True AP Image of the Proximal Radioulnar Joint as a Tool for Assessment of Radial Head Height

Abstract

Replicating the original bony anatomy during radial head replacement or fracture repair is important for both joint stability and prevention of arthritis, but can be difficult to achieve. An important landmark for determining appropriate radial head height that is often overlooked is the proximal radioulnar joint (PRUJ). We propose a methodology to use an en face view of the lesser sigmoid notch as a fluoroscopic reference point to establish appropriate height for the proximal border of the prosthesis in radial head replacement. 5 fresh frozen cadaveric elbows were dissected medially to assess the relationship of the radial head to the lesser sigmoid notch of the ulna while maintaining structural integrity of the lateral stabilizers. Plain radiographs of each elbow were obtained to define a true anteroposterior view of the PRUJ. The radial head and proximal ulna were resected to measure their cartilage thickness. The angle of the lesser sigmoid notch was found to be an average of 28.2 degrees, which correlated to a 30-degree external oblique radiograph of the elbow as providing an en face view of the PRUJ. The average thickness of the radial head cartilage rim in the coronal plane was 1.95 mm, while the middle aspect of the lesser sigmoid notch was 2.5 mm. A 30-degree lateral oblique radiograph of the elbow provides a true anteroposterior view of the PRUJ, which can be a useful adjunct for restoring radial length in situations such as radial head replacement or fracture repair. Level of Evidence: Basic science study.