Dental implants as a promising tooth replacement therapy:
Dental implant therapy has provided us with one of the most promising tooth replacement procedures. Over the last few decades there has been an increasing use of endosseous (in-bone) implants as a means of providing a foundation for intra-oral prosthetic devices 1, from full arch dentures to single crowns, or other devices for orthodontic anchorage 2 or distraction osteogenesis 3, 4.
Although dental implants are known for a long time, but most of the research and developments in this field have occurred in last few decades. In the following articles we shall read about various aspects of implant therapy, but before moving forward we should remember that every effort should be made to save natural tooth because there is nothing like your own body part. In situations where tooth can’t be saved because it has a bad prognosis or cases where teeth have already been extracted we consider implant therapy as one of our options for treatment.
By definition implant dentistry is the area of dentistry concerned with the diagnosis, design, and insertion of implant devices and implant restorations that provide adequate function, comfort, and aesthetics for the edentulous or partially edentulous patient 5.
Before we actually start placing implants in patient, it is important to know the basics of implant therapy. We must be aware of the biological aspect of implants, the clinical aspect of the implants and most importantly the complications that may occur during or after the implant therapy and how to handle them. Because the patient is paying good money for this treatment, he/she expects esthetically and functionally good dentition. Failing in doing that may be problematic. So, the first thing is, tell the patient exactly what you can achieve practically after the treatment. Patient should not have misconceptions in his/her mind.
Many implant companies project implants like a simple procedure where it has to be just placed into the bone and in few months it integrates with the bone. It is not exactly true. In my opinion the most important step in implant therapy is the diagnosis and treatment planning. Proper knowledge of the bone response, osseointegration, implant surface properties, healing around implants and mechanical forces on implants during function are essential before we start doing implant therapy. It is important to note that any implanted implant/biomaterials often induce the formation of a poorly vascularized collagenous capsule that can eventually lead to implant failure. Our aim during implant placement is to achieve an environment where only bone growth takes place with minimal fibrous growth.
In my opinion, surgical therapy is comparatively a smaller part of implant therapy. Major pat is the prosthetic rehabilitation and occlusal harmonization; especially in case of full mouth rehabilitation cases were a stable occlusion is mandatory to achieve a long term stability of the dentition.
Rationale for Dental Implants:
Loss of teeth due to any reason cause compromised chewing ability, speech, facial appearance, and self-confidence. Due to tooth loss, atrophy of the surrounding bone takes place due to lack of use. Dental implants maintain the bone levels by applying direct forces on the bone 6. This is in comparison to removable partial and complete dentures which have opposite effects on bone levels. It has been shown that poorly fitting partial and complete dentures lead to bone resorption as only 10% of the chewing efficiency is achieved by these, causing gradual bone loss 7.
Another important factor is sunken face due to tooth loss. Due to loss of teeth, the muscles of mastication undergo atrophy because of lack of use giving the patient a sunken face appearance. Dental implants allow the patient to use their muscles of mastication properly thereby preventing atrophy of these muscles.
Dental implants are the closest replacement of natural teeth. The patient self confidence is drastically improved when he/she knows that his/her teeth are fixed, functionally and esthetically good. With implants patients are able to smile with confidence, without worry of denture displacement or showing gaps when smiling.
Most importantly, implant becomes a part of body after osseointegration. The prosthesis supported by the implant may get deteriorated over time demanding replacement but a properly osseointegrated implant may last lifelong.
Classification of dental implants:
Based on implant design:
- Endosteal (Ramus fram/Root form/Plate form)
Based on attachment mechanism of the implant
Based on macroscopic body design
- Hollow dental implants
- Cylindrical dental implants
- Vented dental implants
- Threaded dental implants
Based on the surface of the implant
- Machined surface implants
- Textured surface implant
- Coated surface implant
Based on the type of the material
The detailed description of above classification is given in their relevant topics.
