This is a cadaveric study using 12 fresh frozen elbow cadavers. The capitellum of the distal humerus and three different conditions of radial head were tested, the native radial head, a monopolar prosthesis, and a bipolar prosthesis, for radiocapitellar joint stability. The model consisted of a custom mechanical device with no soft-tissues. Simply, the radial head was loaded axially on a distal humeral cadaveric capitellum.The results demonstrated that in this model, the monopolar metallic radial head and the native radial head behave similarly regarding resistance to subluxation. The authors found that the bipolar radial head behaved in an entirely opposite manner than the native and monopolar head and actually acted to facilitate subluxation. The authors conclude that mobility of the bipolar radial head had a compromising effect on the concavity compression stability of the radiocapitallar joint.
This in-vitro model has obvious limitations. While, it appears to show that a monopolar implant is more effective in stabilizing the radiocapitellar joint with direct axial load applied perpendicular to the humerus, the theoretical benefit of a bipolar implant is its ability to accommodate a very complex articulation and more effectively transfer load. In fact, when tested in a more clinically relevant model, it was shown that it is not possible to consistently insert a radial head prostheses anatomically. When the elbow is tested under more physiologic load with the presence of adjacent bones and soft-tissues, marked radiocapitellar subluxation occurs with a monopolar but not with a bipolar implant due to its ability to self align (Yian et. al, JSES 17: 663-70, 2008).