Nonanatomic insertion of radial head prostheses may result in maltracking and capitellar erosion. The purpose of this study was to analyze the ability to perform anatomic radial head replacement and to study radiocapitellar prosthetic subluxation under unstable conditions. In vitro measurements were performed in 10 intact elbows and after insertion of radial head prostheses (rigid uncemented, rigid cemented, bipolar). The diameter and length of the proximal radius were approximately restored. However, prosthesis-shaft malalignment, up to 21 degrees , was observed frequently. A posterolateral rotatory stress after resection of 50% of the coronoid caused a mean radiocapitellar subluxation of more than 30% for the rigid prostheses, whereas bipolar devices self-aligned with a mean subluxation similar to intact elbows. In conclusion, it was not possible to consistently insert radial head prostheses anatomically. Marked radiocapitellar subluxation occurred for the rigid but not for the bipolar implants. Clinical trials are needed to analyze these findings.