EFFECTS OF TWIN BLOCKS
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Twin blocks are simple bite blocks with occlusal inclined planes
DESIGN OF THE TWIN BLOCK The occlusal inclined plane The occlusal inclined plane is the fundamental functional mechanism of dentition. Cuspal inclined planes play an important part in determining the relationship of the teeth
If the mandible occludes in a distal relationship to the maxilla (in class II) the occlusal forces acting on the mandible in normal function have a distal component of force that is unfavorable to normal forward mandibular development.
Twin-blocks constructed in a protrusive bite effectively modifies the occlusal inclined planes by means of bite-blocks
Modification of occlusion
The bite blocks acts as a guiding mechanism causing the mandible to be displaced downward and forward. The unfavorable cuspal contacts of a distal occlusion are replaced by favorable proprioceptive contacts on the inclined planes of twin-blocks to correct the malocclusion & to free the mandible from its locked distal functional position.
STANDARD TWIN BLOCKS Standard twin blocks are essentially for treatment of an uncrowded class II div 1 malocclusion with a good arch form. appliance design Clarkâ€™s Twin Block appliance consists of 1. Base plate 2. Occlusal inclined plane or bite blocks 3. Retentive components - Delta and ball end clasps 4. Active components- screw ,springs and bows
Base plate/bite blocks Appliances may be heat-cure or self cure.heat cure is preferred because of strength CLASPS Delta clasp Routinely placed on upper first molars and on lower first premolars.
Ball-ended clasps Routinely employed mesial to lower canines and in the upper premolar or deciduous molar region.
C-clasps Additional c-clasps may be placed to improve retention
Occlusal inclined planes ď śInitially the angulation between the blocks were made at 90 degrees ď śsince it was difficult to hold the mandible forward at this angle, the angulation was changed to a 45 degree one ď śthe angulation was changed to 70 degrees to the occlusal plane to apply a more horizontal force encouraging a more forward mandibular growth.
Position The inclined plane on lower bite block is angled from the mesial surface of the second premolar or deciduous molar whichever present.the lower bite block does not extend distally to the marginal ridge on the lower second premolar. This allows the leading edge of the inclined plane on the upper appliance to be positioned mesial to the lower first molar so as not to obstruct eruption.
Buccolingually the lower bite block covers the occlusal surfaces of the lower premolars . In canine region it has to be thinner. Upper inclined plane is angled from the mesial surface of the upper second premolar to the mesial surface of the first molar.
Since the upper arch is wider than the lower,it is necessary to cover only lingual cusps of upper posterior teeth rather than the full occlusal surface.. Mistakes in the appliance can lead to treatment failure
CASE SELECTION FOR CLASS II 1. Actively growing patients; M: 13+2; F: 11+2 2. Class II skeletal relationship with retrognathic mandibular position 3. Increased overjet 4. Deep overbite 5. None to very mild crowding in U/L arches 6. Low to average mandibular plane angle 7. Good VTO
BITE REGISTRATION In growing patients, overjet up to 10mm can be corrected on the initial activation by posturing the mandible in edge-to-edge position. The amount of mandibular protrusion depends on the ease with which the patient can posture forward.
As a general rule, the initial activation should reduce the overjet by 5 to 7 mm leaving 3 to 5 mm interocclusal clearance in the first premolar region.
In case there is no adequate posterior clearance for the construction of the bite block then a 2 mm of anterior opening is recommended. Larger overjet can be corrected by progressively reactivation of the appliance during the course of treatment.
summary of bite registration
Inter incisal clearance
In first premolar region
THE TWIN BLOCK TECHNIQUE Twin Block is described in two stages. The first phase is an active phase in which Twin Block appliance is used followed by the support phase
ACTIVE PHASE In the first phase of twin block therapy, the appliance is used to achieve correction of sagittal jaw position. After correction vertical discrepancy is corrected by selectively trimming the posterior bite blocks. The aim of the first phase is to achieve correction to class I occlusion and control of the vertical dimension by a three-point contact with the incisors and the molars. At this stage the overjet overbite and sagittal relationship is full corrected. Time required is 6-9 months
Sequence of trimming
SUPPORT PHASE The aim of the support phase is to maintain the corrected incisor relationship until the buccal relationship is fully interdigitated. To achieve this objective an upper removable appliance is fitted with an anterior inclined plane with a labial bow to engage the lower incisors and canines. Time required is 4-6 months
anterior inclined plane
RETENTIVE PHASE Treatment is followed by retention with upper anterior inclined plane appliance. Appliance wear is reduced to nighttime wear only when the occlusion is fully established.
