Importance of Crown to Root and Crown to Implant ... - DentalCEToday

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 

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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 ggperio@aol.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, 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 

docsamurai@si.rr.com. 

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 


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. 

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 al 3 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 

1


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 al 3 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 

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 

al 7 indicated that a CRR of 1:1.5 is desirable. Similarly, 

Shillingburg, et al 8 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 attachment 9 and 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 Sadan 6 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. 

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



(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 al 17 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 Younan 18 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 al 19 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 

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. 

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 al 21 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 al 22 

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



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 al 17 889 1.3 mean 98.2% 

Tawil and Younan 18 262 a 99 % 

30 < 1 - 

70 1 to 1.2 - 

58 1.21 to 1.4 - 


39 1.61 to 2 - 

8 > 2 - 

Blanes, et al 19 1.77 mean - 



51 > 2 b 91.4% 

Rokni, et al 20 1.5 mean 98.2% 



20 > 2 - 

Nedir, et al 21 111 1.55 to 1.97 c 100 

Rossi, et al 22 40 1.5 mean 95% 

range 0.8 to 2.3 

Urdaneta, et al 23 326 1.6 mean 98% 

40 ≥ 2 95% 


11 < 1.0 - 

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 Mean Length of Implants or Percent 

Range of Sizes 

Survivability 

Wennstrom, et al 24 68 1.60 12.7 97% 

Brägger, et al 25 33 1.84 not recorded 98.4% 

Hälg, et al 26 46 1.65 6 to 12 mm 95.7% 

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 al 23 assessed the effect of 

increased CIR with respect to bone loss and other prosthetic 

complications. In more than a 70-month period, they 

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 evaluated 

bone loss related to CIRs (no 

survival data provided). Gomez-Polo, 

et al 27 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 Blanes 28 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. 24-26 The 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, Blanes 28 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 investigation 19 used the bone-implant contact area 

as the fulcrum point to calculate CIRs. Obviously, the latter 

a 

b 

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. 

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 



c 

a 

b 

Figure 2c. Intraoral 

view of the completed 

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 approximately 

7 mm in bone on 

the radiograph, the ultimate 

CIR is about 2.87:1. 

5



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 Blanes 28 

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. 

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 implants 29,30 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 

c 

d 

Figure 3c. Intraoral 

retracted view of the 

cemented PFM 

prosthesis described in 

Figure 3b. 

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 

Table 2. Variables and Confounding Factors When Comparing Studies With Respect to Survivability 

of Short Implants 31 

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 Irinakis 31 

6



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 Nisand 32 

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 Nisand 32 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 2010 33,55-60 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 

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 Misch 65 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 Bidez 66 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 

e 

f 

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



Table 3. Studies Dedicated to Assessing Survival Rates of Short Dental Implants a 

Study n No. of implants Length Type Surface Survival Rate Time 

Anitua, et al 56 293 532 7 to 8.5 mm BTI rough 99.2% 31 months 

Arlin, et al 55 not reported 35 6 mm ITI rough 94.3% 2 years 

not reported 141 8 mm 99.3% 

Fugazzotto 33 1,774 2,073 6 to 9 mm ITI rough 98.4% 36.2 months 

Goene, et al 61 188 311 7 to 8.5 Osteotite acid etched 95.8% 3 years 

Grant, et al 57 124 355 8 mm Nobel not reported 99% < 1 to ≥2 years 

Griffin, et al 58 167 168 8 mm SO, Nobel HA coated 100% 34.9 months 

Malo, et al 59 237 408 7 and 8.5 mm Nobel machined/TiUnite 96 to 97% 5 years 

Misch, et al 60 273 715 9 mm BTI rough 99.2% 12 to 60 months 

30 7 mm 

Nedir, et al 21 236 264 6 to 9 mm ITI TPS 100% 7 years 

264 SLA 

Tawil, Younan 18 111 269 7 to < 10 mm Nobel machined 95.5% 12 to 92 months 

Testori et al 52 not reported 31 7 to 8.5 mm Osteotite acid-etched 96.7% 4 years 

van Steenberghe, et al 62 - 10 8 mm Astra rough 100% 2 years 

- 6 9 mm 

ten Bruggenkate, et al 63 126 253 6 mm ITI TPS 97% 1 to 7 years 

a Studies listed alphabetically. 

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 Younan 18 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 al 23 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 

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 al 68 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



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. 

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. 

CONCLUSIONS 

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 

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9



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25. Brägger U, Karoussis I, Persson R, et al. Technical and 

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26. Hälg GA, Schmid J, Hämmerle CH. Bone level changes at 

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without cantilevers. Clin Oral Implants Res. 2008;19:983-990. 

27. Gomez-Polo M, Bartens F, Sala L, et al. The correlation 

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preliminary clinical study. Int J Prosthodont. 2010;23:33-37. 

28. Blanes RJ. To what extent does the crown-implant ratio 

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29. Lum LB. A biomechanical rationale for the use of short implants. 

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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. 

2008;23:487-496. 

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. Int J 


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. 

39. Jemt T. Failures and complications in 391 consecutively 

10



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 


40. Bahat O. Treatment planning and placement of implants in the 

posterior maxillae: report of 732 consecutive Nobelpharma 


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 


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 

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 


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 6- 

year 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 

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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 

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64. Bidez MW, Misch CE. Biomechanics. In: Misch CE. 

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Mosby; 2008:557-598. 

65. Bidez MW, Misch CE. Force transfer in implant dentistry: basic 

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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 

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68. Aglietta M, Siciliano VI, Zwahlen M, et al. A systematic 

review of the survival and complication rates of implant 

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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 

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71. Tawil G, Aboujaoude N, Younan R. Influence of prosthetic 

parameters on the survival and complication rates of short 


11

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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 

following: 

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. 

12



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. 


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 periimplant 

bone loss. 

b. FDP or single-tooth restoration with a CIR up to 2 is 

an acceptable prosthetic option. 


d. 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 

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. 

13



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