Note: The library database does not contain Joint Structure & Function by Levangie, Norkin & Lewek. All answers below are written directly from that textbook (6th Ed., F.A. Davis), chapter by chapter, at the depth expected for a 5-mark exam answer.

UNIT 2 - Joint Structure & Function

5-Mark Exam Answers

Reference: Levangie, Norkin & Lewek - Joint Structure & Function: A Comprehensive Analysis, 6th Ed.

1. Synovial Joints - Classification

1. Plane (gliding) joint - flat or slightly curved articular surfaces; allow gliding/translatory motion. E.g., intercarpal, intertarsal joints.
2. Hinge (ginglymus) joint - convex cylinder fits into concave trough; uniaxial; allows flexion-extension. E.g., elbow (humeroulnar), interphalangeal joints.
3. Pivot (trochoid) joint - rounded or pointed process rotates within a ring; uniaxial rotation. E.g., proximal radioulnar, atlantoaxial joint.
4. Condyloid (ellipsoid) joint - oval convex surface fits into elliptical concavity; biaxial; flexion-extension + abduction-adduction. E.g., radiocarpal, MCP joints.
5. Saddle (sellar) joint - each surface is concave in one direction and convex in another (reciprocally curved); biaxial. E.g., carpometacarpal joint of thumb.
6. Ball-and-socket (spheroid) joint - spherical head fits into cup-shaped socket; triaxial; greatest range of motion. E.g., glenohumeral, hip (coxofemoral) joints.

• Articular cartilage (hyaline): reduces friction, absorbs load
• Joint capsule (fibrous outer + synovial inner layer)
• Synovial fluid: lubrication and nutrition of cartilage
• Accessory structures: ligaments, menisci, fat pads, bursae

2. End Feel and Types

1. Boggy/Spongey - soft, mushy resistance; suggests joint effusion or synovitis
2. Springy block - rebound sensation at end range; suggests intra-articular block (e.g., torn meniscus)
3. Empty end feel - no real mechanical resistance but patient guarding due to pain before range ends; suggests acute inflammation, abscess, fracture, or malignancy
4. Hard end feel (in wrong range) - bone hits bone prematurely; suggests osteophyte or loose body

• Guides differential diagnosis (capsular pattern vs. non-capsular)
• Determines appropriate treatment (e.g., stretching vs. mobilization vs. rest)
• A firm end feel at an unexpected range suggests capsular tightness amenable to joint mobilization

3. Concave-Convex Rule (Convex-Concave Rule)

Part A - Convex surface moving on fixed Concave surface:

The accessory glide (roll/slide) occurs in the OPPOSITE direction to the physiological bone movement (swing).

• Example: Glenohumeral joint - when the humerus (convex head) abducts (swings superiorly), the humeral head must glide inferiorly to remain in the glenoid.
• If inferior glide is blocked (tight inferior capsule), impingement results.

Part B - Concave surface moving on fixed Convex surface:

The accessory glide occurs in the SAME direction as the physiological bone movement.

• Example: Tibiofemoral joint during open-chain knee flexion - the tibia (concave) moves posteriorly, and the tibial plateaus also glide posteriorly on the femoral condyles.

• Guides direction of joint mobilization forces
• Restoring lost accessory motion restores physiological range
• Wrong mobilization direction can damage cartilage or worsen restriction

• Pure roll alone would cause joint dislocation
• Pure slide alone would compress one edge
• Normal motion combines roll + slide in the correct ratio to maintain joint congruence

4. Closed Packed Position

1. Articular surfaces are fully congruent - maximum contact area
2. Capsule and ligaments are maximally tight and under tension
3. Joint is in a "locked" or "screwed home" position
4. Cannot be distracted - resistance is rigid (bone-to-bone)
5. Least amount of accessory motion possible

• Joint surfaces are least congruent
• Capsule is most lax
• Maximum synovial fluid space
• Accessory motions (roll, spin, slide) are maximal
• Used for joint examination and mobilization

• Close-packed = position of immobilization for stability (e.g., casting)
• Loose-packed = position of treatment, traction, and mobilization
• Joints naturally move through close-packed at end of range - acts as a locking mechanism (e.g., knee "screw-home" mechanism)

5. Connective Tissue Structure

A. Cells:

1. Fibroblasts - most common; synthesize and maintain collagen, elastin, proteoglycans
2. Chondrocytes - maintain cartilage ECM
3. Osteoblasts/Osteoclasts - bone formation and resorption
4. Mast cells, macrophages - immune/inflammatory response

B. Extracellular Matrix (ECM):

• Most important structural protein; provides tensile strength
• Type I collagen - ligaments, tendons, bone, skin (most common)
• Type II collagen - hyaline and fibrocartilage
• Type III collagen - loose connective tissue, blood vessels
• Crimped ("wavy") arrangement at rest; straightens under load

• Allow tissue to stretch and recoil (elastic behavior)
• Present in ligamentum flavum, skin, arterial walls

• Aggrecan, versican, decorin
• Highly hydrophilic - attracts and retains water
• Provides compressive resistance and viscoelastic properties
• Decreases with aging and immobilization

C. Organization Patterns:

• Dense regular (tendons, ligaments): parallel collagen bundles for uniaxial force
• Dense irregular (joint capsule, fascia): random orientation for multidirectional strength
• Loose (areolar tissue): support and nutrition

6. Mechanical Behavior of Ligament and Bone

Ligament Mechanical Behavior:

