The purpose of this study was to evaluate the effects of 2 extraction patterns on incisor and molar movements in patients with growing Class II Division 1.
The sample included 54 patients 10-17 years of age treated by 2 private practice orthodontists using Tweed directional force mechanics, 4 premolar extractions, J-hook headgears, and Class II elastics or Saif springs. The sample was divided on the basis of having maxillary and mandibular first premolars (4/4) or maxillary first and mandibular second premolars (4/5) extracted. Each group included 27 patients. Treatment lasted 2.8 ± 0.60 years and 2.6 ± 0.54 years for the 4/4 and 4/5 groups, respectively. Pretreatment (T1) and posttreatment lateral cephalograms and dental casts were evaluated. Cranial base, mandibular, and maxillary superimpositions were performed to quantify tooth movements and displacements.
There were no statistically significant T1 between-group differences in crowding or in the SNA, SNB, ANB, and MPA angles. Analyses of covariance, controlling for statistically significant ( P <0.05) differences in T1 mandibular incisor position, showed that mandibular first premolars extractions produced greater (1.6 mm) mandibular incisor retraction than second premolar extractions. The mandibular first molars were protracted significantly more (0.7 mm) after the second premolar than the first premolar extractions. Within-group changes of the MPA, between-group differences in the changes in MPA, and the amount of vertical eruption of the maxillary and mandibular molars were not significantly different between the 2 extraction patterns.
Extraction of mandibular second premolars enhances Class II molar correction, with greater mesial first molar movement and less distal incisor movement. Neither extraction pattern has an effect on the MPA or the vertical dimension (ie, there was no “wedge effect”).
Extraction of mandibular second premolars enhances Class II molar correction.
Mandibular second premolar extraction produces less distal incisor movement.
Both 4/4 and 4/5 extractions do not affect MPA or vertical dimension.
Class II malocclusion is a common problem, affecting approximately 15% of the U.S. population. This form of malocclusion can be treated in a variety of ways, including headgears, functional appliances, surgery, and extractions. Advances in appliance and anchorage systems and our understanding of growth and development have made it possible to better control tooth movements in all 3 dimensions, using a variety of treatment modalities. To this end, orthodontists often have to choose between extracting maxillary and mandibular first premolars (4/4) or maxillary first and mandibular second premolars (4/5) when treating patients with Class II Division 1. Although their effects on the vertical dimension have been established, the effects of these 2 extraction patterns on incisor and molar movements remain controversial.
It was originally thought that the vertical dimension could be better controlled by extracting the mandibular second premolar than the first premolars. This idea was based on the “wedge effect,” which suggested that the mandibular first molars moved more mesially and decreased the vertical dimension by “closing down the wedge.” However, no differences in vertical facial dimensions have been reported between the 4/4 and 4/5 extraction patterns. The only study evaluating the “wedge effect” in Class II Division I patients found no differences between the 2 extraction patterns in either vertical facial growth or in changes of the mandibular plane angle.
Orthodontists believe that extraction of mandibular first premolars produces more incisor retraction, whereas second mandibular premolars extractions result in more mesial mandibular first molar movement. However, these notions are based largely on clinical observations. For example, Campbell was among the first to suggest that the extraction of mandibular second premolars aids in the Class II correction because it allows more mesial movement of the mandibular first molars. Understanding incisor and molar movements is important because it is the basis for the shift from 4/4 to 4/5 extraction patterns when treating patients with Class II Division I malocclusion. There currently is no solid evidence regarding incisor and molar movements when first or second mandibular premolars are extracted.
