Design Considerations in Radial Head Arthroplasty

Radial head arthroplasty design must consider that the implant needs to bear load, articulate correctly, and/or compensate for incorrect articulation. The radial head is elliptical, with a circular articular dish that is offset from the axis of rotation of the forearm, and angled in two planes. Design considerations can be grouped as to whether they relate to the head, the stem, or the head-stem connection. The head has an asymmetrical shape that bears load differently in different degrees of elbow flexion and forearm rotation. Importantly among these is the crescent-shaped rim posteromedially which has a variable radius of curvature that articulates with the lateral trochlear ridge of the humerus. This is the main load-bearing region in pronation/extension. A nonanatomic monoblock prosthesis is at risk of edge loading or failing to bear adequate load in this region. Stems are either fixed or loose-fitting. Fixed stems can be either cemented or press-fitted for bone ingrowth. Getting the height correct is one of the two most important technical variables. Getting the stem direct diameter correct is essential to prevent loosening of a porous ingrowth stem. Broaches and the final stem must be hammered into the canal to ensure reliable ingrowth. Current concerns include symptomatic loosening and osteolysis around ingrowth stems, cartilage and bone erosion due to nonanatomic radial head shapes on press-fitted stems, failure of head-stem coupling mechanisms, and the question of whether or not a loose-fitting stem in the canal truly functions as a load-bearing prosthetic replacement in the long term.