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🦷 OCCLUSION - Complete Lecture for Final Year BDS

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PART 1: What is Occlusion? (The Foundation)

Remember: Occlusion is NOT just about biting. It includes any tooth contact - chewing, swallowing, speaking, even resting contact.

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PART 2: The Key Players - The TMJ

TMJ Anatomy - Here's the actual diagram:

Two Movements in One Joint

• Lower compartment = HINGE (rotation) - mouth opens slightly
• Upper compartment = GLIDE (translation) - condyle slides forward onto the articular eminence for wider opening

Easy memory: "Lower = rotation, Upper = translation" - L comes before U, rotation comes before translation when opening your mouth.

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PART 3: Critical Terminology - Jaw Positions

3A. Centric Relation (CR)

┌─────────────────────────────────────────────────────┐
│  CENTRIC RELATION = Position of the CONDYLE          │
│                                                       │
│  The condyle is in its most superior, anterior,      │
│  and medial position in the glenoid fossa            │
│  WITHOUT any tooth contact                           │
│                                                       │
│  This is a JOINT position, not a tooth position      │
└─────────────────────────────────────────────────────┘

• It is reproducible - you can find it again and again
• It is a border position - at the very limit of the hinge arc
• NOT dependent on teeth - even an edentulous patient has a CR
• This is why CR is used as the reference position when making dentures and occlusal records

3B. Centric Occlusion (CO) = Intercuspal Position (ICP)

┌─────────────────────────────────────────────────────┐
│  CENTRIC OCCLUSION = Position of MAXIMUM TOOTH       │
│                        CONTACT                       │
│                                                       │
│  Upper and lower teeth are in maximum intercuspation │
│  (maximum number of teeth touching each other)       │
│                                                       │
│  This is a TOOTH position                            │
└─────────────────────────────────────────────────────┘

3C. The CR-CO Slide (Most Important Concept!)

         CR                     CO
          |                      |
          |  ← usually 0.5–2mm →  |
          |     anterior slide    |
         [condyle in fossa]    [teeth in max contact]

Why this matters clinically: A large CR-CO slide creates uneven forces, which can lead to TMD (Temporomandibular Disorder), tooth wear, and muscle pain.

3D. Rest Position (Postural Rest Position)

┌──────────────────────────────────────────────┐
│  REST POSITION = Mandible at rest            │
│                                               │
│  Muscles of mastication are at minimum       │
│  activity (physiologic equilibrium)          │
│                                               │
│  Teeth are APART - a small gap exists        │
│  called the FREEWAY SPACE                    │
└──────────────────────────────────────────────┘

• The gap between upper and lower teeth when the jaw is at rest
• Normal = 2-4 mm (average 3 mm)
• Too small = denture feels "cramped," muscles fatigue
• Too large = patient looks "sunken in," speaking difficulty, clicking sounds when talking

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PART 4: Angle's Classification of Malocclusion

The Reference Point: Upper First Molar

Upper 1st Molar: The "KEY OF OCCLUSION" (Angle's term)
     |
     ↓
Mesiobuccal cusp of upper 1st molar sits in...
     |
     ├── ...mesiobuccal groove of lower 1st molar → CLASS I ✓
     |
     ├── ...distal to that groove → CLASS II (lower jaw behind)
     |
     └── ...mesial to that groove → CLASS III (lower jaw forward)

Class I - Normal Occlusion

   Upper: [  |  |  |  |  |  |  ]
              ↓ mesiobuccal cusp lands in ↓
   Lower: [  |  |  |  |  |  |  ]
                   mesiobuccal groove

• The upper and lower molars are in ideal relationship
• All other teeth may still have individual rotations/crowding - that's Class I malocclusion
• The molar key is correct but individual teeth can be problematic

Class II - "Buck Teeth" / Retrognathic Mandible

• Upper incisors are proclined (flared forward)
• Deep overjet (horizontal overlap is increased)
• Usually associated with incompetent lips (can't close without strain)

• Upper central incisors are retroclined (tipped back)
• Upper lateral incisors are proclined and overlap centrals
• Usually deep overbite (vertical overlap is increased)
• Lips are competent

