Reliability of the Bolton tooth-size analysis when applied to crowded dentitions

The Bolton tooth-size analysis is widely taught and used in orthodontics. However, its reliability has not been documented. The purpose of this study was to evaluate the reliability of the analysis when performed with needle-pointed dividers and a Boley gauge. Four clinicians measured the teeth on 15 sets of casts with two instruments at two sessions. The measurements were used to calculate tooth-size excess. To evaluate the measurement error, the difference between the two analyses made by the same investigator on the same set of casts was calculated. More of the same-investigator analyses were significantly correlated when the Boley gauge was used than when the needle-pointed dividers were used. Between-investigator analyses revealed significant correlations for each measurement session with both instruments. Every investigator was found to have at least one measurement error for each analysis and with each instrument that was as large as a clinically significant result of a Bolton analysis. The results of this study demonstrate that clinically significant measurement errors can occur when the Bolton tooth-size analysis is performed on casts with at least 3 mm of crowding. The Boley gauge demonstrated a higher frequency of significantly correlated repeated measures and thus may be somewhat more reliable for this analysis than the needle-pointed dividers.