Lower Incisor Extraction Treatment with the Invisalign System
Single-lower-incisor extractioncases have rarely beenpublished, perhaps because thereare few patients who meet thestandards for such treatment.The following diagnostic characteristicsare usually requiredfor single lower incisor extractions:
In any such case, a full diagnosticsetup should be made toensure the occlusal results willbe acceptable.1 Unfortunately,diagnostic setups usually involvelong and laborious laboratoryprocedures of cutting, setting,and waxing the teeth in place. Inaddition, conventional methodsof tooth repositioning with removableappliances require alterationof the casts by resettingthe teeth, or by scraping awayplaster from the teeth to bemoved and blocking out spacefor them with wax.2
New diagnostic softwarenow makes it simple, quick, andefficient to perform virtual setupsusing the Invisalign System,an alternative to traditionalorthodontic appliances. Thisarticle will show how a series ofclear aligners can sequentiallymove teeth from start to finish ina case involving a single lowerincisor extraction.
A 24-year-old female presentedwith a chief concern of"lower incisor crowding". Shehad undergone orthodontic treatment10 years previously andhad recently had gingival graftson her upper and lower anteriorteeth.
Clinical examination revealedfull incompetent lips withthe chin deviated to the right(Figs. 1A, 1B). The patient had astraight profile with mentalismuscle strain; on smiling, shedisplayed 100% of her incisorsand 1mm of gingiva. The molarand canine relationships wereClass I. The patient had a 10%overbite and 3mm overjet, withthe lower midline shifted 3mm tothe right. Good oral hygiene wasevident, although slight gingivalrecession was found in the areasof the upper first bicuspids andthe lower right lateral incisor.
The maxillary arch waswell aligned, with a peg-shapedleft lateral incisor; Bolton analysisindicated a maxillary toothsizedeficiency of 1mm. Bothcuspids showed occlusal wear,and the right second bicuspidwas lingually positioned and rotatedmesially. There was 5mmof crowding in the lower anteriorregion, with lingually tippedlower cuspids and right secondbicuspid.
The radiographic analysisshowed a full permanent adultdentition with previous extractionof third molars and minorrestorations. The patient hadmild generalized bone loss withnormal root morphology andlength. Cephalometric findingsincluded a well-positioned maxillaand slightly prognathicmandible, resulting in a slightlyexcessive sagittal jaw relationshipor Class III tendency (Table1). The upper and lower incisorswere protrusive and proclined.
The treatment objectives inthis case were primarily to resolvethe lower crowding,achieve good overjet and overbite,and avoid any further proclinationof the upper and lowerincisors with their thin attachedgingivae. Further goals includedimproving the lower midline andresolving the Bolton discrepancy.
There were three treatmentalternatives in this case. The firstwas to expand both arches to alleviatethe crowding and to bondveneers to the upper lateral incisorsat the end of treatment toresolve the tooth-size discrepancy.The problem with this optionwas that the midlines could notbe centered.
The second alternative wasto alleviate the lower crowdingby reproximation. The lower anteriorregion was not suitable forstripping, however, due to theshape and small size of the lowerincisors. Reproximation of theposterior segment was not agood choice because of the ClassI posterior occlusion.
The final alternative was toextract a lower incisor to alleviatethe crowding. The upper midlinecould then be aligned withthe middle of the lower teeth.This plan would minimize proclinationof the lower incisorsand would also address the ClassIII tendency and Bolton discrepancy.A diagnostic setup showedthat the treatment would indeedrelieve crowding and allow thecase to be finished with goodoverbite and overjet. The majordrawback was that a lower incisorwould have to be extracted.The lower right lateral incisorwas selected because it was themost misaligned and thus contributedmost to the crowding,and because its attached gingivawas the least satisfactory of allthe lower incisors'.
The patient was referred tohave the lower right lateral incisorextracted, and upper andlower polyvinyl siloxane impressionswere taken for Invisalignappliances. A vacuum-formedretainer was made to hold the teeth in position until the alignerswere delivered. The patient'sfinal tooth setup and stages oftooth movement were generatedby the three-dimensional AlignTechnology software and reviewedby the orthodontist on acomputer, using the proprietaryClinCheck system (Fig. 2).
