Chapter 8 – Joints (Part C) Study Notes

Joints, also known as articulations, are crucial parts of our skeletal system that allow bones to connect and enable movement. Without them, our skeleton would be a rigid, unmoving structure. This section introduces what joints are and their main roles in the body.

Introduction to Joints

• Terminology: Joints = articulations —> sites where two or more bones meet.

• Primary functions- Provide skeleton with ext{mobility} (range from immovable to freely movable).

  • Hold the skeleton together, maintaining overall structural integrity.

• Reference mention: “Crash Course: Joints” (supplementary video overview).

To better understand joints, we classify them based on how they're built (structure) and how much they can move (function). These two systems help us categorize the many different types of joints in our body.

Classification of Joints

• Two major schemes are used; both appear throughout textbooks & exams.

This classification system looks at what kind of material connects the bones at a joint and whether there's a space or cavity between them. It helps us group joints based on their physical make-up.

1. Structural Classification

(Emphasised in this lecture because boundaries are clearer)

• Fibrous – bones linked by dense fibrous connective tissue; no cavity.

• Cartilaginous – bones united by cartilage; no cavity.

• Synovial – bones separated by fluid-filled cavity; always contain a capsule.

This classification system focuses on how much movement a joint allows, ranging from no movement at all to a lot of freedom. It's about what the joint does.

2. Functional Classification

(Based on the degree of movement permitted)

• Synarthroses – immovable.

• Amphiarthroses – slightly movable.

• Diarthroses – freely movable.

• Relationship tip: the more complex the structural components, the more movement the joint generally allows (fibrous < cartilaginous < synovial).

Fibrous joints are simply bones held together by tough, stringy connective tissue, like strong threads. Because there's no space between the bones and they're tightly bound, most of these joints allow very little to no movement.

Fibrous Joints

• Common features
  

  • Joined by dense fibrous CT
  

  • Lacking a joint cavity
  

  • Movement capacity depends on fibre length (most are immovable)

• Three anatomical sub-types

Sutures are a special type of fibrous joint found only in the skull. They form incredibly strong, interlocking connections between the skull bones, which are very important for protecting the brain.

Sutures (skull only)

• Rigid, interlocking wavy edges – visual “zipper”.

• Youth: short collagen fibres permit slight expansion for brain growth.

• Middle age: fibres ossify → ext{synostoses} (completely fused, immovable) protecting brain.

Syndesmoses are fibrous joints where bones are joined by a ligament, which is a band of fibrous tissue. The amount of movement allowed in these joints varies depending on how long these connecting fibers are.

Syndesmoses

• Bones connected by ligaments (bands of fibrous tissue).

• Fibre length varies ⇒ motion varies
  

  • Short fibres → little/no movement e.g. inferior tibiofibular joint.
  

  • Long fibres → considerable movement e.g. interosseous membrane between radius & ulna (permits pronation/supination).

Gomphoses are a distinct type of fibrous joint, where a peg-shaped bone, like a tooth, fits into a socket. This unique design is specifically seen in how our teeth are held firmly in our jawbones.

Gomphoses

• “Peg-in-socket” design; only example = tooth in alveolar socket.

• Periodontal ligament = short fibrous band anchoring tooth to bone.

Cartilaginous joints are connections between bones made of cartilage, which is a more flexible tissue than bone. Unlike fibrous joints, these don't have a cavity either, but they generally allow for slightly more movement, though some are still completely immovable.

Cartilaginous Joints

• Shared traits
  

  • Bones united by cartilage (hyaline or fibrocartilage).
  

  • No joint cavity.
  

  • Generally amphiarthrotic to synarthrotic (limited motion).

• Two anatomical sub-types

Synchondroses are a type of cartilaginous joint where bones are joined by hyaline cartilage, which is a smooth, glass-like cartilage. These joints are typically very stable and allow almost no movement.

Synchondroses

• Plate/bar of hyaline cartilage joins bones.

• Almost all are synarthrotic (immovable).

• Classic examples
  

  • Temporary epiphyseal plates between diaphysis & epiphysis of long bones (become synostoses after closure).
  

  • 1st costal cartilage ↔ manubrium of sternum.

Symphyses are cartilaginous joints where bones are connected mainly by tough fibrocartilage, which is known for its strength and ability to absorb shock. These joints are often found in areas that need to be strong but also allow for a small amount of movement.

Symphyses

• Fibrocartilage is the main linking material; thin hyaline layers cover bony surfaces.

• Strong, amphiarthrotic (slightly movable).

• Examples: intervertebral discs; pubic symphysis.

Synovial joints are the most common type of joint in the body, famous for allowing a wide range of motion. What makes them unique is the presence of a fluid-filled cavity between the bones, allowing them to move freely against each other. These are the joints you typically think of, like your knees, elbows, and shoulders.

Synovial Joints

• Hallmarks: bones are separated by a fluid-filled cavity → highest mobility ⇒ all are diarthrotic.

• Account for virtually all limb joints; also found elsewhere (TMJ, vertebral facets, etc.).

