Cement-free Implant Retention

Dental News Volume XXII, Number III, September, 2015

by Dr. George Freedman DDS, FAACD, FIADFE, FACD and Dr. Randy Kwon  DT

Implants are very predictable, both in terms of maxillary and mandibular osseointegration and as functional and esthetic dental restorations. They tend to have very high rates of success, and are suitable for most patients. Their widespread, routine use in dentistry over the last 20-30 years has established implants as a mainstream modality. Most importantly, patients are now generally more aware of this restorative opportunity, and are increasingly choosing implant-borne restorations in the place of 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 in some jurisdictions contribute to the separation of treatment interests and responsibilities.

Periodontists, oral surgeons and endodontists generally prefer the surgical elements of implant therapy, readying the hard tissue recipient site for the implant, and then placing the implant in the bone. 

General practitioners, while they typically take the responsibility of overseeing 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. pre-treatment positioning and angular alignment of the implants, essential in the development of the functional and esthetic parameters that guarantee a successful case (computer-generated guides and surgical stents are used to communicate this data between the surgical, restorative and laboratory team members),

2. impression-taking from the abutment phase to the final restorative phase,

3. occlusal and esthetic adjustment of the lab-fabricated implant prosthesis,

4. attachment of the prosthesis to the implant(s) by one of several means, and,

5. maintenance and repair of the implant restoration.

This last responsibility, including maintenance and repair, is commonly underestimated. 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 are in continuous function in patients’ mouths over extended periods of time, years and decades. They are subject to the significant functional and parafunctional forces that are generated by the muscles of mastication acting upon materials of varying hardness as they 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. ^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}  On the other hand, the  6º conical connection found in MIS C1 implants (MoreDent/More Group, Melbourne, Australia) 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, and reducing bone loss at the crestal level. ¹⁷ A 6-position cone index within the C1 connection assists in the correct orientation during abutment insertion.

Fig 1

Fig 1

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 MIS C1 system, the fixation screw between the abutment and the implant does not provide all the retention: the conical C1 connection is rotationally stable and positionally-oriented, 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 appropriately tightened, the screw head is protected by cotton, and the insertion access hole is restored to occlusion with composite. This makes it possible, in case the need arises (due to ceramic fracture or screw loosening), to access the retention screw. The composite is carefully removed to the cotton layer 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 or bridge to be totally retrievable. The location of the occlusal access hole is often rather unasthetic, and is difficult to adapt to patient acceptability with composite.

Cement retention 

The most common approach (often selected for esthetic reasons) 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 the span of several months. 

Thus, the provisionally cemented implant crown can become effectively irretrievable within 3-12 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, the abutment or bridge requires an addition due to the further loss of natural dentition, or the implant retention screw suffers loosening 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 the abutment. It is not likely that the sectioned crown is salvageable; thus, it cannot be repaired adequately for reinsertion, and must be remade.

There are additional risks in cutting off an implant-borne crown to consider. 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, often damage the screw retained abutment as well. These risks are even greater where the retention screw has loosened and the drill bit does not have a secure purchase on the wobbling implant crown assembly.

The question of financial liability is an important consideration. Once an implant-borne crown is sectioned off, it (and all too often, the abutment) must 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 patient’s financial responsibility?

Setscrew retention 

A well-established engineering concept is available for implant restorations, offering a clinical solution that eliminates all of the above risks and potential liabilities. The setscrew is a totally retrievable mechanism for affixing the implant-borne crown to the abutment. It is a high-precision system that is manufactured entirely at the dental laboratory, requiring no additional chairside time or steps. The process adds somewhat to the laboratory cost but significantly reduces clinical chairtime during the insertion phase; the greatest benefits occur in situations where implant-borne crowns/bridges must be removed. Removing the setscrew 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 setscrew does not become more difficult to remove with time, and the anatomy of the head is so shallow that no supplementary 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 conical-index anatomy (MIS C1), a retention screw plus hex, or a retention screw alone. A precision-fit implant crown relying on a conical metal-to-metal interface does not require very much retentive strength. The small, horizontally-inserted setscrew 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 pre-planning required is to ensure that the placement of the setscrew 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 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, Canada) returned the custom abutment, crown, and setscrew 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 on the model and to establish the screw access angulation. FIG 6. The abutment is viewed from the lingual, indicating the milled setscrew entry path FIG 7.

Fig 2

Fig 3

Fig 4

Fig 5

Fig 7

Fig 8

The transitional abutment has maintained the anatomy and the gingival health of the tissue surrounding the implant FIG 8. The abutment is positioned and secured with the fixation  screw FIG 9. The indexed conical taper of the precision-fitting abutment provides a rotationally stable and gap-free attachment to the implant. 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 setscrew access with the abutment setscrew hole Fig 10. The setscrew is carefully positioned on the driver  FIG 11 and the crown is fixed onto the abutment FIG 12. 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 fig6). The tightened setscrew leaves a small space towards the gingival FIG 13. 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 setscrew 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 14 and close-up FIG 15.

Fig 8

Fig 9

Fig 10

Fig 11

Fig 12

Fig 13

Fig 14

Fig 15

Case 2

The “provisionally” cemented implant 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 16. The new abutment required an innovative laboratory approach FIG 17 to create a suitable site for the setscrew and to maintain adequate ceramic space for esthetics (Progenic Dental Laboratory, Oakville ON, Canada). The positioning of the abutment bucco-lingually Fig 18 and mesio-distally FIG 19 are dependent upon the less than ideal arch form. The custom abutment is fixed on the implant Fig 20, and the crown is placed on the abutment and secured with a setscrew Fig 21. The lingual view demonstrates the non-traditional positioning of the crown margins and the setscrew to offer the patient a long-term restoration. FIG 22

Fig 16

Fig 17

Fig 18

Fig 19

Fig 20

Fig 21

Fig 22

Case 3

The implant was osseointegrated and the surrounding tissues healthy FIG 23. The abutment was inserted  and the implant-borne crown (Progenic Dental Laboratory, Oakville ON) was tried in. Once the crown setscrew access was aligned with the abutment setscrew hole, the setscrew was inserted from the lingual FIG 24. The crown is viewed from the occlusal, FIG 25, the buccal, FIG 26 and the lingual. FIG 27

Fig 23

Fig 24

Fig 25

Fig 26 

Fig 27

Conclusion

Setscrew retention provides the practitioner with the option of a restoration that is totally retrievable within seconds. Pre-planning the location of the setscrew access allows uncompromised functional and esthetic results. The required collaboration between the dentist and the laboratory technician is simple and straightforward. Setscrew retained implant restorations make the practice of implant dentistry better, faster, and easier.

Dr. 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. He is a Visiting Professor at the MClinDent programme in Restorative and Cosmetic Dentistry, BPP University in London. His most recent textbook, “Contemporary Esthetic Dentistry” is published by Elsevier. Dr Freedman is the author or co-author of 12 textbooks, more than 700 dental articles, and numerous webinars and CDs and is a Team Member of REALITY. Dr Freedman was recently awarded the Irwin Smigel Prize in Aesthetic Dentistry presented by NYU College of Dentistry. He lectures internationally on dental esthetics, adhesion, desensitization, composites, impression materials and porcelain veneers. A graduate of McGill University in Montreal, Dr. Freedman is a Regent and Fellow of the International Academy for Dental Facial Esthetics and maintains a private practice limited to Esthetic Dentistry in Toronto, 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 CAD/CAM technician at Progenic Dental Lab, where he has trained and worked since 2004. He keeps his industry knowledge up with regular enrollment in accredited implant, esthetic and functional courses across North America.

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