As a treatment option, implants are very predictable, both in terms of osseointegration and as functional and esthetic dental restorations. They tend to have very high rates of success, and are suitable for most patients. Their routine use in dentistry over the last 20-30 years has established implants as a mainstream modality. Most importantly, patients are now generally aware of this restorative opportunity, and are increasingly choosing implant-borne restorations rather than fixed bridges and/or removable appliances.
Some practitioners elect to perform both the surgical and the restorative procedural components that comprise the total implant treatment. Most professional, however, tend to gravitate towards one or the other of these treatment areas. Specialty practice licensure limitations contribute to the separation of treatment interests and responsibilities.
Periodontists, oral surgeons and endodontists have a preference for the surgical elements of implant therapy, readying the bony recipient site for the implant, and then placing the implant in the hard tissues.
General practitioners, while they typically take the responsibility of quarterbacking the total implant case, are more likely to be tasked with the pre-treatment planning and the post-surgical restorative aspects of the implant sequence. These responsibilities include:
1. the pre-treatment positioning and angular alignment of the implants that are essential in the development of the functional and esthetic parameters that guarantee a successful case (the surgical stent can be used to communicate this data between the surgical, restorative and laboratory team members),
2. the impression-taking from the abutment phase to the final restorative phase,
3. the occlusal and esthetic adjustment of the lab fabricated implant prosthesis
4. the attachment of the prosthesis to the implant(s) by one of several means, and
5. the maintenance of the implant restoration
This last responsibility of maintenance (including repair) is commonly overlooked. This task, by default, falls to the restorative implant provider, typically the general practitioner who provides ongoing clinical services to the patient.
Implants are appliances that function continuously in patients’ mouths over extended periods of time. They are subject to the significant functional and parafunctional forces that are generated by the muscles of mastication and materials of varying hardness that are chewed. Considering the mechanical loads on un-splinted implant molar crown restorations, the mechanical strength of many types of screw-retained implant-abutment connections is too low.1-15 On the other hand, the Morse Taper connection found in ANKYLOS implants (DENTSPLY Tulsa, Tulsa, OK) provides an exceptionally strong, gap-free, subgingival tapered connection that relies on surface-to-surface metal friction at the fixture/abutment interface for retention.16 (Fig.1) The friction grip relationship eliminates micro movement between the abutment and the implant, eradicating microbial colonization at the interface.17
Occasionally, masticatory and accidental forces can cause damage to the carefully constructed restorations. The titanium of the implants and the metals utilized for the restorative infrastructures are so strong that that they re rarely affected; porcelain, on the other hand, can chip or fracture, bridge metals can fatigue or require unit additions, and retention screws can loosen. In the ANKYLOS system, the fixation screw between the abutment and the implant is not required for retention: the ANKYLOS connection is rotationally stable, requires a comparatively low torque (15 N/cm), and offers the greatest possible security against screws loosening and breaking.18
There are several methods commonly used to fix implant restorations to osseointegrated implants. The most commonly used approaches to fixating a crown on an implant are:
Screw Retention
The crown is fixed to the abutment, or directly to the underlying implant, with a screw. Additional rotational stabilization such as manufactured or milled hexing may be required. After the screw is tightened, the screw head is protected by cotton, and the insertion hole is restored to occlusion with composite. This makes it possible, in case the need arises (ceramic fracture or screw loosening), to access the retention screw. the composite is carefully removed without damaging the screw head. The cotton that covers the head of the retention screw is picked out, exposing the retention screw. The crown can then be removed for laboratory repair or screw tightening. This approach permits the implant crown to be totally retrievable.
Cement Retention
The most common approach has been to cement the crown onto the implant abutment with a theoretically retrievable temporary or provisional cement. The fundamental concept of this technique is that the crown is removable from the abutment simply by breaking the seal of the provisional cement between the crown and the abutment at the margin. While provisional cements allow the somewhat flexible acrylic and bis-acryl dental provisionals to be separated rather easily from natural abutments (within the first few weeks or months after placement), the precision-fit metal-to-metal crown-abutment implant interfaces are far more gap-free and retentive. These factors contribute to a much greater retention than would be normally expected from a provisional cement. Furthermore, there is ample evidence to indicate that provisional cements mature to the properties of permanent cements over time.
