Presentation on theme: "DENTAL COMPOSITES REVIEW DEFINITION COMPOSITE CHEMISTRY Dental composite is composed of a resin matrix and filler materials. Coupling agents are used."— Presentation transcript:
DENTAL COMPOSITES REVIEW
COMPOSITE CHEMISTRY Dental composite is composed of a resin matrix and filler materials. Coupling agents are used to improve adherence of resin to filler surfaces. Activation systems including heat, chemical and photochemical initiate polymerization. Plasticizers are solvents that contain catalysts for mixture into resin. Monomer, a single molecule, is joined together to form a polymer, a long chain of monomers. Physical characteristics improve by combining more than one type of monomer and are referred to as a copolymer. Cross linking monomers join long chain polymers together along the chain and improve strength.
RESIN MATERIALS BIS-GMA resin is the base for composite. In the late 1950's, Bowen mixed bisphenol A and glycidylmethacrylate thinned with TEGDMA (triethylene glycol dimethacrylate) to form the first BIS-GMA resin. Diluents are added to increase flow and handling characteristics or provide cross linking for improved strength. Common examples are: RESIN:- BIS-GMA bisphenol glycidylmethacrylate DILUENTS:- MMA methylmethacrylate BIS-DMA bisphenol dimethacrylate UDMA urethane dimethacrylate CROSS LINK DILUENTS TEGDMA triethylene glycol dimethacrylate EGDMA ethylene glycol dimethacrylate
COUPLING AGENTS Coupling agents are used to improve adherence of resin to filler surfaces. Coupling agents chemically coat filler surfaces and increase strength. Silanes have been used to coat fillers for over fifty years in industrial plastics and later in dental fillers. Today, they are still state of the art. Silanes have disadvantages. They age quickly in a bottle and become ineffective. Silanes are sensitive to water so the silane filler bond breaks down with moisture. Water absorbed into composites results in hydrolysis of the silane bond and eventual filler loss. Common silane agents are: vinyl triethoxysilane methacryloxypropyltrimethoxysilane
HEAT CATALYST Polymerization of resin requires initiation by a free radical. Initiation starts propagation or continued joining of molecules at double bonds until termination is reached. Heat applied to initiators breaks down chemical structure to produce free radicals, however, monomers may polymerize when heat is applied even without initiators. Resins require stabilizers to avoid spontaneous polymerization. Stabilizers are also used to control the reaction of activators and resin mixtures. Hydroquinone is most commonly used as a stabilizer. Common heat based initiators are peroxides such as benzoylperoxide t-butylperoxide t-cumythydroxyperoxide
PHOTOCHEMICAL CATALYST Early photochemical systems used were benzoin methyl ether which is sensitive to UV wavelengths at 365 nm. UV systems had limited use as depth of cure was limited. Visible light activation of diketones is the preferred photochemical systems. Diketones activate by visible, blue light to produce slow reactions. Amines are added to accelerate curing time. Presently, different composites use different photochemical systems. These systems are activated by different wavelengths of light. In addition, different curing lights produce various ranges of wavelengths that might not match composite activation wavelengths. This can result in no cure or partial cure. Composite materials must be matched to curing lights. Common photochemical initiators are: Camphoroquinone Acenaphthene quinone Benzyl
LIGHT CURING Light curing can be accomplished with:- 1) Quartz-Tungsten-Halogen 2) Plasma Arc Curing 3) Light Emitting Diode
CHEMICAL CATALYST Chemical activation of peroxides produces free radicals. Chemical accelerators are often not color stable and have been improved for this reason. The term self cure or dual cure (when combined with photo chemical initiation) describes chemical cure materials. Chemical composites mix a base paste and a catalyst paste for self cure. Bonding agents mix two liquids. Mixing two pastes incorporates air into the composite. Oxygen inhibits curing resulting in a weaker restoration. Chemical accelerators include: Dimethyl p-toludine N,N-bis(hydroxy-lower-alkyl)-3,5-xylidine
COMPOSITE FILLERS Fillers are placed in dental composites to reduce shrinkage upon curing. Physical properties of composite are improved by fillers, however, composite characteristics change based on filler material, surface, size, load, shape, surface modifiers, optical index, filler load and size distribution. Materials such as strontium glass, barium glass, quartz, borosilicate glass, ceramic, silica, prepolymerized resin, or the like are used.