CLASS II Div 1         CLASS II Div 2
  ┌─┐                    ┌─┐
  │/│ ← proclined        │\│ ← retroclined
  └─┘                    └─┘
  deep overjet           deep overbite

Class III - "Underbite" / Prognathic Mandible

Lower jaw sticks out → lower teeth bite in front of upper teeth

• Can be true Class III (skeletal - large mandible)
• Or pseudo/false Class III (postural - mandible shifts forward to achieve contact)
• Common in certain ethnic populations

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PART 5: Overjet and Overbite - Two Most Confused Terms

         ↕ OVERBITE           ←→ OVERJET
         (vertical)            (horizontal)

     Upper tooth
      ────────
          |   ↕ Overbite = vertical overlap
      ────────
     Lower tooth
                ←→ Overjet = horizontal gap

• Increased overjet = Class II div 1 appearance, proclined upper incisors
• Increased overbite = deep bite, can impinge on lower gingiva
• Reverse overjet = lower teeth in front (Class III or edge-to-edge)
• Open bite = teeth do NOT overlap at all - a space exists when back teeth contact
• Edge-to-edge = incisors meet tip to tip (0 mm overjet AND 0 mm overbite)

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PART 6: Occlusal Contacts - Types and Their Functions

6A. Ideal Contacts in ICP

   Cusp tip → Central fossa or marginal ridge
   
   Upper buccal cusp → contacts lower central fossa
   Lower buccal cusp → contacts upper central fossa
   
   RULE: "Cusp to Fossa" or "Cusp to Marginal Ridge"

6B. Cusp Types - Stamp vs. Shear

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PART 7: Occlusal Schemes - The Big Three

7A. Balanced Occlusion (Bilaterally Balanced)

┌─────────────────────────────────────────────────────┐
│  Contacts on BOTH sides during ALL lateral movements │
│                                                       │
│  Used in: COMPLETE DENTURES                          │
│                                                       │
│  Why: Denture stability - bilateral contacts prevent  │
│  the denture from tipping/rotating                   │
└─────────────────────────────────────────────────────┘

7B. Mutually Protected Occlusion (Canine Protected / Organic Occlusion)

┌─────────────────────────────────────────────────────────┐
│  Posterior teeth protect anteriors in ICP               │
│  Anterior teeth protect posteriors during EXCURSIONS    │
│                                                          │
│  Used in: NATURAL DENTITION, Fixed Prosthodontics       │
│                                                          │
│  In lateral movement → ONLY CANINE contacts on          │
│  working side (canine guidance)                         │
│                                                          │
│  In protrusive movement → ONLY incisors contact         │
└─────────────────────────────────────────────────────────┘

• Strong roots (longest, with dense bone)
• Good proprioception
• Located at corners - ideal for guiding lateral movements

7C. Group Function Occlusion

┌──────────────────────────────────────────────────────────────┐
│  During lateral movements → Multiple teeth on working side   │
│  contact simultaneously (canine + premolars + possibly       │
│  first molar)                                                 │
│                                                               │
│  NO balancing side contacts                                   │
└──────────────────────────────────────────────────────────────┘

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PART 8: Mandibular Movements and Their Guidance

The 4 Border Movements of the Mandible

                    ┌───────────────┐
                    │    CR/ICP     │   ← Maximum intercuspation
                   /               \
                  /                 \   ← Lateral borders (guided by teeth/TMJ)
                 /                   \
                /                     \
               └─────────────────────┘
                  Maximum open

         ICP (Centric Occlusion)
          ●
         /|
        / |
   CR  /  |  ← hinge arc (pure rotation)
      ●   |
       \  |
        \ |
         \|
          ● Maximum open
          |
          ● Maximum protrusion
          
Envelope = the outer boundary of ALL possible jaw movements

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PART 9: Occlusal Discrepancies and Their Effects

Premature Contact / Deflective Contact

    ┌──────────────────────────────────────────────────────┐
    │  One tooth hits before all others during closure     │
    │                                                       │
    │  CR closure → one tooth contacts → mandible SLIDES   │
    │  to reach full CO                                     │
    │                                                       │
    │  This is a DEFLECTIVE contact                        │
    │  (it deflects/redirects the jaw path)                │
    └──────────────────────────────────────────────────────┘

• Tooth mobility
• Bone loss around that tooth
• Muscle hyperactivity
• TMD symptoms (clicking, pain, limited opening)

Interference

Key point: Non-working (balancing) side interferences are the MOST harmful because they create a lever-like effect, concentrating enormous force on those teeth.

