Harshiv Flashcards
What is the difference in maxillary and mandibular resorption rates?
Ridge resorption in the mandibular distal extension
areas can occur much faster (by four times) than the
maxillae due to a smaller bearing area (Tallgren, 1972)
Look into article to understand methodology, results, conclusions. Critically appraise the article. Where did they measure? How did they measure? Why is it 4x faster? What is the absolute amount of loss per year in 1st year and subsequent years? What is the actual quantitive rate of resorption in the first year and subsequently?
What are the limitations of intra-oral scanning for digital complete dentures?
- Current technology still does not permit the
recording of peripheral boundaries and jaw relations
in a truly functional state. (Srinivasan M et al. 2020) - Scanning of mandibular peripheral boundaries for a complete denture is particualarly techncially difficult due to tongue movement. (Goodacre BJ and Goodacre CJ., 2018)
- Digitally determining the depth of the posterior palatal seal area is not possible. These depths need to be palpated intra-orally. (Goodacre BJ and Goodacre CJ., 2018)
- Intra-oral scanning head may cause patient discomfort (Goodacre BJ and Goodacre CJ., 2018)
- Additional research is required to compare conventional and digital impression accuracy of edentulous jaws with varying ridge morphology. (Goodacre BJ and Goodacre CJ., 2018)
What are the principles of tooth preparation?
(Fundementals of fixed prosthodontics - Shillingburg et al. 2012)
1. Preservation of tooth structure
2. Retention and resistance
3. Structural durability
4. Marginal integrity
5. Preservation of the periodontium
(Contemporary fixed prosthodontics - Rosenstiel et al. 2022)
3 groups of principles: Biologic, restoration/tooth interface and esthetic.
Shillingburg et al. - Principles for cast restorations only?
Rosenstiel et al. - Principles are more contemporary and not only for conventional restorations and include adhesion, enamel vs dentine, and esthetics as principles recongnosiing these factors may change tooth preparation design.
What are 10 things to assess for maxillary anterior teeth esthetics?
The Smile Esthetic Index (SEI) has been shown to be a repoducible method with substantial intra- and inter-rater agreement.
- CORRECT SMILE LINE (Do the incisal edges of the maxillary central incisors appear below the tips of the canines showing a convex appearance that can approximate and harmonise with the line of the lower lip?)
- CORRECT FACIAL MIDLINE (Does the facial midline correspond with the interincisive line, without any evident asymmetry between the right and left side of the upper dental arch?)
- CORRECT TOOTH/CROWN ALIGNMENT (Are the exposed teeth aligned correctly, without any malposition on the three dimensions of the space – i.e.: rotation, extrusion, inclination?)
- ABSENCE OF VISIBLE TOOTH DEFORMITY (Are the exposed teeth not abraded and not showing any crown form alteration?)
- ABSENCE OF VISIBLE TOOTH DISCHROMY (Do the exposed teeth show a homogeneous colour, without any dischromy?)
- ABSENCE OF VISIBLE GINGIVAL DISCHROMY (Does the gingiva of the exposed teeth show a homogeneous colour, without any dischromy, such as inflammation, amalgam tattoo, white spot/area from previous free gingival graft?)
- ABSENCE OF VISIBLE GINGIVAL RECESSIONS (Are the gingival margins of the exposed teeth correctly located and covering the cemento-enamel junction?)
- ABSENCE OF VISIBLE GINGIVAL EXCESSES (Is the gingival profile homogeneously integrated with the adjacent area on the buccolingual aspect, without any gingival excess – such as a bulky profile in correspondence with a gingival area treated using a very thick connective tissue graft?)
- ABSENCE OF VISIBLE GINGIVAL SCARS (Is the superficial texture of the mucogingival complex homogeneous, without any scars or superficial clefts?)
- ABSENCE OF VISIBLE DIASTEMA AND/OR MISSING INTERDENTAL PAPILLAE (Is an interdental diastema absent? Do the interdental papillae of the considered smile completely fill in the interdental spaces)
Describe the pink esthetic score (PES)
The pink esthetic score was first described by (Fürhauser et al., 2005) and later modified by (Belser et al, 2009) to correspond more readily to a novel white esthetic score that they had developed to assess the visible part of the implant prosthesis.