Fillers are classified by material, shape and size. Fillers are irregular or spherical in shape depending on the mode of manufacture. Spherical particles are easier to incorporate into a resin mix and to fill more space leaving less resin. One size spherical particle occupies a certain space. Adding smaller particles fills the space between the larger particles to take up more space. There is less resin remaining and therefore, less shrinkage on curing the more size particles used in proper distribution. FILLERS CLASSIFICATION
Classification According to Size:- MACROFILLERS TO 100 um MIDIFILLERS TO 10 um MINIFILLERS TO 1 um MICROFILLERS TO 0.1 um NANOFILLERS TO 0.01 um FILLERS CLASSIFICATION
PLASTICIZERS Dental composite is composed of a resin matrix and filler materials. Coupling agents are used to improve adherence of resin to filler surfaces. Plasticizers are solvents that contain catalysts for mixture into resin. They need to be non reactive to the catalyst & resin.
Physical Characteristics Following are the imp physical properties:- 1) Linear coefficient of thermal expansion (LCTE) 2) Water Absorption 3) Wear resistance 4) Surface texture 5) Radiopacity 6) Modulus of elasticity 7) Solubility
C- FACTOR It is the ratio of the bonded surfaces to the unbonded or free surfaces in a tooth preparation. The higher the C-Factor, greater is the potential for bond disruption from polymerisation effects.
INTERNAL STRESSES Internal stresses can be reduced by, 1) Self start Polymerisation 2) Incremental placement 3) Use of stress breaking liners such as:- a)Filled Dentinal Adhesives b)RMGI.
COMPOSITE CLASSIFICATION Composite is classified by initiation techniques, filler size, and viscosity. Laboratory heat process fillings are processed under nitrogen and pressure to produce a more thorough cure. Core build up materials are commonly self cure. Dual cure composite is commonly used as a cementing medium under crowns. Viscosity determines flow characteristics during placement. A flowable composite flows like liquid or a loose gel. A packable composite is firm and hard to displace.
Composite is classified by initiation techniques, filler size, and viscosity Heat cured composites are polymerized by application of heat. Self cured composite means chemical initiation converting monomer to polymer takes place. Light cured composite means photochemical initiation causes polymerization Dual cure means chemical initiation is used and combined with photochemical initiation so either and both techniques polymerize composite.
Radiospacity One of the requirements of using a composite as a posterior restorative is that it should be radiopaque. In order for a material to be described as being radiopaque, the International Standard Organization (ISO) specifies that it should have radiopacity equivalent to 1 mm of aluminium, which is approximately equal to natural tooth dentine. However, there has been a move to increase the radiopacity to be equivalent to 2 mm of aluminium, which is approximately equal to natural tooth enamel. A majority of the composites described as all-purpose or universal have levels of radiopacity greater than 2 mm of aluminium
INDICATIONS 1) Class-I, II, III, IV, V & VI restorations. 2) Foundations or core buildups. 3) Sealant & Preventive resin restorations. 4) Esthetic enhancement procedures. 5) Luting 6) Temporary restorations 7) Periodontal splinting.
CONTRAINDICATIONS 1) Inability to isolate the site. 2) Excessive masticatory forces. 3) Restorations extending to the root surfaces. 4) Other operator errors.
ADVANTAGES 1) Esthetics 2) Conservative tooth preparation. 3) Insulative. 4) Bonded to the tooth structure. 5) repairable.
DISADVANTAGES 1) May result in gap formation when restoration extends to the root surface. 2) Technique sensitive. 3) Expensive 4) May exhibit more occlusal wear in areas of higher stresses. 5) Higher linear coefficient of thermal expansion.
STEPS IN COMPOSITE RESTORATION 1) Local anaesthesia. 2) Preparation of the operating site. 3) Shade selection 4) Isolation of the operating site. 5) Tooth preparation. 6) preliminary steps of enamel and dentin bonding. 7) Matrix placement. 8) Inserting the composite. 9) Contouring the composite. 10) polishing the composite.
