Importance of Crown to Root
and Crown to Implant Ratios
Authored by Gary Greenstein, DDS, MS, and
John S. Cavallaro Jr, DDSUpon successful completion of this CE activity 2 CE credit hours may be awarded
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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@example.com.
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, Brook-lyn, 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 firstname.lastname@example.org.
Disclosure:Dr. Cavallaro reports no disclosures.
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
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.
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
Importance of Crown to Root
and Crown to Implant RatiosEffective Date: 03/1/2011 Expiration Date: 03/1/2013
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.
CROWN TO ROOT RATIOS AND CROWN TO IMPLANT
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.6When 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.7Dykema, et
al7 indicated that a CRR of 1:1.5 is desirable. Similarly,
Shillingburg, et al8suggested 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 attachment9and extrusion of teeth
to provide additional bone both decrease the CRR, whereas
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.10This 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
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.
PERFORMANCE OF IMPLANTS WITH RESPECT TO
CROWN TO IMPLANT RATIOS
An elevated CIR has been described as a type of non-axial loading, which can detrimentally affect a prosthesis
(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 pros-theses and the data are summarized in Table 1.
Schulte, et al17assessed 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 Younan18evaluated 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: <1 (30), 1 to 1.2 (70), 1.21 to 1.4 (58), 1.41 to 1.6 (29), 1.6 to 2.0 (39) and >2 (8). Blanes, et al19 appraised the impact of CIRs on the
survival of rough surfaced implants (N = 142, Straumann im-plants) 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 di-ameter; 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
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.
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.
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
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 Number of Implants CIR Percent Survivability
Schulte, et al17 889 1.3 mean 98.2%
Tawil and Younan18 262 a 99 %
30 < 1 -70 1 to 1.2 -58 1.21 to 1.4 -29 1.41 to 1.6 -39 1.61 to 2 -8 > 2 -Blanes, et al19 1.77 mean -8 0 to 0.99 -133 1 to 1.99 -51 > 2b 91.4% Rokni, et al20 1.5 mean 98.2% 22 0.81 to 1 -156 1.1 to 2 -20 > 2 -Nedir, et al21 111 1.55 to 1.97c 100 Rossi, et al22 40 1.5 mean 95% range 0.8 to 2.3 Urdaneta, et al23 326 1.6 mean 98% 40 ≥2 95% 206 1.0 to 1.99 -11 < 1.0
-a me-an CIR for -all the impl-ants w-as not provided, surviv-al percent-age 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 Mean Length of Implants or Percent Range of Sizes Survivability
Wennstrom, et al24 68 1.60 12.7 97%
Brägger, et al25 33 1.84 not recorded 98.4%
monitored 326 implants (Bicon) 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 abutments [3/61]).
Another study specifically evalu-ated bone loss relevalu-ated to CIRs (no survival data provided). Gomez-Polo, et al27 assessed the correlation
between crown-implant ratios and bone resorption in 69 patients with 85
implants. After 5.7 years, they reported that CIRs of 0.43 to 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 affect peri-implant crestal bone loss. In addition, they reported that the survival rate of prostheses with a CIR of more than 2 was 94.1%. Three other studies contained data with respect to CIRs when implants were used to support short span cantilevered bridges.24-26The reported CIRs, the
percentages of implant survival after 5 years, the number of assessed implants for each study, and the mean or range of implant sizes are listed in Table 1. Elevated CIRs on abutments for cantilevered bridges associated with successful prostheses further support the concept that implant restorations can successfully tolerate CIR values that were considered risky ratios for teeth.
