The ratio of the lengths for the root and crown of a permanent tooth reflects on its development. A disturbance in tooth development is often manifested by a decrease in the ratio.
Method: The landmarks for each tooth type is shown in Figure 1, page 492, Hollta et al (2004).
root-crown ratio =
= (vertical length of root in mm) / (vertical length of crown in mm)
where:
• Another ratio to consider would be the ratio of root to total tooth length (root + crown).
• The longest root is used when measuring a multi-rooted tooth.
Interpretation:
• Normal teeth show ratios that range from 1.78 to 2.46 depending on the specific tooth (Table 2, page 494, Hollta et al, 2004).
R/C Ratio
Disturbance in Root Development
> 1.6
no disturbance
1.2 - 1.6
mild
0.9 - 1.1
moderate to severe
< 0.9
very severe disturbance or arrested root development
from page 123 Holtta et al (2002)
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