PRINCIPLES OF ANTERIOR COMPOSITE RESTORATION 1. Smile Design 2. Color and Color Analysis 3. Tooth Color 4. Tooth Shape 5. Tooth Position 6. Esthetic Goals 7. Composite Selection 8. Tooth Preparation 9. Bonding Techniques 10. Composite Placement 11. Composite Sculpture and 12. Composite Polishing to properly restore anterior teeth with composite:
1. SMILE DESIGN A dentist must understand proper smile design so composite restoration can achieve a beautiful smile. This is true for extensive veneering and small restorations. Factors which are considered in smile design include:- A. Smile Form which includes size in relation to the face, size of one tooth to another, gingival contours to the upper lip line, incisal edges overall to the lower lip line, arch position, teeth shape and size, perspective, and midline. B. Teeth Form which includes understanding long axis, incisal edge, surface contours, line angles, contact areas, embrasure form, height of contour, surface texture, characterization, and tissue contours within an overall smile design. C. Tooth Color of gingival, middle, incisal, and interproximal areas and the intricacies of characterization within an overall smile design.
2. COLOUR AND COLOUR ANALYSIS Colour is a study in and of itself. In dentistry, the effect of enamel rods, surface contours, surface textures, dentinal light absorption, etc. on light transmission and reflection is difficult to understand and even more difficult replicate. The intricacies of understanding matching and replicating hue, chroma, value, translucency, florescence; light transmission, reflection and refraction to that of a natural tooth under various light sources is essential but far beyond the scope of this article.
3. TOOTH COLOUR Analysis of colour variation within teeth is improved by an understanding of how teeth produce color variation. Enamel is prismatic and translucent which results in a blue gray color on the incisal edge, interproximal areas and areas of increased thickness at the junction of lobe formations. The gingival third of a tooth appears darker as enamel thins and dentin shows through. Color deviation, such as craze lines or hypocalcifications, within dentin or enamel can cause further color variation. Aging has a profound effect on color caused by internal or external staining, enamel wear and cracking, caries, acute trauma and dentistry.
4. TOOTH SHAPE Understanding tooth shape requires studying dental anatomy. Studying anatomy of teeth requires recognition of general form, detail anatomy and internal anatomy. It is important to know ideal anatomy and anatomy as a result of aging, disease, trauma and wear. Knowledge of anatomy allows a dentist to reproduce natural teeth. For example, a craze line is not a straight line as often is produced by a dentist, but is a more irregular form guided by enamel rods.
5. TOOTH POSITION Knowledge of normal position and axial tilt of teeth within a head, lips, and arches allows reproduction of natural beautiful smiles. Understanding the goals of an ideal smile and compromises from limitations of treatment allows realistic expectations of a dentist and patient. Often, learning about tooth position is easily done through denture esthetics. Ideal and normal variations of tooth position is emphasized in removable prosthetics so a denture look does not occur.
6. ESTHETIC GOALS The results of esthetic dentistry are limited by limitations of ideals and limitations of treatment. Ideals of the golden proportion have been replaced by preconceived perceptions. Limitations of ideals are based on physical, environmental and psychological factors. Limitations of treatment are base on physical, financial and psychological factors.
7. COMPOSITE SELECTION Esthetic dentistry is an art form. There are different levels of appreciation so individual dentists evaluate results of esthetic dentistry differently. Artistically dentists select composites based on their level of appreciation, artistic ability and knowledge of specific materials. Factors which influence composite selection include A- Restoration Strength, B- Wear C- Restoration Color D- Placement characteristics. E- Ability to use and combine opaquers and tints. F- Ease of shaping. G- Polishing characteristics. H- Polish and colour stability
8. TOOTH PREPARATION Tooth preparation often defines restoration strength. Small tooth defects which receive minimal force require minimal tooth preparation because only bond strength is required to provide retention and resistance. In larger tooth defects where maximum forces are applied, mechanical retention and resistance with increased bond area can be required to provide adequate strength.
9. BONDING TECHNIQUES Understanding techniques to bond composite to dentin and enamel provide strength, elimination of sensitivity and prevention of micro- leakage. Enamel bonding is a well understood science. Dentinal bonding, however, is constantly changing as more research is being done and requires constant periodic review. Micro-etching combined with composite bonding techniques to old composite, porcelain, and metal must be understood to do anterior composite repairs.
10. COMPOSITE PLACEMENT TECHNIQUE Understanding techniques which allow ease of placement, minimize effects of shrinkage, eliminate air entrapment and prevent material from pulling back from tooth structure during instrumentation determine ultimate success or failure of a restoration. It is important to incorporate proper instrumentation to allow ease of shaping tooth anatomy and provide color variation prior to curing composite. In addition, a dentist must understand placement of various composite layers with varying opacities and color to replicate normal tooth structure.
