Continuing Education Course Number: 135
Importance of Crown to Root and Crown to Implant Ratios Authored by Gary Greenstein, DDS, MS, and John S. Cavallaro Jr, DDS Upon successful completion of this CE activity 2 CE credit hours may be awarded
A Peer-Reviewed CE Activity by
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Recommendations for Fluoride Varnish Use in Caries Management abutments in dental prostheses. However, guidelines were based upon empiricisms rather than scientific data. This resulted in teeth being extracted that could have been retained. With respect to dental implants, the desire to reduce crown to implant ratios (CIRs) could result in an area being unnecessarily bone augmented to provide additional support for a longer implant. Therefore, it would be advantageous if an assessment of the dental literature provided guidance as to the survival of restored teeth and implants with elevated crown to root or CIRs. Fixed denture prostheses (FDPs) can be retained by teeth, dental implants, or a combination of both. In either scenario, a FDP needs an adequate number and size of abutments to provide support for a restoration. The number of abutments to retain a prosthesis is dependent on numerous clinical variables, including size and design of the prosthesis, number of missing teeth, occlusal and parafunctional forces, opposing dentition, amount of available bone around abutments, and cost. With respect to providing adequate support for a prosthesis, the issue of root or implant length needs to be considered. The objective of this article is to discuss the importance of CRRs and CIRs based on a search of the literature focusing on 4 overlapping topics: (1) relevance of Ante’s law to reconstructive dentistry, (2) CRR and CIRs, (3) the utility of short versus long dental implants, and (4) the concept of vertical cantilevers.
Importance of Crown to Root and Crown to Implant Ratios Effective Date: 03/1/2011
Expiration Date: 03/1/2013
LEARNING OBJECTIVES After reading this article, the individual will learn: • The clinical importance of crown to root ratios (CRRs) and crown to implant ratios (CIRs). • Suggestions for restoring teeth and implants based on a search of the literature focusing on CRRs and CIRs.
ABOUT THE AUTHORS Dr. Greenstein is clinical professor, Department of Periodontology, College of Dental Medicine, Columbia University; Private Practice, Surgical Implantology and Periodontics, Freehold, NJ. He is a board Diplomate and Fellow of the American Academy of Periodontology, and has authored more than 100 articles on periodontal and implant therapy. He can be reached via e-mail at
[email protected]. Disclosure: Dr. Greenstein reports no disclosures. Dr. Cavallaro is clinical director of the Implant Fellowship Program, College of Dental Medicine, Columbia University, New York, NY; private practice, surgical implantology and prosthodontics, Brooklyn, NY. He is a member of the Academy of Osseointegration and a former Fellow of the Greater New York Academy of Prosthodontics. He can be reached at
[email protected].
ANTE’S LAW In 1926, it was suggested that the total periodontal membrane area of abutment teeth must equal or exceed the membrane area of teeth to be replaced.1 This was referred to as Ante’s Law, but it actually was an opinion that was never scientifically validated. Unfortunately, application of this concept precluded employing numerous teeth as abutments, because they had lost periodontal support. Thus, teeth were extracted that might have had a better prognosis than expected.2 In this regard, a systematic review (6 publications) by Lulic, et al3 surveyed the literature from 1966 to 2006. They evaluated the survival of FDPs when periodontally healthy teeth whose periodontium was severely reduced were incorporated into prostheses. To be included in the review the restorations had to be in
Disclosure: Dr. Cavallaro reports no disclosures.
INTRODUCTION Historically, crown to root ratios (CRRs) of teeth were used as a parameter to help decide if teeth should be restored with or without splinting to adjacent teeth or employed as 1
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Importance of Crown to Root and Crown to Implant Ratios function for 5 years. While not specifically stated in the text, it is reasonable to presume that teeth with reduced periodontiums incorporated into FDPs often demonstrate increased CRRs. Lulic, et al3 reported that among 579 FDPs, the survival rate was 96.4% and 92.9% after 5 years and 10 years, respectively. These findings are similar to data concerning survival of FDPs fabricated on teeth with good periodontal support.4,5 In summary, considering the data from long-term clinical trials, it can be concluded that the concept referred to as Ante’s Law with respect to teeth has been refuted. Furthermore, since teeth and implants are retained by 2 different mechanisms (periodontal ligament versus osseointegration), the concept of extrapolating guidelines for patient care with one type of support to the other would be inappropriate.
an increase in vertical dimension of occlusion and surgical crown lengthening or ridge reduction to make room for an implant abutment increase the CRR.6 After assessing the literature, Grossmann and Sadan6 concluded that no definitive recommendations could be established for an optimal CRR concerning teeth. To compensate for increased forces on prostheses due to increased CRRs, clinicians have splinted teeth together.10 This may shift the center of rotation and transmit less horizontal forces to individual abutments or alter the response to the applied forces.11,12 However, no objective criteria exist to define the need or extent of splinting to assuage the effect of an increased CRR. 13 Dental Implants A consensus conference defined a desirable crown height space for a fixed prosthesis to be between 8 to 12 mm (bone level to opposing dentition).14 This height leaves 3 mm for soft tissue (includes biologic width and soft-tissue coverage of implant collar), 2 mm for occlusal porcelain, and an abutment ≥ 5-mm high. However, it was cautioned as the height of the prosthesis increases there is increased risk of component and material fracture due to elevated forces on the restoration.14 Therefore, increased crown height should be considered as a factor that can affect clinical outcomes both technically and biologically. In this regard, a projected increased CIR can be reduced by increasing the bone height surgically with ridge augmentation or distraction osteogenesis. On the other hand, if increased CIRs were proven to be safe to use, it would provide a variety of advantages including avoiding some guided bone regeneration procedures, abridged treatment time, facilitating a greater number of patients to be treated, and decreased fees. Therefore, the literature was searched to determine if increased CIRs enhance therapy or induce additional stress that eventually has a deleterious effect on bone adjacent to implants supporting prostheses or prostheses’ components.