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PART 10: Occlusal Plane and Curves

The Curve of Spee

     Side view of lower arch:
     
     Incisors            Molars
        *                    *
         *                  *
          *                *
           *              *
            * * * * * * *
               
    The curve following the buccal cusp tips 
    from canine back to the last molar = CURVE OF SPEE
    (also called the "anteroposterior curve")

The Curve of Wilson

     Frontal view (cross-arch):
     
          Upper teeth
        \      |      /
         \     |     /
          \    |    /   ← Each posterior quadrant tilts
    Left   \   |   /    lingual = Curve of Wilson
    Lower   *  |  * Right Lower
              \|/

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PART 11: Occlusion in Prosthodontics - Clinical Application

In Complete Dentures

1. Always use CR as the reference - because there are no teeth, you can't use ICP
2. Record the freeway space to correctly set vertical dimension of occlusion (VDO)
3. Bilateral balanced occlusion is the goal - prevents denture tipping
4. Set teeth on the residual ridge - occlusal plane should be at the level of the tongue in rest

In Fixed Partial Dentures (Bridges)

1. The restoration must not create new interferences
2. Should follow the existing curve of Spee
3. Contact in ICP should be light and equal to adjacent teeth
4. Canine guidance or group function must be preserved or restored

In Implants

1. Implants lack the periodontal ligament - no proprioception
2. Must not receive heavy occlusal forces
3. "Implant-protected occlusion" - slightly lighter contact than natural teeth
4. Avoid lateral forces (cantilever effect is dangerous)

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PART 12: Occlusal Examination - How to Check in Clinic

1. Articulating paper - have patient bite on blue paper → marks where teeth hit
   • Red/blue marks show contact points
   • Large marks = heavy contact, small dots = light contact
2. Check ICP - ask patient to clench, look at marks, should be bilateral and even
3. Check for premature contact in CR - use a leaf gauge or bimanual manipulation to guide mandible to CR, then have patient close
4. Lateral excursions - move jaw side to side, check:
   • Is there canine guidance (canine alone rises the posterior teeth)?
   • Or group function (multiple contacts)?
   • Any balancing side contact?
5. Protrusive movement - move jaw forward, check:
   • Only anterior teeth should contact
   • All posterior teeth should be free (posterior disclusion)

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PART 13: Quick Summary Table

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PART 14: TMJ Anatomy (Labelled Diagram from Textbook)

• A = Articular eminence (the ridge the condyle slides over during mouth opening)
• B = Articular disc (fibrocartilage "pillow" - divides the joint)
• C = Mandibular fossa (temporal bone socket)
• D = Condyle (mandibular head)
• E = Upper lamina (fibro-elastic - allows disc to spring back)
• F = Lower lamina (non-elastic collagen)
• H = Lateral pterygoid muscle (pulls disc and condyle forward)

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High-Yield Exam Points (Don't Miss These!)

1. CR is reproducible; CO/ICP is not always the same as CR - average slide is 0.5-2 mm
2. Freeway space = 2-4 mm (if lost = patient has excess VDO; if more = insufficient VDO)
3. For complete dentures - always record in CR, never in CO
4. Non-working side interference is the most destructive type of occlusal interference
5. Canine has the best bone support = best tooth to carry lateral guidance
6. Bilateral balanced occlusion is ONLY indicated for complete dentures - harmful for natural dentition
7. Angle's classification is molar-based - always check the first molar relationship first
8. Posselt's envelope - any restoration violating this will cause occlusal trauma
9. Articular disc surface = fibrocartilage, NOT hyaline cartilage (unlike other joints)
10. Lower joint compartment = rotation; Upper = translation - separate movements in one joint

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