11. COMPOSITE SCULPTURE Composite sculpture of cured composite is properly done if appropriate use of polishing strips, burs, cups, wheels and points is understood. In addition, proper use of instrumentation maximizes esthetics and allows minimal heat or vibrational trauma to composite resulting in a long lasting restoration.
12. COMPOSITE POLISHING Polishing composite to allow a smooth or textured surface shiny produces realistic, natural restorations. Proper use of polishing strips, burs, cups, wheels and points with water or polish pastes as required minimizes heat generation and vibration trauma to composite material for a long lasting restoration.
DIRECT POSTERIOR COMPOSITES Composites are indicated for Class 1, class 2 and class 5 defects on premolars and molars. Ideally, an isthmus width of less than one third the intercuspal distance is required. This requirement is balanced against forces created on remaining tooth structure and composite material. Forces are analyzed by direction, frequency, duration and intensity. High force occurs with low angle cases, in molar areas, with strong muscles, point contacts and parafunctional forces such as grinding and biting finger nails. Composite is strongest in compressive strength and weakest in shear, tensile and modulus of elasticity strengths. Controlling forces by preparation design and occlusal contacts can be critical to restorative success. Failure of a restoration occurs if composite fractures, tooth fractures, composite debonds from tooth structure or micro-leakage and subsequent caries occurs. A common area of failure is direct point contact by sharp opposing cusps. Enameloplasty that creates a three point contact in fossa or flat contacts is often indicated.
Tooth preparation requires adequate access to remove caries, removal of caries, elimination of weak tooth structure that could fracture, beveling of enamel to maximize enamel bond strength, and extension into defective areas such as stained grooves and decalcified areas. Matrix systems are placed to contain materials within the tooth and form proper interproximal contours and contacts. Selection of a matrix system should vary depending on the situation (see web pages contacts and contours in this section). Enamel and dentin bonding is completed. Composite shrinks when cured so large areas must be layered to minimize negative forces. Generally, any area thicker than two millimeters requires layering. In addition, cavity preparation produces multiple wall defects. Composite curing when touching multiple walls creates dramatic stress and should be avoided.
Composite built in layers replicate tooth structure by placing dentin layers first and then enamel layers. Final contouring with hand instruments is ideal to minimize the trauma of shaping with burs. Matrix systems are removed and refined shaping and occlusal adjustment done with a 245 bur and a flame shaped finishing bur. Interproximal buccal and lingual areas are trimmed of excess with a flame shaped finishing bur. Final polish is achieved with polishing cups, points, sandpaper disks, and polishing paste.
INDIRECT POSTERIOR COMPOSITES Indirect laboratory composite is indicated on teeth that required large restorations but have a significant amount of tooth remaining. It is used when a tooth defect is larger than indicated for direct composite and smaller than indicated for a crown. A common situation is fracture of a single cusp on a molar or a thin cusp on a bicuspid. Force analysis is critical to success as high force will fracture composite, tooth structure or separate bonded interfaces. High force is indicated on teeth furthest back in the mouth for example, a second molar receives five times more force than a bicuspid. Orthodontic low angle cases and large masseter muscles generate high force. Sharp point contacts from opposing teeth create immense force and are often altered with enameloplasty. Indirect composite restorations are processed in a laboratory under heat, pressure and nitrogen to produce a more thorough composite cure. Pressure and heat increase cure while nitrogen eliminates oxygen that inhibits cure. Increased cure results in stronger restorations. Strength of laboratory processed composite is between composite and crown strength and requires adequate tooth support.
TOOTH PREPARATION Tooth preparation requires removal of existing restorations and caries. Thin cusps and enamel are removed in combination of blocking out undercuts with composite, glass ionomer, flowable composite or the like. Tooth preparation requires adequate wall divergence to bond and cement the restoration and ideally, margins should finish in enamel. The restoration floor is bonded and light cured. Bonding agent is light cured to stabilize collagen fibers and avoid collapse during restoration placement. A base of glass ionomer or composite is used if thermal sensitivity is anticipated. Restoration retention is judged by bonded surface area, number and location of retentive walls, divergence of retentive walls, height to width ratio and restoration internal and external shape. Resistance form, reduction of internal stress and conversion of potential shear and tensile forces is accomplished by smoothing sharp areas and creating flat floors as opposed to external angular walls.