CROWN TO ROOT RATIOS AND CROWN TO IMPLANT RATIOS Teeth Frequently, the CRR is used to determine if a tooth could function as a suitable abutment. The term refers to a ratio calculated from a radiograph with respect to length of the tooth not within bone divided by the portion of the tooth in alveolar bone.6 When forces are applied to a single-rooted tooth with a complete periodontium, the tooth’s center of rotation, or fulcrum, is in the center of the root, two thirds down the root within the bone.6 The main reason for increased CRRs is fabrication of taller crowns on teeth or pontics subsequent to alveolar bone loss. This results in the crown portion of the prosthesis providing a greater lever arm and the root providing less resistance. When there is additional bone loss, the center of rotation moves apically.6 With regard to CRRs discussed in the literature, a variety of ratios were reported. Ideally, the ratio should be 1:2 or 0.5; however, this is rarely seen in clinical practice.7 Dykema, et al7 indicated that a CRR of 1:1.5 is desirable. Similarly, Shillingburg, et al8 suggested a 1:1.5 ratio as most favorable for an abutment and a CRR of 1:1 as a minimum for a tooth abutment. Several procedures alter the CRR as follows: overdentures with a small attachment9 and extrusion of teeth to provide additional bone both decrease the CRR, whereas
PERFORMANCE OF IMPLANTS WITH RESPECT TO CROWN TO IMPLANT RATIOS An elevated CIR has been described as a type of nonaxial loading, which can detrimentally affect a prosthesis 2
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Importance of Crown to Root and Crown to Implant Ratios Figure 1. Single Tooth Implant: tooth position No. 19. Radiograph of a restored dental implant with a millimeter ruler superimposed on the film. It should be noted that to compute the crown to implant ratio (CIR) in this and the other figures, the following was recorded: clinical crown length was the restored crown plus the abutment collar height plus the part of the implant body that was not encased in bone; implant length was the part of the implant encased in bone. In Figure 1, the part of the implant encased in bone is 7 mm. The portion of the implant coronal to the bone plus the abutment and crown measure 12 mm. This results in a clinical CIR of 12/7 = 1.7.
(Figure 1).15,16 Ten studies were found in the dental literature that addressed the survivability of implants in relation to CIRs (Table 1).17-26 Seven investigations are succinctly addressed, because they all have different characteristics; the latter 3 pertain to cantilevered prostheses and the data are summarized in Table 1. Schulte, et al17 assessed survival of 889 restorations in more than an average of 2.3 years (range 0.1 to 7.4 years). The average survival rate was 98.2%, and these implants (Bicon) had a mean CIR of 1.3 (range 0.5:1 to 3:1). More than 400 prostheses had a CIR ≥ 1.2. Among the 889 restorations that were monitored, 16 implants failed, and these prostheses manifested a mean CIR ratio of 1.4. Similarity of CIRs between failed and successful implants implies that the crown to implant ratio was not a critical determinant with respect to implant survival. Concerning the relationship between CIR and bone loss around implants, Tawil and Younan18 evaluated 262 machined surfaced implants (Brånemark) in 109 patients in more than 53 months. Bone loss was not related to CIRs and the survival rate of restored implants was 99.9%. The reported CIRs and the number of cases (in brackets) were as follows: 2 (8). Blanes, et al19 appraised the impact of CIRs on the survival of rough surfaced implants (N = 142, Straumann implants) in more than a 10-year period. They divided the patients into 3 groups with respect to CIR: 0 to .99, 1 to 1.99, and ≥ 2. The mean clinical CIR was 1.77 and increased CIRs were not found to be associated with additional bone loss. The success rate with a CIR of >2 was 94.1% (48 of 51 implants). They concluded that implant restorations with a CIR between 2 and 3 could be successfully used in posterior regions of the dentition. However, it should be noted that the majority of these implants were splinted (81.3%), which furnished an improved distribution of forces between implants. The impact of CIRs pertaining to bone loss around sintered porous surfaced implants (Endopore) was evaluated by Rokni, et al.20 They monitored (N = 198) implants for 4 years that were 5- to 7-mm long (short implants) and implants 9- to 12-mm long (long implants). Overall, implant restorations had a 1.5 CIR and demonstrated a 98.2% survival rate. The range of CIRs was 0.81 to 3:1. The percentage of implants with a CIR between
1.1 and 2 was 78.9%. Implants with a CIR >2 (n = 20) were followed for approximately 3 years. Different sized implants had the following CIRs (indicated in brackets): 5 mm (2.6), 7 mm (1.8), 9 mm (1.4) and 12 mm (1.0). There were no statistically significant relationships between CIRs and bone loss. The data underscored that short implants with a sintered surface which manifested elevated CIRs did not result in their failure or bone loss. Nedir, et al21 reported after a 7-year monitoring period that the survival rate of short (< 10 mm) and longer implants (Straumann implants) (10 to 13 mm) (N = 276) used to support single crowns or short prostheses were 100% and 99%, respectively. They provided the CIR for each length of implant and the number of placed implants: 6-mm long, CIR-1.97, n = 6; 8-mm long, CIR-1.59, n = 97; 9-mm long, CIR-1.55, n = 8; 10-mm long, CIR-1.3, n = 194; 11-mm long, CIR-1.17, n = 71. In general, these data corroborated the predictable use of short implants to support prostheses. However, note that 6-mm implants were splinted to longer implants to support prostheses. In contrast, Rossi, et al22 monitored for 2 years 35 patients who received a total of 40, 6-mm long implants (Straumann) (19 with a 4.1-mm diameter; 29 with a 4.8-mm diameter) that were not splinted together. Their mean implant CIR was 1.5 (range 0.8 to 2.3) and the implant survival rate was 95%. The mean bone loss 3
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Importance of Crown to Root and Crown to Implant Ratios Table 1. Crown to Implant Ratios (CIRs) of Implant Supported Prostheses (Single Crowns, Fixed Partial Denture [FPD], Cantilevered Bridges)
CIRs of Implant Supported Single Crowns or FPD Study
Schulte, et al17
Number of Implants 889
Tawil and Younan18 262 30 70 58 29 39 8 Blanes, et al19
Rokni, et al20
Nedir, et al21
Rossi, et al22 Urdaneta, et al23
8 133 51 22 156 20 111 40
326 40 206 11
CIR
Percent Survivability
a
2
99 % -
1.5 mean 0.81 to 1 1.1 to 2 >2
98.2% -
1.3 mean
98.2%
1.77 mean 0 to 0.99 1 to 1.99 > 2b
91.4%
1.55 to 1.97c
100
1.6 mean
98% 95% -
1.5 mean range 0.8 to 2.3 ≥2
1.0 to 1.99 < 1.0
95%
a mean CIR for all the implants was not provided, survival percentage is for all groups combined b survival rates only provided for CIRs ≥ 2. c CIR was computed related to specific implant lengths—6-mm long, CIR-1.97, n = 6; 8-mm long, CIR-1.59, n = 97; 9-mm long, CIR-1.55, n = 8, 10-mm long.
