340

Journal of Maxillofacial & Oral Surgery 2008 Vol. 7 : No. 3

Prospective StudyLong term bone levelstability on Short Implants:
A radiographic follow up
study
Caterina Venuleo, Sung-Kiang Chuang, Meghan Weed, Serge Dibart
Abstract: Objectives: Placement of short endosseous implants represents a valid
treatment in the setting of limited alveolar bone height. This study’s objectives
were: to estimate the 5 year clinical survival of BiconTM short implant and toevaluate radiographic bone level changes around 6 x 5.7mm implants incomparison with longer non-6 x 5.7mm implants.

Caterina Venuleo1, Sung-Kiang
Chuang 3, Meghan Weed4, Serge
Dibart2
1

Postdoctoral Resident
Professor & Clinical Director, Dept. of
Periodontology and Oral Biology
2

Boston University, Goldman School of
Dental Medicine
3

Asst. Professor, Dept. of OMFS,
Massachusetts General Hospital andthe Harvard School of Dental Medicine,Research Associate, Dept. of
Biostatistics, Harvard School of Public
Health, Boston, USA
4
Research Associate, Implant Dentistry
Centre, Boston, MA, USA.

Methods: A retrospective cohort study design was used. The cohort was composedof patients who had at least one 6 x 5.7mm implant placed for 5 years, at least one
non - 6 x 5.7mm implant, and who were willing to return to the dental office forradiographic evaluation. A total of 62 implants, 28 6 x 5.7mm (test group=short
implant) and 34 non - 6 x 5.7mm (control group=non short implant), were placedin 20 patients (12 males and 8 females). Mean length of non - 6 x 5.7mm implants
was 9.7 mm, ranging from 8mm to 14mm, while mean diameter was 4.30 mm(range: 3.5 to 5 mm). Bone loss, defined as the vertical difference in crestal bonelevel measurements, from the baseline (day of implant placement) to the 5 years
follow-up, was digitally determined on periapical radiographs. Generalized linear
mixed models were used for the statistical analysis.
Result: Five years survival rates for test and control groups were 100.0% and96.8% respectively, but this difference was marginally not statistically significant(p=0.35). There was no significant difference between the two groups with regard
to mean changes of radiographic bone levels.
Conclusion: Short implants with large diameter (6 x 5.7mm) have a long-term(>5-years) survival rate and crestal bone level maintenance similar to thatobserved for non - 6 x 5.7mm implants. The results of this radiographic study
support the hypothesis that 6 x 5.7mm implants can be successfully used in
edentulous maxillary and mandibular areas with limited bone height.
Keywords: Short implants and crestal bone levels.
Background
During the past decades, implant
therapy has been shown to be a successful
option for tooth replacement. After toothloss, however, severely atrophic residual
alveolar ridges are quite common,
especially in patients who have been
edentulous for a long period of time.
Posterior areas of the maxilla and the

mandible are areas where clinicians havegreater anatomical limitations. Reduced
alveolar bone height, very often represents
a contraindication to implant therapy,
unless a procedure such as ridge
augmentation or sinus floor elevation is
performed. Although widely utilized, thesetechniques imply greater morbidity, longer
treatment times and higher costs. Sinus

Address for Correspondence:
Serge Dibart
Department of Periodontology & Oral
Biology
Boston University Goldman School of
Dental Medicine
100 East Newton Street
Boston, MA 02118, USA
Received for publication September 2007
Accepted after peer review September 2008
Available online Sep. 2008 at www.jmosi.com

