Reference points for radial head prosthesis size

PURPOSE: Metallic radial head implants are useful when the radial head cannot be repaired reliably and when either the elbow or the forearm is unstable. Problems arise when the radial head prosthesis is too thick, causing erosions of the capitellum and incongruity of the ulnohumeral joint. We used quantitative 3-dimensional computed tomography analysis to investigate the relative height of the radial head relative to the lateral edge and central ridge of the coronoid process as reference points for optimal insertion of a radial head prosthesis. METHODS: Seventeen computed tomography scans of the elbow were analyzed. The anatomic coronal plane of the forearm was determined using 3-dimensional images and a 2-dimensional image bisecting the articular surface of the radial head was created in this plane. The distance between the plane of the articular surface of the radial head and parallel planes at the most proximal aspect of the coronoid (the central ridge) and the lateral edge of the coronoid articular surface were measured. Negative values indicate the radial head is proximal to the coronoid. RESULTS: The average distance between the planes defined by the radial head articular surface and the coronoid central ridge was -0.8 mm. The average distance between the planes defined by the radial head articular surface and the lateral edge of the coronoid articular surface was -0.9 mm. CONCLUSIONS: Because the radial head was on average only 0.9 mm more proximal than the lateral edge of the coronoid process and because the key is to not overstuff the joint a useful general guideline would be to place the plane of the articular surface of the radial head even with or just slightly more proximal than the lateral edge of the coronoid articular surface. Considering the substantial variability of the normal height of the articular surface of the radial head with respect to that of the coronoid, preoperative radiographs of the opposite elbow may be useful to avoid overstuffing the elbow.