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TheEffectofSplintingImplant-SupportedRestorationsonStressDistributionofDifferentCrown-ImplantRatiosandCrownHeightSpaces.pdf

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J Oral Maxillofac Surg 69:2990-2994, 2011

The Effect of Splinting Implant-Supported Restorations on Stress Distribution of Different Crown-Implant Ratios and

Crown Height Spaces Joseph Nissan, DMD,* Ora Gross, DMD,† Oded Ghelfan, DMD,‡

Ilan Priel, DMD,§ Martin Gross, BDS, LDS, MSc,� and Gavriel Chaushu, DMD, MSc¶

Purpose: To assess whether splinting can counterbalance the detrimental effects of varying the crown-to-implant (C/I) ratio and crown height space (CHS) by decreasing nonaxial overload stresses.

Materials and Methods: Three implants were inserted into a photoelastic block model. Two strain gauges were cemented onto the neck of each implant on the buccal and lingual aspects and provided a simultaneous direct reading of strain. Four groups of splinted cement-retained restorations with C/I ratios of 1:1, 1:1.5, 1:1.75, and 1:2 were used. CHSs were 10, 15, 17.5, and 20 mm, respectively. Fifteen static loadings were carried out simultaneously with 20-kg weights via a custom-built loading apparatus at 30° to the vertical axis.

Results: Occlusal force application at 30° showed a statistically significant increase in both buccal (1,911.65 � 110 vs 3,252.06 � 150) and palatal (35.58 � 7 vs 286.85 � 15) microstrain values as the C/I ratio increased from 1:1 to 1:1.5 (P � .001). Force application at 30° in cases with C/I ratios of 1:1.75 and 1:2 resulted in fracture of the abutment screw followed by dislodgement of the crowns. Failures were noted at a CHS of 15 mm or greater.

Conclusions: In this biomechanical mode, splinting does not prevent prosthetic failure when the CHS is 15 mm or greater. Vertical bone augmentation is highly recommended in cases with a CHS of 15 mm or greater. © 2011 American Association of Oral and Maxillofacial Surgeons

J Oral Maxillofac Surg 69:2990-2994, 2011

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he clinical parameters that lead to biomechanically dvantageous conditions after splinting implants in pecific clinical situations are unclear.1 The traditional ationale for splinting teeth was to decrease stresses, esulting in increased prosthesis stability.2 One clas-

sical clinical indication was increased crown-to-root ratio.3 However, evidence-based data to support such n indication are largely missing for teeth and even ore so for implants.1,4 Increasing the crown length

nd degree of nonaxial load over an implant-sup- orted prosthesis increases the risk of excessive oc- lusal overload because of an increased moment

Received from The Maurice and Gabriela Goldschleger School of

Dental Medicine, Tel Aviv University, Tel Aviv, Israel.

*Senior Lecturer, Department of Oral Rehabilitation.

†Instructor, Department of Oral Rehabilitation.

‡Instructor, Department of Oral Rehabilitation.

§Instructor, Department of Oral Rehabilitation.

�Associate Professor, Department of Oral Rehabilitation.

¶Associate Professor, Department of Oral and Maxillofacial Surgery.

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rm.5 A related term is crown height space (CHS), efined as the distance measured from the crest of the lveolar bone to the plane of occlusion. The biome- hanics of CHS is related to lever arm mechanics.6

Nonaxial loading creates a significant lateral moment, which increases proportionally to the increase in CHS, resulting in stress concentration at the implant neck.7

Implants are nonmobile; therefore, when nonaxial forces are applied to an implant, they are concen- trated at the implant abutment junction and the crest of the supporting bone.8,9 Overload leads to stress

Address correspondence and reprint requests to Dr Nissan:

Department of Oral Rehabilitation, The Maurice and Gabriela Gold-

schleger School of Dental Medicine, Tel Aviv University, 4, Klackin

St, Tel Aviv, Israel; e-mail: [email protected]

278-2391/11/6912-0015$36.00/0

oi:10.1016/j.joms.2011.06.210

mailto:[email protected]

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that may cause mechanical failure of the prosthetic components and bone microfractures that could lead to implant loss.10-12 The rationale of splinting in im- plant dentistry is to minimize stress by increasing the resistance area over which the load is distributed.13

However, clinical studies are contradictory. Implant splinting did not significantly improve implant suc- cess rates for implant-supported fixed partial dentures (97.1%) compared with single-implant restorations (94.3%).14 Furthermore, splinting did not have an effect on crestal bone loss at different crown-to- implant (C/I) ratios.15

The purpose of this study was to assess whether splinting can counterbalance the detrimental effects of varying the C/I ratio and CHS by decreasing non- axial overload stresses.

