Using Three Familiar Radiographic Shapes to Improve Dental Implant Longevity

One such shortcut when examining a patient's clinical condition is to simply identify one of three radiographic prosthodontic shapes: types 1 through 3 (Figure 1). These prosthodontic profiles are easily remembered because they resemble three very familiar figures: Snoopy (type 1), E.T. (type 2), and a heart (type 3). Keeping these figures in mind can help the clinical team establish effective treatment plans while setting realistic expectations for the patient.

Type 1: Beware of the Cantilever

Although the character Snoopy of comic strip Peanuts fame can be quite charming, when dental radiographs appear in the shape of his profile, caution must be exercised (Figure 2). Type 1 prostheses tend to have a greater risk for developing complications such as abutment screw loosening, broken crowns, cementation failure, and a fractured abutment or implant.^1-3 Moreover, healthy teeth in close proximity may be prone to developing caries, often due to food traps created by this type of prosthesis.⁴ Many of the complications associated with a type 1 prosthesis shape are related to a principle in mechanical engineering called a "cantilever," which allows linear forces acting at a distance to be converted to rotational or bending forces.

Furthermore, rotational forces challenge practically all aspects related to dental implants. Implants are designed to support loads being applied in an axial direction; thus, when subjected to rotational or bending forces, they can break or fatigue easily.⁵ Similarly, small abutment screws bend due to torsional loads, which can cause screw loosening or fracturing as well. To effectively separate an internal conical connection between abutment and implant, one must distort the interface by applying off-axis or rotational forces.⁶ This will generate an asymmetric load that will do just that. Ceramics and bonding cements tend to be their strongest when exposed to compressive forces; however, the opposite is true when subjected to tension.⁷ When rotational forces are applied, these materials become placed in an environment of tensile stress where they can easily break apart and fail. As a result, it is critical that both ceramic and cement components remain within their most suitable operating conditions for optimal performance.

Thus, if presented with a cantilever, the clinician should watch for potential complications. When a type 1, or Snoopy-like, implant restoration radiograph is noted, the clinician should set the patient's expectations straight and determine an ideal treatment option. For instance, there is a strong likelihood of impending screw loosening with a type 1. Clinically, the risks can be mitigated by ensuring the cantilever end of the prosthesis cannot hold shimstock. Also, by addressing this concern with patients up front and centering them in their care plan accordingly, they are more likely to be better prepared for potential complications down the road.

Type 2: Coronally Placed Implant, Narrow Neck

The type 2 prosthodontic profile, or "E.T." shape (Figure 3), should also prompt precautionary measures. This type of prosthesis can be the result of inadequate communication between the dental office and laboratory, or an implant placed too coronally. Historically, crown-and-bridge labs have shaped their dies on stone models with dentists aiming to keep the crown margin near the gingival border. Consequently, technicians are familiar working supracrestal down to crown margins, as this approach has been reliable for decades. When submitting an implant impression, it is essential for clinicians to include details regarding the position of the implant platform and emergence profile of a stock healing abutment. Omitting this information may lead to incorrect prosthesis construction by lab technicians that mimics what they see in the stone model only (Figure 4). The narrow neck and obtuse emergence angle can potentiate mechanical and biological complications. This seemingly innocuous mistake could lead to parts breaking, like the prosthesis, implant, and abutment, and create an alcove for pathogenic bacterial accumulation resulting in a host of negative biological consequences.⁸

Risks of a type 2 prosthesis can be minimized with the use of a fillet, a rounded corner that is often utilized in mechanical engineering. This approach eliminates sharp edges and improves cleansability while potentially decreasing danger. In dentistry, fillets are used to create gentler emergence profiles that reduce stress in implant solutions. Excessive tension, which is likely to accompany type 2 restorations, can cause higher levels of stress than those with fillets and should be avoided. Following this general engineering principle involving fillets should reduce the chances of complications and enable clinicians to provide safer treatment options to patients.

If a type 2 prosthesis has been designed or manufactured, the clinician can simply remake the ideal prosthesis and inform the lab to disregard the soft-tissue profile (Figure 5 and Figure 6). The clinician can then manage the soft tissue chairside and request a type 3 "heart"-shaped emergence profile. At delivery, the soft tissue can be repositioned utilizing the apical gingival displacement technique to adapt the soft tissues into an ideal functional and esthetic solution.⁹

To decrease risks and promote better results, prevention is vital. A simple, straightforward approach to communicating to the lab the desired emergence profile is to ask for a prosthetic outcome that resembles a type 3, or heart-shaped, design.

Type 3: A Shape Clinicians Can Love

The heart-shaped prosthesis is the third radiographic shortcut and the preferred shape (Figure 7). Its design will typically meet each desired criteria for success. Type 3 prostheses have a gentle emergence profile that is conducive to causing less stress and fewer complications.

To realize the full effect of this shape, the implant should be centered between two adjacent teeth. This minimizes cantilevers, reduces implant-to-tooth distance, and allows placement depth to be close to ideal for attractive esthetics results.¹⁰ With this design, occlusal loads are channeled directly to the implant, putting it under axial compression, which is widely viewed as the safest force that can be applied. Also, a type 3 solution permits use of a screw-retained crown, thereby eliminating the possibility of cement entering the sulcus. Furthermore, this approach significantly reduces costs associated with such cases because stock abutments may be used. With a type 3 radiograph, the chances of complications will be minimized, and the patient's long-term outlook will be excellent.

Conclusion

Remembering the three radiographic shapes discussed-Snoopy, E.T., and a heart-can aid the dental team in identifying possible risk factors associated with implants and help clinicians improve patient outcomes. Utilizing a type 3 prosthesis, with its gentle emergence profile, minimized cantilevers, and screw-retained solutions, can allow dentists to confidently provide patients with successful implant treatments that yield optimal esthetic results and long-term stability.

About the Author

Robert J. Stanley II, DDS, PhD, MS
Adjunct Professor, University of North Carolina School of Dentistry, Chapel Hill, North Carolina; Founder, Stanley Institute, Cary, North Carolina; Private Practice, Cary, North Carolina; Diplomate, American Board of Oral Implantology; Diplomate, International Congress of Oral Implantologists; Fellow, American Academy of Implant Dentistry; Fellow, Academy of General Dentistry