Effects of human relaxin on orthodontic tooth movement and periodontal ligaments in rats

Abstract

Introduction: The rate-limiting step in orthodontic treatment is often the rapidity with which teeth move. Using biological agents to modify the rate of tooth movement has been shown to be effective in animals. Relaxin is a hormone present in both males and females. Its main action is to increase the turnover of fibrous connective tissues. Thus, relaxin might increase the amount and rate of tooth movement through its effect on the periodontal ligament (PDL). The purpose of this study was to measure the effect of relaxin on orthodontic tooth movement and PDL structures.

Methods: Bilateral orthodontic appliances designed to tip maxillary molars mesially with a force of 40 cN were placed in 96 rats. At day 0, the animals were randomized to either relaxin or vehicle treatment. Twelve rats in each group were killed at 2, 4, 7, and 9 days after appliance activation. Cephalograms were taken at appliance placement and when the rats were killed. Tooth movement was measured cephalometrically in relation to palatal implants. Fractal analysis and visual analog scale assessments were used to evaluate the effect of relaxin on PDL fiber organization at the tension sites in histologic sections. The in-vitro testing for PDL mechanical strength and tooth mobility was performed by using tissue from an additional 20 rats that had previously received the same relaxin or vehicle treatments for 1 or 3 days (n = 5).

Results: Both groups had statistically significant tooth movement as functions of time. However, relaxin did not stimulate significantly greater or more rapid tooth movement. Fractal and visual analog scale analyses implied that relaxin reduced PDL fiber organization. In-vitro mechanical testing and tooth mobility assessments indicated that the PDL of the mandibular incisors in the relaxin-treated rats had reduced yield load, strain, and stiffness. Moreover, the range of tooth mobility of the maxillary first molars increased to 130% to 170%, over vehicle-treated rats at day 1.

Conclusions: Human relaxin does not accelerate orthodontic tooth movement in rats; it can reduce the level of PDL organization, reduce PDL mechanical strength, and increase tooth mobility at early time points.

Figures

Fig 1

Cepahalometric analysis for measurement of tooth movement. UP and LP, horizontal pins in roots of upper and lower incisors; IMP, sagittal palatal implants; A, most lateral tip of left UP; B, center of left bonded ligature; C, most posterior tip of the left IMP; D, intersection on line AB created by line drawn from point C perpendicular to line AC. Length B-D represents sagittal distance changes caused by forward tooth movement of molar. Line AB indicates molar-incisor vector (MIV).

Fig 2

Samples of hematoxylin and eosin-stained histologic sections. Cervical region and tension side were chosen for analysis; 5 × 5 mm ROI was cropped from this area. A, Sample from vehicle-treated rat; B, sample from relaxin-treated rat. RT, tooth root; AB, alveolar bone; PDL, periodontal ligament. Small boxes indicate ROI for fractal analysis.

Fig 3

PDL mechanical strength testing apparatus. A, Location of test section; B, resin-embedded testing block; C, loading on testing machine; D, strain-stress curve created from testing. TS, testing section; TR, root; AB, alveolar bone; TB, testing block; APP, custom-made loading apparatus. Definitions in stress-strain curve: B-M, steepest slope, indicating Young's modulus (stiffness); Y, yield point: zero slope, indicating transition from elastic to plastic deformation under compressive loading; F, failure point: <20% drop from maximal stress.

Fig 4

Tooth mobility testing apparatus: A, embedded molars and device; B, setup; C, force gauge; D, raw recording indicating range of molar palate-buccal mobility produced by 2 kg horizontal load. DVRT, differential variance reluctance transducer; CG, custom-made grip; DC, desk clamp; TB, testing block; P, palatal side; B, buccal side; PF, pulling force; F, force stop.

Fig 5

Boxplots of measured molar movement at 4 time points. N, sample size of each group at each time point. Upper and lower limits of box represent 75th and 25th percentiles, respectively. Horizontal line in each box represent median. Outliers and extreme values were excluded (see text).

Fig 6

A, Boxplots of fractal values at 4 time points. Top 2 pictures represent typical ROI selected for fractal analysis from relaxin-treated (left) and vehicle-treated (right) samples. B, Boxplots of VAS values at 2 time points. Top 2 pictures represent typical hematoxylin and eosin-stained pictures for visual test from relaxin-treated (left) and vehicle-treated (right) samples.

Fig 7

Boxplots of values of PDL mechanical strength at 2 time points: A, failure load; B, percentage of strain at failure load; C, percentage of strain at peak load; D, Young's modulus (stiffness). All values were lower in relaxin-treated than in vehicle-treated groups.

Fig 8

Boxplots of values of tooth mobility at 2 time points. Significantly larger mobility in relaxin-treated molars was found at day 1.