The literature evaluating incisor movement associated with these 2 extraction patterns is controversial, with most studies reporting no differences. , , , Steyn et al found that the mandibular incisors of patients with Class I malocclusion were retracted significantly more after 4/4 than 4/5 extractions, but their study was subject to potential selection bias and used reference lines (RLs) to measure incisor movement that are affected by growth and treatment. One of the studies reporting no significant differences between the 2 extraction patterns did not includee mandibular incisor retraction as a part of the treatment plan, whereas others included patients with both maxillary first and second premolars extractions. ,
The effects of 4/4 and 4/5 extractions on molar movements are also controversial. Al-Nimri found significantly more molar mesial movement when the mandibular second premolars of patients with Class II Division I malocclusion were extracted. In contrast, Shearn and Woods, who quantified molar movement of patients with Class I and Class II malocclusions by subtracting the change in arch length from the amount of incisor retraction, found no significant difference between extraction patterns. Wang et al also found no significant difference in molar movements based on the cranial base superimposition.
These inconsistencies are probably due to the study designs. Most of the studies comparing 4/4 and 4/5 extraction patterns did not control for crowding, the vertical skeletal pattern, the anteroposterior skeletal pattern, or molar occlusion. In addition, the RLs used to evaluate tooth movement were often affected by growth and tooth movement.
The present study aimed to evaluate mandibular incisor and molar movements in growing patients with Class II Division I malocclusion who either had 4/4 or 4/5 extraction patterns. Efforts were made to ensure that the samples were similar before treatment, with both groups exhibiting mild to no crowding, a normal range of vertical growth patterns, and the same anteroposterior skeletal and dental relationship.
Material and methods
The sample includes 54 growing extraction patients treated by 2 private practice orthodontists. The treatment mechanics used included Tweed directional force mechanics, headgears, and Class II elastics or Saif springs. Archwires and sometimes headgears were used to derotate the maxillary first molars. Sliding and frictionless (closing loops) mechanics were used for space closure on a 19 × 25-in stainless steel archwire. High-pull J-hook headgears were used to retract maxillary anterior teeth. Class II elastics and Saif springs were used as needed. All of the incisors were upright over the basal bone at the end of treatment.
To be selected, the patients had to meet the following inclusion criteria: (1) ANB >4°; (2) Class II Division I malocclusion with ≥ half-step molar and canine relationships bilaterally; (3) age between 10 and 17 years; (4) treatment with extraction of maxillary first premolars and either mandibular first or second premolars; (5) permanent dentition (excluding second and third molars); (6) successful treatment outcome defined as Class I molar and canine relationships, along with a normal overjet and overbite; and (7) pretreatment (T1) and posttreatment (T2) lateral cephalograms. Patients were excluded if they (1) were treated with functional appliances, quad helix, or rapid palatal expansion before or during fixed appliance therapy; or (2) had congenital anomalies, significant facial asymmetries, or congenitally missing teeth. This study was approved by the Texas A&M University College of Dentistry Institutional Review Board (no. IRB2016-0479).
The sample was divided on the basis of the 2 extraction patterns: maxillary and mandibular first premolars (4/4) or maxillary first and mandibular second premolars (4/5). Practitioner 1 provided 78% of the 4/4 patients and 88% of the 4/5 patients. The 2 groups were matched on the basis of sex, T1 ANB, and T1 MPA. The 4/4 group included 27 patients (15 males, 12 females) treated over 2.8 ± 0.6 years. Their average T1 and T2 ages were 12.9 ± 1.0 years and 15.7 ± 1.0 years, respectively. The 4/5 group consisted of 27 patients (10 males, 17 females) treated over 2.6 ± 0.5 years. Their average T1 age was 12.3 ± 1.3 years, with an average T2 age of 14.9 ± 1.4 years.
Thirteen standard landmarks ( Table I ) were identified and digitized by 1 investigator using Dolphin Imaging and Management Solutions (version 11.8, build 24; Dolphin Imaging, Chatsworth, Calif). Four traditional measurements were used to quantify the anteroposterior (SNA, SNB, ANB) and vertical (S-N/Go-Me) skeletal changes of the maxilla and mandible. Mandibular incisor position was quantified relative to the mandibular plane (IMPA: L1 t -L1 a /Go-Me), the N-B plane (L1 t -L1 a /N-B), and the A-Pg plane (L1 t -L1 a /A-Pg).
