true molar intrusion attained during orthodontic treatment: a systematic review

REVIEW ARTICLE

True molar intrusion attained duringorthodontic treatment: A systematic reviewJulia Ng,a Paul W. Major,b and Carlos Flores-Mirc

London, Ontario, and Edmonton, Alberta, Canada

Introduction: The aim of this systematic review was to quantify the amount of true molar intrusion attainableduring orthodontic treatment. Methods: A literature search was conducted to identify clinical trials thatassessed true molar intrusion through superimposition of lateral cephalogram tracings. Craniofacial growthhad to be factored out when appropriate. Electronic databases (Pubmed, Medline, Medline In-Process &Other Non-Indexed Citations, all EBM reviews, Embase, Web of Science, and Lilacs) were searched with thehelp of a senior health-sciences librarian. Abstracts that appeared to fulfill the initial selection criteria wereselected, and the full-text original articles were then retrieved and analyzed. Only articles that fulfilled the finalselection criteria were finally considered. Their references were also hand-searched for possible missingarticles from the database searches. Results: Thirty abstracts met the initial inclusion criteria, and thesearticles were retrieved. From these, 29 were later rejected because they did not either quantify true molarintrusion or factor out normal craniofacial growth when required. Only 1 article remained, and it showed amean maxillary molar intrusion of 0.96 mm (SD, 0.54) in 12 subjects. Conclusions: True molar intrusionappears to be achievable in the maxillary arch, although the amount of evidence is minimal. The clinicalsignificance of the magnitude of the true intrusion reported is questionable as the sole treatment option tocorrect open-bite malocclusions. Better quantification method of the true intrusion attained has to be utilized.
(Am J Orthod Dentofacial Orthop 2006;130:709-14)
Open-bite malocclusions with skeletal compo-nents are difficult to treat because of their highrelapse tendencies.1-4 Patients with skeletal

open bites often exhibit vertical skeletal-growth dis-crepancies, abnormal muscular and soft-tissue develop-ment, or habits that cause unfavorable tongue andorofacial muscle activity.1,2

Treatment options for open-bite malocclusions in-clude elimination of the etiology, extrusion of theanterior teeth, surgical impaction of the maxilla, inhi-bition of molar eruption in growing patients, intrusionof the molars, and a combination of these.3-7 Althoughextrusion of the anterior teeth is often used to close anopen bite, caution must be used because a patient withan anterior open bite often has shorter roots and lessfacial bone support of the anterior teeth.8 Compromisedesthetics and a less stable outcome than for intrusion of

aResident, Graduate Orthodontic Program, University of Western Ontario,London, Ontario, Canada.bProfessor, Director of Orthodontic Graduate Program, University of Alberta,Edmonton, Alberta, Canada.cClinical Associate Professor and Director, Cranio-facial & Oral-health Evi-dence-based Practice Group, Edmonton, Alberta, Canada.Reprint requests to: Dr Carlos Flores-Mir, Faculty of Medicine and Dentistry,Room 4051A, Dentistry/Pharmacy Centre, University of Alberta, Edmonton,Alberta, Canada T6G 2N8; e-mail, [email protected], March 2005; revised and accepted, May 2005.0889-5406/$32.00Copyright © 2006 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2005.05.049
posterior teeth have been also considered drawbacks ofincisor extrusion in these patients.6 Molar intrusion isdifficult to achieve because molars are large multi-rooted teeth. A patient’s desire to avoid surgery, end-odontic therapy, or extensive prosthetic restoration,however, makes it an available option to correct ananterior open bite and overerupted molars.9

It has been established in incisors that tipping givesthe clinical impression of intrusion because it affectsvertical incisal edge position.10,11 Neither the incisaledges nor the root apices are good reference pointsbecause they are not independent of tooth inclinationchanges.12 Much like incisors, neither cusp tips nor rootapices are ideal reference points to evaluate molarintrusion. The molar centroid, like that of the incisor, isa point on the longitudinal axis of the tooth that isindependent of any changes in inclination; this makes itthe ideal reference point.13 For incisors, the palatalplane for the maxillary molar and the mandibular planefor the mandibular molar are used as the referencestructures relative to the centroid to evaluate whethertrue intrusion has been achieved.13-15