cavity in the maxilla and alveolar nerve
proximity in the mandible are clinicalsituations where short implants may beconsidered as an alternative treatment
option.
The need for long term (>5 years)
studies has been emphasized by many
researchers and deeply stressed by theUnited States National Institute of HealthJournal of Maxillofacial & Oral Surgery 2008 Vol. 7 : No. 3(NIH) and Institute for Dental Research(NIDCR) regarding the clinicalperformance of different types of implants.
In particular, radiographic evaluation of
crestal supporting bone loss was the mainsuggestion of NIH Consensus Conferencein 1988. As one of the criteria for implant
success, stable bone levels are believed tobe critical to the long term maintenance ofan implant (Smith and Zarb 1989). 1
According to these authors the mean
vertical bone loss should be less than 1.5
mm during the first year and less than 0.2
mm annually following the first year ofservice and this value is still considerate asa limit nowadays. The loss of crestal bonehas been reported to be influenced by manyfactors. These include surgical trauma,
implant abutment microgap, bacterial
infection of peri-implant tissues andbiomechanical factors related to loading.
The location of the microgap existing
between the implant neck and the abutment
represents the most common factor
inducing bone loss (Piattelli et al. 2003).2
In fact, 2 mm vertical bone resorption fromthis interface is considered acceptable andphysiological (Hermann et al. 2000; Kinget al. 2002) 3 , 4 as comparable to the
biological width. However, excessive andprogressive bone loss can lead to theeventual loss of the implant (Chou et al.
2004)5 and therefore has to be constantly
evaluated over the years.
The specific purposes of this study
were: 1) to assess the 5 year survival rateof short implants (5.7 mm length and 6 mm
width); 2) to radiographically evaluate thecrestal bone loss associated with the 6x5.7mm implant, compared to longer implantsof the same company (Bicon, Boston MA),
within the same patient and same clinicalscenario. A 5 year radiographic follow up
was performed in order to achieve our aims.
Materials and Methods
Study Design and Sample
The patients for this retrospective
cohort study were recruited from the
population of patients who had at least one
6x5.7 mm (Fig. 2) implant placed bypractitioners at the Faulkner Hospital or at
the Implant Dentistry Centre, Boston,
Massachussets, between February 1997 andFebruary 2006. Subjects who had at leastone 6x5.7 mm implant for at least 5 years
of follow up and at least one non 6x5.7 mm
implant within the same alveolar arch wereconsidered eligible and included in the
study. Exclusion criteria were insufficient
documentations in clinical and radiographic
records.

341(A)

Fig. 2: Example of 6mm (wide) x 5.7mm (long)
implant used in the study.

(B)

(C)
Fig. 1: A) Diagram showing crestal bonemeasurement for implant adjacent to natural
teeth; B) Diagram with measurements for
implant adjacent to implant; C) Diagram withmeasurements for implant not adjacent toimplant or tooth.

Study Variables
The major predictor variable was
implant size. The implant that met theinclusion criteria were divided into two
categories: 6x5.7 mm and non - 6x5.7 mm
implants. Other study variables are listed
below:
Demographics: These included
patient’s gender and age at the time of
implant placement.
Medical History: Patient health status
included smoking, any contributory diseaseaffection and use of medication.
Anatomy: The variables included in thiscategory were implant location (maxilla,mandible, anterior, posterior), dentition
status (partially or fully edentulous), implantproximity to teeth or other implants (noadjacent teeth, one adjacent implant, one
adjacent tooth, two adjacent implants, two
adjacent teeth, one adjacent implant and one
adjacent tooth) and bone quality (types 1 to
4). Bone quality was determined by the same
operator, at time of implant placement upon
examination of the contents of the flutes of
a 3.5 mm reamer extracted from theFig. 3: Digital calibration and radiographic
measurements of crestal bone levels. Here
implants at last follow-up. Adjustment for
magnification error was made using thefollowing equation: Corrected crestal bonelevel = Measured crestal bone level x (actual
implant length ¸ measured implant length). In
this radiograph, short 6x5.7 mm implant in the
maxilla adjacent to one natural tooth and one
non-6x5.7 implant.

osteotomy. Type 1 bone was defined ascompact, bloodless bone that completely
filled the flutes of the reamer. Bone quality
was classified as type 4 when minimal
amount or no bone filled the flutes of thereamer. Intermediate grades were classified
as either type 2 or type 3 bone.
Implant specific variables: They
included implant length (5.7 to 14 mm),diameter (3.5 to 6 mm), well size (2 to 3mm), coating (uncoated, Titanium Plasma
Spray [TPS] or H ydroxyapatite [HA]
coated).
Surgical variables: Implant staging (oneor two stages), immediate post extraction
or delayed placement, bone augmentation
procedures and time when they were
performed, type of bone, use of membrane,
presence, type of complications and timeof occurrence were the study surgicalvariables. For each implant, the date of
implant placement, dates of follow up
radiographs and date of implant removal(if applicable) were recorded.