The Pink Esthetic Score evaluates the following 7 variables:
- Mesial papilla
- Distal papilla
- Level of soft tissue margin
- Soft tissue contour
- Alveolar process deficiency
- Soft tissue colour
- Soft tissue texture
Soft tissue evaluation around implant‐supported single tooth is performed with a score of 0 to 2 (0 representing the poorest and 2 the best result).
The maximum score of 14 points reflects perfect implant esthetics with that of the reference tooth.
The Pink and White Esthetic Score (Belser et al, 2009) assesses implant crown as well as soft tissue esthetics. The modifed PES in this system by evaluates the following 5 variables:
- Mesial papilla
- Distal papilla
- Curvature of the facial mucosa
- Level of the facial mucosa
- Root convexity/soft-tissue color and texture
These variables are scored on a scale from 0 to 2, for an ideal PES result of 10. (The WES part of this system also scores 10 for a maximum PES/WES result of 20).
PES has high intra‐ and well as inter‐rater reliability.
Poor correlation between PES and most other indices to subjective patient satisfaction.
This and most other indices do not address the topic of variable weighting, while common sense dictates that variables may not contribute equally to the overall esthetic outcome.
The WES and PES apply to single-tooth implants adjacent to natural teeth only. They therefore do not allow for objective assessment of prostheses replacing multiple adjacent teeth.
Objective evaluation of the esthetic outcome of implant therapy inherently fails to reflect subjective patient opinions, however, requires consistency of results to enable between‐study comparison and meta‐analysis.
What are 4 theories as to why platform switching preserves bone?
Platform switching is a concept in implant dentistry that involves using an abutment that is smaller in diameter than the implant itself.
The platform-switching connection attempts to reduce the marginal bone loss by shifting the implant-abutment interface horizontally.
The origin of platform-switching abutments was the unavailability of matching wide abutments for restoring implants of wider diameter. Therefore, abutments of a smaller diameter than that of the implant were used in the restorative phase; hence, the evolution of platform-switching abutments. With platform-switching abutments, (Lazzara RJ and Porter SS, 2006) observed a lesser degree of bone loss than with implant restorations with butt-joint connections.
Both histomorphometric studies and three-dimensional finite element models have shown the potential role of the platform-switching configuration to limit peri-implant marginal bone loss.
Several theories attempt to explain the positive outcomes associated with the platform-switching concept:
Platform switching refers to the inward horizontal repositioning of the implant-abutment junction (IAJ) so that it is positioned away from the outer edge of the implant and adjacent bone.
All three mechanical and biologic theories have been proposed, but none has yet been proven.
a. Inflammation at the implant abutment junction theory
- Medializing the implant abutment microgap and inflammatory cell infiltrate away from the crestal bone (Broggini et al. 2006)
b. Biologic width theory
- Optimizing space for the components of biologic width and connective tissue stabilization. Connective tissue covering the implant-abutment interface/microgap.(Cochran et al., 2013)
- More space to establish a proper peri-implant biologic width associated with a reduced apical extension of the junctional epithelium and, consequently, less CBL changes.
c. Biomechanical theory
- Shifting the area of maximum biomechanical stress towards the center of the implant (Maeda et al. 2007)
Has platform switching been shown to be effective?
(Cappiello et al. 2008)
- Vertical bone loss for patients who did not receive a platform-switched abutment was between 1.3 and 2.1 mm.
- Patients who received an abutment 1 mm narrower than the platform had vertical bone loss between 0.6 and 1.2 mm.
- These figures validate the significant role of the microgap between the abutment and the implant in the remodeling of the peri-implant crestal bone.
(Canullo et al. 2009)
- Found that the group that received a narrower abutment had a bone reduction level of 0.30 mm, whereas the control had a bone reduction level of 1.19 mm.