A systematic review is a methodologically soundprocess to search and analyze all available publishedevidence about a specific question. Well-defined searchcriteria, adequate selection of articles clearly related tothe question, and their comprehensive analysis distin-
guishes a systematic review from a narrative review.
709

found i

American Journal of Orthodontics and Dentofacial OrthopedicsDecember 2006

710 Ng, Major, and Flores-Mir

The present trend for evidence-based facts gives sys-tematic reviews a significant importance in currentdental research.16 In a systematic review, it is importantto cover all available evidence; therefore, efforts mustbe made to cover all possible databases—even thosenot in English.17,18

Although there are claims that molar intrusion isattainable during orthodontic treatment, no comprehen-sive review was found in the literature. The purposes ofthis systematic review were to quantify the amount oftrue molar intrusion attainable in orthodontic patientswithout surgical procedures and to consider whether itis clinically significant.

MATERIAL AND METHODS

A computerized search was conducted in the fol-lowing electronic databases: Medline (from 1996 toweek 2 of April 2005), PubMed (from 1966 to week 2of April 2005), Medline In-Process & Other Non-Indexed Citations (April 22, 2005), all EBM reviews(to first quarter of 2005), Embase (from 1988 to week17 of 2005), Web of Science (from 1945 to 2004), andLilacs (April 25, 2005). The main terms used in thedatabase search were orthodontics, intrusion, and mo-lar. The selection and specific use of each term in everydatabase search were made with the help of a seniorlibrarian specializing in health-sciences databasesearches (Table I).

Clinical trials and intrusion of permanent molarswere considered the initial inclusion criteria to selectpotentially appropriate articles from the abstracts in thedatabase searches. Surgically assisted molar intrusionwas considered an exclusion criterion at this stage.

Table I. Database search strategy and sensitivity of the

Database Key

PubMed (1) orthodontic*; (2) intru*; (3) molarhumans

Medline (1) orthodontic$.mp or exp ORTHODor exp MOLAR; (4) 1 and 2 and 3;

Medline In-Process & OtherNon-Indexed Citations

(1) orthodontic$.mp or exp ORTHODor exp MOLAR; (4) 1 and 2 and 3;

Web of Science (TS�orthodontic* AND TS�intru*) atypes; Language�All languages; Da

Embase (1) orthodontic$.mp or exp ORTHODor exp MOLAR; (4) 1 and 2 and 3;

All EBM reviews (1) orthodontic$.mp or exp ORTHODor exp MOLAR; (4) 1 and 2 and 3;

Lilacs (1) molar AND intrusion

*Percentages do not add up to 100% because same references were

Eligibility of the selected studies was determined by

reading the abstracts of the articles identified in theinitial search (Table I). No attempts were made at thisstage to identify studies that did not use adequatecontrol groups to factor out growth changes. It wasconsidered improbable that the abstracts would reportenough information about control groups, and so rely-ing too heavily on them, might exclude some usefularticles.

All article abstracts that appeared to meet the initialinclusion criteria were selected, and the actual articleswere collected. The selection process was indepen-dently conducted by 2 researchers, and their resultswere compared to settle discrepancies through discus-sion, except for the Lilacs database, which was evalu-ated by only 1 researcher (C.F-M.) because of thelanguage limitation. If the article abstract did notprovide enough information to make a decision, theactual article was obtained.

The actual articles from the selected abstracts werethereafter independently evaluated by the 3 researchers.A consensus was reached regarding which articlesfulfilled the final selection criteria and were finallyincluded in the systematic review. Articles that did notreport true molar intrusion and did not factor outgrowth when required (growing samples) were re-jected.

True molar intrusion was considered met when theselected studies used the center of resistance of themolar as the reference point to quantify the verticalmovement of the molar in the dentoalveolar bone. Useof occlusal or apical points for molar intrusion evalu-ation can artificially increase or decrease the amount ofrelative intrusion if tipping is part of the total molar

onic databases used

Results Selected

% of totalselected

abstracts (30)*

and 2 and 3; (5) limit 4 to 115 24 80.0

S; (2) intru$.mp; (3) molar$.mpit 4 to humans

116 27 90.0


0 0 0

(molar*); DocType�All docs)�SCI-EXPANDED

35 10 33.3


0 0 0


5 4 13.3

52 0 0

n several databases.

electr

words

*; (4) 1

ONTIC(5) lim

ONTIC(5) limnd TS�tabase(

ONTIC(5) lim

ONTIC(5) lim

movement; therefore, those articles were rejected.

