deciding on retention in ist and iind class kennedy...

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SummaryThe clinician must plan to ensure that the prosthesis resists movement away from the tissues thatsupport it, irrespective of whether it is to be fixed or removable. Removable prosthesis achieves itsretention in a variety of ways (clasps, precision attachments, planned use of undercuts and guideplanes) and using neuromuscular control. The use of resilient snap attachments in IInd and Ist classKennedy edentulous situations is an esthetic and functional solution, but we need to be aware of thepotential for bulk and to plan common path of insertion and withdrawal. Objectives. The study intended to establish the protocol that is to apply in designing the prosthesiswith a view to securing the best results in retention using snap resilient attachments and the way thedirect retainers’ action is increasing with the presence of the indirect retainers on the R.P.D. (remov-able partial denture) design. Material and methods. We treated and observed a group of 30 patients aged between 45-60 years oldin different situations of IInd and Ist class Kennedy edentulous arches, which had snap R.P.D. withor without indirect retainers.Results. Using snap resilient attachments in Ist and IInd class Kennedy edentulous patients provedto be an inspired long term solution considering that mobility measurements on the abutment teethmade with dentoperiodontal mobile meter device registered no variations during this period of timeif the denture had indirect retainers.Conclusions. The indirect retainers are protecting the edentulous ridge to accelerate resorption; theyare increasing the retention of the R.P.D. with snap attachments and they are precluding the frac-ture of the attachment.

Key words: R.P.D., snap spherical attachments, indirect retainers.

Introduction Planning removable partial denture

retention is an integral part of the treatmentplan that concerns both the fixed parts(retained through parallelism, friction andcement materials) and the mobile prosthesisitself for which the clinician is designingdirect and indirect retainers. The directretainers are the snap spherical resilientattachments while the indirect retainers are

the guide plans, the opposing arms (milledsupport for stress distribution arm) and theinterlocks (proximal connecting elements)milled on the parallelometer in the insertion-withdrawal path.

Material and methodsAccording to The Glossary of Prostho-

dontic terms (Academy of Prosthodontics1999) a precision attachment is “a device

Deciding on retention in Ist and IInd class Kennedy edentoulism

Oana-Cella Andrei1, Mihaela Pauna2

Department of Removable Prosthodontics, Faculty of Dental Medicine,“Carol Davila” University of Bucharest

VVAARRIIAA

1 Assist. Prof., Department of Removable Prosthodontics, Faculty of Dental Medicine, “Carol Davila” University ofBucharest

2 Professor, Department of Removable Prosthodontics, Faculty of Dental Medicine, “Carol Davila” University ofBucharest

OHDMBSC - Vol. VI - No. 1 - Martie, 2007

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which comprises of two or more compo-nents which is machined or fabricated forthe purposes of providing retention to pros-thesis”. The snap-type precision attachmentsmay be resilient or non-resilient. The verti-cal space necessary for the use of snapattachments is generally of about 5 mm butthe system is available in various dimen-sions.

To ensure comfort and functionality tothe patient the prosthesis must be as stableas possible during its current use.Irrespective of whether the prosthesis isfixed or removable the clinician must plan

the way it would withstand the tendency ofmoving away from the support tissues. Thefixed prosthesis gains its retention throughparallelism, friction and cement materials.The mobile prosthesis gains its retention inmany more ways. The mobile prosthesiswith snap attachments uses this type ofextracoronal matrix-patrix precision attach-ments as main mean of retention (Figures 1,2, 3, 4.). This kind of precision attachmentsmay be fabricated totally metallic or havingpolymer components. Using polymer com-ponents has at least two advantages:

Figure 1. Prefabricated castable(patterns) plastic patrix

Figure 2. Polymer changeablematrixes: green for reduced reten-tion, yellow for medium retention,

red for increased retention

Figure 3. Accessories for theparallelometer

Figure 4. Matrixinsertion sprue used

for placing the matrixinto the prosthesis

(www.bredent.com)

- increased resilience therefore protec-tion against overloading the abutment teeth;

- can be replaced when they are looseand the system (and also the prosthesis)regains its initial retention.