FIXED APPLIANCE PHASE Final detailing of the occlusion is completed using fixed appliance therapy
MODIFICATIONS OF TWIN BLOCK Twin Block For Transverse Development
By combining twin-block with schwarz appliance. Screws in upper & lower twin block to develop arch form in mixed dentition.
TWIN-BLOCK WITH SCHWARZ APPLIANCE.
Twin block for sagittal development
For anteroposterior arch development two screws which are aligned antero posteriorly.
Twin block for sagittal development
For both transverse and sagittal 3 way screws
With habit breakers
FOR CLASS III REVERSE TWIN BLOCKS
Magnetic twin blocks
Attachment in TWIN BLOCK for advancement
Twin block bio finisher Extruding lower molars by vertical traction to stabilize the TMJ
Advantages of twin blocks The twin block is the most comfortable , the most aesthetic and the most efficient of all the functional appliances . Twin blocks have many advantages compared to other functional appliances.
Comfort Patient can wear twin blocks 24 hours per day &can eat comfortably with the appliances in place. It is to take full advantage of all functional forces applied to the dentition , including forces of mastication.
Aesthetics Twin blocks can be designed with no visible anterior wires without loosing efficiency Function The occlusal inclined plane is the most natural of all the functional mechanisms.there is less interferences with normal function because the mandible can move freely in anterior and lateral excursion without being restricted by a bulky appliance.
Patient compliance Twin blocks may be fixed to the teeth temporarily or permanently to guarantee patient compliance Facial appearance From the moment twin blocks are fitted the appearance is noticeably improved.The absence of lip,cheek or tongue pads ,places no restriction on normal function & does not distort the facial appearance during treatment
Speech Patients can learn to speak normally with the twin blocks. Clinical management Adjustments and activation is simple.The appliance is robust and not prone for breakage.chair side time is reduced in achieving major orthopedic correction.
Arch development Twin blocks allow independent control of upper and lower arch width.appliance design is easily modified for transverse and sagittal arch development.
Twin blocks achieve excellent control of the vertical dimension in treatment of deep overbite and anterior open bite Facial asymmetry Asymmetrical activation corrects facial and dental asymmetry in a growing child
Safety Twin blocks can be worn during sports activities with the exception of swimming & violent contact sports Age of treatment Arch relationships can be corrected from early childhood to adulthood.However treatment is slower in adults & the response is less predictable
Integration with fixed appliances Integration with conventional fixed appliance is simpler than with any other.
to detail the occlusion
Twin blocks for TMJ dysfunction Effective as splints
TMJ dysfunction Un favorable occlusal contacts eliminated Simultaneously sagittal,vertical ,transverse arch dvp proceeds
MADC: MADC: MADC: MADC:
Do functional appliance have orthopedic effect
Functional appliances have been used for over a century in the treatment of Class II Division 1 malocclusions. Although few clinicians deny their clinical efficacy, proof of their growth modifying effect remains elusive.
The effects of Twin Blocks: A prospective controlled study David Ian Lund
CLAIM There is little evidence to support the claim that functional appliances significantly affected mandibular growth.
Bjรถrk and Pancherz demonstrated only small changes in mandibular growth and said that it was not affected by treatment with functional appliances.
There may be significant influences on mandibular growth after timely intervention. COUNTER CLAIM
As suggested by Harris, DeVincenzo, and Windmiller (The effects of Twin Blocks: A prospective controlled study David Ian Lund 1988 AJO)
Little scientific evidence exists as to the effect of functional appliances appliance on growth of the jaws in humans
The effects of Twin Blocks: A prospective controlled study David Ian Lund, 1998 Jan
BRODIEâ€™S pattern concept Face may get bigger ,but its form never changes
Then is it possible to grow mandible
Studies done on effects of functional appliances Animal studies
Studies on humans
Well-controlled animal studies at the University of Michigan-Ann Arbor and the University of Toronto have shown large amounts of downward and forward glenoid fossa relocation in appliances worn 24 hours a day.
