The acquired midtarsus deformity classification system--interobserver reliability and intraobserver reproducibility.

A radiographic classification (Schon's) divides Charcot midtarsus deformities into four types identified by Roman numerals (I to IV), according to the anatomical location of the pathological process,11 and an objective method of severity staging using radiographic criteria is introduced and tested. A beta stage is assigned if one of the following criteria is met: 1. a dislocation is present; 2. the lateral talar-first metatarsal angle is > or = 30 degrees; 3. the lateral calcaneal-fifth metatarsal angle > or = 0; or 4. the AP talar-first metatarsal angle is > or = 35 degrees. An alpha stage can be assigned when all four features are absent. Clinical features useful in assessing and managing these deformities have been associated with the various types and stages. To determine whether the classification system is valid, a study was performed. Two examination booklets and an instructional booklet designed to teach the method were distributed to 75 orthopaedic surgeons at the AOFAS summer meeting to test for intraobserver reproducibility and interobserver reliability. Information about the participants was recorded, and the tests were scored. The highest scores for correct responses were achieved by foot and ankle fellows, followed by orthopaedic residents. Attending orthopaedic surgeons achieved the lowest scores. The most common error was a type I deformity misidentified as a type II. The interobserver reliability for correctly classifying the deformities was 81%, and the intraobserver reproducibility was 97%. We concluded that this classification system, intended to clarify the patterns of acquired midfoot collapse, permits assignment of both anatomic type (I to IV) and degree of severity (alpha-beta) with high reliability and reproducibility. It can therefore be used as a tool for diagnosis, planning treatment, and assessing the prognosis.