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 130, Number 6

Ng, Major, and Flores-Mir 711

Vertical growth of the dentoalveolar bone wasimportant to factor out to make an accurate assessmentof the amount of true intrusion achieved. Failure toconsider dentoalveolar growth would result in under-estimation of the amount of intrusion attained.

Although measurement error is needed for a correctinterpretation of the clinical significance of the find-ings, it was not considered a reason to reject an articlebut was considered in the interpretation of the data.

These 3 considerations were used in a previoussystematic review to evaluate true incisor intrusion.19

The reference lists of the retrieved articles were alsohand-searched for additional relevant articles that mighthave been missed in the database searches. Also, wemade a hand search using the online resources of the 3main orthodontic journals (American Journal of Orth-odontics and Dentofacial Orthopedics, Angle Orth-odontist, and European Journal of Orthodontics) andany additional potential references. When extra infor-mation that was not specifically stated in the article wasrequired for discussion or statistical analysis, we con-tacted the authors to obtain it.

RESULTS

Different numbers of hits were found depending onthe electronic database selected. Medline identifiedmost of the abstracts with 112, followed by PubMedwith 87, Lilacs with 50, and Web of Science with 33.The remaining databases had only a few hits. The useof the Lilacs database permitted the inclusion of articleabstracts in Spanish that did not appear in Englishdatabases, but none fulfilled the initial selection criteria.From the total abstracts identified in the electronicdatabases, only a relatively small percentage fulfilledthe initial inclusion criteria (Table I).

When we compared the results between databases,Medline obtained all abstracts except 1. Medline iden-tified 27 abstracts that were not found in PubMed, 1 ofwhich was not found in any other database search. Ofthese 27 abstracts, only 4 were selected. PubMedidentified 2 abstracts that were not in the Medlinesearch, of which 1 was selected. In Medline, 85.7% ofthe selected abstracts were included in PubMed. InWeb of Science, 9 abstracts identified were not in-cluded in the PubMed search; none was selected. Allselected abstracts from Web of Science were found inPubMed. Medline In-Process, EBM, and Embase didnot include abstracts not found in PubMed.

Thirty article abstracts fulfilled the initial selec-tion criteria. Of these, 29 were later rejected becauseof methodological issues. The studies were rejectedbecause true intrusion was not evaluated,1-6,20-37

or the authors did not consider the impact of ver-

tical dentoalveolar growth in children or adoles-cents.7,21,24,27,28,30-32,34,35,38 Studies by Dyer et al21 andSherwood et al6 were not selected based on our inabil-ity to determine whether true molar intrusion wasachieved; neither author responded to our request foradditional information (Table II). Only the senior au-thor from the remaining article39 replied and confirmedthat true molar intrusion was used. Also, on thesuggestion of that author, another article and a thesiswere considered, but they were later rejected becausethey did not consider a control group to factor outnormal growth.40,41

The article by Firouz et al39 was the only one thatwas finally selected. Those authors examined the ef-fects of high-pull headgear on Class II Division 1patients. The study compared a nonrandom, noncon-secutive sample of 12 adolescents (age range, 9.5 to12.5 years) who wore the headgear for 6 months for anaverage of 12 hours a day. Twelve other patients servedas controls. The appliance consisted of an Interlandi-type high-pull headgear with head straps. A force of

Table II. Studies that were considered but later rejected

Authors Methodologic limitations

Pearson5 1Pancherz44 1Baumrind et al20 1Barbre and Sinclair1 1Dyer et al21 1, 2Everdi et al2 1, 3Orton et al22 1, 3Pancherz and Anehus-Pancherz23 1Brown et al24 1, 2Everdi and Ozkan25 1Ghosh and Nanda38 2, 3Wilson41 2Uçem and Yuksel28 1, 2Stucki and Ingervall27 1, 2Covell et al29 1, 3Pearson and Pearson30 1, 2, 3Alcan et al31 1, 2, 3Bussick and McNamara32 1, 2, 3Deberardinis et al40 2Sankey et al33 1Heinig and Göz34 1, 2, 3Du et al35 1, 2Stromeyer et al36 1, 2, 3Is can et al45 1, 3Sherwood et al6 1, 3Sugawara et al3 1, 3Trisi et al37 1Everdi et al4 1, 3Gurton et al7 1, 2, 3

1, True intrusion not evaluated; 2, effect of growth not considered; 3,no error of measurement stated.

