Biomechanics of Joints

Joint Basics & Arthrokinematics - Bones Meet Motion

• Classification:
  • Structural: Fibrous (e.g., sutures), Cartilaginous (e.g., symphysis), Synovial (e.g., knee).
  • Functional: Synarthrosis (immovable), Amphiarthrosis (slight movement), Diarthrosis (free movement - all synovial).
• Degrees of Freedom (DoF): Number of planes of motion.
  • Uniaxial: 1 DoF (e.g., elbow - flexion/extension).
  • Biaxial: 2 DoF (e.g., wrist - flex/ext, abd/add).
  • Multiaxial: 3 DoF (e.g., shoulder - flex/ext, abd/add, rotation).
• Arthrokinematics (Joint Surface Motion): Fundamental movements between joint surfaces.
  • Roll: New points on one surface contact new points on another (tire rolling).
  • Slide (Glide): One point on a surface contacts new points on another (tire skidding).
  • Spin: One point on a surface rotates on one point of another (top spinning).
• Convex-Concave Rule: Governs roll and slide.
  • Moving Convex on Fixed Concave: Roll & slide in OPPOSITE directions.
  • Moving Concave on Fixed Convex: Roll & slide in SAME direction. 📌 (Mnemonic: COnvex on COncave = Opposite; COncave on COnvex = COngruent/Same)

⭐ All synovial joints are diarthroses, characterized by a joint cavity, synovial fluid, articular cartilage, and ligaments, permitting free movement.

Kinetics & Levers - Joint Power Play

• Kinetics: Study of forces causing motion.
  • Newton's Laws:
    • 1st: Inertia.
    • 2nd: $F = ma$.
    • 3rd: Action-Reaction.
  • Torque (Moment): $τ = F \times d$ (Force × moment arm). Rotational force.
  • Joint Reaction Force (JRF): Force within a joint due to external/internal loads.
• Levers: Rigid bar rotating around a fulcrum (F).
  • Components: Effort (E), Resistance (R).
  • Mechanical Advantage (MA): $MA = \text{Effort Arm} / \text{Resistance Arm}$.
  • 📌 FRE 123: Lever Classes:
    • 1st Class: F between E & R (e.g., Atlanto-occipital joint). MA varies.
    • 2nd Class: R between F & E (e.g., Standing on tiptoes). MA > 1 (force advantage).
    • 3rd Class: E between F & R (e.g., Elbow flexion - Biceps). MA < 1 (speed/ROM advantage).

⭐ Most levers in the human body are 3rd class, prioritizing speed and range of motion over force.

Major Joint Mechanics - Movers & Shakers

• Knee Joint: Modified hinge; crucial for stability & movement.
  • Screw-home mechanism: Terminal tibial ER (Open Kinetic Chain) locks knee in full extension.
  • Q-angle: ASIS to mid-patella & tibial tuberosity to mid-patella. Normal: Men ~14°, Women ~17°. ↑Q-angle → patellofemoral stress.
• Hip Joint: Stable ball-and-socket; primary weight-bearer.
  • Femoral neck angle: ~125-130°. Coxa vara <120° (↑shear stress); coxa valga >135° (↑instability risk).
• Shoulder Complex: Highly mobile ball-and-socket; dynamic stability via rotator cuff.
  • Scapulohumeral rhythm: Coordinated 2:1 humerus:scapula motion beyond 30° abduction.

⭐ The "unhappy triad" of the knee (ACL, MCL, medial meniscus tear) commonly results from a valgus force to a flexed, externally rotated, planted foot.

Applied Clinical Biomechanics - Fixing Faulty Moves

• Gait Cycle Analysis: Key for identifying deviations.
  • Stance Phase (60%): Heel Strike (HS), Foot Flat (FF), Mid-Stance (MS), Heel Off (HO), Toe Off (TO).
  • Swing Phase (40%): Initial Swing (Acceleration), Mid-Swing, Terminal Swing (Deceleration).
• Common Pathomechanics & Correction:
  • Trendelenburg Gait: Gluteus medius/minimus weakness. Correct: Hip abductor strengthening.
  • Antalgic Gait: Pain avoidance. Correct: Address pain source, gait retraining.
  • Foot Drop: Dorsiflexor weakness. Correct: Ankle-Foot Orthosis (AFO), nerve stimulation.
• Therapeutic Principles:
  • Joint Protection: ↓Load, assistive devices (canes, walkers), ergonomic advice for ADLs.
  • Rehabilitation: Strength, flexibility, proprioception exercises.
  • Orthotics: Custom/prefabricated devices for support/correction.

⭐ Trendelenburg gait: Pelvis drops on swing side due to stance-side gluteus medius/minimus weakness. Positive Trendelenburg sign indicates this weakness.

High‑Yield Points - ⚡ Biggest Takeaways

• Levers: Class 3 (e.g., elbow flexion) is most common, offering ↑ROM & speed.
• Close-packed position: Max joint congruency & stability; ligaments taut.
• Arthrokinematics: Convex-on-concave rule (slide opposite roll); Concave-on-convex (slide same as roll).
• Stress-Strain: Elastic (reversible), plastic (permanent deformation), then failure point.
• Hyaline cartilage: Low friction, shock absorption; avascular, limited repair.
• Wolff's Law: Bone remodels based on applied mechanical stress.
• Joint lubrication: Weeping & boundary mechanisms reduce friction.