Dr. Peter Sanders explores the pros and cons of screw-, cement-, and friction-retained restorations and explains the appropriate methods of all three
When placing implants, choosing the right method of component retention can be a complicated decision. Whether you choose screw-, friction-, or cement-retention, each method has great strengths and weaknesses; therefore, your selection should be made in accordance with each individual clinical situation.
Retrievability, stress, esthetics, and accessibility are just a few of the elements that enter the equation of suitability for optimal implant placement, as the clinical success of the restoration can be significantly influenced by your choice.
Strengths and weaknesses
Over the years, the formula for the ideal method of restoration has divided opinion among dental implantologists, many of whom have a strong preference for a single particular method. These preferences have been known to change depending on current trends, what we read and, of course, the failures we may have experienced in the past.
The truth is, screw-, cement-, and friction-retained implants have many both pros and cons, and while some may be considered to be more advantageous, certain methods may be more appropriate than others in specific circumstances.
For example, while cement retention is often considered to be the most successful method by reducing stress in the restoration and bone, there are still certain disadvantages to this technique. Cement retention, for example, can cause issues that occur with excess cement application leading to bone loss and peri-implant disease — particularly if standard components place the restorative margin deeper than would be ideal. Debonding — which can lead to the repeated loosening of the prosthesis and patient dissatisfaction — using a permanent cement may reduce debonding episodes but makes recovery more difficult or impossible.
Screw-retained implants also present disadvantages in certain cases; achieving a passive fit is significantly more difficult to attain with a screw-retained restoration due to stress being introduced into the restoration through the tightening of screws. If, however, the prosthesis is cemented, a thin layer of the cement will offset a level of the stress offering compensation for any minor discrepancies in the fit, which would ordinarily contribute to lack of passivity. So, cement-retained restorations are more likely to reduce stress and therefore the likelihood of stress related implant failure or peri-implantitis.
Nonetheless, if retrievability is a factor that must be considered, then the likelihood of damaging the crown or bridge during removal is extremely high; a soft or temporary cement can be used if removal is necessary, but if frequent access is required, it is not the best method of retention.
Friction-retained implant systems (where the abutment and/or prosthesis are retained by the force of friction between components) can provide a high level of stability and allow for a tension-free fit, without cement — this method has many advantages when it comes to ease of handling, use, and retrieval.
However, friction retention can also have disadvantages in certain circumstances. If the friction-retained prosthesis is designed to be removable by the patient, then the age or the dexterity of the patient should always be considered. If (for this or any other reason) the patient has ongoing issues with removing the retained prosthesis for hygiene reasons, it then becomes a de facto fixed restoration and will, therefore, need to be redesigned accordingly to facilitate oral hygiene procedures.
Many clinicians consider screw-retained implants to be the most effective solution if easy removal or access is a requirement. Retrievability is an important factor if the patient requires assistance with hygiene, or if the crown is fractured and needs to be repaired. However, while screw-retained implants can be easily accessed and removed, there is a risk that stress-related tension on the superstructure may be caused when re-tightening the screws in the process of replacement.
In terms of retrievability in cement-retained implants, one of the most commonly cited disadvantages is the difficulty in removal of the prosthesis. If the patient requires a great deal of hygienic maintenance, or has a personal preference to be able to easily remove the superstructure to clean the prosthesis, friction-retained implants allow for this kind of access. Using a milled bar with a milled superstructure and by using tight angles (of around 4 degrees), the superstructure will gain retention through the friction applied to the bar. This will allow for the patient to easily access and remove the structure to aid the cleaning process.
One of the biggest advantages of the friction-retained implant prosthesis is that, if the patient has maxillary atrophy and therefore requires volume in the super-structure to deliver soft tissue or lip support, this tissue can be built, providing a more secure placement. If the superstructure is fixed, excess tissue support would normally be avoided due to the fact that the patient would not be able to efficiently clean it. With friction-retained implants, the gums can be overlapped and cleaned with ease.
Esthetics: screw versus cement
While the screw system may have advanced over many years, it is still at a disadvantage when it comes to esthetic results. With an implant angled to optimize bone availability and a system that allows only screw retention in line with the implant, the access hole in the prosthesis needs to be filled, and therefore, the esthetics of the crown are compromised from the outset. In addition, the hole filled in the prosthesis may also deteriorate over time.
Clearly, this is more of an issue in the esthetic zone at the front of the mouth where the flared maxilla inevitably results in the extension of the implant axis emerging through the facial surface of the crown. To overcome this, an angled abutment is screwed into the implant and the crown, and then cemented. The cementation of the implant restoration eliminates the unsightly screw holes, resulting in a much more esthetically pleasing result.
Often, in order to achieve optimum results, combining methods of retention can be most effective. For example, systems such as ANKYLOS® SynCone® (Dentsply Implants) combine the use of screws and friction to provide a tension-free retention with optimum hygiene capability. With such systems, 4-, 5- or 6-degree tapers are used, and here, gold caps are locked on to the abutments. These components are connected in the mouth to ensure passivity, but removal of the prosthesis for daily cleaning can be a challenge for less dexterous elderly patients.
Intraoral welding is also a technique that uses a combination of friction- or screw-retention methods. Here, the abutments are connected to the implants and matching screw- or friction-retained components are tapped on to these. A titanium bar is then welded to these welding components while in the patient’s mouth. This means the fit is completely accurate from the outset, and a more passive, stress-free structure is created.
With intraoral welding, friction components, which are inherently less bulky and easier to fit in the prosthetic envelope, are used around the front area of the mouth. The emergence of unsightly screw holes is also avoided. Farther back in the mouth, where more space is available, screw retention is used, giving the superstructure excellent stability.
Such combinations of retention are ideal if a patient has a fixed restoration but wants the benefits of having smaller volume components that provide friction retention toward the front of the mouth in the esthetic zone. Screws can then be used to lock the superstructure down in an area where there is space, and therefore, the esthetics of the prosthesis are not compromised.
Finding the right solution
As an implant dentist, I am frequently asked which is the best method of post retention. My answer is always the same: No single method is superior to another. A well-experienced and adept implant dentist will understand the features, benefits, and risks of each of the methods of retention, and will apply the most suitable method (or combination of methods) to meet each patient’s specific requirements.
For example, there may be functional and occlusal considerations; lip support and bone volume may need to be considered, as well as the space available for each patient. You may need to consider where the bone is best in relation to the prosthetic envelope, what angles you have to compensate for, and what the patient’s capacities are for hygiene and maintenance.
These all factor into whether screw-, cement-, or friction-retained implants should be selected for optimum placement. Having a broad knowledge and understanding of all three methods will enable you to provide a treatment plan that specifically suits your patient, ensuring successful long-term results are achieved.