(Do functional appliances have an orthopedic effect? Donald G. Woodside 1998 AJO)
study by Mc namara on primates
Changes in muscle activity Posterior head of remporalis
Masseter in activity Superior head of lateral pterygoid
Until new equilibrium is achieved
Proliferation of condylar cartilage
The effects of Twin Blocks: A prospective controlled study David Ian Lund 1998 AJO
This study investigated the net effects of the Twin Block functional appliance taking into account the effects of normal growth in an untreated control group.
The treatment group consisted of 36 subjects, mean age of 12.4 years
The control group consisted of 27 subjects with a mean age of 12.1 years.
These patients were observed for a mean time of 1.2 years
Linear measurements recorded from cephalometric landmark to Sella-Nasion perpendicular
Is mandibular growth increased?
There was a statistically significant increase in mandibular length measured from Articulare-Pogonion
It was not possible to determine whether the increase in Ar-Pog was due to an increase in mandibular length or a repositioning of the mandible.
Baumrind and Korn and Haynes found similar changes in Ar-Pog. (1986 AO,AJO 1981)
No actual measurement of fossa adaptation or relocation was made in this study.
Can a functional appliance (FA) stimulate mandibular growth? Animal studies, with some exceptions, have found significant increases in length In human beings conflicting results have been reported with use of the Herbst and the Fränkel appliances and the activator. (AJO1991 Changes in mandibular length with a functional appliance – DeVincenzo)
Do Twin Blocks restrain maxillary forward growth? When forward growth of the maxilla was assessed by means of conventional angular measurement little change in SNA was observed thus indicating little maxillary restraint.
ď Œ So no headgear like effect as said by DeVinzenzo et al.
it was postulated that some degree of maxillary restraint
ď Š might have occurred but was not detected because of dentoalveolar remodeling disguising the skeletal effects of the treatment.
Is there a beneficial sagittal change? Despite the fact a restraining effect on the maxilla could not be demonstrated,
the forward growth/repositioning of the mandible does result in a significant change in ANB thus the severity of the Class II skeletal pattern is reduced.
Does tooth tipping contribute greatly to correction? There was a significant amount of tipping of the labial segment teeth in both arches. The maxillary incisors were retroclined, mandibular incisors were proclined as a result of treatment, which greatly contributed to correction of the overjet.
Does anteroposterior molar movement aid correction of the malocclusion? A restraining effect on the upper molars was demonstrated to the extent that there was slight distalization along with a statistically significant forward movement of the lower molars. This change in molar position aids the correction of the disto-occlusion.
Do Twin Blocks control the vertical position of the teeth? There was a significantly increased eruption of the lower molars during treatment after judicious trimming of the bite blocks. This differential lower molar eruption is an important feature in Twin Block therapy as it not only contributes to overbite reduction and closure of lateral open bites but also helps with Class II molar correction.
differential lower molar eruption
Pterygoid response Also called as tension zone by HARVOLD. These are clinical signs after fitting functional appliance. The patient experiences adaptation of muscle function immediately on insertion of the appliance, in response to altered occlusal function.. Within few days the patient experiences pain behind the condyle when the appliance is removed.
Cause From the studies of histological changes in animal experiment , it may be deduced that retraction of the condyle results in compression of connective tissue & blood vessels and that ischeamia is the principal cause of pain. A new pattern of muscle behavior is quickly established whereby patient finds it difficult and later impossible to retract the mandible into its former rertruded position.
After a few days it is comfortable to wear the appliance than to leave it out.
This change in muscle action has been described by McNAMRA as the pterygoid response & results from the altered activity of the medial head of the lateral pterygoid muscle
The lateral pterygoid muscle hypothesis Suggests that both postural and functional activity in the masticatory muscles increases after functional appliance insertion. This increased activity, especially in the superior head of the lateral pterygoid muscle, then acts as a stimulus to mandibular growth. (McNamara JA. Neuromuscular and skeletal adaptations to altered function in orofacial region. AJO 1973)
SUMMARY OF TREATMENT EFFECTS Skeletal changes as a result of Twin Block therapy
A mean forward growth/repositioning of the mandible of 2.4 mm, measured at Ar-Pog, was demonstrated after Twin Block therapy.
The most noticeable skeletal change was an increase in the angle SNB.
No significant maxillary restraint could be demonstrated.
There was an increase in lower anterior facial
Dental changes as a result of Twin Block therapy
overjet reduction retroclination of the upper incisors proclination of the lower incisors.
Buccal segment correction occurred by distal movement of the upper molars
lower molar eruption in an anterior and superior direction.
