W hile pressed leucite ceramics have demonstrated

enhanced aesthetics and clinical longevity due

to their natural translucency and adhesive cementation

techniques,1 a lithium disilicate ceramic (Empress2, Ivoclar

Williams, Amherst, NY) was recently developed to

significantly elevate the strength coefficient beyond the

original leucite material and enable the fabrication of

3-unit fixed partial dentures. Utilizing the same fabrication

technology of waxing and heat-pressing the ceramic sub-

structure, Schweiger et al created a microstructure com-

posed of densely arranged lithium disilicate crystals (over

60% volume) uniformly bonded in a glassy matrix.2 The

interlocking structure of the ceramic hinders crack prop-

agation and elevates the fracture toughness and flexural

strength to approximately 340 ± 20 MPa. Although den-

tal ceramics generally experience a significant reduction

in strength properties when exposed to an aqueous environ-

ment,3-7 no statistically significant change in the flexural

strength of the lithium disilicate ceramic was measured

following a 1-week period of water storage. Conse-

quently, a high chemical stability has been achieved with

this chemical composition and ceramic structure.

Since the coefficient of thermal expansion of the sub-

structure ceramic is significantly reduced, the pressed leucite

and the lithium disilicate ceramics cannot be interchanged.

The veneering ceramic (Empress2, Ivoclar Williams,

Pract Periodont Aesthet Dent 1998;11(1):95-106

Figure 1. Preoperative facial view of the patient who presented missingtooth #5 and excessive wear of cusp tips #6 and #11.

*ODA Centennial Professor of Restorative Dentistry; Director,Dental Clinical Research Center, Portland, Oregon; Section Editor,Practical Periodontics & Aesthetic Dentistry.

†Lecturer, Department of Fixed Prosthodontics, Oregon HealthSciences University, Portland, Oregon; private practice,Laguna Niguel, California.

‡Senior Research Associate, Department of Fixed Prosthodontics,Oregon Health Sciences University, Portland, Oregon.

§Ivoclar Italy, Naturno, Italy.llResearch Assistant; private practice, Portland, Oregon.

**Research and Development, Ivoclar, Schaan, Liechtenstein.

John A. Sorensen, DMD, PhDOregon Health Sciences UniversityDental Clinical Research CenterSchool of Dentistry611 S.W. Campus DrivePortland, OR 97201-3097Fax: 503-494-1235

A lithium disilicate glass-ceramic material has recently

been developed for the fabrication of 3-unit fixed par-

tial dentures. Conducted on 60 restorations, this initial trial

attempted to define clinical indications and establish design

principles for fixed partial dentures fabricated of this

ceramic material. The design requisites varied depending

on placement on the arch, and the authors concluded that

lithium disilicate restorations caused reduced antagonist

structure or opposing tooth wear. This investigation demon-

strated that when a novel ceramic system was utilized for

3-unit restorations replacing up to the first premolar and

attained minimal criteria for connector dimensions, an

acceptable clinical success rate was achieved.

95

SO

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111

A CLINICAL INVESTIGATION ON

THREE-UNIT FIXED PARTIAL DENTURES

FABRICATED WITH A LITHIUM DISILICATE

GLASS-CERAMICJohn A. Sorensen, DMD, PhD* • Mark Cruz, DDS† • Wayne T. Mito, CDT‡

Oscar Raffeiner, MDT§ • Hannah R. Meredith, RDHll • Hans Peter Foser, MDT**

C O N T I N U I N G E D U C A T I O N 3NEW YORK UNIVERSITYCollege of DentistryCenter for Continuing Dental EducationNew York City, NY

Amherst, NY) contains fluorapatite that creates apatite

crystals similar in structure and optical properties to natu-

ral teeth,2 and is available in a variety of ceramic shades.

The destruction of antagonist tooth structure by wear

of porcelain occlusal surfaces has challenged clinicians

for years.8-10 The unique microstructural features of the

lithium disilicate system offer several advantages to the

use of metal-ceramic materials. Due to the fine grain struc-

ture and high crystallinity of the lithium disilicate substructure

ceramic, the potential wear of antagonist tooth structure

is reduced. The process of sintering the fluorapatite veneer-

ing ceramic onto the substructure creates apatite crystals

similar to those present in natural tooth structure. An in

vitro testing machine11,12 that evaluated the wear of oppos-

ing enamel cusps against various ceramics in three-body

wear testing recorded less wear with the lithium disili-

cate substructure material and the fluorapatite veneer

ceramic than the bovine enamel control.13 The measure-

ment of clinical wear of antagonist tooth structure has

demonstrated similar low wear characteristics.

In addition to elevating the strength of the substructure

ceramic, its high crystalline content makes the ceramic

extremely machinable, allowing a high polish to be ren-

dered, which further reduces the abrasion potential to

the opposing tooth structure.14 The unique ceramic struc-

ture also has a reduced propensity towards iatrogenic

periodontal disease.15 The tendency to overcontour metal-

ceramic restorations at the margin can be avoided,16,17

and plaque accumulation is diminished due to the smooth

margin that can be achieved with the ceramic material.15 While clinicians have mistakenly believed that

aggressive tooth reduction was required to achieve supe-

rior aesthetics with all-ceramic systems, Sorensen et al

clinically demonstrated that less than 1.3 mm of axial

reduction was necessary with adhesively cemented crown

restorations.1 One potential benefit of a ceramic with

higher strength and translucency is a decrease in the

amount of axial tooth reduction required for full-coverage

crown abutments. While 1.4 mm to 1.7 mm of axial

reduction is recommended for metal-ceramics18-20 and

1.3 mm reduction for adhesively cemented pressed leucite

restorations,1 it was hypothesized that only 1 mm of reduc-

tion is necessary for the Empress2 ceramic.

