Enamel Hypoplasia

The cell responsible for enamel formation is the ameloblast. If the ameloblast is exposed to a stress when it is in its secretory phase it will stop secreting enamel matrix properly and the result will be an enamel hypoplasia. This enamel hypoplasia will then be a feature of the tooth for long after the person it was attached to is dead and buried, unless it is worn away. The permanency of enamel hypoplasias coupled with the fact that teeth are so durable over time makes hypoplasias a very useful indicator of physiological stress.

Enamel hypoplasias can be seen as: no enamel at all, grooves, horizontal lines and pits. Ameloblasts lay down enamel matrix in increments. These increments are separated by striae of Retzius, which under light microscope are seen as perikymata at the crown’s lateral surfaces. Each increment indicates a constant period of growth in an individual of between 6-12 days. This period of growth in an individual can be determined by counting the number of daily growth striations (seen as cross striations) within each increment.

Linear enamel hypoplasias (LEH) are seen as enlargements of the width of the striae. This enlargement is the result of enamel matrix not being secreted by the ameloblast during periods of stress. An LEH is seen as a groove; one side of the groove is called the occlusal wall (closer to the occlusal surface of the tooth) and represents the period of disrupted enamel formation and the other side of the groove is called the cervical wall (closer to the cervical part of the tooth) and represents the period of return to normal enamel formation. The duration of the disruption of enamel formation can be found by multiplying the number of perikymata in the occlusal wall by the daily growth striations within those perikymata. For example if three perikymata are seen in the occlusal wall and each perikymata consists of 8 daily growth increments, then the duration of disruption is approximately 24 days.

Syndromes Relevant to Dentistry

The word syndrome is derived from greek and means 'runs together'. Syndromes refer to medical conditions that have signs and symptoms that often occur together. A syndrome has no known cause, but may still be referred to as a syndrome even after the cause has been discovered, e.g. AIDS. If a dentist is alerted to one or more signs and/or symptoms of a suspected syndrome, they should look out for the other common signs and symptoms of the syndrome.

Neural Crest Syndromes

• DiGeorge Syndrome - remember the description of this syndrome with the mnemonic CATCH 22 - Cleft palate, Abnormal facies, Thymic aplasia, Cardiac defects, Hypocalcaemia, 22q11.2 deletion. May be caused by migration defects of neural crest derived tissues.
  
• Hemifacial microsomnia - lower half of face affected (on one or both sides). Most commonly mouth, mandible and ears.
• Sturge-Weber Syndrome - most obvious feature is a port-wine stain on the face. Neurological abnormalities are also commonly seen as angiomas can develp on the brain (the same side as the port-wine stain). Glaucoma also commonly develops. Increased vascularity of the hard and soft tissue of the jaw on the same side as the port-wine stain may occur and this can cause dental problems such as swelling, premature tooth eruption and periodontal problems.
  
• Thalidomide malformations - phocomelia (very short or absent long bones). (S) enantiomer of thalidomide thought to be the neural crest toxin.
  
• Treacher-Collins - often present with underdeveloped facial bones, micrognathia, microtia, cleft palate. Half have hearing problems due to defects in middle ear bones.