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Multi-Unit Abutments (Dentistry)
Multi-unit abutments (MUA) are used for screw fixation of bridges made of zirconium or metal-ceramic group restorations to the implant. In fact, this is a double screw fixation of the suprastructure to the implant. The first stage, the multi-unit abutment, is screwed to the dental implant and is not removed until the end of the operation of the prosthesis. The second stage is the screw fixation of the prosthesis to the already fixed multi-unit abutment. For fixation with the abutment, special conical sleeves are laid and glued into the prosthesis, which are put on the abutment with the wide side and the screw shaft for fixation passes through the narrow end.

Terminology

 * Abutment (dentistry) -
 * a supporting element fixed in the implant, a prosthesis is directly attached to the abutment: a crown, multiple restoration or a splinted restoration of a complete dentition.


 * A dental implant -
 * is a part of the restoration of the dentition implanted into the bone, most often it is a root implant. The most popular type of implant in the world is a titanium conical or cylindrical implant with threads on the outside and a porous surface for better osseointegration.


 * Osseointegration -
 * biological fusion of bone tissue with non-biological elements, in the context of this article to medical titanium


 * Healing abutment gum former (gingival cuff former) -
 * is a special titanium plug that is installed on the implant. The gingiva former is always placed below the line of occlusion so that it does not participate in the bite. The main purpose of the healing cap is for the soft tissue to form a gingival cuff around the implant so that the future restoration sits in the soft tissue socket just like a natural tooth.


 * Suprastructure -
 * is the collective name of all elements that are attached to the implant at different stages. These are all types of abutments, plugs, gum formers, etc.


 * Torque -
 * the force in Newtons per centimeter with which the thread is screwed when installing the suprastructure or the implant itself into the bone tissue. A torque wrench allows you to precisely apply the force.

History of multi-unit abutments
All modern varieties have a common source. The first system for fixing prostheses using multi-units (abutment, sleeve and screws) was introduced by Nobel Biocare in 2001. This solution turned out to be so thoughtful and convenient that it quickly became the industry standard and has been used for more than 20 years.

Varieties of multi-unit abutments
Based on the development of Nobel Biocare, many variants (MUAs) have been created. Some almost completely copy this development; others have a number of improvements.

Let's take a closer look at a few common options:


 * Multi-unit V-type abutments - with a low profile of 2 mm, suitable for cases where it is necessary to ensure a minimum height of the abutment above the implant. Also, the V-type abutment is better suited for the aesthetic zone of the anterior teeth, the height of the cone in this modification is noticeably lower, which means that titanium elements will not be visible at the border of crowns and soft tissues. There are only straight V-type MUAs.
 * D-type multi-unit abutments - the classic version with a large 4.5 mm taper and high strength is better suited for installation in the area of multi-rooted prosthetics. There are different heights of placement of the platform according to the level of the future gums of the patient, from 1 to 4 mm.
 * Angled multi-unit D-type is designed for cases when the implant is placed at an angle to the plane of the dentition. Angular MUA D-type allows you to compensate for deviation and install prostheses in the correct position. The most common options for angulated abutments are 17° and 30°, but other options are possible. Angled multi-unit abutments are also available in different gingival heights.

Varieties of sleeves for attaching the prosthesi
As illustrated from left to right sleeves for MUA D-type:


 * Long sleeve made of titanium for high MUA prostheses, cut to the desired height if necessary. Compatible with CAM/CAD technology.
 * Standard sleeve for low MUA prostheses. Compatible with CAM/CAD technology.
 * The long sleeve for temporary structures in the manufacture of the prosthesis by analog methods is also cut to the desired height.
 * Sleeve for the manufacture of molded individual designs, the sleeve itself is made of ashless plastic.

As illustrated from left to right sleeves for MUA V-type:


 * Ultra-compact sleeve for low crowns - 1.5 mm.
 * Low sleeve for low crowns - 3 mm.
 * Standard sleeve - 4.5 mm.
 * Long sleeve for massive restorations - 6 mm.
 * Long sleeve for temporary structures.
 * Sleeve for casting individual designs made of ashless plastic.