Another potential application of the adhesively

cemented all-ceramic technology is the utilization of fixed

Occlusal Forces

Opposing cuspcontact

Figure 3. Lingual preoperative view of the site of tooth #5.

Figure 4. Illustration depicts the determination of occlusal contactpoint to establish the occlusal limit of connector height.

Figure 2. Buccal preoperative view. Note the status of the gingivaltissues and excessive wear of canine.

96 Vol. 11, No. 1

Practical Periodontics & AESTHETIC DENTISTRY

partial dentures with inlay and onlay preparations, which

could conserve a large degree of tooth structure. Since

the restorative team would only have to match the pon-

tic to the adjacent natural teeth, superior aesthetics could

theoretically be achieved. No previous clinical studies

have evaluated this mode of treatment, and essentially

no research has been performed to define the clinical

parameters of adhesive preparation design.

It was postulated that the adhesively cemented lithium

disilicate material could be used for the fabrication of con-

servative fixed partial dentures. Based on the amount of

remaining tooth structure, the extent of preexisting restora-

tions, and the anatomical location of the tooth, two con-

servative preparation designs were used. The experimental

designs included half-tooth and full-coverage onlay

restorations as well as two- and three-surface inlay

restorations. As a guideline for the dimensions of the inlay

preparations, the margins would have to be greater in

a buccolingual and occlusogingival dimension than the

minimum connector dimensions in order for the labora-

tory technician to create the proper embrasure form at

all four aspects of the connector.

The fundamental questions to be answered in this

developmental clinical trial were: Does the lithium disil-

icate ceramic have the fracture toughness and strength

to provide clinical longevity for all-ceramic fixed partial

dentures? Since the occlusal forces rapidly increase as

one moves posteriorly towards the temporomandibular

joints, functional demands of a premolar restoration are

considerably greater than those of an anterior fixed par-

tial denture. Hence, one objective of this clinical study

was to evaluate the ability of the system to resist fracture

based on location in the mouth to delineate the extent

of posterior location limits. Available data from clinical

studies on metal-ceramic prostheses indicate a success

rate of 93.5% to 98% at 5 years.21-24 Therefore, the max-

imum acceptable failure rate of multiunit all-ceramic

restorations is approximately 6%.

The purpose of this prospective longitudinal study

was to evaluate the clinical performance of 60 3-unit all-

ceramic fixed partial dentures fabricated with the lithium

disilicate ceramic system. The focus was to measure the

longevity of these all-ceramic fixed partial dentures, to

define minimum preparation criteria for full-coverage and

inlay/onlay restorations, and to evaluate fixed partial

5.0 mm

Periodontal probe formeasuring vertical height

occlusogingivally

Rounded axial-gingivalline angle

2.0 mm

1.0 mm

2.0 mm

1.0 mm

≥1.0 mm

Figure 6. Diagram of the recommended preparation design foranterior full-coverage crown fixed partial denture retainers.

Figure 5. Diagram demonstrates the measurement of potential occlu-sogingival connector height from crest of gingiva to occlusal contactpoint.

Rounded internal line angles

2.0 mm

1.0 mm1.0 mm

1.5 mm 2.0 mmMultiple planes

of reduction

Figure 7. Illustration of recommended preparation design forposterior full-coverage crown fixed partial denture retainers.

P P A D 97

Sorensen

denture design principles — particularly the connector

dimensions. The authors also sought to determine the lim-

its of posterior placement, evaluate the adhesive cemen-

tation technique, and measure the wear to the opposing

tooth structure.

Materials and MethodsSixty 3-unit fixed partial dentures were placed in 57 (24

female; 33 male) patients whose age ranged from 21 to

75 years (mean = 45.7 ±12.8 years). Entrance criteria

for the study included: a) medical-dental history that did

not preclude routine dental treatment, b) a minimum of

20 teeth, c) subject did not wear a removable partial

denture, d) moderate to good oral hygiene, e) no active

periodontal disease, f) missing a single tooth (Figures 1

through 3). The definitive restoration had to be placed

in occlusion and a maximum of two fixed partial den-

tures could be seated in each patient. The margins of

the abutment teeth were placed less than 1 mm subgin-

givally, and the restorations exhibited a minimum occlu-

sogingival dimension of 4.5 mm from the proximal

interdental papilla to the marginal ridge of the abutment

teeth (Figures 4 and 5).

Abutment teeth were prepared with diamond burs

(Sorensen All-Ceramic, Beavers Dental Burs, Sybron

Canada, Morrisburg, Ontario) and high-speed hand-

pieces under water irrigation. Margins on posterior teeth

were placed either equi- or supragingivally when possi-

ble or minimally subgingival to establish a margin on

sound tooth structure. Two groups of restorations were

formed and defined by their preparations: conventional

fixed partial dentures were prepared with a full-coverage

crown design that had a minimum axial reduction of

1 mm and occlusal reduction of 1.5 mm to 2 mm (Figures

6 through 10); experimental restorations were charac-

terized by intracoronal or partial veneer retainers, includ-

ing inlay, distal slice, and onlay preparations. The inlay

preparations had an occlusogingival height of 4 mm and

5 mm in the anterior and posterior, respectively, and a

minimum axial thickness of 1 mm. These restorations also

had a minimum buccolingual dimension of 4 mm. The

abutments for the onlay preparations had axial reduction

of 1 mm and occlusal reduction of 1.5 mm to 2 mm and

a shoulder or chamfer margin design.

