Literature review

Analysis of load transfer and stress distribution by splinted

and unsplinted implant-supported fixed cemented

restorations

J . N I S S A N * , O . G H E L F A N

* , M . G R O S S

* & G . C H A U S H U

† Departments of

* Oral Rehabilitation and

† Oral and Maxillofacial Surgery, The Maurice and Gabriela Goldschleger School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel

SUMMARY Controversy remains over the rehabilita-

tion of implant-supported restorations regarding the

need to splint adjacent implant-supported crowns.

This study compared the effects of simulated occlusal

loading of three implants restored with cemented

crowns, splinted versus unsplinted. Three adjacent

screw-shaped implants were passively inserted into

three holes drilled in a photo-elastic model. Two

combinations of cemented restorations were fabri-

cated; three adjacent unsplinted and three adjacent

splinted crowns. Strain gauges were connected to the

implant necks and to the margins of the overlaying

crowns. Fifteen axial static loads of 20-kg loadings

were carried out right after each other via a custom-

built loading apparatus. Strain gauges located on

the implant neck supporting splinted restoration

demonstrated significantly (P < 0Æ001) more strain

(sum of strains = 3348Æ54 microstrain) compared

with the single crowns (sum of strains = 988Æ57

microstrain). In contrast, significantly (P < 0Æ001)

more strain was recorded on the strain gauges located

on the restoration margins of the single crowns (sum

of strains = 756Æ32 microstrain) when compared with

splinted restorations (sum of strains = 186Æ12 micro-

strain). The concept of splinting adjacent implants to

decrease loading of the supporting structures may

require re-evaluation. The clinical relevance of these

findings needs further investigation.

KEYWORDS: load, stress distribution, splinted,

unsplinted

Accepted for publication 19 March 2010

Introduction

Occlusal load and its distribution is considered to be one

of the principal components that influences the success

and failure of implant-supported restorations over time

(1–4). The traditional rationale for splinting teeth was

to increase retention and resistance resulting in

decreased stresses, improving prosthesis stability (5).

The rationale of splinting in implant dentistry to

minimize stress by increasing the resistance area over,

which the load is distributed, is controversial (6). The

biomechanical advantages following splinting restora-

tions are still unclear (7). Evidence-based data to

support splinting are largely missing for teeth and even

more for implants (7, 8).

Some authors have maintained that occlusal loads

transferred to implants supporting splinted restoration

are larger than those applied to implants supporting

unsplinted restorations because of the development of

moments (6, 9). Clinical studies on the successful

restoration of unsplinted adjacent implant-supported

restorations in partially edentulous individuals have

been reported (10–14). Implant splinting did not

significantly improve implant success rates for

implant-supported fixed partial dentures (97Æ1%) compared to single-implant restorations (94Æ3%) (14). Furthermore, splinting did not have an effect on crestal

bone loss (15).

This study examined load transfer and stress distri-

bution of simulated axial occlusal loading on adjacent

ª 2010 Blackwell Publishing Ltd doi: 10.1111/j.1365-2842.2010.02096.x

Journal of Oral Rehabilitation 2010 37; 658–662

J o u r n a l o f Oral Rehabilitation

implant-supported fixed restorations. The effects on

splinted and unsplinted implant-supported restorations

are compared using strain gauge analysis.

Materials and methods

A photo-elastic block model (PLM-4B*) with modulus

of elasticity 450 ksi [range of human bone (16)]

was constructed. The model dimensions were 15Æ8 · 35Æ2 · 32 mm. Three holes were drilled vertically in a straight line in the mid-axis of the photo-elastic model

at predetermined locations to lengths of 12 mm. Dril-

ling was carried out according to surgical protocols with

successive drill diameters in sequence to minimize

residual stresses in the model. Stresses introduced into

the photo-elastic model by the drilling process were

relieved by placement of the model in an oven on a

teflon surface for 120 min at 70 �C. The model was cooled in the closed oven. Stress relief was verified with

the aid of a circular polariscope, and the model was

found to be stress free. Implants were located 7Æ8 mm from the edge of the model and each implant separated

by 4 mm. Three external hex, screw type titanium

implants of diameter 3Æ8 mm, of length 12 mm† were inserted into the model. The 2-mm implants’ neck

protruded forms the superior surface.