Thus, the provisionally cemented implant crown can become effectively irretrievable within 6-12 months after placement. This is a major clinical problem for the practitioner in cases where the esthetic ceramic covering of the crown or bridge fractures, is compromised, requires an addition due to the further loss of natural dentition, or suffers the loosening of a retention screw that simply needs to be retightened. If the crown cannot be removed from the implant abutment easily (particularly difficult in situations where the retention screw has loosened), without damaging the abutment or compromising the integrity of the underlying implant in the bone, the practitioner is forced to cut the crown off of the abutment.
It is not likely that the sectioned crown can be repaired adequately for reinsertion, and will have to be remade. There are additional risks in cutting off an implant-borne crown, however. The metal-to-metal crown-abutment interface consists of a very thin and often virtually invisible layer of temporary cement. It is easy to inadvertently cross this line during the high speed crown removal process. Thus, sectioning will, in addition to destroying the crown, damage the screw retained abutment as well. These risks are magnified where the retention screw has loosened and the drill bit does not have a secure purchase on the mobile crown.
The question of financial liability is an important consideration. Once an implant-borne crown must be sectioned off, it (and occasionally the abutment) has to be re-fabricated to restore the patient’s dentition. Who should assume part or all of the financial liabilities that arise from the process: removal, re-fabrication, and re-insertion? Is it the patient, the dentist, the laboratory technician, or the manufacturer? And at what point in time after the original insertion of the prosthesis does the “normal use and wear” make the replacement procedure the responsibility of the patient?
Set Screw Retention
A well-established engineering concept is available for implant restorations offering a clinical solution that eliminates all the above risks and potential liabilities. The set screw is a totally retrievable mechanism for fixing the implant-borne crown to the abutment. It is a high-precision system that is manufactured entirely at the dental laboratory. The process ad
ds somewhat to the laboratory cost but significantly reduces clinical chairtime during the insertion phase; the greatest benefits occur in situations where the crown must be removed. Removing the set screw is very simple, very fast, and very easy, and the implant prosthesis is released within seconds at no risk to the osseointegrated implant and no discomfort to the patient. The set screw does not become more difficult to remove with time and the anatomy of the head is so shallow that no additional restorative coverage is required.
The implant bears the occlusal masticatory loads, and lateral forces are transferred at the abutment-implant interface by one of the following: a tapered anatomy (ANKYLOS), a retention screw plus hex, or a retention screw alone. Thus, a precision-fit implant crown does not require very much retentive strength. The small, horizontally-inserted set screw only needs to prevent the vertical movement of the crown on the abutment, and is not exposed to high stress. During the insertion, it also provides the practitioner with a defined registration for the crown, both rotationally and inciso-gingivally.
The only additional planning required is to ensure that the placement of the set screw access does not compromise the patient’s esthetics and that the direction of entry allows adequate clearance for the driver and the practitioner’s fingers in the patient’s mouth during insertion.