CIRs of Implant Supported Cantilevered Prostheses Study
Number of Implants
CIR
Wennstrom, et al24
68
1.60
12.7
Hälg, et al26
46
1.65
6 to 12 mm
Brägger, et al25
33
1.84
Mean Length of Implants or Range of Sizes
Percent Survivability
not recorded
98.4%
97%
95.7%
Recently, Urdaneta, et al23 assessed the effect of increased CIR with respect to bone loss and other prosthetic complications. In more than a 70-month period, they
for implants that lost bone (29 of 38 implants) between loading and the 2-year followup was 0.43 mm. Two implants failed and were removed before restorations were placed. 4
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Importance of Crown to Root and Crown to Implant Ratios monitored 326 implants (Bicon) a b whose mean CIR was 1.6. Forty patients had a CIR > 2. They concluded that an increased CIR did not result in an increased amount of bone loss, implant failure, or crown failures, but there was an increased amount of prosthetic problems associated with an increased CIR (eg, crown loosening [21/326] and fracture of 2-mm wide posterior Figures 2a and 2b. Radiographs of unilateral fixed implant prosthesis (straight line splint). The abutments [3/61]). implant in the No. 21 position is 11.5-mm long and is encased in approximately 9.5 mm of bone. The Another study specifically evalu- clinical crown is 11-mm long plus an abutment which has a 2 mm collar height. This results in a CIR of 11 mm + 2 mm + 2 mm = 15 mm/9.5 mm = 1.57:1. The implant in the No. 19 position is 10-mm ated bone loss related to CIRs (no long and is encased in 8 mm of bone while supporting an abutment with a cuff height of 1 mm plus a crown, which is 12-mm long. This results in a crown to implant ratio of 12 mm + 1 mm + 2 mm survival data provided). Gomez-Polo, clinical = 15 mm/8 mm = 1.87. The implant in the No. 18 position is 8-mm long and is encased in 6 mm of et al27 assessed the correlation bone while supporting an abutment with a 1-mm cuff height and a crown which is 12-mm long. This results in a CIR which is 12 mm + 1 mm + 2 mm = 15 mm/6 mm = 2.5. These implants benefit from between crown-implant ratios and the fact that the definitive PFM prosthesis is splinted—at least in a straight line fashion. bone resorption in 69 patients with 85 Figure 2c. Intraoral c view of the completed implants. After 5.7 years, they reported that CIRs of 0.43 to cemented PFM splint. 1.5 were not associated with peri-implant bone loss. A systematic review by Blanes28 that addressed CIRs included only 2 of the above studies,18,19 because investigations did not meet their inclusion criteria (eg, monitoring for > 4 years). They concluded that CIRs do not Figure 3a. Maxillary Arch affect peri-implant crestal bone loss. In addition, they a Reconstruction. Intraoral view of long custom reported that the survival rate of prostheses with a CIR of abutments attached to more than 2 was 94.1%. Three other studies contained data short implants. The implant lengths are in brackets with respect to CIRs when implants were used to support next to the corresponding 24-26 short span cantilevered bridges. The reported CIRs, the tooth: No. 5 (8 mm), No. 6 (10 mm), No. 7 (8 mm), percentages of implant survival after 5 years, the number of No. 10 (8 mm), No. 11 assessed implants for each study, and the mean or range (10 mm), No. 12 (8 mm). of implant sizes are listed in Table 1. Elevated CIRs on b Figure 3b. The restored lateral incisor is 20-mm long abutments for cantilevered bridges associated with and is supported by an successful prostheses further support the concept that implant which is 8 mm long (as part of a splinted implant restorations can successfully tolerate CIR values prosthesis). The restored that were considered risky ratios for teeth. length of the tooth is in excess of twice the length of From a different perspective, there are still unanswered the corresponding implant (CIR is 2.5). In actuality, the questions. For instance, Blanes28 questioned if the fulcrum abutment adds 2 additional point for prostheses with a large CIR is at the crown implant milli-meters to the “crown” connection or at the most coronal bone-implant contact results in larger CIRs (Figures 1, 2a portion of the equation, rendering the crown to root area. Most studies used the former for measurements, and to 2c, and 3a to 3g). The authors ratio 22/8 = 2.75:1. Since only his investigation19 used the bone-implant contact area suggested that the bone-implant this 8 mm implant is approximately 7 mm in bone on as the fulcrum point to calculate CIRs. Obviously, the latter contact area was probably the the radiograph, the ultimate CIR is about 2.87:1.