(Atieh et al. 2010) in a meta-analysis has confirmed that an implant/abutment offset of ~ 0.4 mm was associated with a more favorable bone response
The effect of platform switching still remains inconclusive, particularly with respect to long-term data and also due to the multifactorial causes of peri-implant bone loss.
Peri-implant bone loss is multifactorial - what factors may contribute to peri-implant marginal bone loss?
Several factors can contribute to peri-implant marginal
bone loss, such as:
- Establishment of a biologic width (Hermann JS et al. 2001)
- Vertical soft tissue thickness (Linkevicius T et al. 2009)
- Surgical trauma (Blanco, J. et al. 2008),
- Interimplant distance (Tarnow et al. 2000) (Rodríguez-Ciurana X et al. 2009)
- Presence of a microgap at the level of the implant–abutment interface (IAI) (Ericcson et al. 1995)
- Implant positioning relative to the alveolar crest (Hermann JS et al. 2000)
- Macrodesign of the cervical area of the implant (i.e., platform-switching and platform-matching implants) (Strietzel FP et al. 2015)
- Type of implant–abutment connection (Palaska I et al. 2016)
- Surface topography of the implant neck (Hammerle CH et al. 1996),
- Micromovements of the abutment (prosthetic components)(Duyck J et al. 2006)
- Repeated connection/disconnection of abutments ( Abrahamsson I et al. 1997),
- Smoking status (Clementini M et al. 2014)
- Peri-implantitis (Zitzmann NU and Berglundh T. 2008)
What is the defintition of MRONJ?
Medication-related osteonecrosis of the jaw (MRONJ) is a rare side effect of anti-resorptive and antiangiogenic drugs.
Definition: Exposed bone, or bone that can be probed through an intraoral or extraoral fistula, in the maxillofacial region that has persisted for more than eight weeks in patients with a history of treatment with anti-resorptive or anti-angiogenic drugs, and where there has been no history of radiation therapy to the jaw or no obvious metastatic disease to the jaws.
How is MRONJ diagnosed?
Analysis of the physical and histological properties of resected necrotic bone from MRONJ patients have failed to demonstrate any unique features that would serve as a reliable biomarker for MRONJ.
Bone inflammation and infection are usually present in
patients with advanced MRONJ, and appear to be secondary
events.
Patient history and clinical examination remain the most
sensitive diagnostic tools for MRONJ: A clinical finding of exposed bone, or bone that can be probed through an intraoral or extraoral fistula, in the maxillofacial region that has persisted for more than eight weeks in patients with a history of treatment with anti-resorptive or anti-angiogenic drugs, and where there has been no history of radiation therapy to the jaw or no obvious metastatic disease to the jaws.
What is the fracture strength of different ceramics (feldspathic, leucite, lithium, zirconia)?
The mechanical behavior of dental materials is mainly characterized by elasticity, flexural strength, fracture toughness, and hardness. These properties are basically given by the type and strength of the bondings between the atoms. Elasticity is the ability of the material to resume its initial shape after loading, measured in GPa (= 10^3 N/ mm2). Stressing a material beyond its limit of elasticity leads to plastic deformation, a permanent distortion. Brittle materials such as ceramics only show minimal or no plasticity, which means they fracture very soon after reaching the limit of elasticity. The stress where fracture occurs is the flexural strength, measured in MPa (= N/ mm^2). The resistance against crack growth is called fracture toughness, measured in MPa √ m.
Flexural strength (MPa)
- Feldspathic porcelain: ~120
- Leucite: ~200
- Lithium silicate: ~620
- Lithium disilicate: ~560
- 5Y Zirconia: ~800
- 4Y Zirconia: ~1000
- 3Y Zirconia: ~1280
Fracture toughness (MPa √ m):
- Feldspathic porcelain: ~1.2
- Leucite: ~1.0
- Lithium silicate: ~1.5
- Lithium disilicate: ~2.1
- 5Y Zirconia: ~2.9
- 4Y Zirconia: ~3.9
- 3Y Zirconia: ~4.7
Describe the physical properties of dental hard tissues
What is Opalescence?
Opalescence, also called the Rayleigh scattering effect, is an optical property of enamel and refers to the ability to transmit a certain range of natural light wavelengths (red-orange tones) and reflect the others (blue-lavender tones).
