::for.education

Proper Timing with class 2 elastics

by Dr. R.G. "Wick" Alexander

As in most things in life, “timing is everything.” Diagnostically, class II malocclusions can be classified as either skeletal or dental. In the Alexander Discipline, treatment of a skeletal class II case begins with headgear wear 8 – 10 hours each night. Almost every such case, however, class two elastics are worn toward the end of treatment to obtain final occlusion. If the case does not need orthopedic correction, this dental class II can be treated only with class two elastics.

Proper timing for elastic wear is critical during orthodontic treatment in the Alexander Discipline. It is very important that the final archwires in both arches, 17x25 stainless steel in an .018 slot, are fully engaged, tied back and have been in the mouth at least one month before class II elastics are initiated. At this point the minus 5 degree torque in the incisor brackets has been established. Also, the minus six degree tip in the lower first molars has uprighted these teeth allowing for additional anchorage. Therefore the posterior force of the class II elastics on the maxillary teeth and the anterior force on the mandibular teeth will not result in unwanted maxillary anterior retraction or mandibular anterior flaring.

It is also important to attach elastics to the appropriate teeth. Ideally, it is beneficial to maximize the horizontal component and minimize the vertical component while wearing these elastics. Most orthodontists attach class 2 elastics from the maxillary cuspids to the mandibular first molars. (Fig. 1) Consider the force vector. The vertical force component is significant. However in our approach, a more horizontal force component is used to achieve the sagital movement. This effect is obtained by attaching class 2 elastics to mandibular second molar brackets and to ball hooks attached to maxillary lateral incisor brackets. (Fig. 2) This more horizontal force vector reduces the bite opening tendencies of traditional Class II mechanics. If elastics attached in this manner are used for only a few months near the end of treatment, when heavier arch wires are in place and mandibular anchorage is preserved, little or no loss of torque or occlusal plane tipping will occur.

In the Alexander Discipline, class 2 elastics are not employed to open the bite. In a case with an extreme deep bite, the overbite will be corrected with reverse curve in the lower archwire and box elastics to the bicuspids. After the lower arch has leveled and the bite has opened, class 2 elastics will then be employed.

Occasionally, second molars are not fully erupted, yet elastics are required. In these instances, the elastics must be attached to the first molars. However, cases are not finished as a rule until the mandibular second molars have erupted and positioned properly.

In addition to skeletal class II correction, class 2 elastics are commonly used to correct the difference between centric occlusion (CO) and centric relation (CR). If early in treatment the patient has worn the extra oral appliance properly and has a good growth response, Class II elastics may not be required. Most often, however during the final stages of treatment, a slight CO/CR discrepancy remains.

There are a number of definitions of CO and CR. One version is as follows: Centric Occlusion- The occlusion of the mandibular and maxillary dentition when the patient bites down normally. Centric Relation- the occlusion created when the mandibular condyle is located in ideal position within the glenoid fossa.

Different clinicians may debate the location of this ideal position. In my thinking, the ideal position has the condyle being superiorly positioned while being centered within the fossa in the A-P dimension.

Most Class II patients exhibit retrusive condylar positions prior to treatment. The class 2 elastics will tend to move the condyle mesially within the fossa. A typical Class II patient will wear Class 2 elastics in the final stages of treatment for one to six months. Following this period, if manipulation of the mandible fails to reveal any centric slide, the patient will wear elastics at night only. One month later, if the slide has not reappeared, “finishing” elastics will be initiated.

What factors causes a Class II dental relation and skeletal relation to be corrected with class 2 elastics? In my opinion, three possibilities could be promoted as causal elements:

1. If elastics are worn conscientiously during a period of intense skeletal growth, orthopedic stimulus is supplied by the sagital force vector of the Class 2 elastics.
2. If torque control has not been established, the class 2 elastics will tip maxillary anterior teeth lingually and mandibular teeth labially, helping to affect the correction. In an unstabilized lower arch, the molars could become extruded, causing the occlusal plane to be tipped to correct the Class II relationship.
3. Assuming stabilizing wires are in place in both arches, it is then feasible to consider that the entire maxillary dentition can move distally with or through the alveolar trough while the entire mandibular dentition is moved anteriorly through or with the bone. This explanation applies to adult patients as well as adolescents. Dental torques are not modified to a great extent during this treatment, nor does the occlusal plane tip appreciably. Little individualized tooth movement occurs, since the finishing archwire is in place and is tied back. Mandibular molar and maxillary anterior extrusion do not occur because of the horizontal force vectors, full size archwires, and short period of elastic wear.

Occasionally, in an extraction case, proper space closure requires special force application. If it is not desired to retract mandibular incisors, and mandibular molars must be in advanced considerably to close the mandibular spaces, class 2 elastics may be added to help advance the mandibular molars in order to “burn anchorage”. This is one instance in which class 2 elastics are used prior to the placement of finishing archwires. It is used when a 16x22 stainless steel with closing loops archwire is activated. However, the class 2 elastics must be used quite judiciously to prevent loss of torque, tipping, and other problems. Our approach is to wear the elastics for the first 72 hours after the archwire has been activated, then at night only. This approach will allow the majority of the space to be closed by the forward movement of the molars, thus slipping anchorage.

Typically, class 2 elastics are sized in ¼ inch six ounce. Occasionally, 3/16 inch, six ounce elastics are later worn if needed. The patient is requested to wear them 24 hours per day. Eating in the elastics offers faster results. Elastics should be changed every 12 hours.

As with most forces used in orthodontics, class 2 elastics must be worn consistently. Intermittent wear will not accomplish success. As always, Effort = Results.