Case 1
The implant has osseointegrated (B14 ANKYLOS Implant, DENTSPLY Tulsa, Tulsa, OK) and the soft tissue is ideally shaped for the restoration. (Fig. 2) After transfer impressions and electronic shade taking (plus custom in-lab color adjustment), the dental laboratory (Progenic Dental Laboratory, Oakville, ON) returned the custom abutment, crown, and set screw assembly. These are viewed on the model from the buccal (Fig. 3), occlusal (Fig. 4), and lingual (Fig. 5). The driver is used to disassemble the lingual attachment and to establish the screw access angulation (Fig. 6). The abutment is viewed from the lingual, indicating the milled set screw entry path. (Fig. 7)
The transitional abutment has maintained the anatomy and the gingival health of the tissue surrounding the implant (Fig. 8). The custom abutment is positioned with the red acrylic jig and secured with the fixation (not retention) screw (Fig. 9). The Morse taper of the precision-fitting abutment provides a rotationally-stable and gap-free attachment to the implant. The custom abutment is seen from the buccal (Fig. 10). The crown is tried in to verify mesio-distal positioning, occlusal clearance, soft tissue relations, and esthetics. The inciso-gingival positioning of the crown is verified from the lingual by the alignment of the crown set screw access with the abutment set screw hole (Fig. 11). The set screw is carefully positioned on the driver (Fig. 12) and the crown is fixed onto the abutment (Fig. 13). A close up view indicates the required angulation of the driver both mesio-distally and inciso-gingivally, an orientation that was predetermined on the model (refer to fig. 6). The tightened set screw leaves a small space towards the gingival (Fig. 14). While food may sometimes settle in this location in between brushings, it is not likely that this debris will have any effect on the exposed smooth metal surfaces. The set screw allows the crown to be removed, for any reason whatsoever, in a matter of seconds. The patient’s smile shows the implant-borne crown in place (Fig. 15) and close-up (Fig. 16).
Case 2
The “provisionally” cemented crown was loose but impossible to separate from the abutment, and the entire crown assembly was mobile during the removal process. The original implant abutment was damaged when the loosened crown was sectioned (Fig. 17). The new abutment required an innovative laboratory approach (Fig.18) to create the site for a set screw and to maintain adequate ceramic space for esthetics (Progenic Dental Laboratory, Oakville, ON). The positioning of the abutment bucco-lingually (Fig. 19) and mesio-distally (Fig. 20) are specifically directed by the less than ideal arch form. The custom abutment is fixed on the implant (Fig. 21), and the crown is placed on the abutment and secured with a set screw (Fig. 22). The lingual view demonstrates the non-traditional positioning of the crown margins and the set screw to offer the patient a long term restoration. (Fig. 23)
Case 3
The implant (ANKYLOS Implant 4.5 x 9.5mm, DENTSPLY Tulsa, Tulsa, OK) was osseointegrated and the surrounding tissues healthy (Fig. 24). The custom abutment was inserted using the lab-provided (Progenic Dental Laboratory, Oakville, ON) acrylic jig (Fig. 25). and the fixation (not retentive) screw. The implant-borne crown was tried in, and once the crown set screw access was aligned with the abutment set screw hole, the set screw was inserted from the lingual (Fig. 26). The crown is viewed from the occlusal (Fig. 27), the buccal (Fig. 28) and the lingual (Fig. 29).
Conclusion
Set screw retention provides the practitioner with the option of a restoration that is totally retrievable within seconds. Pre-planning the location of the set screw access allows uncompromised functional and esthetic results. ANKYLOS is the only implant system that has pre-drilled threading for the set screw in the abutments (other systems require manual threading). The required collaboration between the dentist and the laboratory technician is simple and straightforward. Set screw retained implant restorations make the practice of implant dentistry better, faster, and easier.OH
The author wishes to thank Dr. Cary Galler (North York, ON) for the surgical implant placement and Progenic Dental Laboratory (Oakville, ON) for the precision components and custom esthetics.
George Freedman is a founder and past president of the American Academy of Cosmetic Dentistry, a co-founder of the Canadian Academy for Esthetic Dentistry and a Diplomate of the American Board of Aesthetic Dentistry. Dr. Freedman sits on the Oral Health Editorial Board (Dental Materials and Technology) is a Team Member of REALITY and lectures internationally on dental esthetics and dental technology. A graduate of McGill University in Montreal, Dr. Freedman maintains a private practice limited to Esthetic Dentistry in Markham, ON, Canada
Randy Kwon achieved his Ontario College Advanced Diploma in Dental Technology from George Brown College in 2007. This early in his young career, Randy Kwon has become the head of the implant department and leading gold and CADCAM technician at Progenic Dental Lab, where he has trained and worked since 2004. He keeps his industry knowledge abreast with regular enrollment in accredited implant, esthetic and functional courses across North America.
Oral Health welcomes this original article. References available upon request.