Two strain gauges (Strain-gauge EA-06-015EH-120*)

were cemented (M-Bond 200*) horizontally onto the

neck of each implant on the buccal and lingual aspects

at a 180�-inclination to each other prior to abutment and crown placement. These strain gauges measured

the bending components (tension ⁄ compression) cre- ated from both vertical and horizontal vectors arising

from applied forces. Strain gauges were connected to a

strain indicator (Strain Indicator System 5000*) that

provided a simultaneous direct reading of strain in

microstrain units of all model components for each

loading session. The described design has been used in

previous studies (17).

Impressions were taken by the open tray technique

with acrylic splinted transfer copings ‡

using custom

acrylic trays. Polyether impression material was used § .

A master working model was fabricated on which all

the restorations were fabricated on attached fixed

abutments with 2 mm gingival height.

Two groups of restorations were cast in Remanium

CoCrMo Model Casting Alloy-GM 380:

1 Three unsplinted crowns (Fig. 1).

2 Three splinted crowns (Fig. 2).

The restorations were fabricated with the occlusal

anatomy of upper first molars with the same mesio-

distal and bucco-lingual dimensions using a silicone

index.

Screw-retained abutments were placed at a con-

trolled torque of 35 Ncm. During each loading session,

the restorations were cemented onto the implant

abutments using temporary cement ¶ . No residual stress

was apparent in the photo-elastic model as verified by

visual inspection. Contact points of the individual

restorations were fabricated so that dental floss passed

with slight difficulty according to standard clinical

procedure.

An additional third strain gauge was cemented

horizontally onto each cast restoration at the cervical

margin parallel to the margin in the mid-buccal

dimension (EA-06-032DE-350*). These were designed

to measure the peripheral strain in the margins of each

casting.

Fifteen static loadings were carried out right after

each other with 20 kg weights via a custom-built

loading apparatus. Load was applied simultaneously

through three individual pins to the inner inclines of

the buccal cusps of each set of restorations at 0� to the vertical axis (Fig. 3).

Fig. 1. Unsplinted crowns.

*Vishay Measurement Group Inc., Raleigh, NC, USA. † Nobel Biocare, Zurich, Switzerland.

‡ Duralay Reliance Dental Mfg Co., Worth, IL, USA.

§ Impregum F; ESPE, Seefeld, Germany.

¶ Temp Bond, NE Kerr, CA, USA.

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ª 2010 Blackwell Publishing Ltd

For each loading, strain gauge recordings were made.

Strain gauges measure electrical resistance. During

extension or contraction, the strain gauge records

changes in electrical resistance. The degree of distortion

of the strain gauge is recorded in calculated microstrains

values, where strain = € = DL (change in length of the strain gauge) ⁄ L (lm ⁄ m) = DR (change in electrical resistance in the strain gauge) ⁄ R.

Statistical analysis

Descriptive analysis consisted of mean and standard

deviation of microstrain values for each group. Groups

were compared by the use of the one-way parametric

analysis of variance (ANOVA). P values of <0Æ05 were considered statistically significant.

Results

Strain gauges located on the implant neck supporting

splinted restoration demonstrated significantly (P <

0Æ001) more strain (sum of strains = 3348Æ54 micro- strain) compared with the single crowns (sum of

strains = 988Æ57 microstrain) (Table 1). In contrast, significantly (P < 0Æ001) more strain was recorded on the strain gauges located on the restoration margins of

the single crowns (sum of strains = 756Æ32 microstrain) when compared with splinted restorations (sum of

strains = 186Æ12 microstrain) (Table 2).

Fig. 3. A custom-built loading apparatus.

Fig. 2. Splinted crowns.

Table 1. Microstrain values on implant necks

Implant

Strain Gauge

location

Restoration

modality

Microstrain values

M �SD

1 B* Single 162Æ64 69Æ41 Splint 1884Æ40 59Æ62

P NS

Single 10Æ27 1Æ62 Splint 48Æ40 4Æ86

2 B NS

Single 48Æ53 2Æ66 Splint 31Æ13 1Æ25

P* Single 529Æ20 4Æ21 Splint 965Æ01 19Æ28

3 B* Single 117Æ13 3Æ96 Splint 233Æ60 4Æ01

P* Single 121Æ00 7Æ20 Splint 186Æ00 11Æ77

NS, Not Significant; B, Buccal; P, Palatinal.

*P < 0.001.

Table 2. Microstrain values on crown margins

Crown

Restoration

modalities

Microstrain values

Mean �SD

1* Single 134Æ26 1Æ16 Splint 87Æ00 6Æ33

2* Single 407Æ46 3Æ50 Splint 21Æ26 1Æ03

3* Single 214Æ60 11Æ23 Splint 77Æ86 0Æ91

*P < 0.001.