Opalescence is easily understood if enamel is compared to the atmosphere of the earth. Because of the presence of small particles like water droplets that interact with the sunlight, the sky can appear either blue (at noon) or red (at sunrise and sunset). A similar effect occurs at the incisal edge due to the scattering of light at the level of the microscopic hydroxyapatite crystals
Enamel, especially at the incisal edge and the DEJ, acts similarly as the “atmosphere of the tooth.” It normally displays a bluish transparent effect under direct lighting or an orange opalescent tone under indirect light.
What material is dry milled?
Dry milling uses pressurized air and a vacuum to remove material particles.
Wet mills use distilled water with an additive that works as a coolant for the tools and material that is being milled.
Materials which are dry milled:
1. Presintered Zirconia (wet milling will cause softening)
- PMMA, wax, polyurethane, composite resin can be milled dry or wet but dry is often prefered to reduce undesired residue.
- Cobalt chrome (with soft milling)
How do indirect restorative dental materials often fail?
The long-term success of a restoration mainly depends on its mechanical performance. From the technical side the success of a restoration can be controlled by the durability of the material, the nature of the design, the quality of the processing, and the effectiveness of the finishing.
**Material **
The mechanical behavior of dental materials is mainly characterized by elasticity, flexural strength, fracture toughness, and hardness. These properties are basically given by the type and strength of the bondings between the atoms.
Elasticity is the ability of the material to resume its initial shape after loading, measured in GPa (= 103 N/ mm2). Stressing a material beyond its limit of elasticity leads to plastic deformation, a permanent distortion.
Brittle materials such as ceramics only show minimal or no plasticity, which means they fracture very soon after reaching the limit of elasticity. The stress where fracture occurs is the flexural strength, measured in MPa (= N/ mm2).
The resistance against crack growth is called fracture toughness, measured in MPa √ m.
Elasticity, flexural strength, and fracture toughness are bulk properties.
Hardness in contrast is a surface property, which is defined as the resistance to localized deformation induced by mechanical indentation or abrasion. Harder materials therefore show less risk of surface damage. Flexural strength and hardness are correlated to a certain extent.
The main risk for mechanical failure of restorations are flaws at the surface, which might act as a starting point for microcracks. In case of tensile loading, a microcrack opens and stress develops at the tip of the crack. Stress which exceeds the strength of the material leads to crack propagation. Under cyclic loading − such as mastication − crack growth happens in a micrometer scale. But over time the crack grows significantly.
Finally, catastrophic failure occurs when the residual cross-section is too small to withstand the load. It is important to understand the fracture mechanisms of the different materials.
In metals the crack tip is rounded out by plastic flow and thus the risk of fracture is significantly reduced. In ceramics plastic flow is not possible due to the covalent bonds. The crack tip remains sharp and crack growth is a significantly higher risk than in metals. That is the reason for the well-known brittle behavior of ceramics.
To increase strength and in particular toughness, strengthening mechanisms on the microscopic level to impede crack propagation are employed. In brittle materials this might be achieved by internal compression or by particles, which act as obstacles against crack growth. The objective of such strengthening mechanisms is to stop crack growth or at least to hamper it, like a hurdler who is not as fast as a sprinter.
The term durability includes not only the mechanical characteristics specified above but resistance to wear and aging as well. The degradation of the materials by wear and aging depends on the mechanical properties and also on the susceptibility to the oral environment including humidity, temperature, and loading characteristics. Water for instance may attack the material’s bonds especially at phase boundaries or microcracks, thus promoting degradation.
How can you make a crown look narrower?
- Bring the line angles closer towards the center of the tooth to create the illusion of a longer tooth
- If this is insufficient, the mesioincisal and distoincisal corners of the tooth are rounded to enhance the lengthening effect
- The tooth can also be modified from the gingival third, by breaking the mesiodistal line angles in this aea creating a more triangular shape, giving the illusiion of a narrower tooth.