Proximal contacts were verified with mylar articu-

lating paper, the internal fit of the prostheses was eval-

uated using silicone-based material (Fitchecker, GC

America, Chicago, IL) (Figures 11 and 12) and any

necessary adjustments were made with a diamond bur.

The occlusion was evaluated with silk ribbon articulating

paper and adjusted as necessary until multiple bilateral

simultaneous contacts were achieved. Following the adjust-

ment of contours and occlusion, the dimensions of the

fixed partial dentures were measured at 29 points.

Cementation

A diagram of the teeth was recorded to indicate the

approximate areas of enamel and dentin on the mar-

ginal area as well as the relation of the margin to the

gingival crest. The ceramic fixed partial denture was

Figure 8. Full-coverage crown retainer preparations are completedutilizing a shoulder margin design and 1 mm of axial reduction.

Figure 9. Buccal view of a premolar bridge retainer preparation withshoulder margin placed equigingivally.

98 Vol. 11, No. 1

Practical Periodontics & AESTHETIC DENTISTRY

cleaned and etched with hydrofluoric acid (IPS Ceramic

Etching Gel, Ivoclar Vivadent, Amherst, NY) for 30 sec-

onds and thoroughly rinsed and dried. Once a silane

agent (Monobond-S, Ivoclar Vivadent, Amherst, NY) had

been applied at all internal aspects for 60 seconds,

unfilled composite resin (Heliobond, Ivoclar Vivadent,

Amherst, NY) was applied and air thinned. The fixed

partial denture was placed under a lightproof cover for

cementation (Figure 13).

The teeth were initially cleaned of debris with hydro-

gen peroxide and cotton pellets. When the restoration

margin was too far subgingivally to control sulcular fluids,

a retraction cord was placed. A cotton pellet contain-

ing 0.12% chlorhexidine gluconate (Peridex, Zila Pharma-

ceuticals, Cincinatti, OH) was placed on each tooth for

60 seconds, which was then rinsed and dried. Areas

with enamel margins were etched with phosphoric acid

for 45 seconds and again thoroughly rinsed and dried.

Areas with dentin tooth structure were initially treated with

dentin adhesive primer (Syntac, Ivoclar Vivadent, Amherst,

NY) for 15 seconds and then air dried. Dentin adhesive

material was subsequently applied with a brush for 15

seconds and air dried. The unfilled resin composite was

applied with a brush to the entire tooth and gently air

thinned, after which shaded cement (Variolink II, Ivoclar

Vivadent, Amherst, NY) was placed in a thin layer on

the internal aspect of the abutment teeth. The lithium disili-

cate ceramic restoration was seated, and any excess

cement was removed with a brush. Waxed floss was uti-

lized to remove excess cement from the interproximal

aspects.1

Following a brief initial cure to secure the position

of the fixed partial denture, it was photopolymerized for

60 seconds at all aspects. The restoration was left

untouched for 10 minutes to allow complete polymer-

ization of the resin cement. A #12 scalpel blade and a

fine scaling instrument were used to shear off the excess

polymerized cement. In order to avoid damaging the

ceramic, root dentin, and gingival tissues, every effort

was made to refrain from the use of rotary burs.1 Once

all excess cement was removed, occlusal contacts were

evaluated and verified; at this stage, any additional

adjustments were made and polishing was performed

(Figures 14 and 15). In order to take advantage of the

wear kindness of the lithium disilicate glass-ceramic mate-

rial, the cusp length of tooth #11 was restored by the

adhesive cementation of a ceramic cusp tip that reestab-

lished canine guidance (Figures 16 through 19).

Clinical Parameters Evaluated

Baseline data (ie, photographs, polyvinylsiloxane impres-

sions, radiographs, periodontal parameters, marginal

fidelity, and occlusal analysis) were recorded. These mea-

surements and records were repeated at 6 months

(Figures 20 and 21) and 12 months, and will be con-

tinued for 5 years. The impressions of the opposing den-

tition and the ceramic restoration were poured in an

epoxy material (Epoxy-Die, Ivoclar Vivadent, Amherst,

NY) according to manufacturer’s instructions for in vivo

wear measurements.25

Figure 11. Excessive pressure on tissues from the pontic weredetected with pressure-indicating paste.

Figure 10. Canine crown retainer uses shoulder margins with aminimum of 1 mm of axial reduction placed equigingivally.

P P A D 99

Sorensen

ResultsOf the 41 total conventional fixed partial dentures placed

(Table 1), the restorations were predominately placed

in the maxillary arch (32 maxillary; 10 mandibular). A

total of 19 (12 maxillary; 7 mandibular) experimental

fixed partial dentures were placed (Table 2), utilizing 31

experimental preparations (14 inlays; 14 onlays; 3 dis-

tal slices) and 7 conventional abutments. Of the surviv-

ing restorations, 52 have been in service for a minimum

of 6 months, and range in service from 6 to 18 months

(mean = 12.1 months). The use of retraction cords was

only necessary in two instances.

