How does the clinician apply principles of growth and development in daily practice? Certain discrepancies occur between the maxilla and mandible, as well as between tooth size and jaw size. As mentioned previously, the clinician must be able to distinguish between transient and incipient malocclusions in order to use interceptive mechanics beneficially for the patient. He or she should also know what happens to the dental arch as a child grows and develops into an adult dentition. Does it increase, decrease, or stay constant?
Often a clinician may be able to use various orthopedic appliances to guide the jaws and obtain a normal maxillomandibular relationship. An example of such appliance therapy is the use of a "chin-cup" to redirect the growth of the mandible in Class III and skeletal open bite malocclusions. (Figure 38) A head- or neck-gear appliance is often used to redirect the growth of the maxilla in severe Class II skeletal discrepancies. (Figure 39)
|Figure 38. Chin cup appliance||Figure 39. Neck gear appliance|
Figure 40. Leeway space
Leeway Space: There is a difference between the mesiodistal widths of the deciduous molars and premolars. This is called the "leeway space." (Figure 40) The mandibular deciduous molars are larger than the maxillary deciduous molars, therefore, the leeway space is slightly greater in the lower dental arch than in the upper. Nevertheless, the transition from the "mixed" dentition to the adult dentition causes a decrease in the arch length due to the forward movement of the permanent first molars into the leeway space. The "flush terminal plane" occlusion of the permanent first molars in the mixed dentition is a transient malocclusion and recovers autonomously into a Class I relationship as the lower molar moves forward a greater amount than the upper.
Incisor Liability: What about the anterior region? Here, nature is not so kind to the patient in terms of transition. There is a "liability" between the deciduous incisors and their permanent successors.
Lower arch mesio-distal diameters:
|Permanent centrals||(5.4mm x 2)||10.8mm|
|Permanent laterals||(5.9mm x 2)||11.8mm|
|Deciduous centrals||(4.2mm x 2)||8.4mm|
|(4.1mm x 2)||8.2mm|
Mandibular incisor liability of -6mm overcome by:
1. Interdental spacing of 2.7mm
2. Intercanine growth of 3.0mm
3. Anterior incisor positioning of 1.3mm
4. Combination of 1, 2 and 3 in varying degrees.
Growth occurs at the midpalatine suture of the maxilla and at the symphysis of the mandible to accommodate the larger permanent incisors. But the suture between the maxillae stops growing actively between the ages of 7 and 9 years, whereas the mandibular symphysis "closes" during the first year of life. Therefore, normal sutural growth does not keep pace with the incisor liability.
Interdental spacing in the deciduous incisor region is the most important means for a harmonious transition between the mixed and permanent dentition. (Figure 41) If no spaces exist between the deciduous incisors, it is safe to say that the result will be crowding of the permanent teeth. (Figure 42)
Just as important as space consideration in the dental arch is the sequence of eruption of the permanent teeth. Although it is important to know the calcification and eruption dates of the different teeth, it is more important to know and preserve their proper eruption sequence. It is also important to realize that the
sequence of eruption in the upper dental arch differs from that in the lower dental arch. In the maxillary arch, the canines erupt after the first and second premolars. In the mandibular arch, the premolar teeth follow the eruption of the canines.
There is a difference in the prevalence of tooth impaction between the upper and lower arches. In the maxillary arch, it is common to see an impacted canine since this is the tooth that is "short-changed" if a lack of space exists. In the lower arch, the second premolar is most commonly impacted, as it erupts after the canine and first premolar.
Because the maxillary permanent canines are late in their eruption pattern, the upper incisors often flare distally and cause spacing in the anterior region of the dental arch.
|Figure 41. Interdental spacing||Figure 42. Crowding of deciduous incisors|
Figure 43. Distal flaring of maxillary incisors
This is termed the "ugly duckling" stage of dental development.(12) The apices of the incisor teeth are together because the canines are pressing against these teeth, and the width at the base of the nose has not yet attained its proper dimensions. (Figure 43) As more growth takes place in this region, and as the canine teeth erupt, the incisors will move together and this transient malocclusion will correct itself.
12. Broadbent BH, Ontogentic Development of Occlusion, Angle Orthodontist, 11:223-241, 1941.