From a different perspective, there are still unanswered questions. For instance, Blanes28 questioned if the fulcrum
point for prostheses with a large CIR is at the crown implant connection or at the most coronal bone-implant contact area. Most studies used the former for measurements, and only his investigation19used the bone-implant contact area
as the fulcrum point to calculate CIRs. Obviously, the latter
results in larger CIRs(Figures 1, 2a to 2c, and 3a to 3g). The authors suggested that the bone-implant contact area was probably the
Figures 2a and 2b.Radiographs of unilateral fixed implant prosthesis (straight line splint). The
implant in the No. 21 position is 11.5-mm long and is encased in approximately 9.5 mm of bone. The 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 long and is encased in 8 mm of bone while supporting an abutment with a cuff height of 1 mm plus a clinical crown, which is 12-mm long. This results in a crown to implant ratio of 12 mm + 1 mm + 2 mm = 15 mm/8 mm = 1.87. The implant in the No. 18 position is 8-mm long and is encased in 6 mm of 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 the fact that the definitive PFM prosthesis is splinted—at least in a straight line fashion.
cFigure 2c.view of the completedIntraoral
cemented PFM splint.
Figure 3a.Maxillary Arch
Reconstruction. Intraoral view of long custom abutments attached to short implants. The implant lengths are in brackets next to the corresponding tooth: No. 5 (8 mm), No. 6 (10 mm), No. 7 (8 mm), No. 10 (8 mm), No. 11 (10 mm), No. 12 (8 mm).
Figure 3b.The restored
lateral incisor is 20-mm long and is supported by an implant which is 8 mm long (as part of a splinted prosthesis). The restored length of the tooth is in excess of twice the length of the corresponding implant (CIR is 2.5). In actuality, the abutment adds 2 additional milli-meters to the “crown” portion of the equation, rendering the crown to root ratio 22/8 = 2.75:1. Since this 8 mm implant is approx-imately 7 mm in bone on the radiograph, the ultimate CIR is about 2.87:1.
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 abutment-implant 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.
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
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)31that
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.31-34Recently,
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
retracted view of the cemented PFM prosthesis described in Figure 3b.
Figure 3d.Smile-line of
the definitive prosthesis in place.
PATIENT:Systemic health, smoking status, periodontal status, parafunction, personal hygiene, professional
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
Table 2. Variables and Confounding Factors When Comparing Studies With Respect to Survivability
Historically, studies indicated longer implants had a greater survival rate than shorter implants when a 10-mm length was used as the cut point.36Renouard 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.37-47The 12th study
employed machined and hydroxyapatite-coated implants.48
In contrast, Renouard and Nisand32reported 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 201033,55-60all 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
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.64Conceptually, 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.64However, 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 Bidez66noted 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
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
Figures 3e, 3f, and 3g.
Maxillary right, anterior, and left side radiographs of the patient depicting the CIRs. These implants benefit from the cross-arch splinting of the definitive prosthesis.
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, straight-line 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 al23reported 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
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 al68for review).
While both vertical and horizontal cantilevers may in-crease 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
Table 3. Studies Dedicated to Assessing Survival Rates of Short Dental Implantsa
Study n No. of implants Length Type Surface Survival Rate Time
Anitua, et al56 293 532 7 to 8.5 mm BTI rough 99.2% 31 months Arlin, et al55 not reported 35 6 mm ITI rough 94.3% 2 years
not reported 141 8 mm 99.3%
Fugazzotto33 1,774 2,073 6 to 9 mm ITI rough 98.4% 36.2 months Goene, et al61 188 311 7 to 8.5 Osteotite acid etched 95.8% 3 years Grant, et al57 124 355 8 mm Nobel not reported 99% < 1 to≥2 years Griffin, et al58 167 168 8 mm SO, Nobel HA coated 100% 34.9 months Malo, et al59 237 408 7 and 8.5 mm Nobel machined/TiUnite 96 to 97% 5 years
Misch, et al60 273 715 9 mm BTI rough 99.2% 12 to 60 months
30 7 mm
Nedir, et al21 236 264 6 to 9 mm ITI TPS 100% 7 years
Tawil, Younan18 111 269 7 to < 10 mm Nobel machined 95.5% 12 to 92 months Testori et al52 not reported 31 7 to 8.5 mm Osteotite acid-etched 96.7% 4 years van Steenberghe, et al62 - 10 8 mm Astra rough 100% 2 years
- 6 9 mm
ten Bruggenkate, et al63 126 253 6 mm ITI TPS 97% 1 to 7 years a Studies listed alphabetically.
