Assessing sesamoid subluxation: how good is the AP radiograph?

Subluxation of the metatarsosesamoid joints frequently occurs with the development of hallux valgus deformity, and the restoration of a normal metatarsosesamoid articulation has been proposed as essential for achieving a biomechanically sound operative result. The position of the sesamoid bones on the AP radiograph is used often to assess the pre- and postoperative relationship between the hallucal sesamoids and the metatarsal sulci. We evaluated the validity of this approach. Thirty subjects with hallux valgus and 30 control subjects participated in this study by undergoing both AP and tangential weightbearing radiographs. The sesamoid station on the AP radiographs was compared with the position of the sesamoids on tangential radiographs, using a new continuous measure to estimate subluxation. In approximately half of the cases, we found a difference between the apparent sesamoid station on the AP radiograph and the true position on the tangential one. Increased metatarsal rotation was associated with misclassification of the sesamoid station on the AP radiograph. We conclude that the standard method for measuring the sesamoid station on the AP radiograph is not valid. Surgeons wishing to evaluate the metatarsosesamoid joint should obtain weightbearing tangential radiographs.