Purpose: The purpose of this study was to define the shape of the radial head by identifying the relationship between precisely defined axes of the radial head. Methods: An anatomic study was done to define the shape of the radial head and specifically the relationship between the long and the short axis. Twenty-seven cadaveric upper extremities were used. The x and y axes of the radial head were defined in relationship to the radial notch of the ulna, with the forearm in neutral position, Outer diameters of the x and y axis were measured. These were compared with the actual maximum and minimum diameters of the radial head. X and y diameters of the articulating surface of the radial head also were measured. Paired 1-tailed Student's t-tests were used to compare the x and y diameters of the radial head. Regression analysis of x and y diameters of the radial head was done to identify a correlation between these parameters. Results: Paired 1-tailed Student's t-tests showed a significant difference between X and Y diameters of the radial head. Regression analysis of x and y diameters of the radial head showed a strong correlation between these 2 axes. Conclusions: The radial head is not round. A strong correlation exists between the x and y diameters of the radial head. The orientation of the long axis is perpendicular to the radial notch with the forearm in neutral rotation. This finding will make it possible to approach the anatomy of the radial head more closely when designing radial head prostheses. The definition of the axes can be used as a guide when implanting the radial head prosthesis.
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