It is required that a material for dental implants is biocompatible, it must not be toxic and it may not cause allergic reactions 8. It must have high ultimate strength (Rm) and yield value (Rp) at low density and low modulus of elasticity (E) 9. Although many metal alloys such as alloys of stainless steels, cobalt alloys, titanium (coarse-grained) and titanium alloys have been investigated for use as dental implants, titanium and its alloys are mostly commonly used for this purpose 10. So, why do titanium and its alloys show such good biocompatibility compared with other alloys? Let us try to find out the answer for this question.
Pure Titanium is a highly reactive metal and will react within microseconds to form an oxide layer when exposed to the atmosphere 11. This oxide, which is primarily TiO2, forms readily because it has one of the highest heats of reaction known (ΔH = −915 kJ/mole) (for 298.16°–2,000°K) 12. Formation of its oxide layer makes titanium is one of the most passive metals. Although the standard electrode potential was reported in a range from −1.2 to −2.0 volts for the Ti→Ti3+ electrode reaction 13, due to strong chemical affinity to oxygen, it easily produces a compact oxide film, ensuring high corrosion resistance of the metal. The passive film of TiO2 that forms on titanium is stable, even in biological systems including chemical and mechanical environments 14.
The easiest method for checking whether the oxide layer formed is protective or not is calculating Pilling –Bedworth (P-B) ratio 15. In metals where P-B ratio is less than 1, since oxide occupies small volume than the metal, so that formed oxide is porous and non-protective. But, if it is larger than 2, since oxide occupies a large volume and may flake from the surface, exposing fresh substrate surface and again exhibits non-protectiveness. So, in metals where P-B ratio is between 1 to 2, volume of oxide is similar to that of metal, so that the formed oxide is adherent to substrate, nonporous, and protective. For titanium P-B ratio is 1.76 indicating that TiO2 layer formed on titanium surface is highly protective. The “natural” oxide film on titanium ranges in thickness from 2 to 7 nm, depending on such parameters as the composition of the metal and surrounding medium, the maximum temperature reached during the working of the metal, the surface finish, etc.
It must also be noted that when implant is placed in bone, the oxide layer formed on titanium implants grows and takes up minerals (P, Ca, and S) from the surrounding environment 16, 17. Factors that affect the biological acceptability and integration of implant are dependent on properties of the oxide layer, such as stoichiometry, defect density, crystal structure and orientation, surface defects, and impurities 17, 18.
Titanium oxide has a low solubility and an isoelectric point between 3.5 and 6.7 19. This makes the surface only slightly negatively charged at physiological pH, which is thought to reduce the ability for capsule formation after insertion into bone and is also thought to lead to favorable reactions with biomolecules.
Importance of implant surface properties:
Because implant surface is in a physical and chemical contact with the surrounding environment, it plays a crucial role in biological interactions for four reasons 20-24:
- It is the only part contacting with the bio-environment.
- The morphology and composition of surface region of an implant biomaterial is almost always different rest of the implant bulk.
- For biomaterials that do not release or leak biologically active or toxic substance, the characteristics of the surface governs the biological response (foreign material vs. host tissue).
- Surface properties of implants such as topography affect the mechanical stability of the implant-tissue interface.
How the implant surface is modified and how it improves the biological and mechanical properties of dental implants has been discussed in detail in “Dental implants: Surface modifications”.
Terminologies used in implant dentistry:
Before we go ahead with detailed implantology, we must know the terms that are used in the subject. Following is the description of frequently used terminologies in implantology,
Material inserted or grafted into tissue.
A permucosal device that is biocompatible and biofunctional and is placed on or within the bone associated with the oral cavity to provide support for fixed or removable prostheses.
Metal (chemical symbol Ti) having low density, good strength, and corrosive resistant properties.
A hole in the alveolar bone made by an implant drill for the placement of a dental implant.
Implant fixture or body is the portion of the implant that is designed to be placed into the bone.
It is that part of the implant that lies between the implant and the crown. Abutment screws into the implant fixture.
The implant analogue is used by the lab technician during fabrication of the prosthesis. The abutment is attached to the analogue and impression is poured. Now, analogue replicates the implant in the cast over which the prosthesis is fabricated.