TOOTH PREPARATION Impressions are taken of prepared teeth, models poured and composite restorations constructed at a laboratory. Temporaries are placed and a second appointment made. At a second appointment, temporaries are removed and a rubber dam placed. Restorations are tried on the teeth and adjusted. Manufacturers directions are followed. In general, bonding is completed on the tooth surfaces and bonding resin precured. Matrix bands are placed prior to etching to contain etch within prepared areas. Trimming of excess cement where no etching has occurred is easier. Composite surfaces are silinated and dual cure resin cement applied. Restorations are seated, excess resin cement is wiped away with a brush and then facial and lingual surfaces are light cured. Interproximal areas are flossed and then light cured. Excess is trimmed with hand instruments and finishing flame shaped burs. The rubber dam is removed and occlusion adjusted. Surfaces are finished and polished.
COMPOSITE WEAR There are several mechanisms of composite wear including adhesive wear, abrasive wear, fatigue, and chemical wear. Adhesive wear is created by extremely small contacts and therefore extremely high forces, of two opposing surfaces. When small forces release, material is removed. All surfaces have microscopic roughness which is where extremely small contacts occur between opposing surfaces. Abrasive wear is when a rough material gouges out material on an opposing surface. A harder surface gouges a softer surface. Materials are not uniform so hard materials in a soft matrix, such as filler in resin, gouge resin and opposing surfaces. Fatigue causes wear. Constant repeated force causes substructure deterioration and eventual loss of surface material. Chemical wear occurs when environmental materials such s saliva, acids or like affect a surface.
COMPOSITE FRACTURE Dental composite is composed of a resin matrix and filler materials. The resin filler interface is important for most physical properties. There are three causes of stress on this interface including: resin shrinkage pulls on fillers, filler modulus of elasticity is higher than resin, and filler thermo coefficient of expansion allows resin to expand more with heat. When fracture occurs, a crack propagates and strikes a filler particle. Resin pulls away from filler particle surfaces during failure. This type of failure is more difficult with larger particles as surface area is greater. A macrofill composite is stronger than a microfill composite. Coupling agents are used to improve adherence of resin to filler surfaces. Modification of filler physical structure on the surface or aggregating filler particles create mechanical locking to improve interface strength. Coupling agents chemically coat filler surfaces and increase strength. Silanes have been used to coat fillers for over fifty years in industrial plastics and later in dental fillers. Today, they are still state of the art.
Multifunctional Composites and Novel Microstructures Hierarchical microstructures - Dr H-X Peng The properties of composite materials can be tailored through microstructural design at different lengthscales such as the micro- and nano-structural level. At the micro-structural level, our novel approach creates microstructures with controlled inhomogeneous reinforcement distributions. These microstructures effectively contain more than one structural hierarchy. This has the potential to create whole new classes of composite materials with superior single properties and property combinations. Research also involves tailoring the nano-structures of micro- wires/ribbons for macro-composites.
Shaped fibres - Dr Ian Bond, Dr Paul Weaver Research has shown that shaped fibres can be an effective means of improving the through thickness properties. A set of guidelines for fibre shape and a preferred family of fibres have been generated from qualitative analysis for the role of reinforcing fibres in composites. Methods have also been developed to produce such shaped fibres from glass in order to form reinforced laminates in sufficient quantity for materials property testing using standard methods. Fibre shape has been shown to play a key role in contributing to the bonding force between fibre and matrix, with significant increases in fracture toughness possible. Results suggest that the shaped fibre specimens have a greater throughthickness strength than the circular fibre composites that are currently used.
Self healing - Dr Ian Bond Impact damage to composite structures can result in a drastic reduction in mechanical properties. Bio-inspired approach is adopted to effect selfhealing which can be described as mechanical, thermal or chemically induced damage that is autonomically repaired by materials already contained within the structure. Efforts are undergoing to manufacture and incorporate multifunctional hollow fibres to generate healing and vascular networks within both composite laminates and sandwich structures. The release of repair agent from these embedded storage reservoirs mimics the bleeding mechanism in biological organisms. Once cured, the healing resin provides crack arrest and recovery of mechanical integrity. It is also possible to introduce UV fluorescent dye into the resin, which will illuminate any damage/healing events that the structure has undergone, thereby simplifying the inspection process for subsequent permanent repair.