5
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Importance of Crown to Root and Crown to Implant Ratios fulcrum, because components connected to the implant were stronger than the cortical bone. Another issue not resolved relates to how different prosthetic designs (eg, single crowns (Figure 1), fixed partial dentures, straight-line splinting (Figures 2a to 2c), splinting around a curve of an arch (Figures 3a to 3g) impact on the relationship between CIRs and peri-implant bone loss, technical problems and survival of prostheses. The authors of this review paper agree with Blanes28 with respect to defining the clinical crown portion as the restored crown plus the abutment collar plus the part of the implant not encased in bone (Figures 1 to 3g). In other words, the level of the bone is considered the fulcrum for computing CIRs. This does not preclude that a potential weak point of prosthetic failure may occur at the abutmentimplant interface. However, the most important fulcrum point exists at the level of the most coronal bone to implant contact. This is the location where the applied forces and strains they create are resisted by the bone.
c Figure 3c. Intraoral retracted view of the cemented PFM prosthesis described in Figure 3b.
d Figure 3d. Smile-line of the definitive prosthesis in place.
prosthetic loads to the interface between bone and the implant. Accordingly, it is important to assess the survival of short implants, which usually have increased CIRs when restored. However, there are many variables (Table 2)31 that make it difficult to compare success rates in different studies. Nevertheless, the preponderance of data indicate that short implants are very successful and that survival rates for short textured implants are comparable to longer implants.31-34 In this regard, 4 review papers evaluated survivability of short dental implants and concluded that they could predictably be employed to support prostheses.31-34 Recently, a systematic review concluded that there was no statistically significant difference in survival rates between short (≤ 8 or ≤ 10 mm) and conventional (> 10 mm) rough surfaced implants placed in totally or partially edentulous patients.35
SHORT VERSUS LONG DENTAL IMPLANTS In general, finite elemental analysis studies indicated that forces on dental implants are transferred to the bone crest adjacent to implants29,30 and a small amount of stress is conveyed to the apical region.30 These data provide an important premise supporting the use of short implants. Namely, if stresses are placed at the crest after osseointegration occurred, then the length of the implant might not be the crucial factor with regard to distributing
Table 2. Variables and Confounding Factors When Comparing Studies With Respect to Survivability of Short Implants31 PATIENT: Systemic health, smoking status, periodontal status, parafunction, personal hygiene, professional maintenance
IMPLANTS: Type, length, width, surface characteristics, shape, internal connection, external connection BONE TYPE: I, II, II, IV
SURGERY: Skill of surgeon, bone grafting or not
PROSTHESIS: Splinted or not, crown to implant ratio, fixed partial denture, cantilever SURVIVAL RATE: Size of the study, duration of monitoring period Modified from Morand and Irinakis31
6
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Importance of Crown to Root and Crown to Implant Ratios Historically, studies indicated longer implants had a greater survival rate than shorter implants when a 10-mm length was used as the cut point.36 Renouard and Nisand32 documented that 11 of 12 investigations reported a higher failure rate with short implants than longer implants when machine surface implants were used.37-47 The 12th study employed machined and hydroxyapatite-coated implants.48 In contrast, Renouard and Nisand32 reported that 9 studies indicated that implant length did not affect the survival rate and 6 of the 9 investigations included in their assessment used textured surface implants.49-54 Many older studies and 7 recent investigations published between 2006 and 201033,55-60 all indicated that short implants had a high survival rate and could predictably be used to support crowns and prostheses (Table 3).18,21,33,52,55-63 Currently, textured surface implants are usually employed.33
e
f VERTICAL CANTILEVER FORCES With regards to the biomechanics of increasing the crown to root or implant ratio, as a crown becomes larger, bending moments can detrimentally affect a prosthesis. To compute the magnitude of a force moment, it is necessary to multiply the force by the perpendicular distance (moment arm) from the fulcrum to the endpoint of the lever arm.64 Conceptually, the occlusal height of a crown should not affect the force moment along the vertical axis, because if it is centered, its effective moment arm is nonexistent.64 However, noncentered occlusal contacts or lateral loads are prevalent and can induce considerable moment arms, thereby torquing the prosthesis. Therefore, as the clinical crown to implant length increases, there is a greater lever arm that potentially can amplify stress on the restoration. It is evident that with a larger clinical crown and smaller implant there potentially will be a bigger lever arm. Bidez and Misch65 indicated that when a crown height is increased from 10 to 20 mm there is a 100% increase of force on the implant. In addition, angulation is a force magnifier and Misch and Bidez66 noted that a 12° angle increased the force on an implant by 20%. Increased stress has the potential to decrease survivability of prostheses or create biological and technical problems. Fortunately, prostheses with increased CIRs from one to 2 have demonstrated a high survivability rate (Table 1). Similarly, short implants that support prostheses
g
Figures 3e, 3f, and 3g. Maxillary right, anterior, and left side radiographs of the patient depicting the CIRs. These implants benefit from the crossarch splinting of the definitive prosthesis.
that usually manifest an increased CIR have demonstrated high survival rates (Table 3). The precise height that cannot be exceeded to avoid de-osseointegration or fracture of an implant is unknown and will be affected by factors such as 7
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Importance of Crown to Root and Crown to Implant Ratios Table 3. Studies Dedicated to Assessing Survival Rates of Short Dental Implantsa Study
Anitua, et al56
293
Fugazzotto33
Arlin, et al55
Goene, et al61 Grant, et al57
Griffin, et al58 Malo, et al59
Misch, et al60
1,774
2,073
6 to 9 mm
ITI
124
355
8 mm
Nobel
408
7 and 8.5 mm
not reported not reported 188 167 237
273
111
a Studies listed alphabetically.
