This document discusses biomechanical considerations for partial coverage restorations in fixed prosthodontics. It covers various topics such as: - Advantages and disadvantages of partial coverage restorations compared to full coverage crowns. - Factors that influence the resistance to fracture or biomechanics of partial coverage restorations, such as masticatory pattern, material properties, cavity design, and cementation system used. - Suitable materials for partial coverage restorations including composites, ceramics, and metals alloys and how their elastic modulus compares to tooth structure. - Design considerations for partial coverage restorations like cavity dimensions, extension, taper, and margins. -

1. Biomechanical consideration of
partial coverage in fixed
prosthodontics
By:
Zanyar Mohammed Kareem
B.D.S, M.Sc.

2. Introduction
• An artificial replacement that restores missing tooth structure by
surrounding part of the remaining structure with a material such as
cast metal alloy, ceramics, or resin; it is retained by mechanical or
adhesive means (GPT, 2017).
• An extracoronal restoration that covers only part of the clinical crown
is considered a partial veneer crown. it can also be referred to as a
partial-coverage restoration. an intracoronal restoration is called an
inlay or an onlay if one or more cusps are restored (Rosenstiel et al., 2016)
• Ceramic inlays, onlays, and veneers have gained popularity in recent
years due to increasing Interest in all-ceramic dental restorations.

3. Advantages:
• conserving tooth structure
• more esthetic
• margins are accessible
decreasing periodontal problems
• setting is more easily verified
• electric pulp test can be
conducted on the tooth.
• Reduction of trauma to the
pulpal and circumferential tissues
and avoids tooth weakening
Disadvantages:
• less retention than full
coverage
• elevated cost
• additional patient visits,
• broad and divergent cavity
preparation design
• Needs temporalization
(Shillingburg and Sather, 2012; Abo El-Farag and El Nawawy, 2019).

4. Posterior partial coverage restoration
• The cavities that have to be restored in posterior area
may have the following shapes once cleaned and
prepared:
1. Inlay (a cavity that doesn’t need any cuspal coverage)
2. Onlay (cavity with coverage of one or more cusps)
3. Overlay (a specific onlay with complete cuspal
coverage)
4. Veneerlay (an overlay with the involvement of the
buccal wall and preparation combined with laminate
veneer)
5. Additional overlay, Occlusal-veneer (or “table-top”)
Long-wrap overlay

5. Indication
• In patients which demand excellent aesthetic results, and have good
oral hygiene
• Medium to large sized cavity when one or more cusps are missing
• Multiple medium to large sized cavity in the same quadrant
• Morphological modification and/or raising the posterior (OVD) in case
of oral rehabilitation where full crown is too invasive
• Cracked tooth syndrome
(Ferraris, 2017)

6. Biomechanics
• Indirect restorations are submitted to high occlusal functional
stresses (vertical and horizontal (oblique) )
• Partial coverage should resist fracture under occlusal load
• Resistance to fracture or/and biomechanics of partial coverage is
influenced by many factors such as:
1. Masticatory pattern,
2. Physical properties of restorative material,
3. Dimension and design of cavity,
4. Cementation system used.

7. Occlusal load and stress distribution
• Masticatory loads in the posterior area are
much higher than the anterior area of
dentition
• Concentration of stress is associated with
various forms of clinical failures such as
tooth fracture, rupture of cement seal, and
fracture of restorative body
• If the maximum stress exceeds the range of
stress that the material can withstand,
fracture occurs due to permanent
deformation

8. • Occlusal force (847 N for man, 597 N for women, over 900 N in
bruxism ) is insufficient to cause restoration failure as the fracture
resistance of teeth restored by inlays/onlays is beyond the maximum
occlusal force of the average man or woman (Soares et al., 2006).
• Materials or bonding interfaces normally fail because of repeated
loading from stresses that are too small to provoke spontaneous
failures after only one application (Kois et al., 2013).

9. • Normal tooth structure transfers external biting loads through
enamel into dentin as compression. (small amount of dentin
deformation (strain) results in tooth flexure)
• Once enamel is no longer continuous (restorations are
designed to distribute stresses onto sound dentin)
• It becomes more complicated when the amount of remaining
dentin is thin
• Any force on the restoration produces compression, tension, or
shear along the tooth restoration interface.
• Stress transfer and the resulting deformations of structures are
principally governed by: 1. The elastic limit of the materials 2.
The ratio of the elastic moduli 3. Thickness of the structure
(Kaladevi and Balasubramaniam, 2020).