342

Journal of Maxillofacial & Oral Surgery 2008 Vol. 7 : No. 3
Table 1: Descriptive Statistics
Group 0Non - 6x5.7 mm

VARIABLES

Group 16x5.7 mm

Implants%

Implants

%

Male

18

52.9

20

71.4

Female

16

47.1

8

28.6

No

30

88.2

26

92.9

Yes

4

11.8

2

7.1

Maxilla

18

52.9

5

17.9

Mandible

16

47.1

23

82.1

Ant

13

38.2

1

3.6

Post

21

61.8

27

96.4

Type 2

5

20.0

8

32.0

Type 2.5

1

4.0

1

4.0

Type 3

4

16.0

8

32.0

Type 4

15

60.0

8

32.0

3.5

8

23.5

NA

NA

4

8

23.5

NA

NA

4.5

8

23.5

NA

NA

5

9

26.5

NA

NA

6

1

3.0

28

100.0

5.7

NA

NA

28

100.0

8

15

44.1

NA

NA

11

18

53.9

NA

NA

14

1

3

NA

NA

2

20

58.8

NA

NA

3

14

41.2

28

100.0

Uncoated

6

17.6

13

46.4

TPS

10

29.4

9

32.1

HA

18

53.0

6

21.4

Delayed

23

67.6

26

92.9

Immediate

11

32.4

2

7.1

No

8

24.2

0

0.0

One

7

21.2

12

42.9

Two

18

54.6

16

57.1

Total Subjects = 20
GenderSmoker

Jaw Location

Anterior-Posterior Location

Bone Quality

Implant Specific
Diameter

LengthWell

Coating

Placement

StagingJournal of Maxillofacial & Oral Surgery 2008 Vol. 7 : No. 3

343

Table 2: Radiographic Bone levels
Group 0Non -6x5.7 mm

Group 16x5.7 mm

Overall Crestal Change of Bone Levels*-0.08 mm (k0=27)-0.03 mm (k1=21)

Crestal Change of Bone Levels at 1 year*-0.20 mm (k0=1)

NA mm (k1=0)

Crestal Change of Bone Levels at > 1 year*-0.07 mm (k0=26)-0.03 mm (k1=21)* Average in mm of measurements mesial and distal to the implant.
k = number of implants

Table 3: Univariate Analysis of Factors associated with Change in Bone Levels °Subjects

P value

Mean age20

0.79

Gender (female)

20

0.75

Smoker20

0.72

Medical Compromised

20

0.85

implant

P value

Jaw location

62

0.25

Ant-Post

62

0.20

Quality of bone

62

0.73

Implant diameter

62

0.85

Implant length

62

0.76

Implant well

62

0.73

Coating62

0.88

Staging62

0.42

Immediate placement

0.13

62

0.74

Peri-augment

62

0.98

Post-augment

62

0.80

Augmentation

62
Pre-augment° statistically significant at p < 0.20
Implant failure was defined as removalof the implant. Time between date of
implant placement and patient’s last visit
or implant removal was defined as implantsurvival. To be considered successful, theimplants had to meet the following
requirements: 1) patient and dentist
satisfaction regarding the implant
supported restorations from the esthetic andfunctional point of view; 2) absence of pain,
discomfort, infection attributable to theimplants; 3) stability of the implants whentested clinically; 4) vertical bone loss
inferior to 0.2 mm per year, after the firstyear of function. (Albrektsson et al.
1986).6,7
Change in bone levels over time wereobtained by direct measurements on nonstandardized, digital periapical radiographs(Digora System, Soredex, Helsinki,

Finland). Magnification error was madeusing the following equation: Correctedcrestal bone level = Measured crestal bonelevel x actual implant length ÷ measured
implant length. On each image,measurements in millimeters were
calibrated based on the known implant
length (manufacturing standards) as
represented in Fig. 3. For each implant theradiographic variables were divided
according to the category the implant
belonged to. The categories were: implantadjacent to tooth mesially or distally, tooth
mesial and distal of implant, implantadjacent to implant mesially or distally,
implant mesial and distal of implant,
nothing adjacent described in Fig. 1. Bonelevels were measured vertically andperpendicularly from implant abutment
interface to crestal bone level at both mesial

and distal surfaces. Distance between
implants or natural teeth was measured
horizontally after calibration. For the rateof bone loss data measurements taken fromthe distal and mesial sites were first
averaged for each implant.
Data analysis
A database was created using Microsoft
Excel® (Microsoft, Inc, Redmond, WA).
SAS® PC-version 9.1 (2002-2003) (SASInstitute, Carey, NC) statistical software
was used for data and statistical analysis.
Descriptive statistics were computed for all
study variables.
Some patients might have more than
one dental implant producing clusteredobservations. To adjust for clustered,correlated observations, Generalized Linear
Mixed effects models (GLM models) using