500 g was applied at the level of the buccal trifurcation



of the maxillary first molars (center of resistance). A0.032 � 0.032-in stainless steel transpalatal arch wasincluded to maintain symmetry and arch widths, as wellas to prevent molar rotation and buccal crown tipping.This retrospective study evaluated its subjects beforeand after treatment but did not report measurementerror. The mean maxillary molar intrusion for the 12subjects was 0.96 mm (SD, 0.54).

DISCUSSION

Although several studies claimed that molar intru-sion is possible, they lacked adequate evaluations ofmolar intrusion. Only 1 study fulfilled the final selec-tion criteria for this systematic review.39 This shows thecurrent lack of scientifically sound studies to evaluatetrue molar intrusion.

Although about 30 published studies evaluatedmolar intrusion as the principal or secondary objective,only 1 quantified it correctly.39 True molar intrusioncan be evaluated only when the center of resistance ofthe molar is used as the reference point to quantify thevertical movement of the molar into its alveolar bone.Using the cusps or apices would not allow distinguish-ing true intrusion from tipping of the cusp edges or rootapices; this could create a false perception of intru-sion.12

An adequate plane of reference is also importantbecause changes in these planes during treatment canclinically alter our perception of the intrusion attained.The palatal plane for the maxillary molar and themandibular plane for the mandibular molar are mostcommonly used for molar intrusion evaluation becausethey represent the basal osseous bone for their respec-tive teeth.42

Another point to consider when determining molarintrusion is the impact of normal dentoalveolar growth.Failure to consider dentoalveolar growth would resultin underestimation of the amount of intrusion attainedin growing adolescents.15 Studies that include growingpatients must include nontreated control samples.

From the study that met all selection criteria, it isevident that true molar intrusion with high-pull head-gear is achievable.39 Subjects were included who had3.0 to 7.0 mm Class II molar occlusions, at least2.0-mm interlabial gaps, and increased lower facialheights. All received the same interlandi-type headgearwith a force of 500 g on each side for 6 months of daily12-hour wear. Statistically significant (P �.01) toothmovements were achieved, including a mean of 0.54mm (0.96 mm when growth was factored in) ofintrusion of the maxillary first molars. The significanceof the reported mean intrusion value, however, might
be questionable from a clinical standpoint. The error of
measurement alone might be about that amount. Arelatively large variance (about 50%) in the amount ofintrusion was also found, and the sample was small.

Because only 1 study fulfilled all selection criteria,it is difficult to assess whether comparable values oftrue molar intrusion can be achieved by other appli-ances or in other types of patients. It is also difficult todetermine which type of appliance would be best suitedfor such tooth movement. Likewise, no comparison ofmolar intrusion could be made between adults andchildren from this study alone, and the actual maximumintrusion values attainable could not be determined.

Finally, most rejected studies2-4,6,7,22,29,30-34,36-38

and the selected article39 did not state measurementerrors. This made the interpretation of the clinicalsignificance of the findings and subsequent conclusionsfrom their studies questionable.

We must carefully consider that the amount ofintrusion attained in clinical situations could be influ-enced by characteristics of the dental arch, individualpatient considerations, and chosen mechanics. Thearticle by Firouz et al39 might not represent the actualvalues of molar intrusion that could normally be at-tained in clinical situations. The available evidence isnot from the highest evidence-based hierarchy43; there-fore, randomized clinical studies should be made toprovide irrefutable evidence about obtaining true molarintrusion that is clinically significant with current orth-odontic appliances.

Even though sound methodological scientific evi-dence about the magnitude of molar intrusion attainableis available only for high-pull headgear, several othermechanical options are available for the clinician. Theuse of a transpalatal arch with an acrylic bottom did notshow significant molar intrusion but helped to limit thevertical facial height increase during treatment.40 Useof magnetic appliances1 and micro screws/plates3,4,6

seemed to be a promising area to attain true molarintrusion, although this has to be confirmed with soundscientific evidence. Future studies that evaluate thesemechanical options and compare them would be useful.

CONCLUSIONS

● Limited evidence is available about the quantity ofattainable molar intrusion. True molar intrusion ap-pears to be achievable in the maxillary arch, but theamount of evidence is minimal.

● The clinical significance of the magnitude of the trueintrusion reported for high-pull headgear is question-able as the sole treatment option to correct open bites
in clinical situations.

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 130, Number 6

Ng, Major, and Flores-Mir 713

● Randomized clinical trials should be conducted toevaluate true molar intrusion and consider normalcraniofacial growth.

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