Connecting interface on implant/abutment line
All implant/abutment connection interfaces are divided into external and internal. Internal connections are becoming more widespread due to better sealing of the connection and the creation of a so-called antimicrobial barrier. Of the internal connections, the most common are the 1.5 mm deep hex socket and the 22° Morse taper interface.

Using Healing Abutments with MUA
Unlike cemented abutments, the healing abutment is placed on an already installed multi-unit abutment. As mentioned earlier, MUAs are installed once and can only be removed or replaced if the prosthesis breaks down or is scheduled to be replaced after a few years.

The order of insertion of the shapers may differ depending on the implantation protocol. The protocol was the first to appear where, after the installation of the implant, a plug is installed in it. After that, soft tissues are sutured over the installed implant. After sufficient time for osseointegration, the soft tissues are dissected, the plug is removed and the multi-unit abutment is placed. A gum shaper is installed on the abutment, and soft tissues are sutured in such a way that a full-fledged gingival cuff is formed around the abutment. After complete healing of the soft tissues, the gum shaper is removed, and a temporary or permanent prosthesis with screw fixation is installed in its place. The appearance of the gingiva formers for multi-unit D-type and V-type abutments is shown in the picture below.

Accessories for CAD/CAM technologies
CAD / CAM - technology allows you to speed up the process of manufacturing the prosthesis and do it with an accuracy unattainable using classical technology. The process of digital prosthesis manufacturing consists of the following steps:


 * Taking a digital image of the patient's jaws with the exact location of teeth, soft tissues and implants, including the angle of their deviation. This procedure is carried out using an intraoral scanner that takes a series of high-speed images. Based on the images obtained, special software creates a digital 3D file that replaces the classic cast and plaster model of the jaw.
 * Development, with the help of the same software complex, of the image of the future prosthesis.
 * Production on a CNC machine of a permanent or temporary prosthesis. The restoration is made with the highest precision and practically does not require adjustment and refinement.



Digital dentistry allows you to speed up the process and dramatically reduce discomfort for the patient. The screw retention system on multi-unit abutments was originally designed to be easily integrated into a CAD/CAM system. Therefore, there are special caps for scanning markers for taking a digital impression. Scan markers are made of PEEK plastic, and their surface is contrasting and well displayed on the pictures taken by the intraoral scanner. Scan markers are fixed with screws to the multi-unit abutment and thus clearly indicate the location and angle of deviation of the implants relative to the remaining teeth. There are scan markers for MUA D-type and V-type. Scan markers are reusable products and can withstand at least 50 uses.

Impression accessories (analog technology)
MUA accessories have also been developed for taking classical impressions and then making a plaster model of the jaw.


 * Transfers for taking an impression with a closed and open tray;
 * Implant analogs, and there are analogs of an implant with an open interface where any type of abutment can be installed, as well as analogs of an implant with a head that imitates the conical interface of a multi-unit abutment.