Type
BTI
Tawil, Younan18
ten Bruggenkate, et al63
Length
7 to 8.5 mm
236
van Steenberghe, et al62
No. of implants 532
Nedir, et al21
Testori et al52
n
not reported -
126
35 141
311
168
6 mm 8 mm
ITI 99.3%
7 to 8.5
Osteotite
rough
Survival Rate
Time
99.2%
31 months
rough
98.4%
36.2 months
not reported
99%
< 1 to ≥2 years
rough
acid etched
94.3%
95.8%
2 years
3 years
SO, Nobel
HA coated
100%
34.9 months
9 mm 7 mm
BTI
rough
99.2%
12 to 60 months
6 to 9 mm
ITI
100%
7 years
269
7 to < 10 mm
Nobel
TPS SLA
machined
8 mm 9 mm
Astra
rough
96.7%
12 to 92 months
10 6
acid-etched
95.5% 100%
2 years
ITI
TPS
97%
1 to 7 years
715 30
264 264 31
253
8 mm
Surface
Nobel
7 to 8.5 mm
Osteotite
6 mm
implant diameter, splinting of implants, and magnitude, duration, frequency, and direction of occlusal forces. Studies have indicated that with increased CIRs there was no additional bone loss compared to locations that did not have increased CIRs.17-20,24,26 In general, there is a dearth of information that addresses how often technical complications occur around restorations with increased CIRs pertaining to various prosthetic designs: single tooth, straightline splint, or fixed restorations that have cross arch stabilization. Of the 7 studies listed in Table 1 that discussed CIRs, only 2 reported the occurrence of technical problems.18,23 Tawil and Younan18 noted the incidence of screw loosening (7.8%) and porcelain fractures (5.2%) among teeth with increased CIRs. These data are within the 23% technical complication rate for implant supported FDPs noted in the systematic review by Berglundh, et al.67 Urdaneta, et al23 reported 3 fractures (3/61) of the 2-mm wide abutments used in the posterior areas when the CIR was 1.47 and no fractures among 3-mm wide abutments (184/184) when there was an increased CIR. They also noted that 21 of
machined/TiUnite
96 to 97%
5 years
4 years
326 crowns loosened during the observation period and 90% of the time this occurred in the maxillary anterior region. It was suggested that splinting of multiple adjacent implants might have avoided these undesirable sequelae. Several studies that assessed the utility of horizontal cantilevers also recorded increased CIRs (Table 1).24-26 These investigations usually reported minor technical problems associated with short span cantilevered bridges that had increased CIRs (see Aglietta, et al68 for review). While both vertical and horizontal cantilevers may increase stress on implants and the prosthesis that they support, the resultant problems may not be exactly similar. For instance, a prosthesis with an increased CIR due to a vertical cantilever may be supported on both ends with a terminal abutment and this may alter the types of technical problems that may occur. Other investigations that assessed the survival of short implants did not address technical problems that may occur if an increased CIR was created upon prosthesis fabrication. 32-35 At present, no definitive 8
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Importance of Crown to Root and Crown to Implant Ratios Table 4. Clinical Suggestions for Restoring Teeth With Increased Crown to Implant Ratios 1. In posterior areas, restore the occlusal surfaces of teeth in such a manner that the patient’s incisal or canine guidance discludes the posterior teeth and minimizes lateral contact in mandibular excursions.60
2. Increase the number of implants supporting the prosthesis. This adds to the surface area where occlusal forces are transmitted. 3. Increase the bone implant contact area with wider implants.
4. Reduce the occlusal width of posterior teeth and have centric contacts centered over the implants.31 5. Avoid elevated CIR in bruxers or over engineer the case with additional implants and splint around the turn of the arch with the restoration when possible.
6. Patients can wear a night guard to reduce nocturnal stresses on the prosthesis.
7. Short implants should be splinted together to maximize their support of the prosthesis and provide cross arch stabilization when possible.12,65
8. Use textured-surfaced implants that have a higher percentage of bone implant contact. 9. Flatten cuspal inclines.31
10. Employ implants with decreased thread pitch (distance between the threads), thereby resulting in an increased number of threads per unit length and an increased surface area.60 However, there are no data that the thread pitch altered the survival rate of prostheses with increased CIRs.
short implants often results in increased CIRs, the clinical implication is that short implants (< 10 mm) can be used to support a prosthesis and do not result in early demise of a restoration. Ultimately, when necessary, utilization of short implants and prostheses with increased CIRs provide greater flexibility when treatment planning patients. These conclusions are in agreement with recent statements by the European Association for Osseointegration,69 which indicated the following: Consensus statement: “Restorations with a CIR up to 2 do not induce peri-implant bone loss.” Clinical implications: “A FDP or single-tooth restoration with a CIR up to 2 is an acceptable prosthetic option.” Nevertheless, it is recognized that a prosthesis with an increased CIR is subject to increased occlusal forces. This can result in amplified stress on the prosthesis and the surrounding bone.70,71 Accordingly, it is advantageous to reduce forces on restorations with an increased CIR. Ten suggestions are listed in Table 4 for how to diminish stresses on prostheses with an increased CIR, thereby possibly reducing biological and technical complications and increasing the survivability of these constructs.
REFERENCES 1.
statements can be made about an increased rate of technical problems associated with increased CIRs, since this issue has not been highly documented in studies.
2.
CONCLUSIONS 3.
The available data (Table 1) demonstrate that prostheses with increased CIRs up to 2 have a high survival rate. Furthermore, increased CIRs did not result in additional peri-implant bone loss.17,19,20,23-26 Therefore, when there is a dearth of bone or anatomic structures that preclude placement of longer implants, treatment planning a prosthesis that results in an increased CIR is a reasonable, predictable procedure. Furthermore, since placement of
4.
9
Ante IH. The fundamental principles of abutments. Mich State Dent Soc Bull. 1926;8:232-257. Greenstein G, Greenstein B, Cavallaro J. Prerequisite for treatment planning implant dentistry: periodontal prognostication of compromised teeth. Compend Contin Educ Dent. 2007;28:436-447. Lulic M, Brägger U, Lang NP, et al. Ante’s (1926) law revisited: a systematic review on survival rates and complications of fixed dental prostheses (FDPs) on severely reduced periodontal tissue support. Clin Oral Implants Res. 2007;18(suppl 3):63-72. Pjetursson BE, Tan K, Lang NP, et al. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years: IV. Cantilever or extension FPDs. Clin Oral Implants Res. 2004;15:667-676.
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Importance of Crown to Root and Crown to Implant Ratios 5. 6. 7. 8. 9. 10. 11.
