Malocclusion, Angle Class III Clinical Trial
Official title:
Clinical/Numerical Study of the Effects of Periodontal Ligament Stress Level on the Rate Bodily Tooth Movement
Orthodontic treatment requires application of force systems to individual teeth or groups of
teeth, which results in a cellular response with periodontal ligament (PDL) and alveolar
bone remodeling. The forces applied must be of sufficient magnitude and duration to exceed
the normal physiologic threshold associated with daily oral function. Excessive force levels
will result in areas of tissue necrosis with delayed tooth movement and increased risk of
root resorption. Although orthodontic tooth movement is achieved in a large segment of the
population, the optimum force level has not been defined. The optimum force for tooth
movement depends on individual root geometry as well as biologic characteristics of
surrounding tissue including bone density, periodontal thickness, and fluid dynamics.
Because experimental and clinical techniques are generally limited to known complex force
systems, biomechanical modeling has become a necessity. Such models must be validated with
well-controlled clinical studies that evaluate orthodontic tooth movement over an extended
distance. The ultimate goal would be development of a computer simulation model to predict
tooth movement in the clinical setting.
The primary objective of this study is to test controlled clinical data with a biomechanical
model of the tooth and supporting tissues for distal movement of the human maxillary canine
tooth (of known root geometry) in response to various 3D force systems that produce
different levels of stress in the supporting tissues. Secondary objectives include
evaluation of rate of bodily tooth canine movement with two known compressive stress levels
(13 and 22 kPa), evaluation of three different reference systems to measure rate of tooth
movement, and evaluation of an implant placed in the roof of the mouth (palatal implant) for
orthodontic anchorage in adolescent patients.
The rate of translation (bodily) tooth movement of the maxillary canine tooth will be
significantly greater with 22kPa compared to 13kPa compressive stress applied to the
periodontal ligament, and this difference can be predicted by appropriate
mathematical/numerical models of the tooth and supporting tissues.
The patients will be drawn from those recruited for the Graduate Orthodontic Graduate
Program. Clinical academic staff will examine approximately 500 patients seeking orthodontic
care. Patients are categorized by malocclusion type and placed into a patient pool from
which some cases are selected for treatment based on the program teaching and research
needs. Each year approximately 50% of patients seeking care are ultimately accepted.
Treatment is provided at fees that are approximately 60% of those charged in private
practice.
Patients who meet the selection criteria for this study will be randomly selected from the
University of Alberta Graduate Orthodontic Graduate Clinic patient pool. Upper 1st premolar
teeth will be extracted and full fixed orthodontic appliances (Ormco Orthos prescription)
will be placed. After the initial alignment and leveling phase is completed the retraction
phase will begin. Non-steroidal anti-inflammatory agents will be avoided; only extra
strength Tylenol will be prescribed as necessary for any momentary pain or discomfort.
Maximum posterior anchorage will be attained through a palatal implant (Straumann) according
to the protocol described by Papadakis. The implants will be also used as three-dimensional
movement reference points. Other reference systems to be used include the palatal ruggae and
occlusal templates. The cuspid retraction force delivery mechanisms will consist of
customized activated stainless steel T-loops designed to generate the required forces on the
bracket. Approximate compressive stress (based on approximate root surface area) of 13 kPa
and 22 kPa will be applied on the distal aspect of the cuspids. The design will be a
split-mouth study in which both levels of stress will be applied in each patient. The side
to which the higher and lower stress levels are applied will be randomized. This design will
allow for inter and intra-patient comparisons since each patient will see both stress
levels. After the initial activation the subjects will be seen at day 1, 3, 17 and
thereafter at 4-week intervals until the space between the cuspid and second bicuspid is
completely closed. In the initial activation session and each of the follow-up sessions,
maxillary impressions and appliance adjustments to apply the desired force will be made. At
each clinical visit impressions will be made of the activated T-loop and the teeth. The
geometry of the T-loop will be measured from the impressions which will allow for the
applied force to be calculated using a numerical method developed by Raboud. The impression
of the teeth will be used to construct the occlusal templates required to measure tooth
movement.
Tooth mobility change during the orthodontic tooth movement will be monitored by means of
the Periotest measuring device. CT volumetric scans with the NewTom machine will be made
just before placement of the implant to ensure adequate palatal bone volume. Extra CT
volumetric scans will be repeated in the middle of the retraction period (based on the space
to be closed) and at the end of the space closure. The CT volumetric scan images will also
be used to determine alveolar bone density, changes in root geometry for calculation of root
surface area and center of resistance. The CT volumetric scans will also be used to
determine alveolar bone density, change in root geometry (root resorption) and to assess
movement of the cuspid in relation to other maxillary structures. The series of models from
each subject and a 3-axis measuring system (MicroVal) will be used to measure tooth position
changes relative to the 3 reference systems.
At the completion of cuspid retraction, the balance of the required orthodontic treatment
will continue. The palatal implant will be retrieved and orthodontic retention delivered.
Data gathered in the patient study will be utilized to validate the biomechanical model.
Biomechanical Modeling:
There will be two separate biomechanical tooth models developed. The first will be a
parametric three dimensional model of the tooth and supporting tissues which will allow for
variable biological properties of the tooth, PDL and alveolar bone. This model allows the
inputs to be patient specific for both the force systems applied and the individual
biological parameters. The three dimensional force systems that are applied to the tooth
will be calculated using the numerical method described by Raboud and the measured geometry
of the T-loop. The Finite Element Model (FEM) of the tooth and supporting tissue and the
applied force system will allow for an improved description of the stress fields in the
tissues, in particular the PDL. This improved stress field will be compared to the resulting
bodily tooth movement. In order to compare the results from the biomechanical model and the
clinical study it is important that the posterior segment does not move. The palatal implant
is to ensure the absolute anchorage required.
As previously mentioned, the accuracy of the FEM model is dependant upon the input
parameters. Consequently, there will be a two dimensional dynamic model of the tooth
response to a mechanical impact. This will model instruments such as the Periotest® which is
designed to measure impact response. Since the tooth response is governed by the
characteristics of the periodontal ligament, analysis of the raw signal obtained from the
Periotest® will allow for the mechanical properties of the PDL to be evaluated during the
retraction.
Statistical Analysis:
The final mean rate of tooth movement between the two force levels will be compared through
the Independent Samples T Test. To evaluate significant changes of the tooth movement in the
distinctive time intervals, a repeated measure ANOVA will be used. If the underlying
assumptions for both methods are not satisfied, then the non-parametric alternative will be
considered (U Mann-Whitney Test).
;
Allocation: Randomized, Endpoint Classification: Efficacy Study, Intervention Model: Single Group Assignment, Masking: Single Blind, Primary Purpose: Treatment
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