Crown-to-root dentistry

ratio: Its significance

Robert E. Penny, D.M.D.,

and Jan H. Kraal, D.D.S.,

Rock Hill, S. C., and Lexington,


Professor, College




Department of Dentistry.





oar crown-to-root ratio can result from improper dental treatment as well as from traumatic or pathologic changes that either increase the length of the clinical crown or decrease the length of the clinical root. However, the most common cause of poor (increased) crown-to-root ratio is periodontitis. Its predominant role in the crown-to-root ratio problem can be extrapolated from epidemiologic data on periodontal disease. In 1955, Marshall-Day and associates’ found crestal loss of alveolar bone in 98% or more of a sample of individuals 35 years of age or older. In 1962, examination of a random sample of Americans revealed the increased prevalence of periodontitis and advanced tissue destruction associated with older age groups.’ Since the prevalence and severity of periodontitis increases with age, problems in crown-to-root ratio are usually associated with an adult population. This article considers the problems associated with poor crown-to-root ratio and reviews treatment modalities for teeth with poor crown-to-root ratio. Mobility, as related to crown-to-root ratio, occurs when alveolar support is no longer adequate to withstand the forces encountered in the oral cavity. Tooth mobility becomes significant when the requirements of comfort and masticator-y function are compromised.’ Development of dental caries on exposed root surfaces is a potential problem. A recent study revealed increased amount of caries on exposed root surfaces in the mandibular arch, most frequently in premolars.’ In these patients the prevalence of root surface caries did not correlate with the degree of oral hygiene or with evidence of previous coronal caries. The prevalence of root surface caries in patients with or without periodontitis was not corre*Associate Kentucky,

in restorative

of Periodontics,





lated with oral hygiene levels, but with fermentable carbohydrate intake.” These data suggest that the incidence of root surface caries may be a function of diet rather than an inevitable sequelae of root exposure. The root surface concavities and increased surface area associated with exposed roots also complicate oral hygiene efforts, thus favoring an increased incidence of caries. Sensitivity from exposed root surfaces is also a common problem. A variety of techniques and substances are available for desensitization.” Unfortunately, no one approach is uniformly successful. The early guidelines on crown-to-root ratio for abutment teeth were conservative, but they still serve as a standard in many texts7-ld Ante’s Law; states that “The combined pericemental area of the abutment teeth should be equal to or greater in pericemental area than the teeth to be replaced.” Removal of all teeth or roots that are “unfit” for further service was also recommended. This conservative approach assured successful results if sound operative and prosthodontic principles were applied. However, this approach provided limited treatment alternatives. A better understanding of the etiology of dental caries and periodontal disease and improved treatment capability allow for a more sophisticated consideration of crown-to-root ratio.



Crown-to-root ratio is the ratio of the respective tooth parts. It is important to differentiate between anatomic and clinical aspects of this relationship. The anatomic portions are defined by the location of the cementoenamel junction. This demarcation gives no information on the amount of alveolar support. The clinical portions, however, are defined by the level of supporting alveolar bone as determined


