The first intermetatarsal angle in hallux valgus: an analysis of measurement reliability and the error involved.

In view of the importance placed on the first intermetatarsal angle in the assessment of surgical intervention in hallux valgus, we assessed the reliability with which one measures this angle. The study involved 10 observers of varying experience measuring the angle using a standard technique on 10 weightbearing AP X-rays of the foot on three separate occasions. The margin of error in measuring the angle was +/-3.60 degrees with a 95% confidence interval. Increasing and averaging the number of readings per observer or the readings of a number of observers, reduces the error. Experience doesn't improve reliability. In conclusion, improvement in the reliability of the measurements can be achieved by careful technique, performing the measurements at least twice, and averaging them.