• 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.

CHEWING

• 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:

1. The opening stroke during which the mandible is lowered.
2. The beginning closing stroke during which the mandible is rapidly raised until the entrapped food is felt and
3. The power stroke in which the food is compressed, punctured, crushed and sheared.

CHEWING MOVEMENTS AND MECHANICS

• The chewing process generally acts as a 2^nd order lever system resulting in compression at TMJ.
• The turning moment generated along mandibular body and ramus creates a sheer at TMJ.
• In 2^nd 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
• MPDS
• 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.

–     Parkinsonism

–     Facial paralysis

–     Nerve decompression

–     Secondary involvement of systemic diseases.

Muscle Pains

• 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: