When is it appropriate to link a prosthesis to natural teeth and implants?

This question was submitted by a general dentist: When is it appropriate to link a prosthesis to natural teeth and implants?

Drs. Natalie Wong and Suham Alexander provided this quick initial response.

To be successful, osseo-integrated implants must exhibit:

• Function – ability to chew
• Good tissue physiology – absence of pathophysiology/pain, integration with the bone and a healthy periodontium
• Patient satisfaction – esthetics and no discomfort

The clinical scenario in which a prosthesis is linked to a natural tooth and implant abutment has been the topic of several discussions. There is evidence to both support and discredit this practice; however, detailed information and clinical findings must be considered before finalizing and creating a successful treatment plan. Two basic designs exist: a conventional fixed partial denture or a fixed partial denture with a non-rigid connector. In both situations, the mobility of the natural tooth must be assessed as it influences the decision to join the implant and natural tooth more than any other factor.

When implants are connected to natural teeth, the prosthesis must be designed such that the initial contact and load is applied to the natural tooth and not the implant. Natural teeth are supported by a healthy periodontal ligament and thus, have natural healthy movement associated with them. Some mobility in natural teeth is expected and within normal limits. In contrast, implants have a bone-implant interface which is rigid.

Additionally, one must minimize any lateral forces. Lateral forces increase tooth movement and may damage the implant. Shear (lateral) forces are created at the bone-implant interface and bone is 60% weaker with lateral loads. Additionally, screw-loosening and/or the cement-seal strength is compromised with lateral loading.  The shear forces also augment the stress created at the level of the crestal bone.

When an implant is connected to a natural tooth with a fixed prosthesis in the dynamic oral environment, there is an increased risk to the longevity and stability of both abutments. The cement seal of the natural tooth may be compromised as it experiences movement with loading and as such, there is a risk of recurrent decay as well as intrusion. Additionally, the rigid implant connector now has a cantilever which increases the force on the bone-implant interface and abutment screw.

In general, anterior teeth are unsuitable abutments as they often exhibit more mobility than is tolerable by the rigid implant and any lateral forces applied to the prosthesis during mandibular excursion are translated to both the natural teeth and implants. In these situations, the use of a non-rigid connector (e.g. semi-precision attachments) may be considered.  However, this is associated with a higher cost for the patient. 

Tooth-Implant Advantages

• ↑ tactile perception for patient – increased comfort chewing and efficiency
• Avoid vital anatomical structures
• ↓ cost
• ↓ chance graft is required
• Better patient acceptance

Tooth-Implant Disadvantages

• ↑Risk of intruding natural tooth
• Biomechanical Issues:
  •  Fixture-abutment failure
  •  Implant fracture
  •  Loosening of screw
  •  Cement failure
  •  Tooth fracture
  •  Decay of tooth
  •  Crown fracture
• Natural tooth Issues:
  •  Endo and/or periodontal involvement
  •  Fracture
  •  Decay
• Peri-implantitis

References 

1. Davis, SM, Plonka, AB, Wang, H. Risks and Benefits of Connecting an Implant and Natural Tooth. Implant Dentistry 2014. 23(3):253-257.
2. Misch, CE. Dental Implant Prosthetics. Elsevier Mosby. 2015.
3. Chee, WW, Mordohai, N. Tooth-to-Implant Connection: A Systematic Review of the Literature and a Case Report Utilizing a New Connection Design. Clinical Implant Dentistry and Related Research. 2010. 12(2):122-132.
4. Weber, HP, Zimering Y. Survival and complication rates of fixed partial dentures supported by a combination of teeth and implants. J Evid Based Dent Pract. 2012 Sep;12(3 Suppl):215-6.