344

Journal of Maxillofacial & Oral Surgery 2008 Vol. 7 : No. 3
Table 4: Multivariate Analysis (p < 0.05)
Parameter estimate

Standard Error

P valueGroup 1 (6 x 5.7 mm)-0.0570.25

0.82Group 0 (Non 6 x 5.7mm)

0.00

referencereferenceAge (increase per year)

0.0024

0.016

0.89

Gender (female)-0.0100.40

0.98

Location (Posterior)

0.21

0.28

0.46

Immediate

-0.34

0.33

0.32

regression analysis was applied to identify
risk factors associated with crestal boneloss. Potential risk factors for crestal bonechanges were identified using the univariate
GLM regression model and wereconsidered as potential predictor variables
if p < 0.15. Variables meeting this criterion
were included in the multivariate clustered
GLM regression model to identify variables
statistically associated (p < 0.05) with theoutcome with the addition to the threebiologic important predictors, age at
implant placement, gender, and the main
predictor group (short versus non short
implants).
Results
Between February 1997 and February
2003, 20 subjects had at least one 6x5.7mm implant placed at least for five years
and met the study inclusion criteria. Totalof 62 implants, 28 6x5.7mm (test group =group 1 = short implant group) and 34 non6x5.7mm (control group = group 0 = non
short implant group), were sampled from
20 patients (12 males and 8 females).
Demographic study variables are
summarized in Table 1. Mean length ofnon-6x5.7mm implants was 9.7 mm,
ranging from 8mm to 14mm and with amean diameter of 4.30 mm (range: 3.5 to 5mm). The mean duration of the clinical andradiographic follow up was 55 months(+/- 31.3) for Group 0 and 68.1 months(+/- 18.0) for group 1. Smoking habit wasreported by 11.8% and 7.1% of group 0 and
1 respectively. Fifty three percent of theimplants belonging to group 0 and 17.9%of group 1 were placed in the maxillaryarch. The majority of the implants (61.8%
group 0 and 96.4% group 1) were placedin the posterior segments. Five years
survival rates for group 0 and group 1 were100.0% and 96.8% respectively, but thisdifference was not statistically significant(p=0.35). Table 2 summarizes the changes
in peri-implant crestal bone levels overtime. Overall the mean changes inradiographic bone levels were -0.08 mm forgroup 0 implants and -0.03 mm for group
1 implants (average of mesial and distallevels). Mean change in bone levels forgroup 0 implants at 1 year was -0.20 mm.
Mean changes in bone levels for period
longer than one year were -0.07 mm and-0.03 mm for group 0 and 1 respectively.
Table 3 summarizes univariate analysis
used to identify association between
individual study variables and peri-implant
crestal bone loss. Implant placement inposterior segments of the jaws andimmediate placement were considered
statistically significant (p < 0.20) and
implant placement in the maxillae resulted
near statistically significant. Therefore
these parameters were included in themultivariate analysis.
In the adjusted multivariate model(Table 4) the association between crestal
bone loss and implant location in posterior
segments of maxilla or mandible andimmediate placement were considered nearstatistically significant (p < 0.05).
Discussion
Short implants offer several surgicaladvantages compared to longer implants.
The use of short implants in the posterior
regions reduces the need for boneaugmentation procedures prior to or in
conjunction with implant placement in the
maxilla and the mandible. Shorter implants
reduce the surgical risk of sinus perforationor mandibular paresthesia, with an overall
reduction in surgical complications. Due tothe decreased length of the drills andimplants, the osteotomy preparation implies
less risk of overheating the bone. Insertionof drills and implants results also easier insmall intra arch spaces. In case of apicalroot proximity short implant can be the only
possible choice. From the patient’s pointof view, shorter implants reduce treatment
time, discomfort and overall costs related
to graft procedures. All these factors makeshort implants a highly attractive restorative
option.
The purpose of this retrospective study

