The prosthodontic concept of crown-to-root ratio: A review of the literature

Yoav Grossmann, DMD,a and Avishai Sadan, DMDb

School of Dentistry, Louisiana State University Health Sciences Center, New Orleans, La;Case School of Dental Medicine, Case Western Reserve University, Cleveland, Ohio

Crown-to-root ratio is intended to serve as an aid in predicting the prognosis of teeth. However,controversy persists as to its impact on diagnosis and treatment planning. This article critically reviews theavailable literature on the crown-to-root ratio assessment and criteria for evaluation of abutment use ofperiodontally compromised teeth. A Medline search was completed for the time period from 1966 to2003, along with a manual search, to locate relevant peer-reviewed articles and textbooks published inEnglish. Key words used were ‘‘crown-to-root ratio,’’ ‘‘periodontal compromised dentition,’’ ‘‘mobil-ity,’’ and ‘‘biomechanics.’’ There was a dearth of evidence-based research on the topic. Although the useof the crown-to-root ratio in addition to other clinical indices may offer the best clinical predictors, nodefinitive recommendations could be ascertained. (J Prosthet Dent 2005;93:559-62.)

One of the most common, yet difficult clinicaldeterminations is the prognosis of teeth that may serveas prosthetic abutments. With no definitive criteria toguide the clinician, the treatment plan is based, at best,on heuristic information and clinical experience. Becauseabutment teeth are subjected to higher than usual occlu-sal forces transmitted through the prosthesis, the clini-cian must evaluate the abutment teeth carefully. Somehave attemped to establish objective standards for abut-ment evaluation1,2 but have not presented evidence-based criteria. The crown-to-root ratio (CRR) is one ofthe primary variables in the evaluation of the suitabilityof a tooth as an abutment for a fixed or removable partialdenture (FPDorRPD).1-4However, abutmentmobility,alveolar bone support, root configuration and angula-tion, opposing occlusion, pulpal condition, presence ofendodontic treatment, and the remaining tooth struc-ture have also been cited as predictors for abutmentlongevity.5-9

This literature review investigates the assessment andprosthodontic impact of the crown-to-root-ratio, par-ticularly in regard to periodontally compromised teeth.A search of the peer-reviewed English dental literaturefrom 1966 to 2003 was performed using Medline as wellas a hand search of pertinent dental textbooks. Key wordsused were ‘‘crown-to-root ratio,’’ ‘‘periodontal compro-mised dentition,’’ ‘‘mobility,’’ and ‘‘biomechanics.’’

Definition of crown-to-root ratio

The CRR represents the biomechanical concept ofClass I lever for evaluating abutment teeth. The ratio

aMaxillofacial Prosthetics Resident, Department of Prosthodontics,School of Dentistry, Louisiana State University Health SciencesCenter.

bAssociate Professor and Chairman, Department of ComprehensiveCare, Case School of Dental Medicine, Case Western ReserveUniversity.

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is defined as ‘‘the physical relationship between theportion of the tooth within the alveolar bone comparedwith the portion not within the alveolar bone, as deter-mined radiographically.’’10 The fulcrum, or center ofrotation, of the Class I lever is in the middle portion ofthe root that is embedded in alveolar bone.11,12 TheCRR may increase over time, primarily as a result ofloss of alveolar bone support; the crown portion of thefulcrum (effort arm) would then increase, and the rootportion (resistance arm) would decrease. In addition,the center of rotation moves apically, and the tooth ismore prone to the harmful effect of lateral forces.2,6

Increasing the vertical dimension of occlusion to restorethe dentition would also cause an increase in the CRR,without altering the root support. Therefore, someauthors have suggested that teeth that may serve asabutments and be subjected to increased occlusal loads,such as in patients with extreme vertical overlap andbruxism, should be evaluated with other parameters aswell as the measurement of CRR.6,12

It is imperative not to confuse the measurements ofthe anatomical crown, the clinical crown, and the crownfor determining the CRR.While the anatomical crown isthe portion of the natural tooth that extends from thecemento-enamel junction (CEJ) to the occlusal/incisaledge, the clinical crown is the portion of the crownthat extends from the free gingival margin to theocclusal/incisal edge.10 These definitions provide noinformation about the amount of the alveolar sup-port, whereas in the CRR, the crown portion ismeasured in relation to the alveolar bone support.