10. Materials used for partially coverage
restoration
There exist three major classes of materials: metal alloys, composite materials
and ceramics.
• Dietchi et al. (2015) had elaborated a list of specifications for materials
dedicated to indirect partial restorations as for example, they must:
1. Allow a conservative preparation,
2. Restore the natural morphology, aesthetic and mechanical strength of the
dental structure,
3. Ensure an optimal adaptation of the interfaces,
4. Be biocompatible
5. Radio-opaque, and
6. Ensure a good longevity

11. • For the longevity of the restoration and for more uniform stress
distribution, the elastic properties of the material should be similar to
the tooth.
• Enamel and dentin have different elastic properties . composite resins’
elasticity moduleses are similar to dentin, ceramics’ elasticity moduleses
are similar to enamel (Yamanel et al., 2009; Özkir, 2018).
Materials Elastic
modulus
(E) (GPa)
Poisson’s
ratio
(n)
Density (r)
(g/cm3)
Enamel 84.10 0.30 2.92
Dentin 18.60 0.31 2.65
Bone 13.80 0.26 1.85
Feldspathic ceramic 65.00 0.19 1.00
Leucite-reinforced ceramic 65.00 0.23 1.01
Lithium disilicate-reinforced
ceramic
95.00 0.24 2.50
Resin-based composite 16.60 0.24 2.10
Resin luting cement 8.30 0.35 1.10
Gold alloy 96.6 0.35

12. • High EM material,(ceramics) behave more rigid showed higher stress
values inside the material (without much strain) did not transfer the
stress to the tooth structure. The risk of tooth fracture may be
minimized because the restorative material is likely to fracture before
crown or root fractures.
•
• Lowe EM material (composite) showed more flexibility (undergo
dangerous strains) and more occlusal force was transferred to the
remaining tooth structure, which resulted in greater risk of tooth
fracture (Kois et al., 2013).
• High elastic modulus materials tend to accumulate stresses and low
elastic modulus materials absorb stresses (Kaladevi and Balasubramaniam, 2020).

13. • Metal alloys (Gold): good mechanical properties but (cost & demand for esthetics have
resulted in declining use ) (Gupta et al., 2021)
• Indirect composite resin ICR: Acceptable clinical performance, low modulus of elasticity
(shock absorber) but poor marginal fit, poor wear resistance and difficulty to obtain
good polish
• ceramic materials: high strength and high elastic modulus, resistance to fracture , better
anatomic form, exhibit a better marginal integrity and colour stability than composite.
• Disadvantage: Abrasiveness and Masticatory fatigue that cause early fractures of
ceramic partial coverage restorations related development of micro-crack growth
(flaws) (Stappert et al., 2007)

14. • Feldspathic ceramics provide excellent esthetic. the fabrication process
(sintering) is laboratory dependent (Microporosities and
inhomogeneties)
• Lithium disilicate could be typically fabricated through heat pressed
techniques which was developed to overcome inhomogeneities and
porosity during the sintering
• The flexural strength of lithium disilicate (360 MPa–400 MPa) is ideal for
clinical use, survival rate (92%-97% in 5 years)

15. • Ma et al. (2013) who explained that zirconia showed greater fracture resistance
(2.5 times) than other types of ceramic (ideal for posterior)
• Zirconia are strong enough but not favorable to be bonded.
• Airborne particles abrasion associated with retentive features helps to strengthen
the bond between zirconia and tooth structure. However, this is not applicable to
partial restorations with no-retentive preparation (Quigley et al., 2021).
• Hybrid materials form a new class of CAD/CAM materials. They are new
ceramic/polymer materials, or ceramic-polymer infiltrating network materials
(PICN), that associate the positive aspects of both ceramics and composites with
a lower cost than ceramics, and has similar properties to natural tooth structure.
• Low hardness levels, high levels of flexural strength, acceptable marginal integrity
of the restoration and acceptable optical properties for the posterior sector.
(Jorquera ., et al 2018)

16. Review
• When the adhesion concerns enamel, we should choose glass-ceramic
restorations (elastic modulus of 69-95 GPa close to enamel) (Ferraris, 2017)
• Substance loss is localized in dentin, composite and hybrid restorations are
priviliged. For overlays, ceramics are the best option (Nasri et al., 2020).
• composite restorations for vital teeth should be reserved to inlays and
onlays of reduced volume; and whenever we need cusp coverage we
should choose ceramic materials (Kois et al., 2013).
• For endodontically treated teeth, in vitro studies of Magne and Knezevic
(2009) indicated that the fatigue resistance of composite and hybrid
material overlays realized on molars are more important than ceramic
restorations

