Courtesy: Rishi Dhir, FRCSOrth, Consultant Orthopaedic and Upper Limb Surgeon, Princess Alexandria Hospital, Harlow, UK

Overview

Biomechanics forms the foundation of understanding movement, load transmission, and joint function in Orthopaedics.

These principles are essential for:

• Injury analysis
• Surgical planning
• Implant design
• Clinical decision-making

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Force

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Definition

A force is a load acting across a particular area.

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Unit

• Measured in Newtons (N)

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Clinical Relevance

• Related to stress within tissues
• Determines tissue deformation and injury patterns

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Newton’s Laws of Motion

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First Law – Law of Inertia

• A body remains at rest or in uniform motion unless acted upon by an external force

Clinical Insight

• When:
  • Sum of forces = 0
  • Sum of moments = 0
    The body is in equilibrium

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Second Law – Law of Acceleration

F=ma  

• Force is proportional to mass and acceleration

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Third Law – Action and Reaction

• Every action has an equal and opposite reaction

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Orthopaedic Examples

• Ground reaction forces during walking
• Joint reaction forces

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Moments (Torque)

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Definition

A moment is the turning effect of a force acting at a distance from a pivot.

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Formula

M=F×d

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Example

• Seesaw principle:
  • Larger force – shorter lever arm
  • Smaller force – longer lever arm

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Couples

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Definition

A couple consists of:

• Two equal
• Opposite
• Parallel forces
• Acting at different points

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Examples

• Turning a steering wheel
• Opening a bottle cap

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Clinical Applications of Force Couples

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Shoulder Force Couples

Coronal Plane

• Deltoid – upward pull
• Rotator cuff – downward & medial pull

 Maintains centering of humeral head

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Rotator Cuff Tear

• Loss of downward force
   Superior migration – rotator cuff arthropathy

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Transverse Plane

• Subscapularis (anterior)
• Infraspinatus + Teres minor (posterior)

 Balance internal and external rotation

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Wrist Force Couples

Lunate Function

• Acts as a torque converter

Balances:

• Scaphoid (flexion tendency)
• Triquetrum (extension tendency)

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Scapholunate Injury

• Leads to:
  • Scaphoid flexion
  • Lunate extension
    DISI deformity

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Levers

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Definition

A lever consists of:

• Fulcrum (pivot)
• Force (effort)
• Load (resistance)

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Types of Levers

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1. First-Class Lever

• Fulcrum between force and load

Examples:

• Seesaw
• Atlanto-occipital joint

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2. Second-Class Lever

• Load between fulcrum and force

Example:

• Tiptoe standing (MTP joint)

 Most efficient lever

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3. Third-Class Lever

• Force between fulcrum and load

Examples:

• Elbow
• Shoulder

 Most common in human body

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Equilibrium

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Definition

A system is in equilibrium when:

• Sum of forces = 0
• Sum of moments = 0

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Free Body Diagrams (FBD)

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Definition

A simplified representation used to analyze:

• Forces
• Moments

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Key Assumptions

Forces

• Joint forces are compressive

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Bones

• Treated as rigid structures

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Joints

• Considered frictionless hinges

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Muscles

• Act only in tension

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Additional Assumptions

• No antagonistic muscle activity
• Line of action passes through muscle center

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Clinical Applications of Free Body Diagrams

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Commonly Applied To

• Hip
• Shoulder
• Elbow
• Knee
• Spine
• Foot

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Hip Biomechanics

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Lever Type

• First-class lever

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Components

• Fulcrum – Femoral head
• Load – Body weight
• Force – Hip abductors

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Example Calculation

500×15=F×5

 Abductor force – 1500 N

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Joint Reaction Force

• 2000 N

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Clinical Application

Trendelenburg Gait

• Weak abductors
   Patient shifts body – reduces joint load

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Walking Stick (Opposite Hand)

• Reduces joint reaction force
• Decreases pain

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Charnley Arthroplasty

• Medializes hip center
   Improves abductor efficiency

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Shoulder Biomechanics

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Lever Type

• Third-class lever

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Components

• Fulcrum – Humeral head
• Force – Deltoid
• Load – Arm weight

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Reverse Shoulder Arthroplasty

• Medializes and inferiorizes center of rotation
   Improves deltoid function

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Elbow Biomechanics

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Lever Type

• Third-class lever

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Key Point

• Increasing weight in hand:
  • Increases muscle force
  • Increases joint reaction force

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Foot Biomechanics (MTP Joint)

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Lever Type

• Second-class lever

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Components

• Fulcrum – MTP joint
• Load – Body weight
• Force – Gastrocnemius–soleus

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Spine Biomechanics

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Lever Type

• First-class lever

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Key Insight

• Squatting:
  • Increases muscle lever arm
  • Reduces spinal load

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Patellofemoral Joint Biomechanics

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Forces Involved

• Quadriceps force
• Patellar tendon force

 Produce compressive joint reaction force

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Effect of Patellectomy

• Loss of mechanical advantage

 Increased:

• Quadriceps force
• Joint reaction force

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Clinical Correlation

• Pain worsens during:
  • Stair climbing
  • Deep knee flexion

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Key Take-Home Points

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Core Concepts

• Force, moments, couples, and levers form the basis of biomechanics
• Most joints function as third-class levers

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Clinical Relevance

• Helps explain:
  • Joint loading
  • Injury mechanisms
  • Surgical principles

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Exam Focus

• Understand:
  • Free body diagrams
  • Lever systems
  • Clinical applications