All you need to know about an implant to abutment to crown connection
A root form dental implant is surgically placed into the site to serve as the root component of a missing tooth. An abutment is a component that is attached to the implant body and serves as a base for the crown, or other supra-gingival structure. The connection between the implant and the abutment is a crucial aspect of the dental implant system, as it must provide a secure and stable foundation for the crown at the spot where the free gingival margin touches the restoration. The abutment is often overlooked middle piece. There are two connections on the abutment. The connection to the implant fixture and connection to the visible part of the complex, the crown. The Vast majority of the implants systems we use today secure the abutment into implant body with a prosthetic screw. There is a rare connection where the abutment has a friction fit into the implant body and is tapped in. Tapped connection does not have anti-rotational feature and is extremely difficult to retrieve. should any problem with supra-structure occur.
The science behind securing an implant
Historically, the problem of securing the crown to the abutment has been the toxicity of the cement, movement of the crown on the supporting piece also known as a micro-movement. Solubility and temperature of the chewed food adds a degree of complexion to constantly changing environments of the mouth. In vitro research shows these cements have different degree of toxicity to the hard and soft tissue of the jaw. This however is extremely hard if not impossible to translate into such a complex environment as oral cavity is
Screw retained crown
The choice of connection depends on the placement of the implant body in the alveolar bone and the long axis of the prosthetic screw. Ideally, such should have an access always on lingual/palatal/occlusal surfaces. If this is the case, then the crown is always bonded/cemented to the abutment in the laboratory. The technician ideally cleans the connection of the crown to the abutment in such manner, that there is no cement present to the outside of the crown. Such abutment / crown complex has an access hole for the prosthetic screw, which then secures the abutment/crown complex to the body of the implant. The access channel is on lingual/palatal/ surfaces of the implant restoration, to be esthetically acceptable for the patient. 
Cemented Retained Crown
When the crown is secured to the abutment by dentist, we refer to it as Cement Retained crown. Currently the preferred connection of a crown/abutment complex to the implant body is via prosthetic screw, which holds the abutment tightly to anti-rotational component of the implant fixture. However, when the screw access hole would have to be in the esthetic portion of the crown (usually anterior crown), laboratory technician creates screw access channel through the abutment only and uses the crown to cover the entire supra-gingival part of the abutment material. We treat such abutment like a tooth prepped for a crown and ready to be “permanently cemented”. Since there has been so much written about toxicity of the material used to secure the crown, we prefer the connection of the abutment margin to crown margin to be at or slightly above the free gingival margin, so that the line of connection – cement line does not touch the gum tissue. But let’s take this issue a little deeper. Since we use these material to secure crowns on permanent teeth, would it make sense to expect the same issue around permanent crowns? And when we do not see such changes, is it fair to blame the cement or composite material for potential failure of the implant? I would argue that the toxicity of the material is to blame to an extent. Bigger issue is that the cement has a chance to flow towards the implant body, which is usually placed at the bone crest, about 3-5mm below the free gingival margin. The hemi-desmosomal attachment of the soft tissue to the polished titanium does not prevent the cement or composite from flowing deeper within the peri-abutment sulcus. When such is allowed and creates rough surface on the abutment, the constant friction of the gingival margin causes inflammatory and foreign body reaction, which suffer from secondary bacterial inflammation. T
his is not material, but an operator error. When we see on radiograph a cement in vicinity of an implant body it is safe to assume, that correct procedure for cementation has not been followed. Dentist must create a barrier which serves as a flow stopper to the cement material.
But what about screw-retained crowns and marginal bone loss?
The Expert Panel of Academy of Osseointegration recommends in the posterior region supra-gingival connection. The reason why we have connection to the implant is to properly orient crown of the implant within the oral cavity. if teeth were all cylindrical and straight, we would not be talking about a connection. Unfortunately they tend to have many different shapes, which require precise orientation.
The issue arises when the connection fails. From fractured screw or non-axial cyclic loading of the abutment, where the forces translate to the screw which rubs on the screw channel and threads within the body of the implant. This results in titanium debris which is removed by macrophages of the gingival environment. Unfortunately, this action starts an inflammatory process known as peri-implant mucositis, which if left unattended may result in further destructive and proliferative changes which we now call a peri-implantitis.
Current research supports the mechanical, microbial origin theory, or both. Depending where the shavings traveling from the implant body leave the protective environment of the implant prosthetic connection, we see more of an inflammatory changes (peri implant mucositis) and or destructive changes of the surrounding tissue (peri-implantitis). But how do the shavings get there? Where are they coming from? Ninety five percent of implants placed in the United States use an internal hex connection. This connection assures, that you can only connect an abutment to implant body 6 different ways. There is also tri-lobe connection (3 way seating only is possible). The connection is stable against the long axis rotation, but teeth don’t meet that way all the time.