Materials and Methods

A photoelastic block model (PLM-4B; Vishay Measure- ment Group, Raleigh, NC) with a modulus of elasticity of 450 kilopounds per square inch (range of human bone) was constructed. The model dimensions were 15.8 � 35.2 � 32 mm.

Three holes were drilled vertically in a straight line n the mid axis of the photoelastic model at predeter-

ined locations to lengths of 12 mm. Implants were ocated 7.8 mm from the edge of the model and each mplant separated by 4 mm. Three external hex, crew-type titanium implants with a diameter of 3.8 m and length of 12 mm (Nobel Biocare, Gothen-

urg, Sweden) were inserted into the model. Implants rotruded 2 mm from the superior surface. Two strain gauges (EA-06-015EH-120; Vishay Mea-

urement Group) were cemented (M-Bond 200; Vi- hay Measurement Group) horizontally onto the ma- hined neck of each implant on the buccal and lingual spects at a 180° inclination to each other before butment and crown placement. These strain gauges easured the bending components (tension/com- ression) created from both vertical and horizontal ectors arising from applied forces. Strain gauges were onnected to a strain indicator (System 5000; Vishay easurement Group) that provided a simultaneous di-

ect reading of strain in microstrain units of all model omponents for each loading session (Fig 1). Impressions were taken by the open-tray technique ith acrylic splinted transfer copings (Duralay; Reli-

nce Dental Mfg Co, Worth, IL) by use of custom crylic trays. Polyether impression material was used Impregum F; ESPE, Seefeld, Germany). A master orking model was fabricated, and all the restora-

ions were fabricated on attached fixed abutments ith 2 mm of gingival height. Four groups of restorations were cast in Remanium

obalt-chromium-molybdenum Model Casting Al-

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loy-GM 380 (Dentaurum, Ispringen, Germany) (Fig 2): ) 3 splinted crowns with a C/I ratio of 1:1, 2) 3 plinted crowns with a C/I ratio of 1:1.5, 3) 3 splinted rowns with a C/I ratio of 1:1.75, and 4) 3 splinted rowns with a C/I ratio of 1:2. All the restorations were fabricated with the occlu-

al anatomy of the upper first molars with the same esiodistal and buccolingual dimensions by use of a

ilicone index. Occlusocervical dimensions were 6, 1, 13.5, and 16 mm in length to compensate for 2 m of the implant neck and 2 mm of the abutment

ingival height. CHSs were 10, 15, 17.5, and 20 mm, espectively.

Cement-retained abutments were placed at a con- rolled torque of 35 N-cm. During each loading ses- ion, the restorations were cemented onto the im- lant abutments with TempBond NE (Kerr, Orange, A). Cementation with provisional cement to allow

etrievability is indicated because of abutments of ufficient height and acceptable retention. Fifteen tatic loadings were carried out simultaneously with 0-kg weights via a custom-built loading apparatus. oad was applied simultaneously through 3 individual ins to the inner inclines of the buccal cusps of each et of restorations at 30° to the vertical axis (Fig 3) to imulate the mean off-axis interval in the clinical sit- ation. For each loading, strain gauge recordings were made.

train gauges measure electrical resistance. During ex- ension or contraction, the strain gauge records changes n electrical resistance. The degree of distortion of the train gauge is recorded in calculated microstrain values, here strain � € � �L (change in length of strain

auge)/L (in micrometers per meter) � �R (change in

FIGURE 1. Master model with strain gauges connected to cervical part of implant. (Reprinted from Nissan J, Ghelfan O, Gross O, et al: The effect of crown/implant ratio and crown height space on stress distribution in unsplinted implant supporting restorations. J Oral Maxillofac Surg 69:1934, 2011.)

Nissan et al. Splinting and Stress Distribution. J Oral Maxillofac Surg 2011.

electrical resistance in strain gauge)/R.