|S||Sella: the center of the pituitary fossa|
|N||Nasion: the most anterior point on the frontonasal suture|
|Me||Menton: the most inferior point on the symphyseal outline of the mandible|
|Go||Gonion: the midpoint of the angle of the mandible|
|A||A point: the most posterior midline point in the concavity between ANS and prosthion|
|B||B point: the most posterior midline point in the concavity between infradentale and pogonion|
|U1 t||The incisal cusp tip of the most facial maxillary incisor|
|U6 o||The mesial buccal cusp tip of the maxillary first molar|
|U6 m||The mesial contact point of the maxillary first molar|
|L1 t||The incisal cusp tip of the most facial mandibular incisor|
|L1 a||The apex of the mandibular incisor root|
|L6 o||The mesial buccal cusp tip of the mandibular first molar|
|L6 m||The mesial contact point of the mandibular first molar|
Cranial base, maxillary, and mandibular superimpositions of the T1 and T2 cephalograms were performed for each subject using naturally stable structures. , To quantify the horizontal and vertical treatment, a rectangular coordinate system was used. A horizontal RL was constructed on the T1 cephalometric tracing, registering on T1 sella and orienting 7° below the SN to approximate natural head position ( Fig 1 , A ). The horizontal and vertical changes of the teeth were measured parallel and perpendicular to RL, respectively ( Fig 1 , B ). Changes in molar position were measured at the mesial buccal cusp tip and the mesial contact point. Horizontal and vertical incisor movements were measured at the cusp tip. Displacements were defined as differences between the changes measured on the cranial base and regional (maxillary and mandibular) superimpositions. Maxillary and mandibular displacement was based on the average displacements of the 3 incisor and molar landmarks. Anterior and superior changes were recorded as positive; posterior and inferior measurements were recorded as negative.
All dental casts were scanned using Ortho Insight 3D scanner (version 7.0; MotionView Software, Chattanooga, Tenn) and imported into Dolphin and Ortho Insight 3D software. Tooth-size to arch-length discrepancy (TSALD) was determined by subtracting the space required (size of maxillary and mandibular teeth) from the space available (arch perimeter). The mesiodistal diameters of the maxillary and mandibular teeth were measured from their distal to mesial anatomic contact points. Arch perimeters were measured by placing a curve over the arch extending from the distal contact point of the right first molar to the distal contact point of the left first molar. , The curve was centered over the contact points of the posterior dentition and where the incisors would be upright over basal bone. The Ortho Insight 3D MotionView software calculated TSALD using the sum of the tooth diameters and arch perimeter, with a negative TSALD indicating crowding and a positive TSALD indicating spacing.
The skewness and kurtosis statistics showed that the distributions of the measurements were all normal. Means and standard deviations were used to describe the 2 groups. An independent sample t test was used to evaluate between-group differences. Paired t tests were used to evaluate within-group differences. Analysis of covariance was used to control for T1 between-group differences. SPSS statistical software (version 23; IBM Armonk, NY) was used to perform the statistical analysis. The significant level was set at P <0.05.
There were no statistically significant T1 between-group differences for SNA, SNB, ANB, MPA, maxillary TSALD, and mandibular TSALD ( Table II ). Both treatment groups exhibited Class II maxillomandibular relationships (ANB, 5.52°) and normodivergent (MPA, 34.5°) skeletal patterns. There were statistically significant ( P <0.05) differences in IMPA, L1:NB, and L1:Apg at T1.
|Measurements||4/4 (n = 27)||4/5 (n = 27)||Mean difference||P value|
The SNA, SNB, ANB, and MPA angles showed no statistically significant between-group treatment differences in the treatment changes that occurred ( Table III ). Both groups showed statistically significant improvements in the ANB angle, primarily because of decreases in the SNA angle. There were no statistically significant between-group differences for SNA, SNB, ANB, or MPA angles at T2 ( Table IV ).
|T1 − T2 changes||4/4 (n = 27)||4/5 (n = 27)||Mean difference||P value|