CLASS III MALOCCLUSION
- In this we deal with dominant bone dysplasia, with adaptive muscle function and tooth irregularities reflecting a severe basal dysplasia.
- It has got strong hereditary pattern the upper lip is short and lower lip is hypertrophic.
- During deglutition cycle there is greater mobility of the hyoid bone as the suprahyoid and infrahyoid muscles demonstrate activity.
- Two separate acts are recognized in the chewing process.
- First is a combination of prehension and incision in which the food is secured by the lips and bitten by the front teeth.
- The second is mastication, the major activity during which the food is mashed between the back teeth.
- The total chewing cycle occurs through three phases:
- The opening stroke during which the mandible is lowered.
- The beginning closing stroke during which the mandible is rapidly raised until the entrapped food is felt and
- The power stroke in which the food is compressed, punctured, crushed and sheared.
CHEWING MOVEMENTS AND MECHANICS
- The chewing process generally acts as a 2nd order lever system resulting in compression at TMJ.
- The turning moment generated along mandibular body and ramus creates a sheer at TMJ.
- In 2nd order lever system resistance is present between lever and fulcrum.
- Chewing in humans is actually asymmetrical and unilateral.
- At the working side:
- It possesses the greatest adductor force, but articular emminence is less substantially loaded.
- At the balancing side:
- It possesses the less adductor force and the articular emminence is substantially loaded.
- At the initial action, contraction of inferior head of lateral pterygoid muscle occurs to initiate mandibular deviation to working side.
- Masticatory Muscle Disorders
Some of the common masticatory muscle disorders involve:
- Congenital hyperplasia/ hypoplasia
- Hypermobility/ hypomobility of the muscle
- Muscle pains
- Myositis ossificans etc.
CONGENITAL HYPOPLASIA/ HYPERPLASIA
- It occurs very rarely, and is more common in masseter and orbicularis oris.
- Its oral symptoms include enlargement or decreased size of the affected muscle, which may show an asymmetric facial pattern and stiffness in the temporo-mandibular joint.
- It may or may not be associated with hypermobility/ hypomobility of the muscles.
MUSCLE HYPERMOBILITY/ HYPOMOBILITY
- This disorder involves extreme or diminished activity of the masticatory muscles.
- Its etiology includes various factors such as:
– Decreased/ increased threshold potential of neural activity.
– Facial paralysis
– Nerve decompression
– Secondary involvement of systemic diseases.
- It usually occurs as a result of reflex protective mechanism and myofacial triggers.
- It is usually felt as a non-pulsatile variable aching sensation, with a boring quality. It may also present with tightness, weakness, swelling or tenderness.
- It includes 3 types:
1. local muscle soreness: