Third molars

This controversial subject has been debated ad infinitum over the years. Personally, I do not believe that third molar eruption is responsible for mandibular anterior relapse, but the potential impact cannot be ignored.

Another equally important factor relating to third molars is future nonorthodontic considerations. If unerupted or partially erupted third molars are left in the mouth, serious consequences can develop. In postorthodontic years, partially erupted third molars can become infected. Inflammation of the gingival tissue can create discomfort for the patient. Tumors can develop around impacted third molars. Therefore, in my mind, orthodontic treatment is not complete until the third molar issue has been resolved.

Over the years, a number of methods have been proposed to determine in advance whether third molars will need extraction. However, I prefer to wait until the patient is at least 16 years old to make the decision. By this time, the posterior segments of the dentition are nearing final development.

My procedure for determining the need for third molar extraction is as follows: First, the amount of space between the distal of the mandibular second molar and the medial plane of the ramus is calculated. Then the mesio-distal crown length is measured. If the crown is too long for the existing space, extraction is usually required. Next, the angulation of the unerupted third molars is observed in the panoramic radiograph. If the third molars are in good eruptive position in a borderline case, they will be allowed to erupt. My philosophy is that they can always be removed later. Conversely, if in a borderline case the third molars are erupting at an inclined angle in the direction of the second molars, they will be removed.

The panoramic radiograph is also checked to see whether the third molars are well shaped. Occasionally, third molars have the appearance of supernumerary teeth and should be removed on that basis alone.

Mandibular third molars can be the key to the extraction decision. If room is available for the mandibular third molars, the maxillary third molars should erupt normally. However, if grounds for removal of one third molar are present, all four will be removed. Rarely are third molars removed from only one arch or only one side of an arch. The functional occlusion seems better in an all-or-nothing situation.

Because more than 80% of our cases are treated with nonextraction (premolar), the majority of the patients have third molars removed. Only in a few instances is treatment completed with all 32 teeth remaining. Premolar extraction does reduce the incidence of third molar extractions. When premolars are removed, no matter how much anchorage is attempted, the first and second molars are going to drift forward slightly, creating more space for third molars. The majority of our cases in which third molars are retained are premolar extraction cases.

Dental memory

Rotated teeth, when corrected, seem to have a memory. They have a tendency to return to their original positions. The transseptal fibers are often the specific cause of dental relapse. These fibers are stretched like a rubber band during treatment. After appliances are removed, the fibers tend to contract, returning the periodontal membrane and hence the dentition toward its original configuration.

My experience with adolescent patients in typically crowded pretreatment conditions is that, as these teeth are held in place with retainers after treatment, the fibers seem to reshape to conform to the new dental configuration. Therefore, posttreatment and postretention relapse are minimized. An exception to this pattern is that when maxillary lateral incisors are positioned palatally prior to treatment, a significant tendency for relapse remains.

Adult patients cannot expect significant reconfiguration of the transseptal fibers during active treatment and retention. Therefore, on adult patients, the circumferential supracrestal fiberotomy (CSF) is a routine procedure. This surgery is performed at least 6 weeks prior to bond removal. As a result of the surgery, the transseptal fiber memory is reduced, preventing significant relapse.

Mandibular intercanine width expansion

If the mandibular intercanine width is expanded beyond its pretreatment width, trouble will usually ensue. Any time this width is expanded, the clinician must accept the fact that some relapse may occur.

A study by Dr Gayle Glenn at Baylor University in which she analyzed 28 nonextraction cases randomly selected from our practice revealed some interesting findings.¹ The patients (14 Class I and 14 Class II, Division 1) averaged 12 to 17 years old pretreatment, 14 to 19 years old posttreatment, and 26 to 27 years old postretention. The mean postretention time was 7 years 11 months. Because these patients were treated in the late 1960s and early 1970s, all cases were fully banded with no pretorqued brackets. The philosophy of treatment, however, was basically the same as that practiced today. Intercanine width was expanded by an average of 0.6 mm in the sample during treatment. The average intercanine width decreased 1 mm postretention. Therefore, the postretention decrease was greater than the increase obtained during treatment! These slightly expanded cases ultimately experienced a slight contraction from the original arch form.

Mandibular intermolar width expansion

In her study, Glenn found that intermolar width had been increased and that this expansion was maintained posttreatment. ¹ Clinically, this can be called buccal uprighting. Other studies have confirmed this.²

Reducing overjet and overbite

Long-term reduction of overjet in Glenn’s study was 3.3 mm.¹ The original overbite averaged 4.6 mm and was corrected to 2.7 mm. No significant change took place postretention.

Interincisal angle

The interincisal angle averaged 129.7 degrees pretreatment. After treatment, it had changed to 132 degrees.¹ There were no significant changes postretention.

Irregularity index

Pretreatment incisor irregularity in this study ranged from 1 mm to 6 mm.¹ Twenty-four cases exhibited minimal incisor irregularity postretention. Three had moderate relapse and one had severe relapse (6.5 mm). Why this case failed so badly is unknown. It is that one case that keeps the orthodontist humble.

Mandibular incisor position

The Tweed concept is that mandibular incisors should be upright over basal bone. I believe wholeheartedly in this philosophy. Many orthodontists today do not pay sufficient attention to torque control of the mandibular incisors. When the mandibular incisors are indiscriminately tipped or advanced, the clinician is creating a danger for potential relapse, especially for a tight-lipped person. The health of the dentition and gingival tissue are also affected adversely when the mandibular incisors are not positioned over basal bone.

On the opposite end of the spectrum, some orthodontists retract the teeth lingually until they are too far upright. This retraction creates an esthetic problem with the facial profile. The resulting dished-in appearance is unacceptable in my opinion. The–5-degree mandibular anterior torque built into the long-term stability brackets allows a “happy medium” to be established quite easily in most cases.

From the stability standpoint, if mandibular incisors are placed too far lingually, the teeth do not tend to advance spontaneously. The balance between the lip pressure and tongue pressure is not violated to a large enough degree to cause a great amount of relapse. In this case, however, the interincisal angle may be too obtuse, causing a tendency for relapse of the overbite as well as a concave soft tissue profile.

If the teeth were placed in the appropriate positions during active treatment, interproximal reduction was properly performed, and third molars were resolved, then the chance of the result remaining stable after retainer removal is excellent.

In the Glenn study, the incisor mandibular plane angle (IMPA) averaged 94.5 degrees pretreatment.¹ IMPA averaged 94.8 degrees after treatment. Postretention 8 years later showed IMPA at 95.1 degrees.

Mandible

The mandible can be influenced during growth, but as orthodontists, we cannot control growth nonsurgically. All we can do is watch it grow. In Class III skeletal patterns, excessive mandibular growth can create minor to severe posttreatment relapse. Orthodontically, because we cannot grow or inhibit growth of the mandible, we must adapt to the amount of mandibular growth and work with or around it.

Maxilla

Orthopedically, the maxilla can be considered our best friend. A facebow can help expand the maxilla transversely, inhibit sagittal forward growth, and help control vertical growth; a face mask can pull the maxilla forward by 2 to 3 mm.

The maxilla can be altered in all three planes of space: transverse, vertical, and sagittal. However, these alterations may or may not be stable.

Class II relapse

When a Class II skeletal pattern has been successfully corrected orthopedically, Class II relapse will only result if condylar resorption has occurred.

Why, then, does this seem to be a common occurrence in Class II treatment? The logical answer is that the patient has worn an appliance that protrudes and holds the mandible forward over a long period of time. This allows the muscles of mastication to adapt to this forward position, giving a false bite when the appliance is removed. With time and mandibular manipulation, the muscles will relax and the mandible will move back with the condyles into their fossae, thus the Class II “relapse.” The reality is that the problem was never corrected in the first place.

Open bite

An interesting question is whether open bites are inherited or acquired. How often do you see a vertical growth pattern in patients whose permanent first molars have not erupted? Unless the patient is a thumb sucker, this phenomenon does not seem to present itself often. This is strictly my observation.

However, as the permanent first molars erupt, if the patient does not swallow properly, the first molars can “super” erupt, creating a vertical skeletal pattern and possibly an anterior open bite.

Is environment a factor? Pollution from the air we breathe? Artificial flavoring and coloring in the food we eat? These could cause allergies and nasal congestion, creating the need to breathe through the mouth. This can create reduced occlusal forces, allowing the molars to continue erupting and creating a high-angle open bite malocclusion.

Clinically, I believe the greatest factor in the development of high-angle, open bite occlusions is the lack of proper muscular function. Inadequate occlusal forces can allow teeth to drift vertically. Could this have a negative effect on growth, causing vertical growth and open bites? What would happen if we taught our patients to get their muscles of mastication in shape?

Doug Thompson placed dental students on a training regimen of clenching isometrically on a 1-mm thickness soft splint.³ Prescriptions consisted of five 1-minute exercise periods per day in which the students were supposed to squeeze their teeth together, hold for 15 seconds, release, and repeat four times. Maximum bite force changes were measured at 3-week intervals. Over a period of 6 weeks, maximum bite forces increased 18% more in the experimental group, and resistance to fatigue was significantly increased clinically. Does it not make sense then that strengthening the muscles of mastication in open bite patients could have a positive effect on the open bite problem?

Forces at work

When I think of Newton’s third law—for every action there is an equal and opposite reaction—I think mostly of horizontal forces, such as those with a facebow. Concerning stability, however, vertical forces have a significant influence on final occlusion.

Archwires and elastics create extrusive forces that can produce vertical increases. In low-angle deep bite cases, this is good treatment. In high-angle open bite cases, this can be a disaster.

Intrusive forces can be generated by mini-implants, a high-pull facebow, or occlusal muscular forces such as squeezing into an Invisalign tray or chewing gum.

Mouth breathing

Mouth breathing results when a patient has an airway and/ or nasal blockage, forcing him or her to breathe through the mouth. Protruding anterior teeth can also force mouth breathing because they do not allow the lips to touch when relaxed.

One must then ask: Which comes first, the breathing problem or the dental/skeletal problem? Most would suggest that muscles are stronger than bones in the long run, so the muscles must be retrained i/>