12. 13.
14.
15. 16. 17.
18. 19.
20. 21. 22.
Tan K, Pjetursson BE, Lang NP, et al. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years: III. Conventional FPDs. Clin Oral Implants Res. 2004;15:654-666. Grossmann Y, Sadan A. The prosthodontic concept of crown-to-root ratio: a review of the literature. J Prosthet Dent. 2005;93:559-562. Dykema RW, Goodacre CJ, Phillips RW. Johnston’s Modern Practice in Fixed Prosthodontics. 4th ed. Philadelphia, PA: WB Saunders; 1986:8-21. Shillingburg HT Jr, Hobo S, Whitsett LD, et al. Fundamentals of Fixed Prosthodontics. 3rd ed. Chicago, IL: Quintessence Publishing; 1997:89-90. Langer Y, Langer A. Root-retained overdentures: Part I— Biomechanical and clinical aspects. J Prosthet Dent. 1991;66:784-789. Schluger S, Yuodelis RA, Page RC, et al. Periodontal Diseases: Basic Phenomena, Clinical Management, and Occlusal and Restorative Interrelationships. 2nd ed. Philadelphia, PA: Lea & Febiger; 1990:405-412. Faucher RR, Bryant RA. Bilateral fixed splints. Int J Periodontics Restorative Dent. 1983;3:8-37. Guichet DL, Yoshinobu D, Caputo AA. Effect of splinting and interproximal contact tightness on load transfer by implant restorations. J Prosthet Dent. 2002;87:528-535. Grossmann Y, Finger IM, Block MS. Indications for splinting implant restorations. J Oral Maxillofac Surg. 2005;63:1642-1652. Misch CE, Goodacre CJ, Finley JM, et al. Consensus conference panel report: crown-height space guidelines for implant dentistry—part 2. Implant Dent. 2006;15:113-121. Rangert BR, Sullivan RM, Jemt TM. Load factor control for implants in the posterior partially edentulous segment. Int J Oral Maxillofac Implants. 1997;12:360-370. Glantz PO, Nilner K. Biomechanical aspects of prosthetic implant-borne reconstructions. Periodontol 2000. 1998;17:119-124. Schulte J, Flores AM, Weed M. Crown-to-implant ratios of single tooth implant-supported restorations. J Prosthet Dent. 2007;98:1-5. Tawil G, Younan R. Clinical evaluation of short, machinedsurface implants followed for 12 to 92 months. Int J Oral Maxillofac Implants. 2003;18:894-901. Blanes RJ, Bernard JP, Blanes ZM, et al. A 10-year prospective study of ITI dental implants placed in the posterior region. II: Influence of the crown-to-implant ratio and different prosthetic treatment modalities on crestal bone loss. Clin Oral Implants Res. 2007;18:707-714. Rokni S, Todescan R, Watson P, et al. An assessment of crown-to-root ratios with short sintered porous-surfaced implants supporting prostheses in partially edentulous patients. Int J Oral Maxillofac Implants. 2005;20:69-76. Nedir R, Bischof M, Briaux JM, et al. A 7-year life table analysis from a prospective study on ITI implants with special emphasis on the use of short implants. Results from a private practice. Clin Oral Implants Res. 2004;15:150-157. Rossi F, Ricci E, Marchetti C, et al. Early loading of single
23. 24. 25. 26. 27. 28. 29.
30.
31.
32. 33. 34. 35. 36. 37. 38. 39. 10
crowns supported by 6-mm-long implants with a moderately rough surface: a prospective 2-year follow-up cohort study. Clin Oral Implants Res. 2010;21:937-943. Urdaneta RA, Rodriguez S, McNeil DC, et al. The effect of increased crown-to-implant ratio on single-tooth locking-taper implants. Int J Oral Maxillofac Implants. 2010;25:729-743. Wennström J, Zurdo J, Karlsson S, et al. Bone level change at implant-supported fixed partial dentures with and without cantilever extension after 5 years in function. J Clin Periodontol. 2004;31:1077-1083. Brägger U, Karoussis I, Persson R, et al. Technical and biological complications/failures with single crowns and fixed partial dentures on implants: a 10-year prospective cohort study. Clin Oral Implants Res. 2005;16:326-334. Hälg GA, Schmid J, Hämmerle CH. Bone level changes at implants supporting crowns or fixed partial dentures with or without cantilevers. Clin Oral Implants Res. 2008;19:983-990. Gomez-Polo M, Bartens F, Sala L, et al. The correlation between crown-implant ratios and marginal bone resorption: a preliminary clinical study. Int J Prosthodont. 2010;23:33-37. Blanes RJ. To what extent does the crown-implant ratio affect the survival and complications of implant-supported reconstructions? A systematic review. Clin Oral Implants Res. 2009;20(suppl 4):67-72. Lum LB. A biomechanical rationale for the use of short implants. J Oral Implantol. 1991;17:126-131. Rieger MR, Mayberry M, Brose MO. Finite element analysis of six endosseous implants. J Prosthet Dent. 1990;63:671-676. Morand M, Irinakis T. The challenge of implant therapy in the posterior maxilla: providing a rationale for the use of short implants. J Oral Implantol. 2007;33:257-266. Renouard F, Nisand D. Impact of implant length and diameter on survival rates. Clin Oral Implants Res. 2006;17(suppl 2):35-51. Fugazzotto PA. Shorter implants in clinical practice: rationale and treatment results. Int J Oral Maxillofac Implants. 2008;23:487-496. Misch CE. Short dental implants: a literature review and rationale for use. Dent Today. 2005;24:64-68. Kotsovilis S, Fourmousis I, Karoussis IK, et al. A systematic review and meta-analysis on the effect of implant length on the survival of rough-surface dental implants. J Periodontol. 2009;80:1700-1718. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: an art form based on scientific principles. J Prosthet Dent. 2001;85:363-376. van Steenberghe D, Lekholm U, Bolender C, et al. Applicability of osseointegrated oral implants in the rehabilitation of partial edentulism: a prospective multicenter study on 558 fixtures. Int J Oral Maxillofac Implants. 1990;5:272-281. Friberg B, Jemt T, Lekholm U. Early failures in 4,641 consecutively placed Brånemark dental implants: a study from stage 1 surgery to the connection of completed prostheses. Int J Oral Maxillofac Implants. 1991;6:142-146. Jemt T. Failures and complications in 391 consecutively
Continuing Education
