LECTURE 4-TOOTH DEVELOPMENT, ERUPTION AND OCCLUSIONThe development of the teeth begins prior to birth and continues on until about age twenty. In this lecture we will be discussing the way in which the teeth develop, the way they fit together and some approaches to the more common orthodontic problems.

4:1 Development and EruptionThe teeth begin their formation deep within the bone and gums in a small sac called a follicle. The first portion of the tooth to be formed is the occlusal surface. As mentioned before, there are three major hard tissue components of a tooth and each is formed by a different type of cell.

The enamel is a crystalline like substance and is formed by cells called ameloblasts. Each crystal is formed nearly perpendicular to the surface of the tooth. The formation of the enamel is a one time event. Once it is completed for a single tooth enamel will never be formed again for that particular tooth.

The dentin is formed by cells called odontoblasts. As with the enamel, the formation begins near the occlusal portion of the tooth and proceeds towards the root ends. When the root is roughly two thirds formed then the tooth usually erupts through the gums. The process of the tooth moving up into the mouth is called active eruption. Each tooth proceeds on its own timetable in this process. It is important to know the average age when teeth erupt because it allows one to better plan treatment for patients. The following table gives the approximate times for tooth eruption.

PRIMARY TEETH ERUPTIONMandibular

Maxillary

Central-------6 mo

Central-------7 mo

Lateral-------7 mo

Lateral-------8 mo

Canine-------18 mo

Canine-------18 mo

1st molar----14 mo

1st molar----14 mo

2nd molar---24 mo

2nd molar---24 mo

PERMANENT TEETH ERUPTIONMandibular

Maxillary

Central--------6 yrs

Central--------7 yrs

Lateral--------7 yrs

Lateral--------8 yrs

Canine--------10 yrs

Canine---------11 yrs

1st premolar-10 yrs

1st premolar--10 yrs

2nd premolar-11 yrs

2nd premolar-11 yrs

1st molar------6 yrs

1st molar------6 yrs

2nd molar----12 yrs

2nd molar----12 yrs

3rd molar----20 yrs

3rd molar----20 yrs

Considerable variation exists in tooth eruption; however, people tend to vary consistently. For example a child who is slow to get his first tooth will likely be delayed with all subsequent teeth. As a general rule the mandibular teeth come in before the maxillary. For example, the primary first molars are both listed at 14 months however in reality the mandibular molar will usually precede the maxillary molar slightly.

After the teeth have completed their growth into the mouth they will continue to grow slowly through a process called passive eruption. This is seen most prominently following an extraction. When a tooth no longer has anything to bite against, the tooth and it's supporting structures, will begin to grow. On a younger person this process occurs fairly rapidly and on older people it may move so slowly that it may not be detectable. This can lead to restorative challenges. The time may come when you want to replace a missing tooth with an implant or a bridge only to discover that the opposing teeth have grown down so much that there is no longer space for the proposed restoration. In this case you would need to either orthodontically move the opposing tooth back into its place or maybe cut down and crown the opposing tooth and thus reduce its height.

4:2 Proximal ContactsThe way that the teeth bite together after they finish eruption into the oral cavity is called occlusion. In studying occlusion we identify different ways in which the teeth fit together and group these by type.

The first contacts that a tooth usually establishes are those where it touches teeth on either side. These are called proximal contacts. When all of the teeth are positioned side by side they form an arch. The tooth arch, like a stone arch, has each member of the arch depending on the other members to maintain the shape and integrity of the arch. When a tooth is lost for an extended period of time the other teeth tend to collapse into the place left by the extracted tooth. The majority of this movement is towards the midline. This process is therefore called mesial drift. Sometimes the tooth drifts forward uniformly, especially if the tooth were lost at an early age. More often the tooth tips forward at an angle to fill the space. This makes for a less than ideal occlusion.Where the teeth touch one another on the sides they form small triangular spaces around the contact point called embrasures. These are formed facial, lingual, incisal/occlusal, and gingival to the proximal contact. Each of these spaces is named by what it is nearest. The triangular space nearest the facial surface is called the facial embrasure and so forth.

In the anterior the facial embrasures are small and the linguals are quite large. Also the incisal embrasures are very small and the gingival embrasures are large. As you move towards the posterior the size and shape of these embrasures become more equal.