+ 05$00.50/O

0 1979 The C. V. Mosby




radiographically. The level of supporting bone is rarely coincident with the cementoenamel junction or dentogingival junction (Fig. 1). Evaluation is best performed using the clinical crown-to-root ratio. Further use of the term crown-to-root ratio will refer to the clinical ratio unless otherwise specified. Jepsen”’ compared root surface areas and radiographic root areas and established that they could be correlated within a 10% to 15% margin of error, thereby demonstrating the validity of radiographic evaluation. One textbook recommends the use of Ante’s Law when allowances for a 15% to 20% variation in computations of the pericemental area are made.” Other textbooks proposed the use of actual crown-to-root ratio in determining prognosis. Presumably these are based on linear measurements from radiographs. A ratio of 1:2 was considered ideal, 1: 1.5 was acceptable, and a crown-to-root ratio of 1: 1 was considered minimal or doubtful.+” Crown-to-root ratio was also discussed in terms of the linear amount of bone loss although the importance of this approach varies with root form and length.“-‘” Teeth exhibiting extensive bone loss, with pocket depth greater than 6 to 7 mm from the cementoenamel junction, are sometimes considered hopeless because of the compromises encountered in periodontal surgery. Accurate and thorough probing of each tooth is required to determine the bony topography around the tooth. Bone loss accompanying a poor crown-to-root ratio has also been expressed as the fraction of alveolar support remaining. Tylman” recommended that teeth with a normal amount of bone be used for abutments. However, he stated that teeth lacking one third to one half of their normal periodontal attachment, when judiciously selected, could render satisfactory service. Beube,” discussing the retention or extraction of teeth, assigned a poor prognosis to teeth with only one third of the apical bone remaining, advanced mobility, and poor root morphology. Goldman and Cohen” advocated the retention of teeth based on their ability to return to health and maintain themselves in function.



Plaque. Plaque control and adequate oral hygiene are of primary concern in teeth having poor crownto-root ratio. Continued progression of periodontitis due to inadequate plaque control invites treatment failure.’ !, Ii Structural changes resulting from restorative and periodontal treatment make successful oral








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Fig. 1. Mandibular second premolar with severe bone loss demonstrating the contrast between anatomic and clinical crown-to-root ratio. hygiene efforts more difficult. Examples are the addition of margins and solder joints and the exposure of less accessible, concave crown and root surfaces.” Periodontal surgery. Periodontal surgery can affect the crown-to-root ratio. Complete osseous resection of periodontal bony defects to create physiologic contours may result in loss of surrounding bone. This applies to the teeth involved with the defect and adjacent teeth if ideal hard and soft tissue architecture is to be established. The significance of the increase in clinical crown length may only apply in severe osseous defects. Selipsky’” noted that the decreased mobility obtained in initial therapy was not compromised in the long-term (1 year) by definitive surgery within “clinically operable limits.“ls Periodontal support regeneration. Regeneration of lost periodontal support is the most logical approach to improve poor crown-to-root ratio, and bone grafting is the most reliable method. ‘IJnfortunately, contradictory findings in published reports of this approach obviate strict comparison of the different techniques.‘” Ingber”’ presented the rationale and technique of forced eruption as a method of treating oneand two-wall infrabony defects. Improved crown-to-root ratio and osseous architec-



ture result from occlusal reduction (crown shortening) and ensuing eruption. Such changes in crownto-root ratio and osseous contours also have been reported in the treatment of osseous defects of periodontosis.” Occlusal reduction. Reducing clinical crown length by occlusal reduction of extruded teeth is a valid approach to improving the crown-to-root ratio. Bohannan and Abram? discussed crown shortening in conjunction with intentional pulp extirpation. They noted an improved crown-to-root ratio but encountered complications. For each millimeter of posterior tooth reduction and resultant decrease in the vertical dimension of occlusion, an increase of 3 mm of anterior vertical overlap (overbite) will occur. Overdentures represent an extreme approach to crown shortening and crown-to-root ratio improvement, providing a new treatment alternative. Increasing stability. The mobility seen in teeth with poor crown-to-root ratio can be reduced by selectively grinding occlusal surfaces and minimizing horizontal forces in the existing dentition.” In a theoretical computer model that related applied occlusal forces to measured root surface areas, Hillam”” found that in teeth subject to horizontal forces, pressures on the periodontal ligament were rapidly increased when bone loss exceeded 55% of the alveolar bone height. In teeth that were loaded axially, the rapid increase of pressure on the periodontal ligament did not occur until 80% of the supporting bone had been lost. Although this was a theoretical model, the results coincide with clinical observations of teeth with poor crown-to-root ratio. Teeth which have poor crown-to-root ratio and exhibit mobility can be retained through splinting. Initially, teeth may be temporarily or provisionally splinted as a diagnostic test of their ability to function in mastication and return to health.” However, mobile teeth that do not respond (decreased mobility) to removal of local irritants and selective grinding should be carefully evaluated as to the cause of this mobility. Also, care must be taken to determine the patient’s commitment to the final restorative therapy prior to permanent changes in tooth structure. Some dentists feel that splinting is indicated in periodontal therapy only when individual teeth no longer withstand functional stresses.‘“’ Ifi Dawson” emphasized the difficulty in maintaining good oral hygiene in splinted areas and suggested splinting only when it is needed. Many authors recommend multiple abutments for favorable force distribution when treating teeth with poor crown-to-root ratio.‘“-19. lj. 16. “w% Photoelastic 36