17. Thickness of partial coverage restoration
• Thick restorations show higher static fracture strength compared to conservative
ones, (Fennis et al., 2004) although they present more drastic and irreversible failures
(thicker restorations may be stronger but simultaneously imply thinner and weaker
dental tissues underneath them)
• Extremely thin material is not systematically and unconditionally recommended
• Move away from the blind application of “minimally invasive dentistry” to a more
realistic concept of “minimally hazardous dentistry” (Skouridou et al., 2013; Rocca et al., 2015).
• Thickness between 1.0 and 1.5 mm seems to be advisable for all modern “white”
restorative materials, (except traditional feldspathic and leucite reinforced ceramics
which require 2 mm thickness).
• metals should be at least 0.5 mm thick with a greater thickness on the occlusal
surface (Rocca et al., 2015)

18. Extend and design of preparation
• The cavity design in partially indirect restorations
(inlay, onlay and overlay), has no exact
preparation pattern.
• Practically, preparation starts with the removal of
the existing restoration and decayed tissues
without initially finishing the enamel margins.
• The preparation must be carefully designed to
cover the weak structure and with no undercut
formation (Rocca et al., 2015).

19. The following parameters that influence and lead the
cavity design :
• Thickness of remaining walls (in order to maintain them) has to be ≥ 2.0
mm in vital teeth, (some articles report values of 1 mm (Rocca et al., 2015),
and ≥ 3.0 mm in endodontically treated teeth
• Generally the if the cuspal thickness of vital tooth is < 2 mm cuspal &
For non vital tooth the thickness is 3 mm, coverage is suggested.
• Width of occlusal isthmus has to be ≥ 2 mm
• Interproximal overjet has to be possibly ≤ 2 mm. The fracture risk
of the restored marginal ridge increases when the overjet is too
large

20. Preparation design
• Absence of undercut>>> 6-10-degree divergent
wall,(according to axis of insertion)
• Internal round corners
• smooth and well-defined walls
• Sharp finishing lines
• Presence a substrate favorable for adhesion
• Restoration margins do not have to coincide with
occlusal contacts.

21. Review
• Conservation of healthy tooth structure is an
important However, protecting the remaining tooth
should be considered even if removal of additional
dental tissue is necessary (Yang et al., 2018).
• Yang et al., (2018) study results showed that the MOD
inlay group generated higher values of stresses to both
inlay body and tooth structures than (O, MO, ONLAY).
• High risk of fracture inlay cavity might be wedge effect
that can produce horizontal stresses on cavity walls

22. • Yang et al., (2018) concluded that the onlay
reduce the maximum von Mises stresses in
amount of 10 times than inlay.
• As the restorative material covered the
functional cusp, forces were absorbed by
the restorative material and partially
transferred to the abutment tooth.

23. • Oblique occlusal forces imparted high stresses to large MOD inlays. the peak von
Mises stress values in the dentine increased with the width of the inlay cavity
increased.
• When the inlay was replaced by an onlay with the same cavity width, most of the
stress transferred to the onlay restorations and stress on dentine decreased
• onlay restorations offer protection to the underlying tooth structure. Therefore, an
onlay with cusp coverage should be used for restoration when the width of the
cavity reaches 4 mm (Mei et al., 2016).

24. • Regarding the stresses that occurred in the restorative materials, it
was revealed that VM and compressive stresses were higher for the
onlay cavity design.
• On the contrary, tensile stress was approximately two times higher
for the inlay cavity design than for the onlay cavity design (Yamanel et al.,
2009)
• Complete cuspal coverage (overlay) showed maximum
reinforcement of the tooth than only palatal coverage design
regardless of used material(specially maxillary premolar tooth)
(Abo El-Farag and El Nawawy, 2019)

25. Resin cement
• All ceramic restorations are brittle must be supported by elastic buffering layer
such as adhesive resin, dentin bonding agent, and human dentin that is able to
absorb stress during load application.
• Cements with higher flexural modulus exhibits higher values of fracture
resistance for the ceramic inlays/tooth assembly (Addison et al., 2008).
• cement/tooth interfaces are weaker and of greater biologic importance than the
cement/restoration interfaces, the elastic modulus of cements should be closer
to that of dentin
• Jiang et al, (2010) reported that ceramic inlays reduced tension at the dentin-
adhesive interface, thus able to provide better resistance against debonding
when compared with composite resins.
(Özkir, 2018).

26. Dental veneers
• Thin coverings that are placed over the front (visible) part of
the tooth
• Introduced by Pincus in the late 1930s
• Many studies reported positive clinical outcomes veneers,
with a survival rate of 91% in 20 years (Layton and Walton, 2012)
• Indicated to cover staining and discoloration, diastema
closure, correction abnormal tooth morphology and
malposition, restoring of an incisal edge and improveing
esthetics (Ustun and Ozturk, 2018).
• Unfavourable conditions of dental veneers include 1)
parafunctional habits such as bruxism 2) edge to edge
relation 3) poor oral hygiene 4) insufficient enamel

27. • In anterior dentition mechanical load act in bucco-lingual plane, induce bending
of tooth (shear stress at crown-root junction)
• On the loading side, tensile stresses and on the nonloading side, compressive
stresses
• Intercuspal position doesn’t cause significant stress, while moving toward edge
to edge position tensile stress concentrate in the palatal fossa of upper incisors.
And facial surface of mandibular incisors but because favorable facial geometry
this stress is less detrimental
• Its better to avoid margins of veneer at palatal fossa
• Massive compressive stresses will be present in edge-to-edge occlusion
• Less stress are found in surfaces of maximums convex curvature such as:
cingulum, marginal ridges
Stress distribution

29. General concepts for preparation
• Restricting the preparation to enamel is considered to be a critical
(favourable bonding strength >> more durable)
• Enamel thickness of anterior teeth was (1.0 to 2.1 mm incisal, 0.6 to
1.0 mm middle third and 0.3 to 0.5 at the gingival third)
• The amount of labial reduction is 0.4-0.7 mm for ceramic veneers
• Preserving the interproximal contact is recommended, this is due to
preserving more enamel and tooth structure, allowing a positive
seat for cementation

30. Preparation design
A) Window preparation B) Feather preparation C)Butt-joint (bevel) (05-1mm
overlap) D)Palatal Chamfer (2mm overlap)

31. REVIEW
• Window preparation, decrease porcelain fracture and wear of opposing teeth,
and will not interfere with incisal guidance. However, difficulty in masking the
ceramic finish line, and the risk of experiencing chipping of the unsupported
enamel on the incisal edges (Walls et al., 2002; Chai et al., 2018).
• Hui and colleagues showed the least stresses were found in ceramic veneers
with window incisal preparation design, followed by feathered-edge and
overlapped design (Hui et al., 1991).
• feathered-edge incisal preparation design may result in a weak veneer, high
risk of experiencing ceramic chipping, and difficulty with seating of the
veneers. Other problems reported also include marginal discoloration and poor
marginal adaptation. also may be subjected to peel and shear forces during
protrusive guidance (Chai et al., 2018).

32. • A study conducted by Ustun and Ozturk (2018), The incisal bevel preparation
design created less stress compared with the featheredge and overlapped
techniques in all the 0°, 60°, and 120° loading condition
• According Ustun and Ozturk, an incisal bevel design could be a more favorable
option for porcelain veneers as it has a high clinical success rate, more uniform
stress distribution in the cement layer, and lower maximum principle stress

33. • Li et al. (2014) stated that the butt-joint design for PLVs is less tolerable
to stress than the palatal chamfer design, in which 60° and 125° loading
angles were applied
• Similarly, Zarone et al. (2005) found that the palatal chamfer
preparation design tolerated stress better than the window preparation
for both 60° and 125° load angulations and Placement of palatal finish
lines at centric stops increases the risk of experiencing incisal edge
chipping

34. Thickness of overlap
• An incisal reduction between 0.5-2 mm with slight overlap is recommended.
More than 2 mm incisal reduction is rarely required for material thickness or
esthetic reasons (Chai et al., 2018).
• Wall and colleagues (1992), stated that no significant difference in the strength of
ceramic veneers with different thicknesses between 0-2 mm. Their study also highlighted
that the loading angulation affected the fracture strength of ceramic veneers
• Friedman (1998) suggested that the incisal extension of ceramic veneer should be
limited to 0.5 mm or less. If incisal reduction is more than 1 mm, the ceramic is
not supported by tooth structure and is at risk of experiencing fracture.
• Other authors recommend placement of palatal finish lines according to the
morphologic and functional requirements (Chai et al., 2018).

35. • Loss of facial enamel or palatal enamel make
the tooth more flexible and high stress
concentration during function
• The material used for veneer should have
elastic modulus close to the enamel
• When a more flexible material like composite
resin replaces the enamel shell only partial
recovery of crown rigidity expected
• It was demonstrated that nearly 100% of
tooth rigidity replaced if feldspathic porcelain
used (EM 70 GPa, enamel=80GPa)
(Magne and Belser, 2013)

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•