- Flatten the labial surface to obtain a large and flat mid-labial segment gives the illusion of a logner tooth
- The crown can also be shaped to bein a slightly more palatal/lingual position making it appear narrower than its actual size.
(Goldstein RE. Esthetics in Dentistry. 2nd ed. Pg. 133-186.)
(Mange P. Biomimetic restorative dentistry. Volume 2. Pg. 170.)
(Gurel G. The Science and Art of Porcelain Laminate Veneers. Pg. 87-89.)
What is the difference between milling and grinding?
Grinding: Uses frictional force to obtain desired shape (usually with diamond burs)
Milling: Uses rotary cutting to obtain desired shape (usually tungsten carbide burs)
The image below shows which materials are ground and which are milled.
How do you avoid metamerism?
Metamerism is a phenomenon where the color of an object appears different, depending upon the light source. When viewed together under the same light source, two objects may appear to have the same color; however, each appears to have a different color when viewed under different light sources. For instance, a crown may be matched under incandescent light; however, when the crown is viewed under color-corrected or fluorescent light, the crown will appear different in color. In dentistry, this phenomenon occurs predictably and frequently if the shade selection environment is not controlled and neutral.
To avoid or minimize metamerism, it is of utmost importance to control the lighting conditions when shade is being determined:
- Ideal color corrected light with temperature of 5500 degrees K
(Gurel G. The Science and Art of Porcelain Laminate Veneers. Pg. 170.)
What are clinical indications of Y-TZP?
5Y-TZP - Single crowns and with restrictions 3 unit FDPs
4Y-TZP - Single crowns, 3 unit FDPs
3Y-TZP - Single crowns, 3, 4, 5 unit or full arch FDPs
What is transformation toughening?
- Zirconia shows three different crystal modifications:
- At room temperature zirconia has a monoclinic structure.
- Heating zirconia leads to a phase transition from monoclinic to tetragonal structure at 1170 ° C.
- And finally, above 2370 ° C a cubic structure is stable (Fig 1-1-19).
- Replacing 3 mol% of ZrO2 by Y2O3 stabilizes the tetragonal phase down to room temperature due to oxygen voids in the crystal lattice and the larger atomic radius of Y compared to Zr. The abbreviation of this material is 3Y-TZP (TZP stands for “tetragonal zirconia polycrystals”). The tetragonal phase of this material is metastable and only occurs when the grain size of zirconia is less than 1 µm.
Transformation toughening
- When energy is brought into the material the phase transition to the monoclinic structure is triggered, even at room temperature.
- This phenomenon is used to reinforce zirconia: the phase transition from tetragonal to monoclinic (t 🡢 m) is associated with a volume increase of about 4– 5%.
- Microcracks under tensile stress lead to stress concentration at the crack tip. In this area, the mechanical energy is sufficient to provoke the t 🡢 m phase transition. For the phase transition only a slight movement of the atoms in the crystal lattice is necessary.
- The increase in volume associated with the phase transition leads to an intrinsic compressive stress at the crack tip, opposing the external tensile stress and thus increasing the materials strength.
- This effect is not reversible. When the monoclinic phase is established, the strengthening mechanism in this area is consumed; like a match, once lit it cannot be lit again.
What can a deficient 2nd plane of reduction impact?
- The labial surface of teeth have a curve.
- Since crown preparation needs to preserve the anatomy of the crown while creating space for the bulk of the material, our prep should also have this curve as well. To simplify things this curve can be divided into two parts: Gingival 2/3 and Incisal 1/3. While thinking that this curve has two parts, these are actually coherent to each other with a smooth transition in between.
The consquences of lack of a 2nd plane of reduction include:
- If the plane is only following the gingival plane -> there will be reduced space avaialle for the material in the mid and incisal third.
- May be less than minimal thickness required for the material leading to insufficient mechanical properties.
- Would lead to lack of translucency being able to be developed.
- Laboratory technican may compensate by making labial surface bulker reducing esthetic outcome.
- If the plane is only following the incisal plane -> there will be overpreparation carried out in the middle and gingival thirds, leading to higher risk of inflammatory pulpal response and associated signs/symptoms.