Advantages of multi-unit abutments

 * It is easier for a dentist to work with screw-retained prostheses. Especially if the prosthesis is made entirely using CAD / CAM technology and does not need to be adjusted. The prosthesis is fixed onto the screws, after which the screw shafts are sealed with a composite composition. If it is necessary to remove the prosthesis, the composite is removed; and access to the screws is opened. The prosthesis is removed quickly, without significant effort and deformation. Whereas in the case of cemented fixation of the prosthesis to the abutment, it would be necessary to physically destroy the bridge structure or crown.
 * Contact of cement residues with soft tissues of the gums is excluded, because all operations with cement are performed outside the patient's oral cavity. It is not an easy task to qualitatively remove the remnants of cement in the oral cavity. With a high probability, small particles of adhesive remain on the line between the crown and the abutment. Direct contact of the cement with soft tissues leads to chronic inflammation of the gums, which can even lead to the loss of the implant.
 * Resorption of bone tissue around the implant with MUA is less. The multi-unit abutment is fixed once and for all with significant torque. Only the screw-retained prosthesis is removed. In the case of cemented abutments, they are removed and installed several times, and each time it causes an injury to the gingival cuff. A new cycle of healing causes the body to dissolve some of the bone tissue around the implant.
 * More uniform load distribution from dentures to the implant, especially the side load. When screw-retained on multi-unit abutments, the load is distributed over a greater depth of the implant. However, when cemented, there is a significant overload in the upper part of the implant. This is especially true for thin implants with a diameter of ≤ 4 mm, where a rather thin wall of the implant body of 0.4-0.5 mm remains in the upper part.
 * No or almost no micro motion at the implant/abutment level. There is always a gap between the implant and the abutment, and the smaller it is, the better the antimicrobial barrier. Achieving minimal clearance and immobility with screw fixing is much easier because the multi-unit is installed once and tightened with significant torque.
 * It is possible to obtain good results and a passive fit even in very difficult cases, when there is a significant bone deficiency and the implants have to be placed at significant angles relative to the line of the dentition. With straight and angled multi-unit abutments, even very complex restorations can be easily achieved with a passive fit.
 * With the help of MUA, it is possible to obtain high-quality fixation at a very small crown height, this is especially true for MUA V-type. There are cases in which the crown should rise only 4-5 mm above the level of the implant. If cemented, the abutment would have sunk no more than 3 mm into the crown. Which is very small and after six months, a maximum of a year, the cement would crumble. In the case of screw fixation, even a small crown height and a short sleeve give a good and durable result.

Disadvantages of multi-unit abutments

 * The multi-unit system is not suitable for single restorations. The conical interface between the abutment head and the sleeve does not have anti-rotation elements and will inevitably rotate during operation. For single restorations, there are other screw-retained prosthesis systems where there are also no operations with cement in the patient's oral cavity. For example, the screw-retained system on T-base abutments.
 * High price. The multi-unit abutment is several times more expensive than the similar one for cement fixation. Against the background of the total cost of treatment, starting with the operation to install the implant and ending with the manufacture and installation of the prosthesis, the difference in price will be noticeable, but not fundamental.
 * In terms of patient comfort and soft tissue alignment accuracy, the MUA is still inferior to the individually cast abutment.

Materials
For the manufacture of implants and abutments of all types, titanium and its alloys are used. Here are the most common options:


 * Grade 4 - commercially pure titanium, or conditionally pure titanium, where the amount of impurities is ≤ 1%, the tensile strength is 550 MPa. This strength is sufficient for the manufacture of sufficiently large parts, but not enough for screws and other parts with a thickness of less than 1.5 mm.
 * Grade 5 is an alloy of titanium with vanadium (4%) and aluminum (6%). Tensile strength 895 MPa. It is much stronger; therefore, some manufacturers combine this material with Grade 4. For example, the implant body and abutment from Grade 4, and the fixing screw from Grade 5. Grade 5 alloy did not replace commercially pure titanium because osseointegration with it is somewhat slower. In the end, the quality of integration is not worse; it just needs a few more weeks.
 * Grade 23 is also an alloy of titanium with vanadium (4%) and aluminum (6%), but with a reduced content of oxygen and iron in impurities. The tensile strength is 840 MPa, but this alloy has a much greater capacity for elastic deformation (Young's modulus). This means that where a part from Grade 5 or Grade 4 breaks, the same part from Grade 23 will deform, and when the load is removed, it will return to its original state without damage. This is one of the most promising materials for the manufacture of dental implants and accessories for them.
 * Roxolid® is an alloy of titanium 85% with zirconium 15% the highest tensile strength among the described materials - 1000 MPa. This is a promising material, but has not yet received wide distribution due to the high price. So far, only one company is promoting Roxolid® alloy implant systems.