• Six morphological classes (know the names; specifics covered in later chapters): plane, hinge, pivot, condylar, saddle, ball-and-socket.

Synovial joints have several key components that work together to allow smooth, free movement and protect the bones. Understanding these features is essential for grasping how these highly mobile joints function.

Six General Structural Features (memorise)

1. Articular cartilage – smooth hyaline caps prevent bone crushing.

2. Joint (synovial) cavity – small potential space containing fluid.

3. Articular (joint) capsule – two layers
   

   • External fibrous layer: dense irregular CT (strength)
   

   • Inner synovial membrane: loose CT secretes synovial fluid.

4. Synovial fluid – viscous filtrate of plasma + hyaluronic acid
   

   • Lubricates; nourishes cartilage (avascular); contains phagocytes.

5. Reinforcing ligaments
   

   • Capsular (intrinsic) = thickened fibrous layer.
   

   • Extracapsular = outside capsule.
   

   • Intracapsular = deep to capsule, covered by synovial membrane.

6. Nerves & blood vessels
   

   • Sensory fibres detect pain, joint position, stretch.
   

   • Capillaries feed synovial membrane → fluid production.

Beyond their main features, synovial joints often have extra structures that help them function even better. These adaptations provide additional cushioning, improve fit between bones, distribute pressure, and reduce rubbing, making the joints more durable and efficient.

Additional Structural Adaptations

• Fatty pads – cushioning (hips, knees, etc.).

• Articular discs / menisci – wedges of fibrocartilage improving fit, stability, load distribution, & wear reduction.

• Bursae – flattened fibrous sacs lined with synovial membrane; reduce friction between moving structures.

• Tendon sheaths – elongated bursae wrapping around tendons subject to high friction (e.g. carpal tunnel).

While joints are designed for movement, they also need to be stable enough not to dislocate or fall apart. Several factors contribute to this stability, ensuring our bones stay properly aligned even during complex movements.

Joint Stability (Why joints don’t fall apart)

1. Articular surface shape – deep sockets & large heads increase stability (minor contributor relative to others).

2. Ligaments – the more, the better (but they can stretch up to 10\% of length before snapping).

3. Muscle tone – constant, low-level contraction keeps tendons taut across joints; MOST IMPORTANT (particularly shoulder, knee, foot arches).

Understanding how our body moves involves knowing the basic principles of biomechanics related to muscles and joints. Every movement we make, from walking to lifting, involves muscles acting across at least one joint, pulling bones in specific directions.

Basic Biomechanics of Movement

• Every skeletal muscle crosses at least one joint.

• Origin (fixed/less movable) vs Insertion (movable); contraction pulls insertion toward origin.

• Movements are described relative to 3 anatomical planes: transverse, frontal (coronal), sagittal.

• Three overarching categories: gliding, angular, rotation (plus “special” movements unique to certain joints).

Gliding movements are the simplest type of joint motion, where flat or nearly flat surfaces of bones just slide past each other. There's no significant bending or rotation involved, just a subtle shifting.

Gliding

• Flat/near-flat surfaces slide past each other.

• Locations: intercarpal, intertarsal, articular processes of vertebrae.

Angular movements change the angle between two bones. These are the common bending and straightening motions you see at joints like your elbow or knee, and they also include movements that take a limb away from or towards the body's midline.

Angular Movements

(occur mainly in sagittal & frontal planes)

• Flexion – decreases angle between bones.

• Extension – increases angle; returns bone toward anatomical position.

• Hyperextension – extension beyond anatomical position.

• Abduction – movement away from midline (frontal plane).

• Adduction – movement toward midline.

• Circumduction – limb outlines cone; sequential flexion → abduction → extension → adduction.

Rotation is a movement where a bone spins or pivots around its own central axis. Think of turning your head from side to side; that's an example of rotation occurring at your neck joints.

Rotation

• Bone pivots around its own longitudinal axis.
  

  • Medial (internal) – toward midline.
  

  • Lateral (external) – away from midline.

• Key examples: atlanto-axial joint (C1–C2), humerus & femur rotations.

Beyond the general categories, some joints have unique movements that are specific to their structure and function. These are often named based on the specific anatomical action or the joint involved:

• Dorsiflexion – lifting the foot to decrease the angle between the foot and shin (e.g., pointing toes up).

• Plantarflexion – depressing the foot to increase the angle between the foot and shin (e.g., pointing toes down).

• Inversion – turning the sole of the foot medially.

• Eversion – turning the sole of the foot laterally.

• Protraction – anterior movement in the transverse plane (e.g., jutting out the jaw).

• Retraction – posterior movement in the transverse plane (e.g., pulling the jaw back).

• Elevation – lifting a body part superiorly (e.g., shrugging shoulders).

• Depression – lowering a body part inferiorly (e.g., dropping shoulders).

• Opposition – movement of the thumb to touch the tips of other fingers.

• Reposition – movement of the thumb back to its anatomical position.

• Supination – rotating the forearm laterally so the palm faces anteriorly or superiorly.

• Pronation – rotating the forearm medially so the palm faces posteriorly or inferiorly.