models have been used to analyze the stresses placed on the dentition in splinted and nonsplinted teeth.‘“, Z’ A better distribution of forces is achieved with splinted teeth than with free-standing teeth surrounded by edentulous spaces. The applicability of these studies to living systems is not conclusive, although these observations do coincide with those obtained by Glickman and associates’” in their histologic study of splinted and nonsplinted teeth.



Cast restorations for teeth with poor crown-to-root ratios place greater demands on the dentist. Ideal margins of restorations are essential, since inflammation has been associated with restorations having excellent margins.17 However, margins may be kept away from the gingiva. Design of the preparations for cast restorations are dictated by the anatomy of the root surfaces, which may necessitate endodontic therapy. Root anatomy exerts further influence in making castings. Contours must be consistent with existing root contours and clinical crown form to permit essential hygiene. Obtaining laboratory services that include proper contours, margins, and prescribed occlusion also may be difficult.” EXTRACT’ION Extraction must be considered as a treatment alternative. Removal or retention of molar teeth related to furcation. involvement was reviewed by Saxe and Carmen.“’ These considerations also apply to teeth with poor crown-to-root ratio. These authors suggested that the indications for removal of problem teeth are (1) an unopposed terminal tooth in an arch, (2) a periodontally involved tooth with sound adjacent teeth providing other treatment alternatives, and/or (3) a solitary distal abutment that exhibits mobility. Generally, any noncritical tooth with serious periodontal liability should be removed. Some seriously involved teeth may be retained if (1) an involved terminal tooth in an arch is the antagonist for a sound tooth and (2) a solitary tooth will serve as an abutment.



The problem of crown-to-root ratio should be approached from the standpoint of the health of each individual tooth. The primary consideration is maintaining a noninflammed periodontium through meticulous patient oral hygiene. This provides opportunity for the use and retention of teeth with JULY








reduced periodontal support even in the presence of mobility.‘” Mobility, in the absence of gingival inflammation, has been demonstrated to be a reversible process that does not result in the loss of connective tissue attachment or the formation of periodontal pockets?” The widened periodontal ligament space that is observed is regarded as a physiologic adaptation..“‘!’ Obviously, mobility is the second major clinical problem associated with poor crown-to-root ratio. Considering the previous data relative to mobility, trauma from occlusion, and control of inflammation, the problem of mobility is narrowed. If a tooth can function in mastication and not be a source of discomfort or distraction for the patient, then mobility can be acceptable, since the prognosis of the tooth is not certain. Nyman and associates,,’ evaluating the role of occlusion for the stability of fixed partial dentures, demonstrated the functional demands that can be placed on the severely reduced, noninflammed periodontium. Patients were selected for their ability to maintain meticulous oral hygiene, rehabilitated with fixed restorations and periodontal surgery, and evaluated yearly for 2 to 6 years. None of the patients exhibited further bone loss. Mobility was present when the restorations were inserted but was considered acceptable, since it did not increase. Bone height on the abutments was unaltered, and the radiographic width of the periodontal ligament space did not widen and was narrowed in some patients. This study demonstrated that secondary occlusal trauma in the noninflamed peridontium is not harmful. Additionally, it demonstrated a continuing stability that can be obtained with fixed restorations in patients where there is a minimum of remaining periodontal tissue support, even in combination with marked mobility of individual abutment teeth. It should be evident at this point that there must be more than one guideline for evaluating teeth with bone loss and poor crown-to-root ratio. Crownto-root ratio alone is not a realistic criterion. Assessment of the teeth is predicated on the elimination of inflammation by plaque control and the removal of all other etiologic agents and contributing factors. With this, one can proceed more confidently in making a definitive treatment plan and predicting the prognosis of the treatment.