The CRR definition has several inherent shortcom-ings. The ratio is based on linear measurements only;however, when evaluating abutment teeth, the clinicianshould assess the status of the alveolar bone height andthe total supported root surface of the abutmenttooth.13 Since most roots have conical shape and theroot length is only a 1-dimensional linear measure-ment, other criteria should be used to evaluate the total

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alveolar support of the abutment. For example, it wasfound that if one half of the height of attachment tothe root was lost due to periodontal disease, a mean of61.5% of the actual attachment area to the root wouldbe lost.14 Furthermore, if a mean of 5.72 mm of rootattachment height is lost, or if a mean of 60.6% of thesame root height remains, only one half of the totalroot attachment area would remain to provide toothsupport. 14 Mowry et al15 studied the root surface areaof mandibular canines and premolars and found that athalf of the root length of these teeth, only 38% of attach-ment remained. The authors also found that increasingattachment loss is related to, but not directly propor-tional to, decreasing root surface area. In multirootedteeth, the relatively limited area of the root trunkprovides more extended surface area than what wouldappear for bone and fiber attachment.16 For the maxil-lary first molar, the mean distance from the CEJ to thepoint at which the roots separate from the root trunkwas 5.0 mm for the mesiobuccal root and 5.5 mm forthe distobuccal root.17,18 However, this area averaged32% of the total root surface area of the tooth and wassignificantly greater than that of any of the 3 individualroots.19 The CRR does not express the actual area ofbone support and, therefore, might underestimate theseverity of bone loss around the abutment.

Radiographic evaluation has been the most widelyused technique in clinical practice for assessing bonelevel around teeth.However, theCRRdefinition, as pre-viously stated, does not recommend a preferred radio-graphic method for determining the ratio. Pepelassiand Diamanti-Kipioti20 evaluated methods of conven-tional radiography for detecting periodontal osseousdestruction and suggested that periapical radiographyis more successful in assessing periodontal osseousdestruction than panoramic radiography. Periapicalradiography was more successful than panoramic inthe detection of especially small osseous destruction(1-4 mm). Panoramic radiography underestimated theosseous destruction, whereas periapical radiography wasrelatively accurate in the assessment of this destruction,regardless of the location.21 Therefore, the radiographicevaluation of the CRR should be based on periapicalradiography rather than panoramic.22 In addition, whenusing panoramic radiographs to assess bone loss and todetermine the CRR, the clinician should use direct mea-surement from the CEJ to alveolar bone rather than theassessment of the proportion of the tooth length withinthe bone. 23

The value of CRR

When describing and discussing the CRR, prostho-dontic literature tends to use vague terms that areopen to interpretation, such as ‘‘favorable,’’ ‘‘appropri-ate,’’ ‘‘satisfactory,’’ ‘‘unfavorable,’’ ‘‘poor,’’ and‘‘unsatisfactory.’’24 A prosthodontic textbook considers

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a CRR for an FPD abutment of 1:2 to be ideal, but inpractice this is rarely observed.3 This ratio is based onstudies of periodontally healthy subjects for whom theroot length and the alveolar bone height are 60% to70% of the tooth length and the alveolar bone heightis 90% or more of the root length.25,26 Dykema et al3

suggested a ratio of 1:1.5 as an acceptable and desireableCRR for abutments, although the authors state that theless favorable proportion may be acceptable when theperiodontium is in healthy condition and the occlusionis controlled. Teeth with a normal amount of bone sup-port should be used for abutments; however, cliniciansshould consider teeth with loss of more than one thirdof the periodontal support to be of questionable valueas abutments.24 Shillingburg et al1 suggested a 1:1.5CRR as optimum for an FPD abutment, or a 1:1 ratioas a minimum ratio for a prospective abutment undernormal circumstances. The authors also indicated thatif the opposing occlusion is composed of tissue-supported prosthesis, a crown-to-root ratio greaterthan 1:1 might be adequate because of the diminishedocclusal forces. Others have suggested that the original1:2 CRR guideline in the selection of abutments isexceptionally conservative and limits treatment.27

Crown-to-root ratio in clinical practice

Clinical procedures directly affect the CRR.Abutment preparation for overdentures has the mostdramatic effect on the ratio, reducing the crown to1 to 2 mm above the free gingival margin,28 which canimprove the CRR from 1:1 to 1:2 or 1:3. The decreasein crown height shortens the corresponding lever armlength, and therefore, less lateral force is applied to theattachment apparatus, with an apparent reduction ofthe abutment horizontal mobility.29 In a longitudinalstudy of overdenture patients, Renner et al30 demon-strated that over a 4-year period, 50% of the roots re-mained immobile, 25% of the roots that were initiallymobile exhibited no mobility, and 25% of the roots de-creased in mobility. Abutment mobility was correlatedto general periodontal health, as well as to the improvedbiomechanical CRR. Conversely, any increase in thevertical dimension of occlusion (VDO) increases theCRR. No clinical study was identified that evaluatedCRR measurements or tooth mobility after theVDO was increased with prosthetic or orthodontictreatments.