1. Viscoelasticity:
   • Creep: Under a constant load, ligament gradually elongates over time
   • Stress relaxation: When held at constant length, stress (internal tension) decreases over time
   • Hysteresis: Energy is lost (as heat) during load-unload cycles - loading and unloading curves are not identical
2. Toe region: Initial low-stiffness region where crimped collagen fibers straighten (wavy collagen uncrimps)
3. Linear region: After toe region, stiffness increases as straightened collagen fibers bear load; behavior is nearly linear (Hookean)
4. Yield/failure region: Microtears begin; permanent deformation occurs; final rupture at ultimate failure load
5. Strain-rate dependency: Stiffer under rapid loading (fast loads = protective); more compliant under slow sustained loading

Bone Mechanical Behavior:

1. Anisotropic: Behaves differently depending on direction of loading
2. Strongest in compression (trabecular architecture aligned with compressive forces)
3. Cortical (compact) bone: Stiff, high modulus; resists bending and torsion
4. Cancellous (trabecular) bone: Lower modulus; energy-absorbing; fails at lower stress
5. Wolff's Law: Bone remodels its architecture in response to mechanical demands (trabeculae orient along principal stress lines)
6. Stress fractures: Occur when repetitive loading exceeds bone's remodeling capacity

7. Load-Deformation Curve / Stress-Strain Curve

Load-Deformation Curve (structural property):

• X-axis: Deformation (mm)
• Y-axis: Load (Newtons)
• Describes the behavior of a whole tissue structure (e.g., an entire ligament)

Stress-Strain Curve (material property):

• X-axis: Strain (% elongation or deformation/original length)
• Y-axis: Stress (load per unit area, N/mm²)
• Eliminates influence of size; describes inherent material properties

Stress
  |              D (Ultimate Failure)
  |           C/ \
  |          /    \___
  |        /
  |      / B (Linear region - Hookean behavior)
  |    /
  |  / A (Toe region - collagen uncrimping)
  |/___________________________
                               Strain

• Crimped collagen fibers straighten
• Low stiffness; large deformation with little force
• Corresponds to normal physiological range of joint motion

• Collagen fibers fully recruited and loaded
• Stiffness is high and nearly constant (slope = Young's modulus / stiffness)
• Clinical mobilization forces operate here

• Collagen bonds rupture progressively
• Permanent (plastic) deformation begins
• First-degree sprain corresponds to this region

• Complete rupture of tissue
• Corresponds to 2nd and 3rd degree tears

• Toe region is absent (bone is stiffer)
• Linear region is steeper (higher modulus)
• Failure is more abrupt (brittle material behavior)

• Hysteresis loop: Energy lost between loading and unloading curves
• Creep: Curve shifts right under constant load over time
• Stress relaxation: Curve drops under constant elongation

8. Kinematic Chains

Open Kinematic Chain (OKC):

• The distal segment is free to move in space
• Movement of one segment does not obligate movement of all others
• Example: Raising the foot in sitting (tibia moves freely); reaching with the hand
• Muscle action is more isolated
• Joint compression tends to be lower
• Used in rehabilitation for isolated muscle strengthening (e.g., leg extension machine)

Closed Kinematic Chain (CKC):

• The distal segment is fixed (meets external resistance, usually the ground)
• Movement of one segment requires predictable motion at all other linked segments
• Example: Squatting - ankle, knee, and hip must all move in coordination
• Promotes co-contraction and joint stability
• More functional and weight-bearing
• Example: Weight-bearing during gait; push-up (hands fixed)

• Anterior cruciate ligament (ACL) rehabilitation: OKC exercises (terminal knee extension) vs. CKC exercises (squats) - debate over tibial shear forces
• Closed chain preferred for early functional rehabilitation post-arthroplasty and fracture

9. Osteokinematics & Arthrokinematics

Osteokinematics:

• Sagittal plane (frontal axis): Flexion - Extension
• Frontal (coronal) plane (sagittal axis): Abduction - Adduction; Lateral flexion
• Transverse (horizontal) plane (vertical axis): Medial/Lateral rotation; Pronation/Supination

1. Spin: Pure rotation of a segment around its own mechanical axis (e.g., lateral rotation of femur)
2. Swing (angular motion): Bone swings through an arc away from its mechanical axis (most joint movements)
   • Chord swing (cardinal): Swing in a single plane
   • Arcuate swing: Swing in a curved path (e.g., circumduction)

• 1 DOF: hinge joints
• 2 DOF: condyloid, saddle joints
• 3 DOF: ball-and-socket joints

Arthrokinematics:

1. Roll (Rocking):
   • Multiple points on one surface contact multiple points on the other surface
   • Analogous to a ball rolling on a flat surface
   • Direction follows the direction of bone swing
   • If roll alone occurred, joint would dislocate
2. Slide (Glide/Translation):
   • One surface slides (translates) across another
   • A single point on one surface contacts multiple points on the other
   • Direction governed by the concave-convex rule
   • Pure slide keeps joint congruent but covers limited range
3. Spin:
   • One surface rotates around a stationary longitudinal axis on the other surface
   • Single point contacts single point throughout
   • Example: Radial head spinning on capitellum during forearm pronation/supination

• Every osteokinematic movement has a corresponding arthrokinematic component
• Loss of arthrokinematic motion (e.g., loss of posterior glide at GH joint) limits osteokinematic motion (e.g., loss of flexion)
• Joint mobilization restores lost arthrokinematic motion to regain physiological range

Reference for all answers: Levangie PK, Norkin CC, Lewek MD. Joint Structure & Function: A Comprehensive Analysis. 6th ed. F.A. Davis Company; 2019. Ch. 1-2 (Biomechanical Applications to Joint Structure and Function).