4:3 Occlusion and OrthodonticsThe second way that teeth contact is with opposing teeth. Normally the upper arch has a slightly greater width than the lower arch. This means, that in a normal occlusion, the upper teeth fit slightly outside the lower teeth and overlap them. On occasion the upper arch is smaller that the lower arch. This leads to the upper teeth being positioned inside the lower ones. This condition is referred to as a crossbite. If the upper arch is only slightly smaller than normal, the upper teeth will be on the inside of the lower teeth on one side only. This is called a unilateral crossbite. If the upper arch is significantly smaller than it should be, then the maxillary teeth on both sides will be inside the lower arch. This would then be called a bilateral crossbite. You can also have a single tooth positioned on the outside; this is called a single tooth crossbite Because the upper anterior teeth are wider than the lower anterior teeth the whole upper dentition is shifted distal in comparison to the lower dentition. This means that the upper and lower molars are not going to line up exactly with one another. In a normal occlusion the upper molar is shifted posterior about 1/2 of a tooth width as it relates to the lower molar. This places the MB cusp of the upper first molar in the middle of the lower molar or centered on the facial groove. This type of bite is called a class I bite or class I molar position. If there is a shift which causes the lower molar to be shifted back farther than normal, in relation to the upper molar, then the bite is called class II. If the lower molar is shifted more mesially than normal then the bite is called a class III bite. These are the three bite types which describe the forward growth of the mandible and maxilla. The prevalence of these three bite types vary among different races. In the Caucasian population 85% of people have a class I bite, 14% have a class II bite and less than 1% have a class III bite.

Class I bites dont usually require orthodontic movement of the upper or lower jaw. Class II and Class III orthodontic cases require the upper, the lower, or both jaws to be moved. This can be done by exerting force in various ways. These might include rubber bands, various pushing appliances like a Herbst appliance, or through external forces like headgear. Sometimes the problem is so severe that surgery is indicated. This can be done by detaching the upper or lower jaw, moving the jaw to the desired position and then screwing the jaw back together.

In a person with a class I bite the upper front teeth will usually be about 2 mm forward of the lower anterior teeth. This is called the horizontal overlap. This is usually measured in millimeters. In a class II bite this distance increases and in a class III bite the measurement may be a negative number.

Another parameter that we look at is the amount of lower front teeth that are covered by upper front teeth when the patient is biting. This is called the vertical overlap. In a normal bite the upper anterior teeth will cover about 2-3 mm of the lower teeth (about 30%). In certain people the lower teeth bite much deeper than normal so that the upper anterior teeth nearly cover the lower teeth. This is called a deep bite. In other people the teeth don't overlap at all. There may even be a gap in the front when the posterior teeth are in occlusion. This is called an anterior open bite. Anterior open bites are often caused by thumb sucking or by an abnormal swallowing pattern. When a person sticks their tongue out between their front teeth during swallowing it is called an anterior tongue thrust. Because the teeth encounter the tongue they will cease to erupt, causing the open bite.

Another orthodontic problem often encountered is crowding. This occurs when the total arch length is not great enough to allow all the teeth to line up side by side. When this happens the teeth may twist or come in behind or in front of the others. The most common place to find crowding is in the lower anterior. There are two ways to correct an occlusion that does not have enough space. The conventional way is to extract several of the permanent teeth, usually the first premolars, and use the extra space gained to line up the remaining teeth. Other treatments may include expanding the arch or pushing all of the teeth toward the distal to gain the necessary space. Determining which treatment is best is done case by case. Treatment, like Invisalign, may be used to resolve slight crowding. When using Invisalign it is common to polish between the teeth to gain space and allow the teeth to more easily slide against one another better.A patient can have more than one orthodontic problem at a time. For example they might be Class II and crowded. The most difficult part of an orthodontic case is correctly diagnosing and treatment planning. The actual execution of the correction is time consuming but not as difficult.Whenever a force is exerted on a tooth, that tooth will begin to move through the bone. Bone is a dynamic, living substance that can be moved and formed. When a pushing force is exerted on a tooth, bone will begin to be formed on the side of the tooth closest to the force and bone will be dissolved on the side of the tooth furthest away from the force. This allows the tooth to move through the bone. Bone formation is carried out by cells called osteoblasts, and bone degradation is performed by cells called osteoclasts. The periodontal ligament is very important in this process. It is the stretching and compressing of the periodontal ligament that communicates forces to the bone. Some times, due to trauma, a tooth will become fused directly to the bone. The PDL no longer exists. When this happens it is called ankylosis. An ankylosed tooth cannot be moved orthodontically.

PAGE 24