was to estimate the 5 year survival andsuccess rate of the 6x5.7 mm short implant,
and to evaluate the crestal bone levels of
the same implant in comparison with longer
implants from the same company. The
survival rate at 5 years for a sample of 28
short implants was 100.0% and the meanbone loss was 0.03 mm. These values werecomparable to the values calculated for thenon -6x5.7 implant group.
A 1998 study by ten Bruggenkate andassociates reported a 6 year survival rateof 94% for a 6 mm long Straumannimplants.8 Similarly, Friberg and coworkers
found the 5 years survival rate to be 95.5%
for a cohort of short Brånemark system
implants. 9 Davarpanah and colleagues
found a success rate at 3 years of short
Osseotite implants of 98.4%.10
Fugazzotto in 2004 evaluated 7 to 9 mmlong implants placed in posterior region of
the maxilla and showed a success rate of
95.1 % up to 84 months of function.11
Hagi and coworkers in their recent
study, concluded that dental implant surface
geometry is a major determinant in how
well this implants perform in short lengths,
defined in that study as < 7mm.12 While
threaded implants showed higher failure
rates in short versus longer length, sinteredporous surfaced implants performed wellin short lengths. Moreover, various
researchers using Finite Elemental Analysis(FEA) have demonstrated that horizontal
and vertical occlusal forces placed on
implants were distributed primarily in the
crestal bone, rather than along the entire
implant/bone interface. These findings led
the group of Lum to conclude that short
implants serve as well as longer ones.13,14,15
However, finite element method is acomputer simulation technique which
considers human bone of the jaws as a
uniform structure material; dense corticalbone and trabecular marrow space which
form human bone, are not considered in thistype of analysis and therefore results stillseem of limited relevance.
Implant diameter should also beJournal of Maxillofacial & Oral Surgery 2008 Vol. 7 : No. 3
considered as an important clinical variable.
It has been suggested that increasing
implant diameter could compensate fordecrease of length. Himmlova andcolleagues showed that an increase in theimplant diameter decreases the stressaround the implant neck more than anincrease in the implant length, as a resultof a more favorable distribution of thesimulated masticatory forces.16
Although several studies in theliterature have shown that short implants
have risk factors and therefore higher
failure rate compared to longer
implants,17,18,19 several recent studies seem
to prove the good long term prognosis of
the short implants. It has been shown that
also the crown/implant ratio do not seemto be a major risk factor in case offavorable force orientation and load
distribution. Tawil and coworkers in 2006
evaluated the bone loss around short
implants (>10mm) and concluded thatthese implants are a long term viable
solution in sites with reduced bone heighteven when the prosthetic parameters
exceed the normal values but under force
parafunction control.20
All the work previously cited confirmed
the results presented in this study and offer
promising and further applications for the
6x5.7 mm short implant. Prospective follow
up study and larger sample size are in
progress.
Conclusion
Within the limitations of the present
study, the short implant (6x5.7 mm)showed a survival rate at 5 years of
100.0% which was not statisticallysignificant different when compared tolonger implants (>8 mm) of the same
company. Radiographic bone levels
measured mesially and distally to the
6x5.7 mm implants at 5 years werecomparable to the bone levels around
longer implants. Clinical parameters,
including gingival and restorativeevaluations were also comparable between
the two groups. However, this study
showed that placement in posterior
maxillary regions and immediate
placement could affect the long term
crestal bone stability. 6x5.7 mm implants
in case of limited bone height are a
predictable option for the treatment ofedentulous patients.
Acknowledgments
The authors would like to thank theclinicians, in particular Dr. Rainier
Urdaneta, and the support staff of Implant