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.
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.17,19,20,23-26Therefore, 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
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,69which 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 accep-table 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 inTable 4for how to diminish stresses on prostheses with an increased CIR, thereby possibly reducing biological and technical complications and increasing the survivability of these constructs.
1. Ante IH. The fundamental principles of abutments. Mich State Dent Soc Bull. 1926;8:232-257.
2. Greenstein G, Greenstein B, Cavallaro J. Prerequisite for treatment planning implant dentistry: periodontal prognosti-cation of compromised teeth.Compend Contin Educ Dent. 2007;28:436-447.
3. 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.
4. 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.
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
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
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.60However, there are no
data that the thread pitch altered the survival rate of prostheses with increased CIRs.
Table 4. Clinical Suggestions for Restoring Teeth With Increased Crown to Implant Ratios
5. 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. 6. Grossmann Y, Sadan A. The prosthodontic concept of
crown-to-root ratio: a review of the literature.J Prosthet Dent. 2005;93:559-562.
7. Dykema RW, Goodacre CJ, Phillips RW.Johnston’s Modern Practice in Fixed Prosthodontics. 4th ed. Philadelphia, PA: WB Saunders; 1986:8-21.
8. Shillingburg HT Jr, Hobo S, Whitsett LD, et al.Fundamentals of Fixed Prosthodontics. 3rd ed. Chicago, IL: Quintessence Publishing; 1997:89-90.
9. Langer Y, Langer A. Root-retained overdentures: Part I— Biomechanical and clinical aspects.J Prosthet Dent. 1991;66:784-789.
10. 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. 11. Faucher RR, Bryant RA. Bilateral fixed splints.Int J
Periodontics Restorative Dent. 1983;3:8-37.
12. 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.
13. Grossmann Y, Finger IM, Block MS. Indications for splinting implant restorations.J Oral Maxillofac Surg. 2005;63:1642-1652. 14. 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. 15. 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.
16. Glantz PO, Nilner K. Biomechanical aspects of prosthetic implant-borne reconstructions.Periodontol 2000.
17. Schulte J, Flores AM, Weed M. Crown-to-implant ratios of single tooth implant-supported restorations.J Prosthet Dent. 2007;98:1-5. 18. Tawil G, Younan R. Clinical evaluation of short,
machined-surface implants followed for 12 to 92 months.Int J Oral Maxillofac Implants. 2003;18:894-901.
19. 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.
20. 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. 21. 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. 22. Rossi F, Ricci E, Marchetti C, et al. Early loading of single
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.
23. 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. 24. 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.
25. 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.
26. 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. 27. 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. 28. 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.
29. Lum LB. A biomechanical rationale for the use of short implants. J Oral Implantol. 1991;17:126-131.
30. Rieger MR, Mayberry M, Brose MO. Finite element analysis of six endosseous implants.J Prosthet Dent. 1990;63:671-676. 31. 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.
32. Renouard F, Nisand D. Impact of implant length and diameter on survival rates.Clin Oral Implants Res. 2006;17(suppl 2):35-51.
33. Fugazzotto PA. Shorter implants in clinical practice: rationale and treatment results.Int J Oral Maxillofac Implants.
34. Misch CE. Short dental implants: a literature review and rationale for use.Dent Today. 2005;24:64-68.
35. 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.
36. 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.
37. 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. IntJ Oral Maxillofac Implants. 1990;5:272-281.
38. 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.
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.
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) machined-surfaced 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
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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
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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 6-year case series study.J Periodontol. 2006;77:1340-1347. 61. Goené R, Bianchesi C, Hüerzeler M, et al. Performance of
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71. Tawil G, Aboujaoude N, Younan R. Influence of prosthetic parameters on the survival and complication rates of short
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POST EXAMINATION QUESTIONS
1. Factors which influence the number of abutments to support a fixed dental prosthesis (FDP) include which of the following?
a. Number of missing teeth. b. Anticipated occlusal forces.
c. Amount of available bone around abutments. d. All of the above.