Fibre Reinforced Dental Resins - Dr Ian Bond and Professor Daryll Jagger The material most commonly used in the construction of dentures is poly (methyl methacrylate) and although few would dispute that satisfactory aesthetics can be achieved with this material, in terms of mechanical properties it is still far from ideal. Over the years there have been various attempts to improve the mechanical properties of the resin including the search for an alternative material, such as nylon, the chemical modification of the resin through the incorporation of butadiene styrene as in the "high impact resins" and the incorporation of fibres such as carbon, glass and polyethylene. The use of self-healing technology within dental resins is a novel and exciting approach to solve the problems of the failing dental resins. Methods are currently being developed to translate the self healing resin technology into dental and biomaterials science.
Nanofibres and Nanocomposites - Dr Bo Su An electrospinning technique has been used to produce polymer, ceramic and nanocomposite nanofibres for wound addressing, tissue engineering and dental composites applications. The electrospun nanofibres have typical diameters of nm. Natural biopolymers, such as alginate, chitosan, gelatin and collagen nanofibres, have been investigated. Novel nanocomposites, such as Ag nanoparticles doped alginate nanofibres and alginate/chitosan core-shell nanofibres, have also been investigated for antimicrobials and tissue engineering scaffolds. Zirconia and silica nanofibre/epoxy composites are currently under investigation for dental fillings and aesthetic orthodontic archwires.
Nanocomposites - Dr H-X Peng Carbon fibre composite components are susceptible to sand and rain erosion as well as cutting by sharp objects. The use of nanomaterials in coating formulations can lead to wear- resistant nanocomposite coatings. Work is developing novel fine-particle filled polymer coating systems with a potential step-change in erosion resistance and exploring their application to composite propellers and blades. These tailored materials also have potential applications in lightning strike protection and de-icing. The nano-structure of magnetic micro-ribbons/wires is being investigated and optimised to obtain the Giant Magneto-Impedance (GMI) effect for high sensitivity magnetic sensor applications.
Composites with Magnetic Function - Dr Ian Bond, Prof. Phil Mellor and Dr H-X Peng The main aim of this work is to examine methods ofincluding magnetic materials within a composite whilst maintaining structural performance. This has been achieved by filling hollow fibres with a suspension of magnetic materials after manufacture of the composite component. Research is continuing to tailor the magnetic properties of the composite to other applications. In another approach, magnetic microribbons and microwires are being tailored and embedded into macrocomposite materials to provide magnetic sensing functions.
Auxetics - Dr Fabrizio Scarpa Auxetic solids expand in all directions when pulled in only one, therefore exhibiting a negative Poissons ratio. New concepts are being develope for composite materials, foams and elastomers with auxetic characteristics for aerospace, maritime and ergonomics applications. The use of smart material technologies and negative Poissons ratio solids has also led to the development of smart auxetics for active sound management, vibroacoustics and structural health monitoring.
Diamond Fibre Composites - Dr Paul May and Professor Mike Ashfold Researchers in the CVD Diamond Film Lab based in the School of Chemistry are investigating ways to make diamond fibre reinforced composites. The diamond fibres are made by coating thin (100 mm diameter) tungsten wires with a uniform coating of polycrystalline diamond using hot filament chemical vapour deposition. The diamond-coated wires are extremely stiff and rigid, and can be embedded into a matrix material (such as a metal or plastic) to make a stiff but lightweight composite material with anisotropic properties. Such materials may have applications in the aerospace industry.
Novel Multifunctional Fibre Composites - Professor Steve Mann New types of composites with a combination of strength, toughness and functionality are being prepared by combining research in the synthesis of inorganic non-particles with that in the synthesis of organic polymers. This interdisciplinary approach has been used to produce flexible fibres of magnetic spider silk as shown in the photograph (left). Silk fibres are coated by a dipping procedure using dilute suspensions of inorganic nano-particles that are prepared with specific surface properties. Similar methods are being investigated with swellable polymer gels and bacterial supercellular fibres to produce novel hybrid composites.
COMPILED & PRESENTED BY, Dr. Amol A. KhapareDr. Amol A. Khapare