We treated and observed a group of 30patients aged between 45-60 years old indifferent situations of IInd and Ist classKennedy edentulous arches, which had snapR.P.D. with or without indirect retainers.The prosthesis has an efficient retention if itdoes not move away involuntarily from thesupport structures when the patient is chew-ing sticky aliments (e.g. white bread). Thepassage from clasps to precision attach-

ments is being determined by esthetic crite-ria considering the excessive visibility of theclasps and not the retention’s efficiency thatis sufficient in their case. Precision attach-ments are more esthetic, but tend to achievea stronger than needed retention, to thedetriment of the sustaining system of theprincipal abutment tooth. If the R.P.D.’sdesign is correct (in the sense of the maximfunctional extension of the free-end saddles)and if the indirect retainers have beenincluded in the design, it will not require forretention more than two snaps attachmentsbilaterally distributed [1].


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If these design conditions are notaccomplished, some inconveniency occurs,such as rapid deactivation of the systemthrough premature usage of plastic compo-nents, exaggerate rotational movement offree-end saddles with fracture of some of theprosthesis components and also decompen-sate functional stress on principal abutmentteeth. Normally, a correctly designed pros-thesis on a stabilized subjacent bony tissuedoes not require the change of the plasticcomponents and the relining of saddles ear-lier than two years since application.

It appears that the resilient sphericalsnap attachments are the less soliciting sim-ple precision attachments due to the spheri-cal form of the patrix allowing an insensi-tive movement of the free-end saddles with-out soliciting the abutment teeth, thereforefunctioning as stress breakers. Generally,the use of these retainers requires two abut-

ment teeth limiting the edentulous space.The abutment teeth should be free of inflam-mation, capable of bearing loads and havesufficient length. In the Ist class Kennedyedentoulism these teeth are medially located(Figure 5). In the IInd class Kennedy eden-toulism the most advantageous for the long-term periodontal resistance is the placementof the snap on a bar that would function as abridge between the medial and distal abut-ment teeth (Figure 6).

There are various clinical situations ofedentoulism and the number of designedabutment teeth can be higher or lower.When in the case of the terminal eden-toulism we have only the frontal group asremaining teeth, for periodontal and biome-chanical reasons it is strongly necessary totake all these teeth as abutments in a func-tional block (Figures 7, 8).

Figure 5. RPD for the Ist class Kennedyedentoulism; unified crowns on the medial abutments

Figure 6. RPD for the IInd class Kennedyedentoulism; the snap is placed on a bar

Figure 7. The remaining frontal teeth unified in afunctional block in a large Ist class Kennedy

edentoulism situation

Figure 8. The RPD appearance in a large Ist classKennedy edentoulism situation


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If we have to take the canine as abut-ment tooth in IInd class Kennedy eden-toulism, while the lateral incisor has a rotat-ed position on the arch or has reduceddimensions it is inappropriate to use it asabutment tooth. The dentist can use thecanine as unique abutment if on the oppos-ing part of the arch there is a sufficient num-ber of remaining teeth (Figures 9, 10).

A R.P.D. with resilient snap attachmentmust be simple in design, using the mini-mum number of precision attachments need-ed for direct retainers and the minimumnumber of saddles. It is obvious that the useof a lateral bar functioning like a bridgebetween the limiting teeth of a lateral eden-tulous space is a much more advantageoussolution, inclusively from the point of viewof the periodontal system of these teeth, thanthe placement of a snap on each tooth and

planning a saddle with two matrixes. Thissolution also complicates achieving of theguide plans and selecting the common pathof insertion and withdrawal. Though havingsaid that, when the distal abutment tooth ofa lateral edentulous space has a morereserved prognostic compared to the medialteeth, but not to the extent of requiring theextraction, two medial teeth can be heldtogether while a separate snap can be placeddistally on the molar. Though usually losinga principal abutment tooth leads to the lossof mobile prosthesis itself, in this situationthe extraction of the molar would not affectthe retention as the resilient snap attachmentmedially positioned is sufficient and the lat-eral bounded saddle can be prolonged sothat it transforms to a free-end saddle(Figures 11, 12).