Analysis of Failed Restorations

In reviewing the clinical data, three modes of failure were

evident. The conventional fixed partial dentures failed due

to minor chipping of the veneer ceramic or fracturing of the

connector between abutment and pontic. The minor fail-

ures occurred primarily on anterior teeth at a rate of 1.1%

(2 of 180 units). In a study on 2,181 metal-ceramic units,

Coornaert et al determined that the majority of failures

occurred within 12 months postcementation,21 and most

often within the porcelain layers rather than between

the metal and porcelain. The most frequent cause of

failure was determined to be occlusal, and the majority

of these patients demonstrated distinct signs of bruxism.21

Catastrophic failure was defined as fracture through

the core material, and occurred at a rate of 6.7% (4 of

61 units) through the connector between abutment and

pontic. One anterior fixed partial denture suffered a

catastrophic failure. Measurement of the connector

dimensions revealed that it had a triangular configuration

with dimensions of approximately 3.6 mm 3 2.5 mm,

which were significantly less than the recommended min-

imum dimensions (4 mm 3 4 mm). The 3 remaining cat-

astrophic failures occurred in the premolar region. Two

premolar fixed partial dentures had connector heights

of less than 4 mm (3.62 mm; 3.80 mm), and 1 failed

premolar restoration had a connector height of 4.29 mm.

No fracture of the conservative designs with inlay and

onlay preparations occurred. The conservative fixed par-

tial dentures were predominately in the posterior region

(18 of 19 sites). The catastrophic failures occurred at

5, 6, 7, and 9 months.

The experimental restorations failed at a rate of

10.5% (2 of 19 units) due to the debonding of distal

slice preparations on maxillary canines at the cement-

tooth interface; these failures occurred at 2 and 4 months.

The ceramic fixed partial dentures did not fracture dur-

ing the 3 to 4 week period when the canine abutments

were debonded, and the premolar inlay or onlay abut-

ments continued to support the restoration. Since the

debonding occurred in 2 of 31 experimental prepara-

tions, the authors hypothesized that the distal slice prepa-

ration design did not provide sufficient mechanical

retention. In one subject, inlay preparation designs were

used in maxillary central and canine abutment teeth. Due

to the mechanical retention provided by the opposing

walls of the inlay preparations, the restoration continues

to function well. The mechanical retention features of a

preparation are critical to the success of these experi-

mental abutment designs.

Ten of the abutment teeth were nonvital and 110

were vital. Following cementation of the restorations,

patient complaints were filed for 10 abutment teeth,

which resulted in a 9.1% incidence of postcementation

symptoms (eg, sensitivity to cold, pain on mastication,

or general ache). Of the abutment teeth that exhibited

symptoms, 4 were anterior and 6 were posterior. While

the symptoms in 6 of these abutment teeth were com-

pletely resolved in 1 week to 5 months, symptoms have

persisted in 4 of the teeth since cementation. The major-

ity of these patients’ symptoms were more severe for a

period following cementation and then diminished in

severity but remain present. Consequently, 3.6% of

100 Vol. 11, No. 1

Practical Periodontics & AESTHETIC DENTISTRY

Table 1

Distribution of Conventional Fixed Partial Denturesby Location of Retainers and Arch

Retainer — Retainer Maxilla Mandible TotalIncisor — Incisor 9 0 9Incisor — Canine 13 0 13Incisor — Premolar 2 0 2Canine — Premolar 6 4 10Premolar — Premolar 0 0 0Premolar — Molar 2 5 7Total 32 9 41

patients’ abutment teeth have persistent symptoms. None

of the abutment teeth in the present study have required

endodontic therapy.

As of this report, interfacial microleakage was

detected on only one margin at the cement-tooth inter-

face on tooth #30 at the mesiolingual aspect of an onlay

abutment. Since measurements were recorded at six

points for each of 120 abutments, the estimated rate of

microleakage is 0.14% (1 of 720 points) of measure-

ment points or 0.83% of teeth (1 of 120 units).

Wear Potential

The epoxy replicas were profiled with the MTS Tooth

Profiling System and compared with the AnSur Program

(University of Minnesota) to determine the location of

wear regions and quantify in vivo wear of either the

opposing dentition or the ceramic restorations.25 Six

months postoperatively, half of the opposing natural teeth

exhibited no measurable wear and approximately 40%

of the ceramic occlusal services demonstrated wear

facets. The mean volume (mm3) of opposing tooth struc-

ture and ceramic surface wear was 0.0701 ± 0.121

and 0.0268 ± 0.370, respectively. Since the profiling

of all the subjects is not complete, these are preliminary

results only.

DiscussionThe challenge for dental ceramic manufacturers has been

to optimize the combination of strength and aesthetics.

Typically, the increase of crystalline content to achieve

greater strength results in greater opacity for a ceramic

material. With a crystallinity of approximately 60%, the

lithium disilicate ceramic system (Empress2, Ivoclar

Williams, Amherst, NY) maintains a relatively high trans-

lucency, but it is not as translucent as the original leucite-

reinforced ceramic.

In a study of 75 resin cemented leucite-reinforced

crown restorations prepared with 1.3 mm of axial reduc-

tion, Sorensen et al recorded a failure rate of 2.7% at

4 years.1 In the present study on lithium disilicate ceramic

restorations with only 1.0 mm of axial reduction, no ceramic

fractures occurred through the full-coverage crown retain-

ers. Metal-ceramic restorations generally require approx-

imately 1.5 mm of facial axial tooth reduction in order

to achieve optimum aesthetics. Shillingburg et al stated

that an absolute minimum of 1.2 mm and 1.4 mm of

facial reduction was required for a base and a noble

metal alloy coping, respectively.18 Chiche and Pinault

recommended 1.4 mm to 1.7 mm of facial reduction for

the porcelain margin of metal-ceramic crown restora-

tions.19 Rosenstiel et al mandated 1.5 mm of facial reduc-

tion for a metal-ceramic crown restoration.20 Since less

axial tooth reduction is required, the lithium disilicate

ceramic system provides a more conservative restoration.

