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Virtually all dental implants placed today are root-form endosseous implants, i.e., they appear similar to an actual tooth root (and thus possess a “root-form”) and are placedwithin the bone (endo- being the Greek prefix for “in” and osseous referring to “bone”). The bone of the jaw accepts and osseointegrates with the titanium post. Osseointegrationrefers to the fusion of the implant surface with the surrounding bone. Dental implants will fuse with bone, however they lack the periodontal ligament, so they will feel slightly different than natural teeth during chewing.
Prior to the advent of root-form endosseous implants, most implants were either blade endosseous implants, in that the shape of the metal piece placed within the bone resembled a flat blade, or subperiosteal implants, in which a framework was constructed to lie upon and was attached with screws to the exposed bone of the jaws.
Dental implants can be used to support a number of dental prostheses, including crowns, implant-supported bridges or dentures. They can also be used as anchorage for orthodontic tooth movement. The use of dental implants permits undirectional tooth movement without reciprocal action.
A typical implant consists of a titanium screw (resembling a tooth root) with a roughened or smooth surface. The majority of dental implants are made out of commercially pure titanium, which is available in 4 grades depending upon the amount of carbon and iron contained. More recently grade 5 titanium has increased in use. Grade 5 titanium, Titanium 6AL-4V, (signifying the Titanium alloy containing 6% Aluminium and 4% Vanadium alloy) is believed to offer similarosseointegration levels as commercially pure titanium. Ti- 6Al-4V alloy offers better tensile strength and fracture resistance. Today most implants are still made out of commercially pure titanium (grades 1 to 4) but some implant systems are fabricated out of the Ti-6Al-4V alloy. Implant surfaces may be modified by plasma spraying, anodizing, etching, or sandblasting to increase the surface area and osseointegration potential of the implant.
Prior to commencement of surgery, careful and detailed planning is required to identify vital structures such as the inferior alveolar nerve or the sinus, as well as the shape and dimensions of the bone to properly orient the implants for the most predictable outcome. Two-dimensional radiographs, such as orthopantomographs or periapicals are often taken prior to the surgery. Sometimes, a CT scan will also be obtained. Specialized 3D CAD/CAM computer programs may be used to plan the case.
Whether CT-guided or manual, a ‘stent’ may sometimes be used to facilitate the placement of implants. A surgical stent is an acrylic wafer that fits over either the teeth, the bone surface or the mucosa (when all the teeth are missing) with pre-drilled holes to show the position and angle of the implants to be placed. The surgical stent may be produced using stereolithography following computerized planning of a case from the CT scan. CT guided surgery may double the cost compared to more commonly accepted approaches.
In its most basic form, the placement of an implant requires a preparation into the bone using either hand osteotomes or precision drills with highly regulated speed to prevent burning or pressure necrosis of the bone. After a variable amount of time to allow the bone to grow on to the surface of the implant (osseointegration), a crown or crowns can be placed on the implant. Unlike conventional dental implants, Mini dental implants may be loaded immediately and still have a high survival rate (94%). The amount of time required to place an implant will vary depending on the experience of the practitioner, the quality and quantity of the bone and the difficulty of the individual situation.
At edentulous (without teeth) jaw sites, a pilot hole is bored into the recipient bone, taking care to avoid the vital structures (in particular the inferior alveolar nerve or IAN and the mental foramen within the mandible). Drilling into jawbone usually occurs in several separate steps. The pilot hole is expanded by using progressively wider drills (typically between three and seven successive drilling steps, depending on implant width and length). Care is taken not to damage the osteoblast or bone cells by overheating. A cooling saline or water spray keeps the temperature of the bone to below 47 degrees Celsius (approximately 117 degrees Fahrenheit). The implant screw can be self-tapping, and is screwed into place at a precise torque so as not to overload the surrounding bone (overloaded bone can die, a condition called osteonecrosis, which may lead to failure of the implant to fully integrate or bond with the jawbone). Typically in most implant systems, the osteotomy or drilled hole is about 1mm deeper than the implant being placed, due to the shape of the drill tip. Surgeons must take the added length into consideration when drilling in the vicinity of vital structures.
Traditionally, an incision is made over the crest of the site where the implant is to be placed. This is referred to as a ‘flap’. Some systems allow for ‘flapless’ surgery where a piece of mucosa is punched-out from over the implant site. Proponents of ‘flapless’ surgery believe that it decreases recovery time while its detractors believe it increases complication rates because the edge of bone cannot be visualized. Because of these visualization problems flapless surgery is often carried out using a surgical guide constructed following computerized 3D planning of a pre-operative CT scan.
The amount of time required for an implant to become osseointegrated is a hotly debated topic. Consequently the amount of time that practitioners allow the implant to heal before placing a restoration on it varies widely. In general, practitioners allow 2–6 months for healing but preliminary studies show that early loading of implant may not increase early or long term complications. If the implant is loaded too soon, it is possible that the implant may move which results in failure. For conventional implants, the subsequent time to heal, possibly graft, and eventually place a new implant may take up to eighteen months. For this reason many are reluctant to push the envelope for healing.
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One-stage, two-stage surgery
When an implant is placed either a ‘healing abutment’, which comes through the mucosa, is placed or a ‘cover screw’ which is flush with the surface of the dental implant is placed. When a cover screw is placed the mucosa covers the implant while it integrates then a second surgery is completed to place the healing abutment.
Two-stage surgery is sometimes chosen when a concurrent bone graft is placed or surgery on the mucosa may be required for esthetic reasons. The latter is usually important where an implant is placed in the “esthetic zone”. This allows more control over the healing and as a result the predictability of the final result. Some implants are one piece so that no healing abutment is required.
In carefully selected cases, patients can be implanted and restored in a single surgery, in a procedure labeled “Immediate Loading”. In such cases a provisional prosthetic tooth or crown is shaped to avoid the force of the bite transferring to the implant while it integrates with the bone.
There are different approaches to place dental implants after tooth extraction. The approaches are:
- Immediate post-extraction implant placement.
- Delayed immediate post-extraction implant placement (2 weeks to 3 months after extraction).
- Late implantation (3 months or more after tooth extraction).
According to the timing of loading of dental implants, the procedure of loading could be classified into:
- Immediate loading procedure.
- Early loading (1 week to 12 weeks).
- Delayed loading (over 3 months)
An increasingly common strategy to preserve bone and reduce treatment times includes the placement of a dental implant into a recent extraction site. In addition, immediate loading is becoming more common as success rates for this procedure are now acceptable. This can cut months off the treatment time and in some cases a prosthetic tooth can be attached to the implants at the same time as the surgery to place the dental implants. Because one of three implants requires a minimum addition of bone tissue, surgical techniques for underlying bone augmentation are currently under a large scale development.
Most data suggests that when placed into single rooted tooth sites with healthy bone and mucosa around them, the success rates are comparable to that of delayed procedures with no additional complications.
Teeth in a Day
“Teeth in a day”, “All-on-four”, “Fast and fixed” are similar surgical concepts whereby implants are placed on the same day and a fixed prosthesis is attached to them. This allows the patient to leave with a fixed solution as opposed having to make do with a removal temporary prosthesis whilst the implant osseointegrate with the bone. This concept is appropriate for completely edentulous jaws where either the teeth are to be extracted or have already been removed.