Importance of Crown to Root and Crown to Implant Ratios system. Int J Oral Maxillofac Implants. 2004;19:247-259. 55. Arlin ML. Short dental implants as a treatment option: results from an observational study in a single private practice. Int J Oral Maxillofac Implants. 2006;21:769-776. 56. Anitua E, Orive G, Aguirre JJ, et al. Five-year clinical evaluation of short dental implants placed in posterior areas: a retrospective study. J Periodontol. 2008;79:42-48. 57. Grant BT, Pancko FX, Kraut RA. Outcomes of placing short dental implants in the posterior mandible: a retrospective study of 124 cases. J Oral Maxillofac Surg. 2009;67:713-717. 58. Griffin TJ, Cheung WS. The use of short, wide implants in posterior areas with reduced bone height: a retrospective investigation. J Prosthet Dent. 2004;92:139-144. 59. Maló P, de Araújo Nobre M, Rangert B. Short implants placed one-stage in maxillae and mandibles: a retrospective clinical study with 1 to 9 years of follow-up. Clin Implant Dent Relat Res. 2007;9:15-21. 60. Misch CE, Steignga J, Barboza E, et al. Short dental implants in posterior partial edentulism: a multicenter retrospective 6year case series study. J Periodontol. 2006;77:1340-1347. 61. Goené R, Bianchesi C, Hüerzeler M, et al. Performance of short implants in partial restorations: 3-year follow-up of Osseotite implants. Implant Dent. 2005;14:274-280. 62. van Steenberghe D, De Mars G, Quirynen M, et al. A prospective split-mouth comparative study of two screw-shaped self-tapping pure titanium implant systems. Clin Oral Implants Res. 2000;11:202-209. 63. ten Bruggenkate CM, Asikainen P, Foitzik C, et al. Short (6-mm) nonsubmerged dental implants: results of a Multicenter clinical trial of 1 to 7 years. Int J Oral Maxillofac Implants. 1998;13:791-798. 64. Bidez MW, Misch CE. Biomechanics. In: Misch CE. Contemporary Implant Dentistry. 3rd ed. St. Louis, MO: Mosby; 2008:557-598. 65. Bidez MW, Misch CE. Force transfer in implant dentistry: basic concepts and principles. J Oral Implantol. 1992;18:264-274. 66. Misch CE, Bidez MW. Implant-protected occlusion: a biomechanical rationale. Compendium. 1994;15:1330-1334. 67. Berglundh T, Persson L, Klinge B. A systematic review of the incidence of biological and technical complications in implant dentistry reported in prospective longitudinal studies of at least 5 years. J Clin Periodontol. 2002;29(suppl 3):197-212. 68. Aglietta M, Siciliano VI, Zwahlen M, et al. A systematic review of the survival and complication rates of implant supported fixed dental prostheses with cantilever extensions after an observation period of at least 5 years. Clin Oral Implants Res. 2009;20:441-451. 69. Sanz M, Naert I; Working Group 2. Biomechanics/risk management (Working Group 2). Clin Oral Implants Res. 2009;20(suppl 4):107-111. 70. Akça K, Iplikçio lu H. Finite element stress analysis of the effect of short implant usage in place of cantilever extensions in mandibular posterior edentulism. J Oral Rehabil. 2002;29:350-356. 71. Tawil G, Aboujaoude N, Younan R. Influence of prosthetic parameters on the survival and complication rates of short implants. Int J Oral Maxillofac Implants. 2006;21:275-282.
inserted fixed prostheses supported by Brånemark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants. 1991;6:270-276. 40. Bahat O. Treatment planning and placement of implants in the posterior maxillae: report of 732 consecutive Nobelpharma implants. Int J Oral Maxillofac Implants. 1993;8:151-161. 41. Jemt T, Lekholm U. Implant treatment in edentulous maxillae: a 5-year follow-up report on patients with different degrees of jaw resorption. Int J Oral Maxillofac Implants. 1995;10:303-311. 42. Wyatt CC, Zarb GA. Treatment outcomes of patients with implant-supported fixed partial prostheses. Int J Oral Maxillofac Implants. 1998;13:204-211. 43. Lekholm U, Gunne J, Henry P, et al. Survival of the Brånemark implant in partially edentulous jaws: a 10-year prospective multicenter study. Int J Oral Maxillofac Implants. 1999;14:639-645. 44. Bahat O. Brånemark system implants in the posterior maxilla: clinical study of 660 implants followed for 5 to 12 years. Int J Oral Maxillofac Implants. 2000;15:646-653. 45. Naert I, Koutsikakis G, Duyck J, et al. Biologic outcome of implant-supported restorations in the treatment of partial edentulism. Part I: a longitudinal clinical evaluation. Clin Oral Implants Res. 2002;13:381-389. 46. Weng D, Jacobson Z, Tarnow D, et al. A prospective multicenter clinical trial of 3i machined-surface implants: results after 6 years of follow-up. Int J Oral Maxillofac Implants. 2003;18:417-423. 47. Herrmann I, Lekholm U, Holm S, et al. Evaluation of patient and implant characteristics as potential prognostic factors for oral implant failures. Int J Oral Maxillofac Implants. 2005;20:220-230. 48. Winkler S, Morris HF, Ochi S. Implant survival to 36 months as related to length and diameter. Ann Periodontol. 2000;5:22-31. 49. Buser D, Mericske-Stern R, Bernard JP, et al. Long-term evaluation of non-submerged ITI implants. Part 1: 8-year life table analysis of a prospective multicenter study with 2359 implants. Clin Oral Implants Res. 1997;8:161-172. 50. Ellegaard B, Baelum V, Karring T. Implant therapy in periodontally compromised patients. Clin Oral Implants Res. 1997;8:180-188. 51. Brocard D, Barthet P, Baysse E, et al. A multicenter report on 1,022 consecutively placed ITI implants: a 7-year longitudinal study. Int J Oral Maxillofac Implants. 2000;15:691-700. 52. Testori T, Wiseman L, Woolfe S, et al. A prospective multicenter clinical study of the Osseotite implant: four-year interim report. Int J Oral Maxillofac Implants. 2001;16:193-200. 53. Feldman S, Boitel N, Weng D, et al. Five-year survival distributions of short-length (10 mm or less) machinedsurfaced and Osseotite implants. Clin Implant Dent Relat Res. 2004;6:16-23. 54. Romeo E, Lops D, Margutti E, et al. Long-term survival and success of oral implants in the treatment of full and partial arches: a 7-year prospective study with the ITI dental implant 11
Continuing Education
Importance of Crown to Root and Crown to Implant Ratios POST EXAMINATION INFORMATION To receive continuing education credit for participation in this educational activity you must complete the program post examination and receive a score of 70% or better.