Robertson suggested that the principal changes that occurred with functional appliance therapy were DENTOALVEOLAR, I.e
distalization of the upper buccal teeth
retroclination of the upper labial segments,
mesial movement of the lower buccal segments
proclination of the lower labial segments. VERTICAL CHANGES
delay of eruption of the upper maxillary molars enhanced eruption of the mandibular molars.
Effects of force on bone The challenge of functional therapy is to maximize the genetic potential of growth & guide the growing face & developing dentition towards a pattern of optimal development. In the dentition the force of occlusion of the teeth is the most natural functional mechanism that can be used to influence the structure of the supporting bone. This natural process of bony remodelling forms the basis of functional correction with the twin-block technique (W, J CLARK IN GRABER, RAKOSI, PETROVIC)
Wolffâ€™s law of transformation of bone The internal & external structure of bone is modified by functional demands to withstand the physical demands made on it with the greatest degree of economy of the structure
functional-appliance therapy can achieve correction of Class II malocclusion through the following factors: (1) dentoalveolar changes (2) restriction of forward growth of the midface (3) stimulation of mandibular growth beyond that which would normally occur in growing children (4) redirection of condylar growth from an upward and forward窶電irected growth to a posterior direction, Do functional appliances have an orthopedic effect? Donald G. Woodside 1998 JAN AJO
(5) horizontal expression of mandibular growth from downward and forward to horizontal. (6) changes in neuromuscular anatomy and function that would induce bone re-modeling (7) adaptive changes in glenoid fossa location to a more anterior and vertical position.
He concludes that , There is still convincing evidence supporting the concept that functional appliances do create an orthopedic effect in specific individuals
Do functional appliances have an orthopedic effect? Donald G. Woodside
Treatment effects produced by the Twin-block appliance and the FR-2 appliance compared with an untreated Class II sample Linda Ratner Toth, and James A. McNamara, Jr AJO 99
cephalometric study compares the treatment effects produced in 40 patients treated with the Twin-block appliance 40 children treated with the FR-2 appliance 40 untreated Class II controls
significant increases in mandibular length were observed in both treated groups. The Twin-block achieved an additional 3.0 mm of mandibular length, whereas the Fr채nkel 1.9 mm more than did the controls. No restriction of midfacial growth in either appliance group relative to controls
A increase in lower anterior facial height in both treatment groups. more dentoalveolar adaptation was observed in tooth-borne Twin-block appliance than with the tissue-borne FR-2.
The Twin-block and FR-2 samples both showed significant retroclination and extrusion (eruption) of the maxillary incisors. The Twin-block patients exhibited distal movement of the upper molars; however, there was no extrusion. Slight lower incisor proclination was noted & greater in the Twin-block group compared with the other 2 samples.
Treatment effects of the twin block appliance cephalometric study Christine M. Mills, and Kara J. McCulloch
A clinical study was done to investigate the treatment effects of a modified Twin Block appliance. Pretreatment and posttreatment cephalometric records of 28 consecutively treated patients with Class II malocclusions were evaluated and compared with untreated Class II control subjects.
Results indicated that mandibular growth in the treatment group was on average 4.2 mm greater than in the control group over the 14month treatment period In addition, some dentoalveolar effects in both arches contributed to the overjet correction.
effects Intermittent wear
full time wear
Muscle response to the Twin-block appliance: An EMG stu Preeti Aggarwal, Kharbanda,, Rashmi Mathur,AJO 1999
An EMG study was performed on 10 young growing girls in the age group of 9 to 12 years with Class II Division 1 malocclusion who were under treatment with Twin-block appliances. Bilateral EMG activity of elevator muscles of the mandible (ie, anterior temporalis and masseter) was monitored for 6 months.
The changes were noted at the start of treatment within 1 month end of 3 months, end of 6 months.
The results revealed increase EMG activity in masseter anterior temporalis activity during the 6 month period of treatment. The increased electromyographic activity can be attributed to an enhanced stretch (myotactic) reflex of the elevator muscles, contributing to isometric contractions.
functional appliances that are worn full-time elicits a greater and more rapid neuromuscular response than those worn only part-time.
orthodontists still are searching for the most effective means of stimulating mandibular growth preferentially. Barring surgical correction, functional appliances seems to be the most direct approach to treatment of a mandibular deficiency problem.
I would like to thank my guide Dr. triveni for helping me out with this topic
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