October 1999
History
Dental History
Indications
Advantages
Disadvantages
Mandibular Anterior Crowding
Deficient Anterior Premaxilla
Deficient Mandibular Anterior Segment
Deficient Posterior Mandible
Movement of Misaligned Dental Implants
HISTORY
Distraction Osteogenesis was first used in orthopedic medicine in the early 1900's, but the current concepts evolved from the ideas of Dr. Gavriel
Ilizarov, who practiced medicine in Kurgan, Siberia. Dr. Ilizarov, who had great understanding of the biophysiolgy of bone, developed techniques to
move bone fragments in controlled vectors using a system of wires and fixed rings joined together with threaded rods and hinges. This technique
allowed slow transport of bone segments without invasive surgery and was especially practical in the treatment of fractures in children and in
lengthening of bones in the legs where there was a discrepancy between right and left bone lengths.
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DENTAL HISTORY
The transfer of techniques involving D.O. in medicine to those used in dentistry was not an easy task. The reason being that the shape, size, location
of bones is much different. The primary boost in the development of distraction osteogenesis techniques in the dental field came from the
conceptualization and construction of miniature devices that could move small bone fragments in a controlled vector. In 1992, McCarthy, was the first
to publish on the use of distraction osteogenesis to lengthen a human mandible. Dr. Martin Chin, a
maxillofacial surgeon in San Francisco, was and still is a primary leader in this process.
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INDICATIONS
Mandibular Lengthening
Anterior-posterior deformity
Trauma reconstruction
Cancer reconstruction
Craniofacial syndrome
Redo
Mandibular Widening
Crowding with A-P deformity
Brodie Syndrome
Asymmetry
Craniofacial Syndrome
Maxillary or mandibular alveolar distraction
Insufficient alveolar height and/or width
Previously failed bone graft sites
Insufficient soft tissue coverage
Insufficient dona bone available
Patient is not a candidate for a bone graft
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Advantages of Distraction Osteogenesis
Little relapse
Bigger movements possible
Ability to mold the regenerate
Out-patient surgery
No need to extract teeth
Generation of soft tissue
Less likelihood of nerve injury
Less likelihood of idiopathic condylar resorption
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Disadvantages of distraction osteogenesis
Technique sensitive surgery
Equipment sensitive surgery
Possible need of second surgery to remove distraction devices
Patient compliance
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Mandibular Anterior Crowding
Growth discrepancies often present themselves with anterior mandibular crowding as a major feature. There have been orthodontic solutions to these
problems as well as extraction therapy and sometimes a combination of both has been used. A newer concept involves distraction osteogenesis. An
osteotomy is performed near the midline of the mandible and a plate is secured to either side of the alveolar segments. A threading rod is placed
horizontally in the mucobuccal fold and often an 8-10 day “rest period” the rod is turned 360° each day which results in a movement of separation of
0.4mm on a daily basis. A pretreatment model, which has been modified to the intended final result, should be used to determine the endpoint of
treatment. At this time, the segments are immobilized by fixing the threading rod for a period of 6-8 weeks. A retainer should be constructed so that
there will be no relapse.
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The Deficient Anterior Premaxilla
Patients often come to us with missing maxillary anterior teeth as their chief complaint, however, on examination it is noted that posterior molars are
missing as well. This posterior bite collapse has caused 700-900 psi forces that were originally intended to be dissipated by the molars and not to be
transferred to the anterior teeth. The activation of the lateral pterygoid muscles causes protrusion of the mandible and without molar guidance, the
anterior teeth are moved in an anterior direction. The resultant, splaying and tipping of the maxillary anterior teeth is often the reason that patients
seek help. Unfortunately, when the teeth are moved in this fashion, the pre-maxillary bone housing moves anteriorly as well. If implants are placed,
they will be in the wrong location for a harmonious, equilibrated occlusion. The entire pre-maxillary boney housing needs to be moved posteriorly and
inferiorly. This can be accomplished with distraction osteogenesis.
Deficient Mandibular Anterior Segment
Often due to trauma, periodontal disease or congenital defects, we are faced with a height/width deficit of the anterior mandible. If there are teeth
adjacent to the defect, it can be very difficult to graft these sites due to bacterial contamination of the exposed cementum and/or dentin. The design
of flaps that will ensure complete tissue coverage of the graft for one sigma (four months in humans) and the compromised blood supply in mandibular
bone. Distraction osteogenesis can overcome all three of these concerns for the following reasons:
Slow movement of the alveolar segment allows adequate blood supply for osteogenesis
Threading rod allows adequate fixation of alveolar segment during movement, thereby
preventing fibrous tissue growth at bone interface
Contaminated cementum/dentin are isolated from moving alveolar segment during the process
After the final movement of the alveolar segment has been accomplished. The threading rod is fixated for ½ sigma ( 8 weeks) to allow union of
the bone interface. Implants can be placed at this time. Each 360° turn of the threading rod correlates to 0.4mm movement of the alveolar segment.
The rod is usually turned 360° per day after an 8-10 day “rest period” after the initial fixation of plates and insertion of the threading rod. The “rest
period” allows the bone to activate a resorption phase of the “A-->R-->Q-->F” cycle.
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The Deficient Posterior Mandible
When mandibular molars are lost, there is a rapid loss of the buccal plate and then in gradual loss of height of the mandibular posterior alveolar
segment. If this site is then loaded with a partial or full denture, there is an increased loss of bone, both in rate of loss and content. This is due to the
fact that bone cannot grow against a pressure gradient. The end result is that the crest of the mandibular posterior ridge becomes located inferiorly
and m to it's original position. If implants we placed in this location, a cantilever force will occur as the maxillary molar teeth occlude with the
implant/crowns. This results in a shear force which is the weakest component of bone. This will occur even if a nerve repositioning procedure is
performed. A much better concept involves using distraction osteogenesis to move the mandibular posterior alveolar segment superiorly and laterally
in a vector which will align the occlusal forces of the maxillary and mandibular molar teeth. This will result in a compressive force on the bone instead
of a shear force. Bone is strongest during compression.
Movement of Misaligned Dental Implants
Sometimes, due to miscellaneous factors, an implant will achieve complete bone integration and yet not be in the correct location for esthetics or
loading. Distraction osteogenesis can be used to relocate the alveolar segment, including the misaligned implants, to a new location. There are a
couple of ways to achieve this. First, the segment must be disease free and stable prior to movement. A model should be made to determine the
vector component required to move the alveolar segment into the correct location. Once this is done, a choice must be made from these concepts:
To push the
alveolar segment from a point superior to the apical section
of the implants
To pull the
alveolar segment from a point superior to the head of implants
In the first concept, two horizontal bone plates are inserted at the time of the osteotomy and a jack screw device is used to move the alveolar
segment after a “rest period” of 8-10 days. In the second concept, a rod that fits the thread of the implant is inserted through a plate embedded in
plastic that is fixed to the teeth After a 8-10 day rest period, post osteotomy, the rod is activated and the alveolar segment is moved to a new location
at a rate of 0.4mm per day. After the correct location is achieved, it is very important that the alveolar segment is fixated with no movement for a
period of 6-8 weeks. Any less time than this can result in a relapse of the segment into an incorrect location. Following this fixation period, the
implants can be restored in a conventional manner.
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Written by R. Michael Keenan, D.D.S.--October 1999
*Special acknowlegdement given to Dr.'s Lee Whitesides, KLS Martin and Martin Chin.
For more information on this topic, visit www.distraction.net
