Definition:
Viscoelasticity describes the combination of viscous (fluid-like) and elastic (solid-like) properties exhibited by biological tissues. It primarily applies to elastic tissues such as muscles, but the concept extends to all non-calcified tissues.

Key Concepts:

• It concerns both viscosity and flow of synovial fluids and elasticity of soft tissues including:
  • Retrodiskal tissues
  • Fibrous capsule
  • TMJ ligaments and tendons
  • Lateral pterygoid muscle (LPM) perimysium
  • Other non-muscular, non-mineralized soft tissues
• Essentially, it explains how these tissues deform under stress and recover when the stress is removed, with a time-dependent response.

Historical Notes:

• The concept faced opposition from Herren (1953), Harvold (1974), and Woodside (1973) to the original Anderson–Haupl theory, which had a different interpretation of joint tissue adaptation.

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Stages of the Viscoelastic Reaction

The viscoelastic reaction proceeds through five sequential stages:

1. Emptying of blood vessels – initial vascular response to stress.
2. Pressing out interstitial fluid – displacement of tissue fluids to redistribute pressure.
3. Stretching of fibres – collagen and elastic fibers undergo elongation.
4. Elastic deformation of bone – bone matrix responds elastically under load.
5. Bioplastic adaptation – long-term remodeling and adaptation of supporting tissues.

      VISCOELASTIC REACTION

             ┌────────────────────┐
             │ Functional load /  │
             │   condylar stress  │
             └─────────┬──────────┘
                       │
                       ▼
          ┌────────────────────────┐
          │ 1. Emptying of         │
          │    blood vessels       │
          └─────────┬──────────────┘
                    │
                    ▼
          ┌────────────────────────┐
          │ 2. Pressing out        │
          │    interstitial fluid  │
          └─────────┬──────────────┘
                    │
                    ▼
          ┌────────────────────────┐
          │ 3. Stretching of       │
          │    fibres              │
          └─────────┬──────────────┘
                    │
                    ▼
          ┌────────────────────────┐
          │ 4. Elastic deformation │
          │    of bone             │
          └─────────┬──────────────┘
                    │
                    ▼
          ┌────────────────────────┐
          │ 5. Bioplastic          │
          │    adaptation          │
          └────────────────────────┘

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Clinical Implications

• To avoid condylar compression, clinicians may use a Herbst appliance combined with a thin posterior bite block and a rapid maxillary expander (RME).
• The RME widens the upper arch, reduces occlusal interferences, and permits a stable forward positioning of the mandible without excessive TMJ strain.

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