Chapter 4: Articular System and Arthrokinematics

Introduction

• Joints allow motion and bear weight, providing stability

• Key concepts:

  • osteokinematics (movement of bones)

    • planes and axes

  • arthrokinematics (motion of joint surfaces)

    • at the joint level

Classification of Joints

• Fibrous (synarthrodial) joints-

  • Types:

    • Syndesmosis

      • slight movement, allows a bit of twisting

    • Gomphosis

      • happens between a tooth and the mandible

    • Suture

  • Characteristics: generally immovable or very limited motion

  • Examples: Skull sutures, Mandible attachment, Fibula-tibia syndesmosis

  • stable joints

• Cartilaginous (synarthrodial) joints-

  • Permit small amounts of bending, twisting, compression

  • Joined by hyaline or fibrocartilage

  • Examples: Intervertebral disks (fibrocartilage), synchondroses, symphyses

• Diarthodial (synovial) joints-

  • Mobile joints with no direct union between bone ends

  • Components: joint space, joint capsule, synovial membrane, synovial fluid, articular cartilage, ligaments

Table 4-2 Characteristics of Synovial Joints

Joint Movements and Joint Structures

• Nonaxial Joint

  : Plane joint; Gliding motion; Examples: Carpals

• Uniaxial Joint

  : One plane of motion about one axis

  • Hinge: Example

    – Humerus vs. Ulna

  • Pivot: Allows rotation about a single longitudinal axis; Examples:

    – Atlantoaxial joint

    : Located between the atlas (C1) and axis (C2) vertebrae, permitting head rotation (e.g., shaking head 'no').

    – Proximal and Distal Radioulnar joints

    : Connect the radius and ulna, allowing pronation and supination of the forearm (e.g., turning a doorknob).

• Biaxial Joint

  : Two planes of motion about two axes; Two degrees of freedom (DOF)- Example: Wrist joint; Carpals vs. radius/ulna; Finger MCP joints

• Triaxial Joint: Three planes of motion about three axes; $3 \text{ DOF}$- Examples: Ball-and-socket joints

  – Glenohumeral (shoulder), Acetabulofemoral (hip)

• Structure of a Joint:

  • Bone

  • Ligament: connects bone to bone and supports it

    • Ex: ACL

  • Capsule (outer layer; inner layer is the synovial membrane)

    • surround around ligament

  • Synovial fluid: lubrication, cushion, and nutrient to hyaline cartilage

  • Cartilage: between bones; reduces friction between bones

    • Hyaline: doesn’t repair itself

    • fibrocartilage: shock absorption; structural integrity; fills space between bones

      • found in interverbral disks

    • elastic:

  • Tendon: Connect muscles to bone

    • (e.g., Thoracolumbar fascia for latissimus dorsi; Linea alba in abdominal wall)

    • various shapes: cylindrical cords or flat band

    • can be encased in tendon sheaths;

    • Aponeuroses

      • broad, flat, tendinous sheet of connective tissue

      • thoracolumbar fascia attachment for latissimus dorsi

      • linear alba: midline of abdominal wall

  • Bursa: Fluid-filled sacs reducing friction between tissues

    • lined with synovial membrane and fluid

  • Supporting structures (examples): Pubofemoral ligament, Iliofemoral ligament; Joints ligaments and capsule emphasis

• Tendon sheath: Reduces friction during movement

Osteokinematic Motion—End Feel

• Normal end feels:-

  • Bony (hard)

  • Soft tissue stretch (firm)

  • Soft tissue approximation (soft)

• Abnormal end feels:-

  • Bony

  • Boggy (soft, wet, spongy feelings)

  • Muscle spasm

  • Empty

  • Springy block (rubbery sensation)

Joint Surfaces and Congruency

• Surface shapes determine arthrokinematic motions

Joint Surface Shapes

Joint Surface Congruency

• Maximum contact; surfaces tightly compressed; difficult to distract

• Close-packed position: joints have their maximal congruence

  • more stable

• In all other positions, joint surfaces are incongruent (open-packed/loose-packed) and resting

  • allows more mobility

Types of Arthrokinematic Motion

Convex-Concave Rule

• Concave joint surface

  • moves in the same direction as the body segment motion

  • Ex: Metacarpal (concave surface) glides posteriorly while proximal phalanx moves posteriorly through sagittal plane (extension)

• Convex joint surface

  • moves in the opposite direction of the body segment motion

  • Ex: Convex surface glides inferiorly while body segment moves superiorly through frontal plane (abduction)

Kinetic Chain and Concave-Convex Rule

• When determining the direction of arthrokinematic motion:-

  • Determine which joint surface is moving

  • Determine if the moving surface is concave or convex

  • Open kinetic chain: distal segment is non-fixated

  • Closed kinetic chain: distal segment is fixated

Clinical Implications

• Joint play movements

• Joint mobilization as a therapeutic technique

Common Pathologies and Capsular Patterns

Capsular Patterns (Table 4-5)

Summary of Key Concepts

• Joints classified as fibrous, cartilaginous, or synovial; synovial joints are the primary functional joints with a joint capsule and synovial fluid

• Joints vary by axis and DOF: nonaxial (plane), uniaxial (hinge, pivot), biaxial (condyloid, saddle), triaxial (ball-and-socket)

• Arthrokinematics describe how joint surfaces move relative to each other (roll, glide, spin) and follow the convex-concave rule

• Kinetic chain context (open vs closed) affects direction and type of arthrokinematic motion

• Understanding joint congruency, close-packed vs open-packed positions, and end feels is essential for assessment and treatment planning

• Pathologies impact capsule and joint mechanics; recognizing capsular patterns guides rehabilitation

$\text{Key formulas and numerical references:}$

• Two planes of motion about two axes: $2 \text{ planes}, 2 \text{ axes}$

• Two degrees of freedom: $2 \text{ DOF}$

• Three degrees of freedom: $3 \text{ DOF}$

• Specific joint angles (examples):

  • $55^\circ$ abduction, $30^\circ$ horizontal adduction

  • $$70