lems were reviewed. Treatment possibilities were discussed in terms of plaque control. periodontal surgery, occlusal adjustment by selective grinding, splinting, restorative considerations. and extraction. The original guidelines for crown-to-root ratio in the selection of abutments were found tcj be exceptionally conservative and treatment limiting. New treatment modalities were considered in light of increased understanding of periodontal inflammation and its control. With inflammation controlled and with a carefully designed occlusion, some degree of mobility may be tolerated, thereby permitting the retention of teeth with minimal alveolar support. REFERENCES I.





6. 7. 8. 9

10 11 12





The definition of the crown-to-root ratio, its manifestation as a clinical problem, and associated probTHE JOURNAL



Marshall-Day, C. D., Stephens, R. G., and (.&igley, I,. I;., Jr.: Periodontal disease: Prevalence and incidewr. ,J. Periodontol 26:185, 1955. Johnson, E. S., Kelly, J. E., and Van Kirk. I,, E.: Srlected dental findings for adults. National Centrr for Health Statistics. Series 11, No. 7, Washington. I).(‘.. 1965, U.S. Public Health Service. Nyman, S., Lindhe, J., and Lundgren. D.: The rolr of occlusion for the stability of fixed bridges in patients with reduced periodontal tissue support. J Clin Periotfontol 2:53, 1975. Sumney, D. L., Jordan, H. V., and Englandcr H. V.: The prevalence of root surface caries in selected populations. ,J Periodontal 44:500, 1973. Hix, J. 0.. and O’Leary, T. J.: The relationship between cemental caries, oral hygiene and fermentable carhohydrare intake. ,J Periodontol 47:398, 1976. Goldman, H. M., and Cohen, D. W.: Periodontal Therapy, cd 5. St. Louis, 19’73, The C. V. Mosby Co. Ante, I. I-I.: The fundamental principles of abutmenrs. Mich Dent Sot Bull 8~14, 1926. Smith, G. P.: Objectives of a fixed partsal denture. .J PROSTHE?. DENT 11:463, 1961. Johnston, J. E., Phillips, R. W.. and Dykema, K. W.. Modern Practice in Crown and Bridge Prosthodontics. t:d 3. Philadelphia, 1971, W. B. Saunders Co. Reynolds, J. M.: Abutment selection for fixed prosthodontics. J PROSTHET DENT 19~483, 1968. Dykema, R. W.: Fixed partial prosthodontics .J ‘fenn Dent Assoc 43:309, 1962. Tylman, S. D.: Theory and Practice of Crown and Fixed Partial Prosthodontics. ed 6. St. I.ouis, I9iO. The C. 17. Mosby Co. Jepsen, A.: Root surface measurement and a method for x-ray determination of root surface area. Acra Odontol Stand 21:35. 1963. Beube, F. E.: Correlation of the degree of alwolar bone loss with other factors for determining the removal or retention of teeth. Dent Clin North Am 13:801, 1969. Amsterdam, M.: Periodontal prosthesis: L5 years in retrospect. Alpha Omegan 67:8, 1974. Prichard, ,J, F.: .4dvanced Periodontal Disease: Surgical and Prosthetic Management, ed 2. Philadelphia. 1972. W. B. Saunders Co. 37