Due to the enhanced optical properties of the ceramic

material, this is achieved without compromising aesthetics.

Three of the 4 restorations that catastrophically failed

had occlusogingival connector heights that failed to

achieve the recommended design standards. Although

the subject qualification criteria included a minimum con-

nector height of 4 mm, the authors would recommend

caution in the use of ceramic material for premolar fixed

partial dentures unless an occlusogingival connector

height of 5 mm can be accomplished.

Biomechanical Considerations and

Diagnostic Procedures

The biomechanical engineering principles and the Law

of Beams are fundamental considerations in treatment

planning for all types of fixed partial dentures. The deflec-

tion of a beam varies directly with the cube of the length

of the span and inversely with the cube of the height.26,27

Therefore, of the two dimensions of the FPD connector,

vertical height has a radically greater effect on the flexure

or strength of the restoration than does the buccolingual

P P A D 101

Sorensen

Table 2

Distribution of Experimental Fixed Partial Denturesby Location of Retainers and Arch

Retainer — Retainer Maxilla Mandible TotalIncisor — Incisor 0 0 0Incisor — Canine 1 0 1Incisor — Premolar 0 0 0Canine — Premolar 4 1 5Premolar — Premolar 0 1 1Premolar — Molar 7 5 12Total 12 7 19

width.28 A connector with a given occlusogingival dimen-

sion will bend eight times as much if the thickness is

halved, while a one-half reduction in the buccolingual

dimension only results in a twofold increase in flexure.

Since the occlusogingival connector height is the criti-

cal dimension, the clinical determination of the ability

to achieve this dimension is the primary determinant of

the ability to use the lithium disilicate ceramic system for

three-unit fixed partial dentures (Figure 22). The occlusal

contact and the gingival tissues define the limits of the

connector height. A gingival embrasure must be main-

tained for oral hygiene access and avoidance of iatro-

genic periodontal disease. If the minimal vertical height

dimension is not available the clinician may consider

performing electrosurgery to remove the soft tissue to

gain space for the connector height, although the extent

of tissue removal is limited, and biological width must

be respected. If this minimum vertical dimension cannot

be achieved, then use of the lithium disilicate ceramic is

contraindicated for fabrication of a fixed partial denture.

The placement of a pontic in a posterior location

increases the functional requirements of the occlusogin-

gival connector dimension (Figure 22). In order to deter-

mine these requisites, the occlusion should be marked

(Figure 4) or the distance from the opposing cusp con-

tact or incisal embrasure to the gingival crest should

be measured with a periodontal probe (Figure 5). For

a first premolar pontic, the connector dimension between

the second premolar retainer and pontic should be

5.0 mm occlusogingivally and 4.0 mm buccolingually.

The connector dimension between canine and lateral

incisor pontic should be 4.0 mm occlusogingivally and

3.0 mm buccolingually. The maximum length of the pon-

tic span is the mesiodistal width of a premolar, or approx-

imately 9 mm in the posterior area and 11 mm in the

anterior region (Figure 23).

Secondary treatment planning considerations include

factors that might limit the occlusogingival connector height

(eg, lack of posterior support or posterior group func-

tion). Parafunctional habits such as bruxism are a contra-

indication to the use of this all-ceramic system for FPDs.

While the inherent strength of the lithium disilicate

ceramic has been significantly improved, the overall

strength of the fixed partial denture is dependent on

Figure 13. The lithium disilicate glass-ceramic (Empress2, IvoclarWilliams, Amherst, NY) fixed partial denture was polished andautoglazed to permit cementation.

Figure 14. Buccal view of the cemented lithium disilicate glass-ceramic fixed partial denture in lateral excursion, which demon-strates the restoration of canine guidance.

Figure 12. The ceramic fixed partial denture was then tried-in topermit evaluation of contours and occlusal adjustments.

102 Vol. 11, No. 1

Practical Periodontics & AESTHETIC DENTISTRY

several factors that can degrade the strength. The

occlusogingival dimension or vertical height of the con-

nector is critical. Consequently, this vertical dimension

should be maximized in the heat-pressed core material

and every effort should be made by the laboratory tech-

nician to minimize flaw content. These flaws act as the

origin of crack propagation and can grow to critical size

in the oral environment with cyclic fatigue loading and

stress corrosion fatigue.29-32

The authors predict that the veneered Empress2

(Ivoclar Williams, Amherst, NY) fixed partial denture

would behave similarly to the In-Ceram system (Vident,

Brea, CA), where the highest tensile stresses occur in the

connector areas between core and veneer ceramics.33

Consequently, since the core ceramic is significantly

stronger than the veneer ceramic, it is recommended that

little or no veneer ceramic be applied at the gingival

embrasure or lingual embrasure of the connectors. This

will also maximize the strength conferred by the core

material. Care must be taken to avoid inducing micro-

cracks and critical flaws in important connector areas,

and it is highly recommended that no rotary instruments

be used on the connector areas once the ceramist has

fabricated the restoration.

In clinical practice there are often instances where

the existing preparation retention and resistance form are

lacking and require augmentation. A clinical advan-

tage of the heat-pressed Empress2 ceramic system is that

it can reproduce auxiliary retention and resistance forms

(eg, boxes and grooves), which expands the clinical

applications of this system closer to those of metal-ceramic

materials. Since no conservative bridges failed by cat-

astrophic fracture, it appears that the lithium disilicate

ceramic system may be utilized in this application.