Figure 9. The canine used as medial abutment Figure 10. The free-end saddle - final situation

Figure 11. The last molar and the position of theattachment

Figure 12. The lateral bounded saddle can betransformed in a free-end saddle


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These examples of apparent exceptionsare not recommendations to refrain fromapplying classic principles of prosthesisdesign. On the contrary, these are coming tostress once again that the design of aresilient snap attachment is mandatory to beadapted to each clinical situation by the cli-nician, and the cooperation with the dentaltechnician is of course only in the sense ofthe precise execution of the establisheddesign.

Beside snaps, that are responsible withthe direct retention, the following elementsare also interfering in achieving the indirectretention [2]:

anterior or posterior bony undercuts; the major mandible double bar connec-

tor (designed as a lingual bar plus a contin-uous clasp) (Figures 13, 14);

the neuromuscular control (difficult tobe quantified and also difficult to beobtained in some patients);

use of guide plans, of the opposingarms and the interlocks milled on the paral-lelometer in the insertion-withdrawal path(Figure 15);

in the case of a partial denture it is, forsure, impossible to obtain a peripheral seallike the one obtained with a total prosthesis,though in some cases and on certain areaswe can obtain it if we are extending the free-end saddles to the maximum limits of thesupporting tissues.

The indirect retainers are the compo-nents of the partial denture helping thedirect retainers to foresee and prevent thelift of the distal area of the saddles. Theseare also working as a stop point of the leveron the opposite side of the fulcrum linewhen the saddle rotates around this line [3].

Figure 13. The major mandible double bar connec-tor on the working cast

Figure 14. Mandibular RPD with doublebar connector

Figure 15. Milling and positioning the attach-ments on the parallelometer in the insertion

path


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Figure 16. Milling of the opposing arms andinterlocks on the milling base

Figure 17. The RPD with the elements of toothsupport

Figure 18. The esthetic appearance of the saddleswhen the guide plans are used in the design

Figure 19. When we use the guide plans we canput the first artificial tooth in the closest position

The milled crown offers the potentialfor good support down the long axis of thesupporting tooth and, if planned well, pre-vents coverage of the gingival mucosa bythe major connector. A part of the indirectretainers are represented by some of thetooth-support elements (the opposing armson the stress distributors and the interlocks)also serving to obtain an increased stabilityof the denture (Figures 16, 17).

The absence of the indirect retainersfrom the denture design shall have at leasttwo major disadvantages as regards theretention:

1. we would not have a stopper oppos-ing the rotation of the free-end saddle,movement that cannot be avoided due to thedifference between the two support types(the saddle is tooth-supported medially and

the mucosa and bone supported distally);2. due to this movement, the prosthesis

shall traumatize the tissues.The use of guide plans for the resilient

snap attachment supposes realization andmilling of the crowns on the abutment teethin the insertion-withdrawal path, thereforethe modification of the coronary preparationto include the guide plan at a certain pathand the stress distributors for the opposingarms. Obviously, we have to accentuate thereducing of the abutment tooth in theseareas. This maneuver shall improve both theretention and the aspect of the denture, aswe know that its esthetic limit is betweenthe last abutment tooth and the first artificialtooth. The use of guide plans shall shrink tothe maximum this less esthetic space(Figures 18, 19).


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It is necessary to analyze the boneundercuts on the study and diagnosis castsusing the parallelometer and to harmonizethem with the insert-withdrawal path of thefuture denture [4]. Obtaining the retention ofthe partial denture with resilient attachmentsin the Ist and IInd class Kennedy of eden-toulism is of course complicated by the dif-ferent type of support of the saddles in theabsence of the distal abutment tooth.Obtaining a retention that besides being effi-cient is to also be esthetic is even more dif-ficult as the dimension of the free-end sad-dle is increasing in anterior direction(Figure 20).

If retention and stability are not suffi-cient, the patient shall use prosthesis adhe-sives, this leading to accumulation of bacte-rial plaque, thus to the increase of the cavi-ties and periodontal disease risks on theremaining teeth as well a poor prosthesishygiene.

If the patient is selecting a mobile pros-thesis with resilient snap attachment astreatment plan, he has to have good oralhygiene and mobility within the physiologi-cal limits of the abutment teeth [5]. Oralcavity draining is surely necessary. If thepatient already wears a partial prosthesis,examining the old prosthesis proves its util-ity, offering valuable information regardingthe retaining, support and esthetics.Preparation of study and diagnosis casts is

also mandatory, as well as examination bothindividually and mounted in correct inter-maxillary relationship. This allows theexamination of the occlusion and occlusionplan, usually unleveled due to vertical andhorizontal migrations consecutive to incor-rect fixed treatments or long time untreatedpartial edentoulism.

