Primary Teeth Morphology and Function

The Role of Primary Teeth

• Aesthetics
• Phonation: Necessary to produce (F, V, S, Z, T) sounds
• Function: Chewing
• Guidance of eruption

Morphological Properties of Primary Teeth

• Maxillary Central Incisor: Mesio-distal dimension is greater than inciso-cervical dimension, labial surface convex and flat, mesial corner is a right angle, distal face rounded, prominent cingulum on the palatal face, narrow collar from all sides, no developmental grooves.
• Maxillary Lateral Incisor: Similar to central but smaller, distal and mesial margins less protruding, distal corner angle rounded, less pronounced lingual anatomy, collar appearance similar to central, root shorter than central but with the same morphological features, higher root-crown ratio than central incisors.
• Mandibular Central Incisor: Both mesial and distal corners are sharp, forming an angle of 90 degrees, labial face is straight, no developmental groove, prominent cingulum on the lingual face, narrow and conical root, almost twice the length of the crown, slightly pointed apex.
• Mandibular Lateral Incisor: Maxillary central is larger than lateral, mandibular central is smaller than lateral, root is conical and longer than the central incisor, shows a definite slope distally at the apex.
• Maxillary Canine: Larger than maxillary centrals, mesio-distal diameter significantly larger than the crown'Ys height, wide crown view similar to primary centrals.
• Mandibular 1st Primary Molar: Crown is different from other primary and permanent teeth, mesial half of the crown is larger than the distal half, four cusps (two vestibular and two lingual), rhombic view from the vestibular side, mesial and distal edges parallel, distal edge curved, marked convexity seen in the cervical 1/3 of the crown from the mesial aspect.
• Mandibular 2nd Primary Molar: Miniature of the mandibular 1st permanent molar, distinct protrusion only on the vestibular face, five cusps (three vestibular and two lingual), square form viewed from the occlusal perspective, mesio-distal length of the crown is twice its height, two roots similar to the mandibular 1st permanent molar.

Properties of Primary Tooth Enamel

• Structure of Enamel: 92-93% inorganic, 4% organic, 3-4% water
• Inclinations of Enamel Prisms: Primary teeth have prisms perpendicular to the tooth axis in the cervical 2/3 of the crown, unlike permanent teeth where this occurs in the occlusal 2/3.
• Aprismatic Layer: More mineralized outer layer with no interprismatic region, thicker in primary teeth.
• Organic Structure: Consists of protein complexes, free amino acids, and lipids.
• Maturation: Enamel matures with age, water content decreases, crystals thicken.
• Prenatal Enamel Distribution: Most of primary incisors, half of primary canine, occlusal surface of 1st molar, and tubercle top of the 2nd premolar.

Differences Between Primary and Permanent Teeth

• General Features:
  • Primary teeth are bluish white, permanent teeth are yellowish white.
  • Primary teeth are generally smaller than permanent teeth, except for the MD diameters of primary canines and molars, which are greater than permanent counterparts.
  • Primary incisors and canines are more rounded than permanent incisors and canines, with more convex vestibular surfaces.
  • Primary molars have more convex vestibular surfaces and interfaces than permanent teeth, forming a "tuberculum molare".
  • Primary teeth enamel narrows significantly toward the collet.
  • Primary tooth crowns are shorter in the occluso-cervical direction compared to permanent teeth, with a greater mesio-distal diameter than the cervico-occlusal height.
  • Vestibular and lingual surfaces of primary molar crowns converge occlusally, making the occlusal surface narrower than permanent teeth.
  • Fissures in primary teeth are less frequent and shallower.
• Pulp Differences:
  • The first molar's crown pulp is larger than that of the second molar.
  • Mandibular deciduous pulp chambers are larger than maxillary ones.
  • The mesial pulp horn of primary molars is closer to the surface, particularly in the lower IV (1.6 mm) compared to the upper IV (1.8 mm).
  • Root canals in mature deciduous teeth are wider than in mature permanent teeth.
  • The height of pulp chambers differs between primary and permanent molars: decreasing from first to second molars in primary teeth, while increasing from first to third molars in permanent teeth.
  • Pulp-periodontal canals opening into the periodontal space at the pulpal bases of primary molars are more common than lateral canals in permanent teeth, leading to increased risk of infection spreading to the furcation area in primary teeth.
• Root Differences:
  • Primary incisor roots are in the same plane as their crowns, while those of permanent incisors incline towards the oral side.
  • Primary molar roots are split due to the presence of the premolar germs beneath them.
  • Primary incisor roots are narrower and straighter mesio-distally compared to their permanent counterparts.
  • Primary molar roots appear longer than permanent teeth when comparing crown sizes.
• Approximate Area Differences:
  • Primary teeth show more wear than permanent teeth due to thinner enamel at the interface.
  • The septum separating primary teeth is generally wider than that of permanent teeth.