When the clinical study was initiated the investi-

gators did not know the exact amount of mechanical

retention that was required for these restorations. The distal

slice preparations — which were essentially hollow ground

bevels with very minimal box form on the distal of the

canine — relied extensively on the adhesion to tooth

structure of the etched enamel and dentin bonding agents

for retention. The authors concluded that this prepara-

tion design did not provide sufficient mechanical reten-

tion as an abutment. Apparently, the opposing vertical

Figure 16. Attrition of canine has rendered length inadequate forcanine guidance.

Figure 15. Lingual view of the posterior fixed partial denture. Theceramic material utilized in the restoration minimizes wear of theopposing dentition.

Figure 17. The ceramic cusp was fabricated and etched for adhesivecementation.

P P A D 103

Sorensen

walls of an inlay are necessary to provide mechanical

retention in addition to the adhesive cementation mech-

anism. The proximal walls of the boxes should maxi-

mize the cross-section of enamel prisms for adhesive

bonding. None of the inlay or onlay retainers failed.

One manufacturer stated that facial veneer prepa-

ration could be utilized with its ceramic system for the fab-

rication of fixed partial dentures.34 Christensen and

Christensen35 tested 40 bridges with a variety of retainer

designs and determined an 80% failure rate for posterior

fixed partial dentures at 2 years. Anterior fixed partial den-

tures with veneer and full-coverage crown preparations

demonstrated a 38% and 22% failure rate, respectively.

Since the ceramic material of this system had a flexural

strength of only 105 MPa, these results were rather pre-

dictable.36 While the concept of conservative veneer prepa-

ration is appealing, a substructure that has the strength

of metal in thin sections has not yet been developed.

While the elimination of postcementation sensitivity

remains a clinical objective, all types of dental cements

cause side effects. Johnson et al recorded a 32% inci-

dence of immediate postcementation sensitivity for zinc

phosphate cement and 19% for glass ionomer cement37;

the incidence of these symptoms continued at the 2-

and 12-week measurement points. The 9.1% incidence

of postcementation symptoms compares favorably to

the incidence of symptoms for conventional cements.

With the lithium disilicate ceramic and cementation sys-

tem used in the present study, 3.6% of patients’ abutment

teeth had persistent unresolved symptoms. This incidence

is not acceptable, however, and the authors are striving

to achieve a zero incidence.

The incidence of microleakage at the tooth-cement

interface was minimal since margin placement extended

no more than 1.0 mm subgingivally, and was placed in

enamel whenever possible. The use of the two-stage

dentin bonding agent also contributed to the minimal

microleakage. The authors also determined that the adhe-

sive cementation procedure was pulpally compatible and

maintained a reliable adhesive seal with relatively few

complications in terms of postcementation sensitivity.

A disadvantage of the ceramic system evaluated

in this study was that fixed partial dentures fabricated

from this material required adhesive cementation, which

Figure 18. Buccal view of the cemented restoration. Note the inte-gration of the adhesive ceramic with the natural tooth structure.

Figure 19. Lingual view of the adhesive cusp tip, which is virtuallyindistinguishable from the adjacent natural dentition.

Figure 20. Three months postoperatively, the interdental papillae hadfilled in the gingival embrasure.

104 Vol. 11, No. 1

Practical Periodontics & AESTHETIC DENTISTRY

is more time consuming and technique sensitive. It is the

adhesive cementation technology, however, that enables

one to use conservative inlay and onlay abutments and

minimize the axial wall reduction for full-coverage crown

preparations. This presents a viable alternative to con-

ventional metal-ceramic restorations, which require

approximately 50% more tooth reduction in order to

achieve similar aesthetics.18-20 Additional institutions are

currently evaluating the conventional cementation of the

lithium disilicate ceramic system.

ConclusionThis clinical trial defined preparation parameters, fixed

partial denture design requisites, and posterior limits of

placement for a novel lithium disilicate ceramic system

(Empress2, Ivoclar Williams, Amherst, NY). The unique

structures of the heat-pressed substructure ceramic and

the fluorapatite veneering ceramic offer clinical benefits

in terms of machinability, polishability, and reduced wear

of opposing tooth structure. Half the antagonist teeth eval-

uated in the trial exhibited no wear from the opposing

ceramic surfaces, and 42% of the ceramic surfaces

demonstrated wear facets, which indicated a strong ten-

dency to reduce the destructive nature characteristic of

conventional ceramic materials.

The authors also determined that 3 of 4 fixed partial

dentures that catastrophically failed had occlusogingival

connector heights that did not achieve recommended

design parameters. The required occlusogingival con-

nector height varied depending on its location on the

arch; between the premolars this height should measure

5 mm and should be 4 mm between the incisors. None

of the conventional full-coverage crown preparations with

1 mm of axial reduction experienced catastrophic fail-

ure through the abutments; the ceramic of the experi-

mental fixed partial dentures did not fracture. Although

2 of the experimental restorations with the distal slice

design failed due to debonding, none of the inlay or

onlay abutments failed clinically, which indicated that

mechanical retention features are required for the prepa-

rations. Consequently, it was determined that the lithium

disilicate possesses sufficient strength for conservative

fixed partial dentures when sufficient retention is achieved

with inlay and onlay abutments.

Occlusal For ce

4.0 mm

4.5 mm4.0 mm 4.0 mm 4.0 mm

4.0 mm3.0 mm 3.0 mm

3.0 mm

5.0 mm

Figure 22. Diagram exhibits minimum occlusogingival and buccolin-gual connector dimensions as a function of position of bridge con-nector and occlusal forces.