(

Surg 2

2992 SPLINTING AND STRESS DISTRIBUTION

STATISTICAL ANALYSIS

Descriptive analysis consisted of mean and standard deviation of microstrain values for each group. Groups were compared by use of 1-way parametric analysis of variance. P � .05 was considered statistically significant.

FIGURE 2. The 4 groups of restorations with different C/I ratios (Reprinted from Nissan J, Ghelfan O, Gross O, et al: The effect unsplinted implant supporting restorations. J Oral Maxillofac Surg

Nissan et al. Splinting and Stress Distribution. J Oral Maxillofac

FIGURE 3. Loading apparatus with 3 individual pins oriented to inner inclines of buccal cusps. (Reprinted from Nissan J, Ghelfan O, Gross O, et al: The effect of crown/implant ratio and crown height space on stress distribution in unsplinted implant supporting resto- rations. J Oral Maxillofac Surg 69:1934, 2011.)

Nissan et al. Splinting and Stress Distribution. J Oral Maxillofac

Surg 2011.

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Results

Occlusal force application at 30°, in implant-sup- ported splinted restorations with different C/I ratios, showed a statistically significant increase in both buc- cal (1,911.65 � 110 vs 3,252.06 � 150) and palatal 35.58 � 7 vs 286.85 � 15) microstrain values as the

C/I ratio increased from 1:1 to 1:1.5 (P � .001). Force application at 30° in cases with C/I ratios of 1:1.75 and 1:2 resulted in fracture of the abutment screw followed by dislodgement of the crowns. The CHS ranged from 10 to 20 mm. Failures were noted at a CHS of 15 mm or greater when force application was at 30° (Table 1).

:1.5, 1:1.75, and 1:2) and CHSs (10, 15, 17.5, and 20 mm). n/implant ratio and crown height space on stress distribution in 34, 2011.)

011.

Table 1. MICROSTRAIN VALUES FOR EACH GROUP

Crown/ Implant

Ratio

Crown Height Space (mm)

Microstrain Value With Force Application at 30° (Mean � SD)

Buccal Palatal

1:1 10 1,911.65 � 110 35.58 � 7 1:1.5 15 3,252.06 � 150 286.85 � 15 1:1.75 17.5 — — 1:2 20 — —

Nissan et al. Splinting and Stress Distribution. J Oral Maxillofac

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Discussion

Most of the fixed implant-supported prostheses nowadays are cemented because of superior occlu- sion, esthetics, passivity, and loading characteris- tics.16 Therefore resistance and retention forms may be a major indication for splinting, although some current clinical protocol trends would show a return to the use of screw-retained restorations for reasons of retrievability. An additional rationale for splinting implant crowns together is to favorably distribute the nonaxial loads, minimize their trans- fer to the restoration and supporting bone, and increase the total load area.17 Splinting the crowns reduced the peri-implant bone stress under hori- zontal load in a finite element analysis model espe- cially recommended for implants surrounded by poor-quality bone.18 A photoelastic study of a par- tially edentulous mandible examined the effect of splinting and interproximal contact tightness on the load transfer to implant restorations.19 It showed that plinted restorations shared the occlusal loads and istributed the stresses more evenly between the im- lants when force was applied. The literature addressing the concept of C/I ratio is

imited. A retrospective cohort study questioned the ole of the C/I ratio in the potential failure of implant estorations. The C/I ratio of implants in function 1:1.3) was similar to that in those implants that failed 1:1.4).20 Another study concluded that a C/I ratio of

1:1.5 (range, 1:0.8 to 1:3.0) did not affect crestal bone levels.21 An additional study with C/I ratios in the ange of 1:1 to 1:2 showed no correlation of peri- mplant bone loss with C/I ratio.22 Similarly, a 10-year rospective study concluded that implant restora- ions may be successful even with C/I ratios between :2 and 1:3.15

A potentially more significant factor related to C/I ratio is CHS.6 The biomechanics of CHS is re- ated to lever mechanics. Nonaxial loading of 30° enerated a proportional increase in stress distribu- ion when prosthetic height increased from 6 mm 17.72 MPa) to 12 mm (30.09 MPa).23 CHS of 15 m or greater is regarded as biomechanically unfa-

orable, resulting in increased stress at the crestal one/implant area.24

Blanes et al15 concluded that implant restorations ith C/I ratios between 1:2 and 1:3 may be success-

ul. The mean reported CHS in this study was less than 5 mm. In another study a mean C/I ratio of 1:1.5 did ot increase crestal bone loss. When 4 implant

engths were used, the CHS was always less than 15 m.21 Thus these findings can again explain the fact

that the C/I ratio is not the most detrimental for

alveolar bone loss.