2. With respect to teeth, the following procedures reduce the CRRs:
a. A low profile overdenture abutment. b. Tooth extrusion.
c. Both A and B. d. Occlusal onlays.
3. Strategies to consider when fabricating fixed dental prostheses with increased CIRs include the
a. Minimize lateral excursions on posterior prostheses and increase the number of supporting implants. b. Increase clinical crown height.
c. Employ textured surfaced implants. d. Both A and C.
4. In this review paper, the authors define the clinical crown as:
a. The sum of the restored crown plus the abutment collar plus the portion of the implant coronal to the bone.
b. The height of the titanium abutment minus 2 mm for biologic width space.
c. The sum of the interarch space plus the height of the restored crown plus the height of the abutment. d. The height of the restored crown.
5. In the study of increased CIRs by Rossi, et al the assessed implants were:
a. The 6-mm long implants.
b. The 4.1-mm diameter implants and the 4.8-mm diameter implants.
c. Splinted implants. d. Both A and B.
6. To calculate the bending moment, it is necessary to multiply the force by what?
a. The perpendicular distance (moment arm) from the fulcrum to the endpoint of the lever arm.
b. The perpendicular distance (moment arm) from the fulcrum to the apex of the tooth.
c. The perpendicular distance (moment arm) from the fulcrum to the cemento-enamel junction.
d. The perpendicular distance (moment arm) from the fulcrum to closest tooth.
7. In the study pertaining to increased CIRs by Blanes, et al the assessed implants had which features?
a. Monitored for 2.5 years.
b. Restored with CIRs of 6 or more. c. Smooth surfaced implants. d. None of the above.
8. In this review paper, the authors define the location of the fulcrum between the clinical crown and the implant as:
a. Occurring at the junction of the restoration and the implant abutment.
b. Occurring at the level of the first bone to implant contact.
c. Occurring at the implant-abutment interface. d. Occurring within the body of the implant half way
from the osseous crest to the apex of the implant.
9. Factors which will affect an implant’s ability to resist biologic and technical complications include:
a. Increasing implant diameter.
b. Use of porcelain fused to metal crowns.
c. Splinting of implants, particularly across the turn of the arch.
d. Both A and C.
10. Which procedures increase the CRR with respect to teeth?
a. Increasing vertical dimension. b. Surgical crown lengthening. c. Both A and B.
d. Ridge augmentation.
11. Urdaneta, et al demonstrated that an increased CIR resulted in what effect on bone around the dental implants?
a. Additional bone loss. b. No additional bone loss. c. Bone apposition.
d. Increased osseous density of bone.
12. The preponderance of data in Table 3 indicate that short implants demonstrate which characteristic?
a. Are very successful and that survival rates for short textured implants are comparable to longer implants.
b. Are very successful and that survival rates for short smooth surfaced implants are comparable to longer implants.
c. Are not very successful and that survival rates for short textured implants are not comparable to longer implants.
d. Are not very successful.
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?
a. 25%. b. 50%. c. 75%. d. 100%.
14. The European Association for Osseointegration reached which of the following conclusions?
a. Restorations with a CIR up to 2 do not induce peri-implant bone loss.
b. FDP or single-tooth restoration with a CIR up to 2 is an acceptable prosthetic option.
c. Both A and B.
d. Restorations with a CIR up to 1 do not induce peri-implant bone loss.
15. Techniques to reduce forces on restorations with an increased CIR include which of the following?
a. Restore the posterior occlusal surfaces of teeth so canine guidance discludes the posterior teeth and minimizes lateral contact in mandibular excursions.
b. Increase the number of implants supporting the prosthesis.
c. Increase the bone implant contact area with wider implants.
d. 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. Unknown. b. 1.
c. 1.5. d. 2.
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