Prior to delivery of the firstaligner, 1mm × 3mm compositeattachments were bonded vertically to the lower incisors andright cuspid to prevent tippingduring space closure. Lower-archtreatment was initiated withthe teeth adjacent to the extractionsite moving first (Fig. 3).Upper aligners were not used foreight weeks, until sufficientoverjet was achieved to enableincisor alignment (Fig. 4). Theteeth were programmed to undergono more than .8mm netmovement per stage.
The patient was seen everyfour weeks for delivery of newaligners and monitoring of treatmentprogress and aligner fit(Fig. 5). Aligners were changedby the patient weekly at first, andlater at two-week intervals.Twelve stages were required inthe upper arch and 22 in thelower, with the last five mandibularaligners used in finishing toadd mesial root tip of the lowerright cuspid.
Total treatment time was11 months. The patient was thengiven Hawley-type upper andlower retainers to be worn atnight.
Post-treatment facial photographsshowed little change infacial profile (Figs. 6A, 6B). Althoughthe patient was protrusive beforetreatment, her profile was acceptableto her, and there was noplan to change it. The Class Imolar and canine relationshipwas maintained, and the mandibularspaces were completelyclosed. The overjet was slightlyexcessive due to the thick marginalridges of the upper incisors;otherwise, good overjetand overbite were achieved despitethe extraction of a lower incisor.The gingival recession inthe lower right central incisor regionincreased during treatment.
Both arches showed goodalignment, with the upper midlinecentered on the middle ofthe lower incisors. Comparisonof the post-treatment occlusalphotographs with the computerimages of their final stagedemonstrated the accuracy of theappliance in achieving the desiredresult (Fig. 7).
The post-treatment panoramicx-ray revealed excessivedistal root tip of the lower rightcuspid and a slight mesial roottip of the lower right incisor,along with mild root resorptionof the lower right central incisor.The lateral cephalogram showedreduction of the overjet and increasedproclination of the lowerincisors (Table 1). Because thepre- and post-treatment lateralcephs were taken on differentmachines, the tracings were notsuperimposed. Superimpositionof the occlusograms, however,revealed minor alignment withbuccal expansion of the upperright second bicuspid, a shift ofthe mandibular midline to theright, and buccal expansion inthe lower right second bicuspidregion (Fig. 8).
Small interocclusal gapsbetween the first and second molarscould be attributed to excessiveforces placed on the posteriorocclusion during aligner wear.Such spaces are usually transientin nature, but patients should bemade aware that bite settlingwith appliances such as Hawleyretainers, positioners, or up-and-downelastics may be requiredtoward the end of treatment. Inthis case, the patient was givenHawley instead of vacuumformedretainers, and the posteriorocclusion settled, as evidencedby photographs takenone year after retention (Fig. 9).
The Invisalign System requirespolyvinyl siloxane impressionsfor longer shelf life,better accuracy, and multiplepours. Full-arch impressions aredifficult to take with this material,but are critical to the technique.
Invisalign treatment requiresthe clinician to plan outsequential movements for everytooth from beginning to end--asomewhat different diagnosticprocess than with conventionalappliances. ClinCheck allowsthe clinician to evaluate the entiretreatment carefully and criticallyin all three planes of space.In the present case, the increasedproclination of the lower incisorswas overlooked by the orthodontistduring the ClinCheck procedure.This proclination may haveexacerbated the gingival recessionin the lower incisor region.
Tipping of the teeth into theextraction site may have been theresult of overly aggressive toothmovement--as much as .38mmper week. That is more than thecurrent Align Technology recommendationof .33mm perstage, with each aligner worn fortwo weeks.
All in all, however, it appearsthat lower incisor extractionwas an appropriate choice inthis case. The slight Class IIItendency, Bolton discrepancy,well-aligned upper arch, andcrowded lower arch all contributedto a good result.
This was the first lower incisorextraction case treated withthe Invisalign System. The treatment time was comparable tothat of fixed appliances, andtherefore offers evidence of a viablealternative to conventionaltechniques.ACKNOWLEDGMENTS: The authorswould like to thank Amanda Ramirez for herhelp and Justin Tindall for his assistance withimaging.