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Silttess, J.: Periodontal considerations in patients treated with dental bridges. J Pcriodontol Rcs 9:50, 1974. Selipsky, H.: Osseous surgery-How much need we compromise. Dent Clin North Am 20:79, 1976. Ellegaard, B.: Bone grafts in periodontal reattachment procedures. J Clin Periodontol 3: I, 1976. Ingber, J. .S.: Forced eruption I: A method of treating isolated one and two wall infrabony osseous defects-Rationale and case sport. J Periodontol 45:199, 1974. Everett, F. G., and Baer, P. N.: A preliminary report on the treatment of the osseous defects in periodontosis. J Periodontol 35:429, 1964. Bohannan, H. M., and Abrams, L.: Intentional vital pulp cxtripation in periodontal prosthesis. J PROSTHET DENT 11:781, 1961. Hillam, D. G.: Stresses in the periodontal ligament. J Periodontol Res 8:51, 1973. Lemmerman, K.: Rationale for stabilization. J Periodontol 47:405, 1976. Dawson, P. E.: Evaluation, Diagnosis and Treatment of Occlusal Problems. St. Louis, 1974, The C. V. Mosby Co. Kronfeld, M.: Mouth Rehabilitation: Clinical and Laboratory Procedures, cd 2. St. Louis, 1974, The C. V. Mosby co. Clickman, I., Stein, S., and Smulow, J. B.: The effect of increased functional forces upon the pcriodontium of splinted and non-splinted teeth. J Periodontol 32:290, 1961. Hood, J. A., Farah, J. W., and Craig, R. G.: Modification of

ARTICLES Fabricating



stresses in alveolar bone induced by a tilted molar. J PROSTHET DENT 34:415, 1975. Glickman, I., Roebcr, F. W., Brion, M., and Pameijer, J. H. N.: Photoelastic analysis of internal stress in periodontium created by occlusal forces. J Pcriodontol 41:30, 1970. Glickman, I., Stein, S., and Smulow, J. B.: The effect of increased functional forces upon the periodontium of splinted versus non-splinted teeth. J Pcriodontol 32:290, 1961. Saxe, S. R., and Carmen, D. K.: Removal or retention of molar teeth: the problem of the furcation. Dent Clin North Am 13:783, 1969. Svanberg, G., and Lindhc, J.: Vascular reactions in the periodontal ligament incident to trauma from occlusion. J Clin Periodontol 1:58, 1974. Poison, A. M.: Trauma and progression of marginal periodontitis in squirrel monkeys. II. Co-destructive factors of pcriodontitis and mechanically produced injury. J Periodontol Res 9:103, 1974. Waerhung, J.: Pathogenesis of pocket information in traumatic occlusion. J Periodontol 26:107, 1955. Wcntz, F. M., Jaraback, J., and Orban, B.: Experimental occlusion trauma imitating cuspal interferences. J Periodonto1 29:117. 1958.






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Reprint requests to: DR. ROBERT E. PENNY 1144 INDIA Hook RD., STE. D ROCKHILL, S. C. 29730



a post and core to fit an existing


N. Beheshti, D.M.D.

The significance of articulator immediate side shift Neal D. Bellanti,


D.D.S., MS.,


Bruce J. Crispin,

The Micro-Ring


and Kenneth

to posterior


Part II: The prevalence

R. Martin,





D.D.S., MS.

for full subperiosteal


and prosthesis


David D. Dalise, D.D.S.

A measuring procedure shape of dentures

for the determination

Anton J. de Gee, Ph.D., Emmy C. ten Harkel,

and Care1 L. Davidson,

Stress analysis


of disjunct


Ph.D., F.D.S.R.C.S.,

Root retention


and removable




J. W. Farah, D.D.S., Ph.D., A. R. MacGregor,

David N. Firtell, D.D.S., M.A., Thomas W. Her&erg,


of the three-dimensional

and T. P. G. Miller

design D.D.S., and Jeremiah

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JULY 1979





Crown-to-root ratio: its significance in restorative dentistry.

Crown-to-root dentistry ratio: Its significance Robert E. Penny, D.M.D., and Jan H. Kraal, D.D.S., Rock Hill, S. C., and Lexington, P Professor,...
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