Publication of one recent article that demonstrates stress
distribution within tooth structure has improved clinicians' understanding of
subtle compression and stress fracture presentations in teeth.1 Until
the publication of this benchmark article, numerous fracture presentations
observed clinically have been difficult to explain. Strain distribution within
the tooth is related to its structure. Enamel acts as a stress distributor,
transferring the load vertically to the root, and horizontally via the
dentinoenamel junction (DEJ) to the dentin of the crown. A thick zone (~200 µm) in the dentin at the DEJ undergoes
greater stress than the central coronal dentin.
Recently discovered structures within the occlusal surface of
molars2,3 dicate that conventional cavity designs are disharmonious
with the tooth's natural mechanical stress distribution system.4-7 is
understanding has resulted in the development of a discipline termed
microdentistry. This philosophy urges the use of modern methods of caries
detection for early accurate minimal intervention in the caries process to
preserve internal mechanical structures within the tooth that are vital to its
long-term mechanical viability.
To understand the various presentations of tooth fracture caused
by the disruption of the natural stress distribution mechanism within the
tooth, the significance of the Moiré fringes must be considered. To date,
stress studies that utilize polarized light have generally been conducted with
plastics to show stresses that occur when loads are applied. This technique is
not effective in natural dentition due to their inability to transmit light, so
these studies have most often focused on stress distribution associated with
intracoronal pins and posts.
The Moiré fringes study allows the visualization of stress
distribution within natural tooth structure (Figure 1). Since it has been
postulated that the source of abfraction lesions is stress concentration at the
cervical DEJ, the relationship demonstrated in the Moiré fringes study confirms
this hypothesis. The Moiré fringes show the peripheral rim of enamel
transferring occlusal load directly to the root of the tooth. Compression load
in the enamel transfers via the DEJ into horizontal load in the dentin of the
crown. In this transfer, a stress concentration occurs at the DEJ as it
converts the vertical load in the enamel into the horizontal load in the
The DEJ is a zone approximately 200 µm thick where collagen density and mineral content are each approximately
50%, compared to a 30% collagen / 70% mineral ratio in the body of the dentin.
Since the DEJ is more elastic due to its greater collagen content, it allows
microcompression to occur between the enamel and the dentin, which enables the
enamel rim -- with its high elastic modulus -- to transfer a vertical load
directly to the root structure. Once this initial load transfer occurs, the
sheer load created within the DEJ transfers into a horizontal load in the body
of the crown dentin, with the stress decreasing toward the center of the tooth.
This increase in load in the DEJ and decrease of stress in the central body of
the dentin is evidenced in the Moiré fringes study.
Once this load distribution system is understood, the function of
the enamel rim assumes greater significance. From the perspective of stress
distribution, the occlusal enamel is a separate entity from the peripheral rim
enamel. In simplified terms, the function of the peripheral rim can be
visualized through a comparison to an inverted teacup. Several years ago, an
automobile company successfully balanced a 2-ton car on four inverted china
teacups. When loaded correctly, the fragile teacups were able to support
significant loads and successfully transfer the load to the floor. In the same
manner, the enamel transfers load to the root. The theory that the peripheral
rim of enamel is able to support significant vertical loads becomes evident in
a clinical setting. Clinicians can observe how the teeth fail in function when
their stress distribution system is disrupted by cavity designs.
A simple tin can is also valuable in this discussion. Untouched,
it can support significant vertical and lateral compressive loads. Even with
the lid removed, it can still support reasonable vertical compressive loads.
When lateral compressive loads are applied to the rim, however, it distorts
easily. Squeezed between two forces on opposite sides of the rim, an opened can
becomes ovate with the apex of the deformation occurring 90 degrees around the
rim from the compression forces. This is a simplified image, but the effects
can be observed clinically in teeth that are failing due to the presence of
restorations. An occlusal cavity in a posterior tooth can cause it to flex
under compressive loads on external cusp planes to an extent that the
distortion in the tooth results in structural failure of the peripheral rim.8-10
Considering tooth structure in this manner, the marginal ridge
becomes a part of the peripheral rim rather than a separate morphological
identity. Its significance to the overall stress distribution system increases
when cavity designs are considered for the treatment of primary interproximal
caries. The peripheral rim can be considered a tension ring. When a minimal MOD-type
cavity is prepared in a sound tooth, the cusps spring apart approximately 10 µm, which indicates that the tooth
structure is under tension.11 When the marginal ridge is simply a
part of the peripheral rim of enamel, its removal to treat Class II interproximal
caries has long-term significance for the structural integrity of the tooth.
Evidence and Significance
If the peripheral rim of enamel functions almost independently of
the occlusal enamel and is able to absorb compressive loads without fracturing,
a transfer of this load via the DEJ to a horizontal load in the dentin should
occur. If this redistribution of the compressive load cannot be distributed
through the body of the dentin due to the blocking effect of a cavity, then one
would expect to find clinical evidence of stress concentration between the
peripheral rim enamel and the cavity wall. Differences in elastic modulus
between the enamel and the underlying dentin could cause vertical delamination
and fracturing along the DEJ.12,13 The failure of the stresses to be
distributed through the dentin, due to the blocking effect of the cavity,
should cause stress concentration between the peripheral rim and the cavity
margin. This would have the same effect as stress concentration at the cervical
enamel margin has in causing abfraction lesions. The result would be an
occlusal abfraction lesion. Once the theory is understood, the clinical
evidence becomes clear (Figures 2 and 3).