The dentist cannot design a functional,correct and esthetic prosthesis with resilientsnap attachment unless he first achieves anocclusion plan level. We have to analyzecompletely the study casts on the paral-lelometer where the insertion-withdrawalpath is established. All components of themobile prosthesis with resilient snap attach-ment shall be prepared having into accountthe established insertion-withdrawal path,both the fixed elements cemented on abut-ment teeth and the mobile elements them-selves. In this manner, we will obtain func-tionality and esthetics, as well as superiorbiological integration.

The abutment teeth on which retainingelements are prepared (unified crowns) hasto have a sufficient coronal height to allowobtaining the vertical space necessary bothon the occlusive area and approximately dis-tal on the patrix placement area. The radio-logical exam is mandatory for the teeth andalso a vitality test eventually. When thereare doubts on the vitality of an abutmenttooth, it is preferably to make the endodon-tic treatment, prior to preparation of theprosthesis. Provisory restorations are usefulin the mandatory occlusive plan leveling, onwhich we can make adjustments and inter-ventions to obtain the desired levels. Thecrowns covering the abutment teeth arepoured together with a prefabricatedcastable component (matrix or patrix,depending on the producer) mandatorilybeing provided with stress distributors forthe opposing arms and the interlocks andguide plans to optimize prosthetics stability,retaining and esthetics.

Figure 20. Long free-end saddle


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The clinician must establish the inser-tion-withdrawal path at parallelometer, andmust not leave this task to the dental techni-cian (Figure 21). With the help of the paral-lelometer, the dentist is referring and dimen-sioning all the dental or bony undercuts. Ifthe denture is gaining its retention by usingspherical snaps attachments, to cover theabutment teeth with micro-prosthesis shallbe mandatory (Figure 22). The dental tech-nician must prepare these crowns by paral-lelometer milling the wax in the insertion-withdrawal path, this solving the dentalinterferences with the abutment teeth under-cuts. Conventionally, the dentist shouldselect the insertion-withdrawal path as verti-cally as possible due to the increase com-modity offered to the patient.

In the absence of the guide plan, theinsertion path should be like or coincidewith the withdrawal path. Patients tend toapply and extract the prosthesis vertically,and choosing an insertion path largely devi-ated from the vertical plan can cause diffi-culties, specifically to the disabled patientshaving decreased dexterity. Exaggerate lin-ings of study casts on parallelometer alsocreate false retaining areas, lowering in factthe action of the retaining elements. Inexchange, including the guide plans in thedesign of the prosthesis ease the sliding ofthe denture on the mouth in the insertionpath along those oral milled areas, and only

in this position. Preparation of the guideplans on the crowns placed on the abutmentteeth allows only one insertion-withdrawalpath. In this way, the retainers (the snap)shall resist to the detachment movementsthat may appear only on one direction,namely on the range of the withdrawal path.

Preparation of the crowns shouldensure sufficient occlusal space, reason forwhich the occlusal table of the abutmentteeth shall be anatoformally reduced. Theclinician must bevel the occlusoapproximaledges to secure a better adaptation of thecrown on the abutment tooth. The form ofthe preparation should be cylinder-conicalto ensure friction. The lingual or palatal sur-face of the crown shall have preparations forstress breakers (for the opposing arms) pro-longed to the approximal area (for the inter-locks) to secure tooth support, stability andindirect retention.

The crowns shall have guide plans toincrease retention and to limit the withdraw-al path to only one. The guide plans preventsthe rotational movements and increase snapefficiency. Due to preparation of the guideplans, the tooth shall need a supplementaryreduction in this area. Being an extra-coro-nary system, the snap does not transmitforces along the long axis of the tooth. Forphysiological transmission of occlusal andchewing pressures in the long ax of the abut-ment tooth, it is necessary to prolong the

Figure 21. Milling the crowns and bridges on theparallelometer

Figure 22. Transfer of the precise abutment posi-tion using the spider key


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preparation of lingual or palatal stress distri-butors up to the approximal area. Supple-mentary reduction of crown’s dimension onthe lingual or palatal face (in order to pre-pare the milled stress distributors) is pre-venting the over contouring of the abutmentteeth crown, thus the appearance of an unde-sired undercut area. The patient’s tongue issensing this area as a discomfort. Betweenthe R.P.D. and these guide plans, opposingarms and interlocks also appears a certainfriction degree, this contributing to theretention of the denture.