Figure 21. Postoperative facial view of the patient demonstratedimproved aesthetics and function while reestablishing bilateralcanine guidance.

11.0 mm

9.0 mm

Figure 23. Diagram demonstrates that the maximum length of ponticspan is equal to the width of the premolar tooth.

P P A D 105

Sorensen

The incidence of postcementation symptoms was

9.1%, the majority of which were resolved in 1 week to

5 months, although 3.6% of the patients demonstrated

persistent symptoms. Minimal interfacial microleakage

(0.83% of the teeth) was noted, and none of the abut-

ment teeth required endodontic therapy. The initial results

documented in the clinical trial indicate promise for this

novel lithium disilicate ceramic system as a biocompat-

ible alternative to metal-ceramic materials, although sub-

sequent clinical investigations must be completed to verify

the long-term prognosis of this treatment modality.

AcknowledgmentThe authors mention their gratitude to Valorie Stouffer,CDA, for the coordination and collection of the data forthis clinical study.

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crown system: Three-year clinical trial results. J Cal Dent Assoc1998;26(2):130-136.

2. Schweiger M, Höland W, Frank M, et al. IPS Empress2: Newpressable high-strength glass-ceramic for esthetic all-ceramicrestorations. Quin Dent Tech 1999 (In press).

3. Jones DW. The strength and strengthening mechanisms of den-tal ceramics. In: Dental Ceramics: Proceedings of the First Inter-national Symposium on Ceramics. McLean JW. Carol Stream,IL: Quintessence Publishing; 1983:110 -116.

4. Hillig WB, Charles RJ. High Strength Materials. Zackey VF. ed.New York, NY: John Wiley; 1965:682.

5. Hasselman DPH. Proposed theory for the static fatigue behav-ior of brittle ceramics. Ultra fine grain ceramics. In: Burke, Reed,Weiss, eds. Proceedings 15th Sagamore Army Materials ResearchConference. Syracuse, NY: Syracuse Univ Press; 1968:297.

6. Southan DE, Jorgensen KD. The endurance limit of dental porce-lain. Aust Dent J 1974;19 (1) :7-11.

7. Hornberger H, Marquis PM. The effect of environment on themechanical properties of In-Ceram. In: Proceedings of Conferenceon Lifetime Prediction and Failure Analysis of Restorative Materials.Dent Mater 1994;7:83.

8. Monasky GE, Taylor DF. Studies on the wear of porcelain,enamel and gold. J Prosthet Dent 1971;25(3):299-306.

9. Mahalick JA, Knap FJ, Weiter EJ. Occlusal wear in prostho-dontics. J Am Dent Assoc 1971;82:154-159.

10. Wiley MG. Effects of porcelain on occluding surfaces ofrestored teeth. J Prosthet Dent 1989;61(2):133-137.

11. Condon JR, Ferracane JL. Evaluation of composite wear witha new multi-mode oral wear simulator. Dent Mater 1996;12(4):218-226.

12. Sorensen JA, Dyer SR, Condon JR, Ferracane JL. In vitro mea-surements of fixed prosthodontic composite systems materials.J Dent Res 1998;77:159(Abstract No. 432).

13. Sorensen JA, Sultan E, Condon JR. Three-body in vitro wearof enamel against dental ceramics. J Dent Res 1999;78(Abstract). In press.

14. Seghi RR, Rosenstiel SF, Bauer P. Abrasion of human enamelby different dental ceramics in vitro. J Dent Res 1991;70(3):221-225.

15. Sorensen JA. A rationale for comparison of plaque-retainingproperties of crown systems. J Prosthet Dent 1989;62(3):264-269.

16. Parkinson CF. Excessive crown contours facilitate endemicplaque niches. J Prosthet Dent 1976;35(4):424-429.

17. Frankhauser G. Clinical investigation of metallo-ceramiccrowns. Zurich, Switzerland: University of Zurich. 1979. Thesis.

18. Shillingburg HT, Jacobi R, Brackett SE. Anterior porcelain-fused-to-metal crowns. In: Fundamentals of Tooth Preparations forCast Metal and Porcelain Restorations. Carol Stream, IL:Quintessence Publishing; 1987:259-278.

19. Chiche G, Pinault A. Metal ceramic crowns. In: Esthetics ofAnterior Fixed Prosthodontics. Carol Stream, IL: QuintessencePublishing; 1994:78-94.

20. Rosenstiel SF, Land MF, Fujimoto J. The metal-ceramic crownpreparation. In: Contemporary Fixed Prosthodontics. 2nd ed.St. Louis, MO: Mosby; 1995:180 -192.

21. Coornaert J, Adriaens P, De Boever J. Long-term clinical studyof porcelain-fused-to-gold restorations. J Prosthet Dent 1984;51(3):338-342.

22. Leempoel PJD, de Haan AFJ, Reintjes AGM. The survival rateof crowns in forty Dutch practices. Ned Tijdscher Tandheelkd.1982.

23. Leempoel PJD, Eschen S, de Haan AFJ, Hof MA. Evaluationof bridges and RPD abutment crowns in a general practice.Ned Tijdscher Tandheelkd 1984;91:231.

24. Strub JR, Stiffler S, Schärer P. Causes of failure following oralrehabilitation: Biological versus technical factors. Quint Int1988;19(3):215-222.

25. Sorensen JA, Berge HX. Clinical wear assessment with MTS3D computerized profiling system. J Dent Res 1998;77:272(Abstract No. 1332).

26. Smyd ES. Mechanics of dental structures: Guide to teachingdental engineering at undergraduate level. J Prosthet Dent1952;2:668-692.