2. With respect to teeth, the following procedures reduce the CRRs: a. b. c. d.
Traditional Completion Option: You may fax or mail your answers with payment to Dentistry Today (see Traditional Completion Information on following page). All information requested must be provided in order to process the program for credit. Be sure to complete your “Payment,” “Personal Certification Information,” “Answers,” and “Evaluation” forms. Your exam will be graded within 72 hours of receipt. Upon successful completion of the postexam (70% or higher), a letter of completion will be mailed to the address provided.
3. Strategies to consider when fabricating fixed dental prostheses with increased CIRs include the following: a. b. c. d.
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Minimize lateral excursions on posterior prostheses and increase the number of supporting implants. Increase clinical crown height. Employ textured surfaced implants. Both A and C.
4. In this review paper, the authors define the clinical crown as: a.
b. c. d.
The sum of the restored crown plus the abutment collar plus the portion of the implant coronal to the bone. The height of the titanium abutment minus 2 mm for biologic width space. The sum of the interarch space plus the height of the restored crown plus the height of the abutment. The height of the restored crown.
5. In the study of increased CIRs by Rossi, et al the assessed implants were: a. b.
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c. d.
The 6-mm long implants. The 4.1-mm diameter implants and the 4.8-mm diameter implants. Splinted implants. Both A and B.
6. To calculate the bending moment, it is necessary to multiply the force by what?
POST EXAMINATION QUESTIONS
a.
1. Factors which influence the number of abutments to support a fixed dental prosthesis (FDP) include which of the following? a. b. c. d.
A low profile overdenture abutment. Tooth extrusion. Both A and B. Occlusal onlays.
b.
Number of missing teeth. Anticipated occlusal forces. Amount of available bone around abutments. All of the above.
c. d.
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The perpendicular distance (moment arm) fulcrum to the endpoint of the lever arm. The perpendicular distance (moment arm) fulcrum to the apex of the tooth. The perpendicular distance (moment arm) fulcrum to the cemento-enamel junction. The perpendicular distance (moment arm) fulcrum to closest tooth.
from the from the from the from the
Continuing Education
Importance of Crown to Root and Crown to Implant Ratios b.
7. In the study pertaining to increased CIRs by Blanes, et al the assessed implants had which features? a. b. c. d.
Monitored for 2.5 years. Restored with CIRs of 6 or more. Smooth surfaced implants. None of the above.
c.
d.
8. In this review paper, the authors define the location of the fulcrum between the clinical crown and the implant as: a. b. c. d.
13. According to Bidez and Misch, when a crown height is increased from 10 to 20 mm, the increase of force on an implant is how large?
Occurring at the junction of the restoration and the implant abutment. Occurring at the level of the first bone to implant contact. Occurring at the implant-abutment interface. Occurring within the body of the implant half way from the osseous crest to the apex of the implant.
a. b. c. d.
d.
a. b.
Increasing implant diameter. Use of porcelain fused to metal crowns. Splinting of implants, particularly across the turn of the arch. Both A and C.
c. d.
a.
Increasing vertical dimension. Surgical crown lengthening. Both A and B. Ridge augmentation.
b. c.
11. Urdaneta, et al demonstrated that an increased CIR resulted in what effect on bone around the dental implants? a. b. c. d.
d.
Additional bone loss. No additional bone loss. Bone apposition. Increased osseous density of bone.
Restore the posterior occlusal surfaces of teeth so canine guidance discludes the posterior teeth and minimizes lateral contact in mandibular excursions. Increase the number of implants supporting the prosthesis. Increase the bone implant contact area with wider implants. All of the above.
16. What is the precise crown to implant ratio that cannot be exceeded to avoid deosseointegration or fracture of an implant? a. b. c. d.
12. The preponderance of data in Table 3 indicate that short implants demonstrate which characteristic? a.
Restorations with a CIR up to 2 do not induce periimplant bone loss. FDP or single-tooth restoration with a CIR up to 2 is an acceptable prosthetic option. Both A and B. Restorations with a CIR up to 1 do not induce periimplant bone loss.
15. Techniques to reduce forces on restorations with an increased CIR include which of the following?
10. Which procedures increase the CRR with respect to teeth? a. b. c. d.
25%. 50%. 75%. 100%.
14. The European Association for Osseointegration reached which of the following conclusions?
9. Factors which will affect an implant’s ability to resist biologic and technical complications include: a. b. c.
Are very successful and that survival rates for short smooth surfaced implants are comparable to longer implants. Are not very successful and that survival rates for short textured implants are not comparable to longer implants. Are not very successful.
Are very successful and that survival rates for short textured implants are comparable to longer implants.
13
Unknown. 1. 1.5. 2.
Continuing Education
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