Both the snap and the stress distributorsprevent the exaggerate clogging of the pros-thesis. The clogging is generating undesiredeffects on the subjacent tissues (hard andsoft) well known from the interim partialdenture that loses quickly its tooth support.This way the design of the denture is pre-venting both trauma of the mucosa and rapidresorption of the alveolar bone, as well astorsion and other disorthodontic effects inthe remaining teeth. Due to use of the milledcrowns, the depth of stress distributors’preparations may be so that they would befunctionally efficient, protecting at the sametime the abutment tooth. The angle betweenthe denture and the abutment teeth crownsduring the functional load is very importantto be of 90° or less.

ResultsIf the prosthesis is designed with guide

plans, the path of insertion will be the samewith the path of withdrawal, and the retain-ers should resist to voluntary and involun-tary detachment and dislodgement onlyalong this path, thus in a single direction. Ifthe prosthesis does not have a unique path ofinsertion-withdrawal, the patient wouldinsert and withdraw it in different positionsthat would finally distort and destroy theprecision attachments. The R.P.D.’s incor-rect design can lead, under the functionalload, even to snap’s metallic patrix fracture.

DiscussionUse of the parallelometer in a treatment

plan allows the consideration of guide plans,undercuts and insertion path. In order toobtain a maximum functionality and an opti-mum esthetics, it is necessary to design thedenture using the parallelometer this allow-ing the increase of retention, support andstability.

The snap partial denture is a versatilemethod of treatment, with a correct cost-efficiency rate and recommended forpatients of all ages. The trends in moderndentistry have changed over the last 20-30years in favor of maintaining as long as pos-sible the natural teeth on the arch and thus ofdecreasing the total denture rate. The goalsof partial denture as a method of treatmentare of course the recovery of the functional-ity, the improvement of esthetics and, mostimportantly, keeping the health of the tooth,mucosa and bone structures on a long-termbasis.

Conclusions The indirect retainers are increasing the

retention of the R.P.D. with snap attach-ments and are precluding the fracture of theattachment. If the clinician is designing thedenture without indirect retainers, it willquickly lose part of the tooth support and theplastic components will be prone to changefrequently. The indirect retainers design isprotecting the edentulous ridge to accelerateresorption. Treating the edentulous patientswith different clinical situations (IInd andIst class Kennedy) with a snap resilient par-tial denture (R.P.D. with spherical attach-ments) is at the same time a functional andversatile method of design that also offerslong-term stability. It is necessary for thepatient to return to the dental office on a reg-ular basis to change the used resilient com-ponent of the system and to reline the sad-dles in order to regain the initial functionalcharacteristics.


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References

1. Andrei OC. Restaurari protetice cu caracterfizionomic in edentatiile partiale intinse. EdituraCartea Universitara, Bucuresti, 2005; pp. 122-165.

2. McGivney GP, Carr AB. McCracken’s,Removable Partial Prosthodontics. Tenth Edition, TheC.V. Mosby Company, ST. Louis, Toronto, Princeton,2000; pp. 143-151.

3. The glossary of Prosthodontic terms, Academyof Prosthodontics, 1999.

4. Ionescu A. Tratamentul edentatiei partiale cuproteze mobile. Editura National, 1999; pp. 58-77.

5. Davenport JC, Basker RM, Heath JR, Ralph JP,Glantz OP. A Clinical Guide To Removable PartialDentures. British Dental Association, London, 2000;pp. 3-14.

Correspondence to: Dr. Oana-Cella Andrei, DMD, Assist. Prof., Department of RemovableProsthodontics, Faculty of Dental Medicine, “Carol Davila” University of Bucharest. Address:Brândusilor str., no. 7, bloc G3, sc. 1, ap. 1, sector 3, Bucuresti, Romania. E-mail: [email protected]

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