Premolar Extraction Affects Mandibular Kinematics

Abstract

Objectives: The practice of premolar extraction in orthodontics is controversial for its potential detrimental effects on the stomatognathic system. However, the ways in which premolar extraction affects mandibular function are still poorly understood. The purpose of this study was to assess the influence of premolar extraction on mandibular kinematics by evaluating axiographic tracings.

Materials and methods: Forty-five orthodontically treated patients with premolar teeth extraction were compared with 45 paired untreated controls, selected for the absence of malocclusions. Systematic three-dimensional axiographic recordings of the mandibular movements were performed for protrusive-retrusive movements and speech. The transversal deviations and length of the movements were recorded for both sides along with the rotation angle during speech.

Statistical analysis: Differences between the axiographic variables were analyzed via the permutation test and Wilcoxon rank-sum test. Linear regression was performed to test whether axiographic parameters were predictive of group affiliation. Dot plots were used to explore the distribution of each of the axiographic outcomes, and isometric principal component analysis to assess the differences between the cumulative effects of premolar extraction on jaw motion.

Results: The mandibular lateral translation in protrusion-retrusion and speech, the amount of rotation as well as the length of mandibular movements during speech were significantly higher in the treated subjects than in the controls, while retral stability did not differ. The linear regression yielded significant results for the mandibular lateral translation in protrusion-retrusion. The isometric principal component analysis showed higher values of the axiographic variables for 11 out of 45 individuals in the study sample compared with the control group.

Conclusions: Premolar extraction altered mandibular kinematics in at least 25% of the cases within our sample, and the transversal discrepancy between protrusive and retrusive tracings was even predictive of group affiliation. These results support the notion that the routine practice of premolar extraction as part of the orthodontic treatment should be discouraged. It is compelling to perform further studies to assess whether a disrupted kinematics of the mandible is associated to temporomandibular disorders.

Fig. 1

Photo of double electronic stylus in place, against the flag, kept in constant contact by magnetic tips, ready to measure the translation, rotation, and transversal components of the movement in each joint.

Fig. 2

Protrusion/retrusion movement showing ( a ) normal movement and ( b ) mandibular lateral deviation on the Y -axis (∆YMLT) and ( c ) maximum mandibular lateral translation measurement on the axiographic tracing on the right side (∆YR).

Fig. 3

Axiographic tracings. ( a ) Diagram showing the maximum mandibular lateral translation (SP-∆Y) and the maximum length on the sagittal plane (SP-3D) on the right and left sides during speech. ( b ) Diagram showing retral stability as the distance in mm from the reference position (a red arrow “a”) and the end of the incursive movement ( red arrow “b”) during opening/closing movements.

Fig. 4

Dot plots (using crosses as symbols) showing the ranges of distribution of the various variable for the control group (P4, upper rows ) and for the study group (with premolar extraction, P4ex; lower rows ). ∆Y, distance from the reference position to the maximum point of lateral translation in protrusion/retrusion ; R-ST, distance between reference position and the end point of incursion in open/close movement; SP-3D, length of the movements performed during speech on the sagittal plane; SP-G, maximum rotation in degrees during speech; SP-∆Y, distance from reference position to the maximum point of mandibular lateral translation during speech.

Fig. 5

Isometric principal component analysis. Upper row : summary scores for the control group (P4); lower row : summary scores for the study group (with premolar extraction, P4ex).