Joints - Classification, Types, and Functions

Definition

Overall Classification

I. SYNOVIAL JOINTS

Characteristic Features

1. Articular cartilage - a layer of hyaline cartilage covers the articulating surfaces (bony surfaces do not contact each other directly). This is why a gap appears on X-ray - cartilage is more radiolucent than bone.
2. Joint capsule - composed of:
   • Inner synovial membrane - highly vascular, attaches to margins of joint surfaces, produces synovial fluid that lubricates articulating surfaces. Closed sacs of synovial membrane also form bursae and tendon sheaths outside joints, reducing friction.
   • Outer fibrous membrane - dense connective tissue that surrounds and stabilizes the joint. Parts may thicken to form intrinsic ligaments; extrinsic ligaments outside the capsule provide additional reinforcement.
3. Accessory structures (not universal) within or enclosed by the capsule:
   • Articular discs (fibrocartilage) - absorb compression forces, adjust to contour changes during movement, and increase the range of movement
   • Fat pads - between synovial membrane and capsule; move in and out of regions as joint contours change
   • Redundant folds of synovial and fibrous membrane allow for large movements

Fig. 1.21 - Synovial Joint: Major features (A) and accessory structures (B)

Types of Synovial Joints

• Uniaxial - movement in one plane
• Biaxial - movement in two planes
• Multiaxial - movement in three planes

Hinge joints are uniaxial; ball-and-socket joints are multiaxial.

Fig. 1.22 - Specific types of synovial joints

II. SOLID JOINTS

Fig. 1.23 - Solid Joints

A. Fibrous Joints

B. Cartilaginous Joints

Summary Table

JOINTS
├── SYNOVIAL (cavity present)
│   ├── Plane — gliding (acromioclavicular)
│   ├── Hinge — uniaxial flexion/extension (elbow)
│   ├── Pivot — uniaxial rotation (atlanto-axial)
│   ├── Bicondylar — mainly uniaxial + limited rotation (knee)
│   ├── Condylar/Ellipsoid — biaxial (wrist)
│   ├── Saddle — biaxial (1st carpometacarpal)
│   └── Ball-and-socket — multiaxial (hip, shoulder)
│
└── SOLID (no cavity)
    ├── FIBROUS
    │   ├── Sutures (skull only)
    │   ├── Gomphoses (teeth only)
    │   └── Syndesmoses (ligament-linked bones)
    └── CARTILAGINOUS
        ├── Synchondroses (growth plates)
        └── Symphyses (pubic symphysis, IVDs)

Joints - Classification, Types, and Functions

Overview: The Two Major Categories

I. FALSE JOINTS (SYNARTHROSES)

1. Syndesmoses (Fibrous Joints)

• Interosseous membrane (radius-ulna; tibia-fibula)
• Tibiofibular syndesmosis
• Fontanelles (newborn skull)
• Gomphosis - immovable articulation of a tooth in its bony socket via the periodontal ligament

2. Synchondroses (Cartilaginous Joints)

• Epiphyseal growth plates - allow bone growth; eventually ossify
• Hip bone - growth plates between ilium, ischium, and pubis (prior to closure)
• Costal cartilage - connecting ribs to sternum
• Pubic symphysis and intervertebral discs (if the intervening tissue is mostly fibrocartilaginous, the joint is called a symphysis)

3. Synostoses (Sites of Bony Fusion)

• Sacrum - fused sacral vertebrae
• Hip bone - fusion of ilium, ischium, and pubis
• Closed and ossified epiphyseal plates in adults

Fig. 4.2F - Synchondroses (cartilaginous joints)

II. TRUE JOINTS (DIARTHROSES / SYNOVIAL JOINTS)

Structure of a True Joint

Fig. 4.3A - Structure of a true joint

1. Articular surfaces - variably shaped, covered by hyaline cartilage (1-2 mm thick in phalanges; 5-7 mm in femoropatellar joint). Exceptions: jaw and sternoclavicular joint are covered by fibrous cartilage.
2. Joint cavity - with articular recesses; width varies
3. Joint capsule - a closed capsule with alar folds, synovial folds, and synovial villi; the inner layer (synovial membrane) is highly vascular and produces synovial fluid
4. Synovial fluid - highly viscous; lubricates and nourishes the avascular cartilage (by diffusion and convection)
5. Intra-articular structures (to improve congruency and increase force-absorbing surface):
   • Menisci - crescent-shaped, wedge-shaped in cross-section; found regularly only in the knee; made of collagen, connective tissue, and fibrocartilage
   • Articular discs - divide the joint into two separate chambers; found in jaw, sternoclavicular joint, and proximal wrist joint
   • Articular labra - wedge-shaped fibrocartilage at outer margins of hip and shoulder sockets (acetabular and glenoid labra); enlarge the articulating surfaces
6. Ligaments - intra- and extracapsular; primary joint stabilizers
7. Muscles - agonist and antagonist pairs crossing the joint to move it in opposite directions
8. Synovial bursae - often near the joint, may communicate with the joint cavity

Note: While the synovial membrane can regenerate through adjoining connective tissue, hyaline cartilage cannot regenerate - it is avascular, lacks perichondrium, and must be nourished exclusively by synovial fluid.

Types of True Joints - Classified by Shape and Movement

Key principle: The more congruent the articular surfaces (similar radius of curvature, e.g., shoulder), the more they slide onto each other. The less congruent, the more they roll off each other (e.g., knee). Most joints use a combination - roll-slide.

Amphiarthroses

III. JOINT MECHANICS: Stability and Function

Constraints on Joint Motion (4 types)

1. Bony constraint - shape of articular surfaces
2. Muscular constraint - muscles crossing the joint
3. Ligamentous constraint - ligaments limiting range
4. Soft-tissue constraint - joint capsule and surrounding tissue

Key Muscles for Erect Posture

• Triceps surae + tibialis anterior - stabilize the ankle in the sagittal plane
• Quadriceps femoris - stabilizes the knee
• Gluteal muscles - stabilize the hip (sagittal plane)
• Gluteus medius and minimus - stabilize the hip (frontal plane)
• Intrinsic back muscles - stabilize the spinal column

Summary Classification Tree

JOINTS
├── FALSE JOINTS (Synarthroses) — no joint cavity
│   ├── Syndesmoses (fibrous)
│   │   ├── Interosseous membranes
│   │   ├── Tibiofibular syndesmosis
│   │   ├── Fontanelles
│   │   └── Gomphosis (teeth)
│   ├── Synchondroses (cartilaginous)
│   │   ├── Epiphyseal growth plates
│   │   ├── Costal cartilage
│   │   └── Symphyses (pubic, intervertebral discs)
│   └── Synostoses (bony fusion)
│       ├── Sacrum
│       ├── Hip bone
│       └── Ossified epiphyseal plates
│
└── TRUE JOINTS (Diarthroses) — joint cavity present
    ├── Diarthroses (variable degrees of freedom)
    │   ├── Translational: Plane joints (femoropatellar, vertebral)
    │   └── Rotational:
    │       ├── 1 axis: Hinge, Pivot
    │       ├── 2 axes: Ellipsoidal, Saddle
    │       └── 3 axes: Spheroidal (hip, shoulder)
    └── Amphiarthroses (stiff, e.g., sacroiliac joint)