1. ORIGINAL THEORY (1970s–1980s)
Proposed by: McNamara, Petrovic
Claim:
- Functional appliances → mandible advanced → LPM hyperactivity
- Hyperactive LPM → traction on condylar cartilage → accelerated mandibular growth
Clinical relevance back then:
Cornerstone explanation for functional appliance effects.
2. WHY THE THEORY FAILED
A. Anatomical Evidence
- LPM does NOT attach directly to the condylar disc.
- It attaches to anterior capsule, not fibrocartilage.
- Other muscles (temporalis, masseter) also influence disc region.
➡️ Traction theory anatomically unsupported.
B. Biomechanical Contradiction
- Functional appliances shorten the LPM (mandible forward).
- Shortened muscles do not reflexively hyperactivate.
➡️ Hyperactivity in a shortened muscle = biomechanically illogical.
C. Myectomy (LPM Removal) Studies
- LPM surgically removed (Whetten, Johnston).
- Condylar growth still occurred.
- Some blood supply disruption possible, but:
➡️ Growth did not rely on LPM traction.
D. Modern EMG Findings
Using refined EMG and longitudinal monitoring:
- Functional appliance therapy → decreased LPM postural activity
- ↓ LPM activity → increased, not reduced, condylar growth
Researchers: Auf Der Maur, Pancherz, Ingervall, Bitsanis
➡️ Opposite of original hypothesis.
3. CURRENT UNDERSTANDING (Modern Paradigm)
What actually promotes condylar growth?
- Sustained forward mandibular positioning
- Viscoelastic stretch of posterior periarticular tissues
- Altered joint-space loading
- Improved retro-condylar vascular perfusion
- Functional adaptation of soft tissues
➡️ Growth results from stable repositioning, not muscle hyperactivity.
4. PRACTICAL CLINICAL TAKEAWAYS
✔ Functional appliances work—but not because of LPM hyperactivity
✔ Continuous wear > part-time wear (stability > force)
✔ Condylar remodeling depends on tissue stretch & vascular changes
✔ LPM helps maintain position but does not stimulate growth
✔ Growth is a multifactorial adaptive response, not a muscle-traction effect
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