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This study used the strain gauge technique to in- vestigate the effects of splinting implant-supported restorations with different C/I ratios and CHSs on nonaxial stresses at the implant cervical area. Axial loading of an implant-supported prosthesis produces minimal stress to the supporting bone compared with nonaxial loading. Changes in the angle of force appli- cation resulted in greater stress to supporting bone. Implant design, prosthesis height, and the use of an offset implant may reduce stress, but the reduction cannot compensate for the increase found with non- axial loading.23,25 Furthermore, different prosthesis

esigns, implant designs, or implant configurations hared the same finding in which most stresses were oncentrated in the cervical area when force was ap- lied to implant-supported prostheses.25-27 The results

of our study showed that force application at 30° signif- icantly increased both buccal (1,911.65 � 110 vs 3,252.06 � 150) and palatal (35.58 � 7 vs 286.85 � 15) microstrain values as the C/I ratio increased from 1:1 to 1:1.5 (P � .001). Furthermore, C/I ratios of 1:1.75 and 1:2 resulted in fracture of the abutment screw followed by dislodgement of the crowns, and failures were noted at a CHS of 15 mm or greater. Splinting could not reduce stresses at the cervical area nor compensate for the detrimental effect of C/I ratio and CHS of 15 mm or greater, which resulted in failure of the restoration.

Although the results seem contradictory to previ- ously mentioned in vitro studies,18,19 they are com-

atible with the available clinical studies.14,15,20-22

Those studies show no detrimental effect of 1:1.75 and 1:2 C/I ratios because whenever the CHS was mentioned, it was less than 15 mm.15,21 Splinting did not have an effect on crestal bone loss.15 Moreover, splinting can result in greater crestal bone loss.21

Within the limitations of this experimental model, the following conclusions may be drawn regarding splinted implant-supported restorations:

1. Nonaxial loading of 30° generated a propor- tional increase in stress distribution when the C/I ratio and CHS increased.

2. Prosthetic failure occurred at a C/I ratio of 1:1.75 or greater and CHS of 15 mm or greater.

3. Splinting increased cervical stresses and could not prevent prosthetic failure.

4. The CHS may be more significant than the C/I ratio in assessing biomechanically related detri- mental effects.

On the basis of the findings, our hypothesis is that vertical ridge augmentation resulting in a CHS of less than 15 may decrease the risk of prosthetic failure. The clinical relevance needs to be investigated with

controlled long-term clinical studies testing the ef-

2994 SPLINTING AND STRESS DISTRIBUTION

fects of additional modifiers, such as “offset implant” positioning and an internal hex-head connection im- plant system. Similarly, different grafting procedures should be considered. Because the grafting recom- mendation is based on a theoretic study, it would need a clinical counterpart to have ultimate validity.

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15. 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 18:707, 2007

16. Hebel KS, Gajjar RC: Cement-retained versus screw-retained implant restorations: Achieving optimal occlusion and esthet- ics in implant dentistry. J Prosthet Dent 77:28, 1997

17. Misch CE: Occlusal considerations for implant-supported pros- theses, in Dental Implant Prosthetics. St Louis, MO, Mosby, 2005, pp 472-510

8. Wang TM, Leu LJ, Wang J, et al: Effects of prosthesis materials and prosthesis splinting on peri-implant bone stress around implants in poor-quality bone: A numeric analysis. Int J Oral Maxillofac Implants 17:231, 2002

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1. Rokni S, Todescan R, Watson P, et al: An assessment of crown- to-root ratios with short sintered porous-surfaced implants sup- porting prostheses in partially edentulous patients. Int J Oral Maxillofac Implants 20:69, 2005

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The Effect of Splinting Implant-Supported Restorations on Stress Distribution of Different Crown ...

Materials and Methods

Statistical Analysis

Results
Discussion
References