Dentistry Centre, Boston MA, for theircooperation in this study and the access topatient records. The authors would also liketo acknowledge the support of thecomputing facilities at the Harvard
University, School of Public Health,Boston, MA and at the Harvard – MIT Data
Center Cambridge, MA for access tostatistical software. This project was
supported by the Educational Grant (LLC)by Bicon, Boston MA.
Bibliography
1. Smith DE, Zarb GA. Criteria forsuccess of osseointegrated endosseous
implants. J Prosthet Dent 1989;
62(5):567-72.
2. Piattelli A, Vrespa G, Petrone G, Iezzi
G, Annibali S, Scarano A. Role of theMicrogap Between Implant and
Abutment: A Retrospective Histologic
Evaluation in Monkeys. J Periodontol.
2003;72(9):1146-1150.
3. Hermann JS, Buser D, Schenk RK,
Higginbottom FL, Cochran DL.
Biologic width around titanium
implants. A physiologically formed andstable dimension over time. Clin Oral
Implants Res. 2000;11(1):1-11.
4. King GN, Hermann JS, Schoolfield JD,
Buser D, Cochran DL. Influence of thesize of the microgap on crestal bonelevels in non-submerged dental
implants: a radiographic study in thecanine mandible. J Periodontol.
2002;73(10):1111-7.
5. Chou CT, Morris HF, Ochi S, Walker
L, DesRosiers D. Crestal bone loss
associated with the Ankylos implant:
loading to 36 months. J Oral Implantol.
2004;30(3):134-43.
6. Albrektsson T, Zarb G, Worthington P,
Eriksson AR. The long-term efficacy of
currently used dental implants: a review
and proposed criteria of success. Int J
Oral
Maxillofac
Implants.
1986;1(1):11-25.
7. Weng D, Jacobson Z, Tarnow D,
Hurzeler MB, Faehn O, Sanavi F,
Barkvoll P, Stach RM. A prospective
multicenter clinical trial of 3i machinedsurface implants: results after 6 years offollow-up. Int J Oral Maxillofac
Implants. 2003;18(3):417-23.
8. ten Bruggenkate CM, Asikainen P,
Foitzik C, Krekeler G, Sutter F. 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.
9. Friberg B, Grondhal K, Lekholm U,Brånemark P-I. Long term follow-up

345
of severely atrophic edentulous
mandibles reconstructed with short
Branemark implants. Clin Dent Relat
Res 2000;2:184-189.
10. Davarpanah M, Martinez H, Celletti R,
Alcoforado G, Tecucianu JF, Etienne D.
Osseotite implant: 3 year prospective
multicenter evaluation. Clin Implant
Dent Relat Res 2001 ;3 :111-118.
11. Fugazzotto PA, Beagle JR, Ganeles J,
Jaffin R, Vlassis J, Kumar A. Successand failure rates of 9 mm or shorter
implants in the replacement of missing
maxillary molars when restored withindividual crowns: preliminary results
0 to 84 months in function. A
retrospective study. J Periodontol.
2004;75(2):327-32.
12. Hagi D, Deporter DA, Pilliar RM,
Arenovich T. A targeted review of study
outcomes with short (d”7 mm)
endosseous dental implants placed inpartially edentulous patients. J
Periodontol 2004;75(6): 798-804.
13. Lum LB. A biomechanical rationale forthe use of short implants. J Oral
Implantol 1991;17:126-131.
14. Matsushita Y, Kitoh M, Mizuta K,
Ikeda H, Suetsugu T. Two-dimensional
FEM analysis of hydroxyapatite
implants: diameter effects on stress
distribution. J Oral Implantol.
1990;16(1):6-11.
15. Holmes DC, Loftus JT. Influence ofbone quality on stress distribution forendosseous implants. J Oral Implantol
1997;23:104-111.
16. Himmlova L, Dostalova T, Kacovsky
A, Konvickova S. Influence of implant
length and diameter on stress
distribution: a finite element analysis.
J Prosthet Dent. 2004;91(1):20-25.
17. Winkler S, Morris HF, Ochi S. Implantsurvival to 36 months as related tolength and diameter. Ann Periodontol
2000;5:22-31.
18. Lazzara R, Siddiqui AA, Binon P.
Retrospective multicenter analysis of 3I
endosseous dental implants placet overa five-year period. Clin Oral Implants
Res 1996;7:73-83.
19. Scurria MS, Morgan V, Guckes AD, Li
S, Koch G. Prognostic variables
associated with implant failure. A
retrospective effectiveness study. Int J
Oral
Maxillofac
Implants
1998;13 :400-406.
20. Tawil G, Aboujaoude N, Younan R.
Influence of prosthetic parameters on
the survival and complication rates ofshort implants. Int J Oral Maxillofac
Implants 2006;21(2):275-282.