J . N I S S A N et al.660

ª 2010 Blackwell Publishing Ltd

Discussion

Occlusal loads on osseointegrated implants are cited as a

significant factor in the long-term success of implant-

supported restorations (1, 6). It is common clinical

practice to join adjacent implant-supported restorations

in the restoration of the partially edentulous. Resistance

and retention forms may be a major indication for

splinting. An additional rationale of splinting implant

crowns together is to favourably distribute the non-

axial loads, minimize their transfer to restoration and

the supporting bone and to increase the total load area

(18). This practice is taken from concepts of splinting

teeth, where the assumption is that joined linear and

non-collinear units improve the collective resistance to

forces and alters the centre of rotation of the joined

units (19).

Several in vitro studies reported conflicting results.

Guichet et al. (20) in a 3D photo-elastic study support

this concept reporting that cemented splinted restora-

tions exhibited better load sharing than non-splinted

restorations. Brunsky et al. (6) maintained that loading

of splinted implant-supported crowns generates mo-

ments resulting in greater forces on the implants when

compared to the applied force. Kim et al. (21) compared

provisional and permanent cement retained, and

screw-retained 2-unit splinted restorations using a

photo-elastic and strain gauge bench model. A single

provisionally cemented restoration showed the least

stress compared to splinted and cantilevered modalities.

On the contrary, clinical studies do not seem to

support splinting. Glantz et al. (9) reported on unex-

pectedly high functional bending moments on the

implants in vivo, on maximum biting and chewing in a

conventional cross arch splinted restoration. Bender

(10) in a 4-year clinical study reported higher success

rates for adjacent unsplinted cemented restorations

when compared to adjacent splinted cemented restora-

tions. He maintains that non-splinted restorations allow

the optimal transfer of stress to the supporting struc-

tures. In another clinical study consisting of 199

implants and 74 partially edentulous patients, splinted

implants showed greater crestal bone loss (0Æ2 mm more) than non-splinted ones. These differences were

statistically significant. They concluded that splinted

implants appeared to favour greater crestal bone loss

(22).

This study compared cemented single versus splinted

configurations. The results showed that in single

unsplinted restorations, significantly (P < 0Æ001) less stress was generated in the implant neck when compared

to splinted restorations (988Æ57 versus 3348Æ54 micro- strain). For each single restoration, there is inherent

inaccuracies because of component misfit (crown ⁄ abut- ment and abutment ⁄ implant) resulting in preload stresses. When several adjacent implant restored crowns

are joined, there is a summation of these misfit inaccu-

racies, and significantly increased moments because of

splinting, resulting in transfer of increased loads to the

implants and supporting structures (6). This can also

explain the disparity of microstrain values on implant

necks exhibited between the implants.

In contrast, significantly (P < 0Æ001) more strain was recorded on the strain gauges located on the restoration

margins of the single crowns when compared with

splinted restorations (756Æ32 versus 186Æ12 micro- strain). Therefore, cemented splinted restorations

exhibited better load sharing than non-splinted stora-

tions, however, they transferred more forces towards

the implant neck because of bending moments.

In addition, in this study, implants were loaded in a

vertical inclination, while in the clinical setup, non-

axial loads are also generated. Moreover, occlusal

contacts are most often lateral (lateral function and

parafunction, and asymmetric contraction of the jaw

closing muscles combine). As a result, non-axial

loading is generally the rule. These factors will all

combine to greatly increase the bending moments

observed in this study. Consequently, additional

moments yield significantly higher strain values in

the implant neck and restorations than seen in the

study.

The present discusses axial loading of splinted and

unsplinted implant-supported restorations. Future

studies should asses whether same results will be still

obtained following non-axial loading.

Within the limitations of this experimental model,

the following conclusions may be drawn:

1 Single unsplinted restorations transfer significantly

less load to the implants and supporting structure

than splinted restorations.

2 Splinted restorations transfer significantly less load to

the crown margins than unsplinted restorations.

3 The concept of splinting adjacent implants to decrease

loading of the supporting structures may require

re-evaluation.

4 The clinical relevance needs to be investigated with

controlled long-term clinical studies.

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Correspondence: Dr Joseph Nissan, Department of Oral Rehabilitation,

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

E-mail: [email protected]

J . N I S S A N et al.662

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