Does crown-root ratio matter or not with an integrated implant?
(Content under development)
The following linked questions were submitted by a practising dentist: Does crown-root (C/R) ratio matter or not with an integrated implant and, then more importantly, what do you do in your practice? What does the literature say about C/R ratio for implants with regard to short implants and crowns that are long or longer than the 50/50 guide for natural teeth?
JCDA Editorial Consultants Drs. Reynaldo Todescan of the University of Manitoba, David Chvartszaid of the University of Toronto, and Ken Sutherland of the University of Saskatchewan provided these initial comments for consideration:
According to Dr. Todescan: Unfavourable C/R ratio on implants does not seem to affect the success rate of implants and several companies are now offering short implants (≤ 8 mm). If the implant design is adequate, the bone will respond to the load. However, a long clinical crown (held by a relatively small screw) can be mechanically unstable and aesthetically unpleasing in some situations. It does not matter if you have a long or short implant, a long clinical crown maybe a mechanical problem.
There are implant connections (internal taper, for instance) that may provide better stability for such situations. One cannot offer a general answer because every case is different. As always, the size of crown/abutment, occlusion adjustment, type of implant connection, anatomical (tooth) site, prosthesis design, etc. are important factors to be taken into consideration.
We have produced a few papers (one of which is linked here) on the use of a short implant system and have had no more problems or failures than other authors using longer implants.
To which Dr. Chvartszaid added: C/R ratio doesn’t have the same meaning with implants as it does with natural teeth (see recent systematic review). In short, it is not a major consideration or contra-indication.
However, higher biomechanical complications can be expected, especially in single implant and short-span bridge applications. This is particularly likely in the posterior region (where greater forces can be expected). The most likely complication that may be experienced with excessive C/R ratios is prosthetic screw loosening. To address this concern, dentists can consider (where appropriate):
1) choosing screw-retention in these applications (in anticipation of the occasional need to retighten prosthetic screws that have loosened),
2) taking steps to reduce the overall loading on the prosthesis (by splinting, reducing occlusal surface, placing additional implants etc.), and
3) ensuring a more frequent recall regimen to ensure that minor biomechanical issues (if they arise) are noted and addressed in time.
Dr. Sutherland prefaced the following comment as being “anecdotal” in nature: The longer I follow implant success rates and the more clinicians I speak with about the issue, the more I am concerned about occlusal load leading to bone loss. Although this comment is not directly related to C/R ratio, the longer the crown relative to the root (fixture), the greater the potential lever arm and therefore the force. Reynaldo is right that there are more short “fat” implants being placed with favorable shorter term results. Let’s see what the longer term holds in store.
Follow-up: Readers are invited to comment on this initial response and provide further insights by posting in the comment box which you will find by clicking on “Leave a reply“ below. Comments come directly to me for approval prior to posting. You are welcome to remain anonymous. We will never post your email address in any response. John