Modularity in total knee system design makes it important for the surgeon to know the model of prostheses being revised since components of different designs are not interchangeable. Preoperative information on the manufacturer and model of prostheses implanted may not always be available. This article describes an algorithm that allows surgeons to identify the prosthesis from standard radiographs. Ninety-nine percent of the knee prostheses implanted between 1985 and 1992 can be identified using this algorithm. The algorithm uses the shape of the tibial keel, the type of baseplate, and the shape of the femoral component as branch points to identify the model and manufacturer of 15 primary total knee prostheses. In addition, certain models were found to have radiographic "fingerprints" such as the presence of a spiked tibial keel. This algorithm should be an aid to surgeons performing total knee revisions.