27. Shillingburg HT, Hobo S, Whitsett LD, et al. Biomechanicalconsiderations. In: Fundamentals of Fixed Prosthodontics. 3rded. Carol Stream, IL: Quintessence Publishing; 1987:93-94.

28. Miller L. A clinician’s interpretation of tooth preparation anddesign of metal substructures for metal-ceramic porcelainrestorations. In: Dental Ceramics: Proceedings of the FirstInternational Symposium on Ceramics. McLean JW. CarolStream, IL: Quintessence Publishing; 1983:173-175.

29. Griffith AA. Phenomena of rupture and flow in solids. Phil TransRoy Soc 1920;A224:163-198.

30. Weibull W. A statistical theory on the strengthening of materi-als. Swed Inst Eng Res Proc 1939;151:1-45.

31. Ritter JE. Mechanical behavior of ceramics. In: Vincenzini, P, ed.Fundamentals of Ceramic Engineering. London, UK: ElsevierApplied Science; 1991:121-222.

32. Ritter JE. Crack propagation in ceramics. In: EngineeringMaterials Handbook, Vol. 4, Ceramics and Glasses, ASMInternational, 1991:694-699.

33. Kelly JR, Tesk JA, Sorensen JA. Failure of all-ceramic fixed par-tial dentures in vitro and in vivo: Analysis and modeling. J DentRes 1995;74(6):1253-1258.

34. Optec-hsp advertisement. J Esthetic Dent 1989;1:20a.35. Christensen G, Christensen R. Service potential of all-ceramic

fixed prostheses in areas of varying risk. J Dent Res 1992;71:320(Abstract No. 1716).

36. Seghi RR, Sorensen JA. Relative fracture strength of 6 newceramic materials. Int J Prosthodont 1995;8:239.

37. Johnson GH, Powell LV, DeRouen TA. Evaluation and controlof post-cementation pulpal sensitivity: Zinc phosphate and glassionomer luting cements. J Am Dent Assoc 1993;124(11):38-46.

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1. Leucite ceramics have been widely utilizeddue to the following characteristics:a. Wide availability and ease of preparation.b. Natural translucency and adhesive cementa-

tion techniques.c. Optimal thermal expansion properties.d. None of the above.

2. Crack propagation of the lithium disilicatematerial is considerably reduced by whatproperty?a. The interlocking structure of the material.b. Increased flexural characteristics in an

aqueous environment.c. A high chemical stability.d. All of the above.

3. The occlusal destruction of opposing dentitionnormally caused by porcelain restorations isreduced by which characteristics?a. Fine grain structure and high crystallinity.b. Medium grain structure and high crystallinity.c. Large grain structure and high crystallinity.d. Fine grain structure and low crystallinity.

4. The crystalline content of the lithium disilicatematerial allows for:a. A reduced propensity towards iatrogenic

periodontic disease.b. An elevated strength of the substructure.c. An optimal polish to be rendered, thus

educing abrasive characteristics.d. All of the above.

5. Clinical studies have indicated that an axialreduction of what length is required for thelithium disilicate material?a. 1.4 mm to 1.7 mm.b. 1.3 mm.c. 1 mm.d. None of the above.

6. The authors avoided the use of rotary burs forremoval of excess polymerized cement.a. True.b. False.

7. The experimental restorations failed at a rateof 10.5% due to:a. Excessive bruxism.b. The debonding of distal slice preparations on

mandibular canines.c. The debonding of distal slice preparations on

maxillary canines.d. Inadequate mechanical retention.

8. Metal-ceramic restorations generally require anaxial reduction of what for optimal aesthetics:?a. 1.2 mm.b. 1.5 mm.c. 1.3 mm.d. None of the above.

9. Of the two dimensions that affect the dentureconnector, which has a greater impact in theflexural strength?a. Vertical height.b. Buccolingual width.c. Mesiodistal length.d. None of the above.

10. Which of the following was considered adisadvantage of the all-ceramic system?a. Decreased flexural strength.b. A propensity to crack.c. The necessity for adhesive cementation.d. All of the above.

To submit your CE Exercise answers, please use the answer sheet found within the CE Editorial Section of this issue andcomplete as follows: 1) Identify the article; 2) Place an X in the appropriate box for each question of each exercise; 3) Clipanswer sheet from the page and mail it to the CE Department at Montage Media Corporation. For further instructions,please refer to the CE Editorial Section.

The 10 multiple-choice questions for this Continuing Education (CE) exercise are based on the article “A clinical investi-gation on three-unit fixed partial dentures fabricated with a lithium disilicate glass-ceramic” by John A. Sorensen, DMD,PhD, Mark Cruz, DDS, Wayne T. Mito, CDT, Oscar Raffeiner, MDT, Hannah R. Meredith, RDH, and Hans Peter Foser, MDT.This article is on Pages 95-106.

Learning Objectives:This article describes a novel ceramic system that appears to demonstrate clinical success when utilized in 3-unit restora-tions. Upon reading and completing this exercise, the reader should have:

• A comprehensive overview of the preparation parameters, fixed partial denture design requisites, and pos-terior limits for a novel lithium disilicate ceramic system.

• An understanding of the potential promise of the lithium disilicate ceramic system as a biocompatible alter-native to metal-ceramic materials.

CONTINUING EDUCATION

(CE) EXERCISE NO. 3CE

CONTINUING EDUCATION

3

NEW YORK UNIVERSITYCollege of DentistryCenter for Continuing Dental EducationNew York City, NY

108 Vol. 11, No. 1