Surgical crown lengthening is often necessary torestore teeth that have been compromised by caries,trauma, or extensive wear.31 Crown lengthening re-establishes the dentogingival junction at a more apicallevel on the root to accommodate the junctional epithe-lium and the connective tissue attachment.32 Thisprocedure increases the CRR. Forced eruption can beused in addition to, or as an alternative to, crown length-ening for teeth with sound tooth structure at or below

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the bone crest.33,34 Continued, slow, passive or activeorthodontic eruption, in rates of approximately 2 mmper month, allows the periodontal ligament to repairand the alveolar bone to remodel between orthodonticadjustments. Hence, slow, forced eruption is preferredto surgical removal of supporting alveolar bone becauseit preserves the biologic width and, at the same time,provides better CRR.35-37

Splinting and crown-to-root ratio

Periodontal bone loss around abutments results in anincreased CRR that is associated with increased toothmobility.38 However, increased mobility is not alwaysfound with teeth showing increased CRR.39 Differentperiodontal treatment modalities that resolve the in-flammatory process may result in reduced tooth mobil-ity without changing the previously reduced alveolarbone support.40 Investigators7,38 have demonstratedthe long-term success of periodontally compromisedabutment teeth that supported extensive cross-archFPDs. The exact CRR of these abutments was not calcu-lated, but no mechanical failure of abutment teeth wasattributed to the increased CRR. The prosthodonticconcept of splinting teeth, especially abutments, evolvedfrom the need to compensate for the increased CRR.12

Splinting abutments may enhance stability andmay shiftthe center of rotation and transmit less horizontal forceto the abutments.41 However, some in vitro studies donot support this theoretical model.42,43

Itoh et al,42 using a photoelastic model, evaluated theeffects of periodontal support and fixed splinting on loadtransfer by RPDs. The authors found that increasing thenumber of splinted teeth did not provide a proportionaldecrease in maximum stress levels and stated that rou-tine cross-arch splinting may not be appropriate.Using a photoelastic model, Wylie and Caputo43 evalu-ated the stresses that cantilever FPDs developed in teethand supporting bone where the most distal abutmentshad osseous defects and hence increased CRR. The au-thors found that for a cantilever FPDwith either normalperiodontal support or a distal abutment with a moder-ate degree of mobility and bone loss, the occlusal forceswere significantly distributed to only the 3 teeth closestto the loaded cantilever. Moreover, increasing the num-ber of splinted abutments beyond 3 did not result in aproportional reduction of stress in the periodontium,and no significant cross-arch sharing of occlusal loadswas seen. No objective criteria were identified in the lit-erature to define the need or extent of splinting in rela-tion to the abutment CRR, and the effect of splinting onabutment longevity has not been established. Therefore,when evaluating the need for splinting of periodontallycompromised teeth, the clinician should consider otherpredictive indices, such as the presence of increasingmo-bility, initial probing depth, initial furcation involve-

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ment, patient ability to maintain optimal oral hygiene,presence of a parafunctional habit without the use ofan occlusal stabilization device, and tobacco use.6,7,44

Crown-to-root ratio as a prognostic tool

The primary objective in evaluating clinical criteriafor abutments and periodontally compromised teeth isto determine the best prognosis. The clinician identifiesobjective findings that can predict the prognosis ofteeth as abutments. Abutment teeth may or may not re-quire more rigid standards due to increased functionaldemands.44 McGuire and Nunn45 evaluated 100periodontally treated patients (2,484 teeth) undermaintenance care for 5 years (with 38 of these patientsfollowed for 8 years) to determine the relationship ofassigned prognoses to the clinical criteria commonly usedin the development of prognosis. The authors classifiedteeth as having either a favorable or unfavorable CRR,although no numeric measurment was mentioned.Unsatisfactory CRR and teeth used as fixed abutmentswere among the clinical factors that resulted in worseinitial prognoses. The coefficients calculated from thesuggested model were able to accurately predict the5-year and 8-year prognoses 81% of the time. None ofthe examined factors, including the CRR, was signifi-cant in worsening the prognosis; the assignment ofprognosis was ineffective for teeth with an initial prog-nosis of less than ‘‘good.’’ Nevertheless, the presence ofan unsatisfactory crown-to-root ratio was identified asone of the significant clinical factors for clinicians toconsider.45

DISCUSSION

As a suggested clinical guideline for the evaluation ofabutment teeth, the clinician should use the crown-to-root ratio only with other multiple clinical parameters,such as abutment mobility, total alveolar bone support,root configuration, opposing occlusion, presence of aparafunctional habit, pulpal condition, presence of endo-dontic treatment, and the remaining tooth structure.The total remaining periodontal bone support providesmore accurate information than the linear measurementof the ratio, which is limited even in the prediction ofthe prognosis of nonabutment teeth. Therefore, indica-tions other than the crown-to-root ratio should be usedto determine whether splinting of teeth is appropriate.

Confounders make it impossible to isolate a singleclinical parameter, such as CRR, from others in vivostudies. However, long-term prospective clinical studiesare required to identify the exact prognostic value ofeach clinical requirement for abutments. Futureresearch should concentrate on predictive indices thatwill assist the clinician in deciding whether to preservecompromised teeth or place implants.

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SUMMARY

There is a lack of consensus and evidence-basedresearch on the influence of crown-to-root ratio ondiagnosis and treatment planning for periodontallycompromised teeth. It appears that multiple factorsmay play a role in determining the prognosis of abut-ments considered for support of a fixed or removableprosthesis.

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