The screw that clamps the abutment onto the implant.
A restoration that is cemented onto the abutment.
Screw retained prosthesis:
A prosthesis that is attached to the implants with the help of screws and is not cemented.
The final intra-implant screw placed after first-stage surgery.
A temporary abutment that is placed after the implant has been inserted and removed before the permanent restoration is placed.
Implants that are exposed to the oral cavity after a single surgical procedure.
Implants that are submerged below gingiva immediately after placement. A second surgery is necessary to uncover them.
Exposure of the dental implants which is carried out approximately 4 months after the initial implant placement.
Implant-retained fixed overdenture:
It is a non-removable prosthesis that attaches to 4, 6 or 8 implants.
Implant-stabilized removable ball overdenture:
Here, a removable prosthesis containing ball attachments to hold, or stabilize the denture in place.
Implant-stabilized removable bar overdenture:
In this case a removable prosthesis containing a bar attachment to hold, or stabilize the denture in place.
Machined implant consisting of a lower plate fixated under the mandible with pins projecting through the mandible which penetrate into the oral cavity.
An endosteal implant consisting of an abutment, cervix, and body (or infrastructure) that is thin buccolingually and has fenestrations to permit the in growth of bone/connective tissue for purposes of anchorage.
An endosteal design consisting of a thin, plate-like component placed into a horizontal osteotomy and attached to a post-like vertical component that protrudes permucosally.
A round endosteal implant, either threaded or press-fit.
Root form endosteal dental implant:
Root-shaped implants that derive their support from a vertical expanse of bone; implants are in the form of spirals, cones, rhomboids, and cylinders; they can be smooth, fluted, finned, threaded, perforated, solid, hollow, or vented; they can be coated or textured and are available in submergible and non-submergible forms in a variety of biocompatible materials.
Endodontic endosteal implant:
A smooth or threaded pin implant that extends through the root into periapical bone to stabilize a mobile tooth.
A connector of multiple prosthetic elements designed to lend strength as well as to retain an overdenture or superstructure; special clips are available that grasp these bars.
A splitting open or a break in the covering epithelium, leaving an isolated area of an implant or bone exposed to the oral cavity.
Local conditions that produce an acidic environment at the metallic interface, causing the metallic oxide to break down.
Layers of materials designed to cover implant substrates for purposes of making them more biocompatible.
An invasive surgical technique using specialized instruments which permits bones to become lengthened to as much as 500 μm per day.
A 6-sided screwdriver; a matching screw.
A fixed bar that cormects 2 or more permucosal extensions; in the case of the ramus frame or subperiosteal implant, it can be an integral part of the substructure.
Calcium phosphate complex that is a primary mineral component of bone; sprayed-on coating used on early dental implants.
Contact established without interposition of fibrous tissue between normal remodelled bone and an implant at the light microscopic level, entailing a sustained transfer and distribution of load from the implant to and within the bone tissue.
Plasma spray-coated with titanium:
A technique of reliably coating the surface of a titanium implant with small, irregular particles of the same metal.
An inorganic, particulate, or solid form of relatively biodegradable ceramic that is used as a scaffold for bone regeneration; it can act as a matrix for new bone growth.
A custom-made abutment designed to be placed on single implants that lack antirotational elements.
These are long screws used for attachment of abutment to the implant during open impression technique.
A surgical instrument used for cutting out circular sections. Used to retrieve implant/fractured implant portion.
Trademark name for a dental alloy comprised of cobalt, chromium and molybdenum.
Formation of air cells or cavities in tissue; in this case, when the sinus spaces are increased due to bone resorption.
The equivalent to periodontitis manifesting around dental implants; bone loss around implants due to bacterial pathogens.
The knowledge about above terminologies is important before we go ahead with the discussion on different aspects of implant dentistry. With this information readers are advised to go through different topics on dental implants which include, “Historical aspect of dental implants”, “Dental implant components and Current concepts in implant design”, “Dental implants: Surface modifications”, “Dental implants: Biological aspect” and “Dental implants: Clinical aspect”.
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