement than synchondroses (e.g., pubic symphysis, intervertebral joints).

  • c. Synovial Joints: Bone ends are separated by a fluid-filled joint cavity. These are diarthrotic, freely moveable joints. Most joints fall into this class, including nearly all limb joints.

B. Synovial Joints

1. Features Common to ALL Synovial Joints:

• a. Articular cartilage: Hyaline cartilage covers articulating bone surfaces. It protects bone ends and absorbs compression.

• b. Articular (joint) capsule: Completely encloses articulating bone ends.

  • Fibrous capsule: External, dense irregular CT; continuous with the periosteum; strengthens the joint.

  • Synovial membrane: Internal, loose CT, lines inside of the fibrous capsule and covers all internal joint surfaces. Secretes synovial fluid.

• c. Joint cavity: A small potential space filled with a small amount of synovial fluid.

• d. Synovial fluid: Slippery fluid inside the joint cavity; lubricates and reduces friction, cushions cartilage, nourishes, and cleanses internal joint structures.

• e. Reinforcing ligaments: Dense CT bands that connect the articulating bones; most are attached to the fibrous joint capsule (capsular ligaments); some are distinct from the capsule (extracapsular ligaments); some joints have ligaments within the capsule (intracapsular ligaments, e.g., knee joint).

2. Additional Synovial Joint Features:

• Present in some synovial joints.

  • a. Bursae: Small, flattened fibrous sacs filled with fluid; reduce friction around joints (e.g., shoulder, knee, and hip joints).

  • b. Tendon sheaths: Elongated versions of bursae that completely enwrap a tendon; found in places where a tendon encounters regular friction against a bone or many tendons are crowded together (e.g., wrist).

  • c. Fat pads: Provide extra cushioning in the knee and hip joints; fill in odd spaces.

  • d. Menisci: Fibrocartilage discs that absorb compression and help improve joint fit (e.g., knee and temporomandibular joints).

3. Synovial Joint Stability Factors:

• a. Articular surfaces: The shape of the articular surfaces determines what movements are possible, but play a minor role in joint stability (an exception is the hip joint - deep ball and socket joint, very stable based on its structure).

• b. Ligaments: Reinforce the joint capsule; generally, the more ligaments a joint has, the more stable it is.

• c. Tendons of muscles crossing the joint: Overall, they are the most important joint stabilizing factors. Good muscle tone helps keep the tendons taut and the joints they cross more stable (especially important in the shoulder and knee).

4. Types of Synovial Joint Movements:

• a. Flexion: To bend, or decrease the joint angle (e.g., elbow).

• b. Extension: To straighten, or increase the joint angle - a return from flexion.

• c. Hyperextension: Movement beyond normal extension - beyond the anatomical position (e.g., to extend the neck backward).

• d. Lateral flexion: Side-bending (neck and trunk).

• e. ABduction: Movement away from the midline (like flying - arms out to the sides).

• f. ADduction: Movement toward the midline (to “add back in” - arms back to the trunk).

• g. Circumduction: Forming a “cone” in space (ball & socket joints only - shoulder/hip).

• h. Rotation:

  • Axial: neck, trunk

  • Lateral/External & Medial/Internal: limbs (shoulders & hips)

• i. Pronation: Forearm - palms turn downward (recall prone = lying face down).

• j. Supination: Forearm - palms turn upward (hold “cup of soup”).

• k. Dorsiflexion: Ankle - toes move toward the shin.

• l. Plantarflexion: Ankle - point/stand on toes.

• m. Inversion: Ankle – sole of foot faces inward.

• n. Eversion: Ankle: sole of foot faces outward.

• o. Opposition: Movement of touching the tip of the thumb to the tip of the pinky finger.

5. Types of Synovial Joints:

• Based on the movements they allow.

  • a. Plane: Flat articular surfaces, allow gliding movements (e.g., facet joints of the spine).

  • b. Hinge: Projection of one bone fits into a trough-like depression of another; allow flexion and extension (e.g., elbow, knee, interphalangeal joints).

  • c. Pivot: Rounded end of one bone fits into a sleeve or ring of another; allow rotational movements (e.g., atlas/dens of axis, proximal radioulnar joint).

  • d. Condyloid: The oval convex surface of one bone fits into the oval concave surface of another (e.g., wrist & knuckle joints); allow flexion/extension, Abduction/Adduction.

  • e. Saddle: Similar to condyloid, but a little more movement (e.g., 1st carpal-metacarpal joint of the thumb – CMC joint); oppositional movement of the thumb.

  • f. Ball and socket: The spherical head of one bone articulates with the cup-like socket of another (e.g., hip, shoulder); universal movement – most moveable type of joint.

C. Knee Joint

• Is the largest, most complex joint in the body.

• Is essentially a hinge joint which allows flexion and extension of the knee, and slight rotation when the knee is flexed.

2 Knee Joints:

• 1) Tibiofemoral joints (lateral and medial).

• 2) Patellofemoral joint (patella only articulates with the femur).

4. Knee Menisci:

• Are 2 C-shaped fibrocartilages attached to the tibial plateaus; they improve the fit of the articulating bones and absorb compression.

5. Knee Joint Ligaments:

• a. Intracapsular (cruciate) ligaments: Are internal knee ligaments that prevent excessive anterior-posterior movements of the femur and tibia.

  • Anterior cruciate ligament (ACL).

  • Posterior cruciate ligament (PCL).

• b. Extracapsular ligaments: Are external knee ligaments that prevent excessive lateral-medial movements of the femur and tibia.

  • Lateral (fibular) collateral ligament (LCL).

  • Medial (tibial) collateral ligament (MCL).

• c. Quadriceps tendon: Connects from the quadriceps muscle group to the patella.

• d. Patellar ligament: Connects from the patella to the tibial tuberosity of the tibia.