What is the crown-to-root ratio for short implants and crowns that are long or longer than the 50/50 guide for natural teeth?

This question was submitted by a general dentist:  What does the literature say about crown-root ratio for implants with regard to short implants and crowns that are long or longer than the 50/50 guide for natural teeth?

Dr. Suham Alexander, Oasis Discussions Clinical Editor, and Dr. Effie Habsha drafted and reviewed this quick initial response 

Crown height is an important determinant of long-term success in an implant-supported prosthetic.  The height of the crown sustains lateral forces and acts as a vertical cantilever where the forces are applied to the crest of the rigidly fixated implant. 

In natural teeth, crown:root ratio is an important factor in determining prosthetic treatment.  However, fundamental differences exist in implant-supported prosthetics:

1. Forces are captured at the crestal ridge
2. Implants do not rotate 2/3 of the way down the root
3. For every 1mm of crown height, the force of the moment applied to the crown increases by 20%.  As such, an increase from 10mm to 15mm in root height translates to a 100% increase in forces.

For these reasons, increasing the length of an implant to support an increased crown height will have no positive effect.  Rather, it is more advantageous to increase the area of functional load by increasing the number implants placed, the size of the implant used or the implant design.

With respect to short implants, a study by Birdi et al. showed that crown:root ratios do not affect the success of short-length plateau-design implants.

References

1. Misch CE. Dental Implant Prosthetics.  2005. Elsevier Mosby.
2. Schulte J, Flores AM and Weed M.  Crown-to-Implant Ratios of Single Tooth Implant-supported Restorations. The Journal of Prosthetic Dentistry. 2007: 98(1) pp1-5.
3. Birdi H, Schulte J, Kovacs A, Weed M, Chuang SK. Crown-to-implant ratios of short-length implants. J Oral Implantol. 2010; 36(6):425-33. Epub 2010 Jun 14.

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