The preparation of cavities in tooth structure disrupts the natural
load distribution and creates zones of stress concentration. The clinical
presentation of this stress concentration depends primarily on two factors: the
type of cavity prepared, and whether the applied load is compressive or
(Continued from page 1 )
Extension-For-Prevention Type Occlusal Cavity
Dentin has a moderate elastic modulus, and enamel has a high
elastic modulus. Between them lies the DEJ, which, due to its high collagen
content, has a low elastic modulus in comparison to both dentin and enamel.
While dentin can deform elastically under load (due to its moderate elastic
modulus), enamel will fracture rather than deform.16 Once the
occlusal surface of a mandibular molar has been removed for amalgam placement,
the tooth begins to behave like the tin can with the top removed. First, the
conventional extension-for-prevention cavity design required by amalgam removes
an important occlusal cross-bracing structure - the subocclusal oblique
transverse ridge in mandibular molars (Figure 4), and the maxillary molar
mesial subocclusal enamel web (Figure 5). Second, amalgam provides little
mechanical support when the tooth is under load.5-6,10,11
One of the most common loads on mandibular molars is a compressive
load on the outer face of the buccal cusps. As the unsupported dentin deforms
under load, a compressive distortion in the cusps is created.8,13
The tooth becomes ovate, which places the peripheral rim enamel in the contact
point areas under tension. Eventually, vertical fracturing of the enamel may
occur in the marginal ridge zone of the peripheral rim. These enamel fractures
are not to be confused with naturally occurring stress relief lamellae in
virgin teeth. Continual flexure in this fracture can eventually result in
caries propagation through the enamel crack (Figure 6). This caries
presentation can best be described as "occlusal effect caries." The
interproximal caries is developing only because an occlusal cavity has been
prepared in the tooth.
The proposition that preparation of an occlusal cavity in a tooth
creates the potential for interproximal caries has serious significance for the
development of minimal intervention microdentistry. Radiographically, this type
of caries presentation is difficult to diagnose.17 Apart from a
faint and diffuse graying of the contact point enamel, little enamel
decalcification can be detected. Due to the lack of a conventional
interproximal enamel caries marker, the dentin caries is often overlooked
unless fracture has been noted at clinical examination, and the radiographs are
viewed with that in mind (Figures 7-8-9-10-11-12-13). While these examples are
exemplified in mandibular molars, this effect can occur in any tooth that has a
compressive force applied to the cusp where a moderate-sized restoration has
Fracturing of Cusps
Vertical fracturing in the contact point area of the peripheral
rim and associated caries can occur when cusps are placed under tension loads.
Although the interproximal caries presentation remains the same as with compression
fractures, there is generally an associated dentin fracture under the tension
facet cusp. This is due to the inability of dentin to absorb tensile forces
once it has been separated from the surrounding tooth structure by a cavity
preparation that removed the occlusal enamel cross-bracing structures.9-11
Tooth structure is designed to absorb compressive and tension loads as a total
biomechanical entity and does not adapt well when this force distribution
system is disrupted to any extent by conventional cavity designs (Figures 14-15-16-17a-17b).
If the preparation of an extension-for-prevention occlusal cavity
can cause long-term failure of the tooth, then cavity designs must be addressed
to accommodate the natural stress distribution mechanisms within the tooth. It
could be necessary to conserve the occlusal cross-bracing structures of the
subocclusal oblique transverse ridge and maxillary molar mesial subocclusal
enamel web, and -- if at all possible -- to avoid the cutting of the peripheral
rim of enamel. To conserve these structures, contemporary caries diagnostic
techniques must be understood and utilized so that caries can be treated in its
preliminary stages without unnecessary destruction of these vital anatomical
entities. The utilization of magnification, caries detection dye,18
laser caries diagnosis,19 and minimally invasive cavity designs
utilizing air abrasion to avoid the microfracturing that occurs in enamel when
cavities are prepared with high-speed rotary instrumentation are all promising
new techniques.20-23 Due to the way it selectively cuts compromised
tooth structure, air-abrasion allows conservation of sound tooth structure. It
was this technology that allowed anatomical structures such as the subocclusal
oblique transverse ridge and maxillary molar mesial subocclusal enamel web to
be observed clinically. In contrast, high-speed rotary burs indiscriminately
cut both sound and unsound tooth with equal ease.
In addition to the need for minimally invasive dentistry, the
techniques and materials utilized in restoring minimal cavities should be
considered. To conserve the integrity of the peripheral rim and occlusal
cross-bracing structures, tunnel preparations become an appropriate treatment
option when treating primary interproximal caries in posterior teeth. This
approach will prevent compressive fracturing of the peripheral rim and tension
fracturing of cusps that occur with conventional MOD-type cavity preparations.
As a consequence, the use of autopolymerizing glass-ionomer cement has to be
considered when restoring this form of cavity design.24-27 Current
cavity designs and treatment concepts have evolved from a tradition based on
the mechanical requirements of amalgam, often to the exclusion of the
biomechanical requirements of the tooth. Only by reassessing their restorative
criteria and modalities can clinicians avoid the perpetuation of the treatment
cycle to which patients are currently subjected.28-30
*Private practice, Hamilton, New Zealand.
† Adjunct Associate Professor, University of Texas Health Science
Center School of Dentistry, San Antonio, Texas.
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