Radiology of the Sacroiliac Joint

Study Design. Radiology of the sacroiliac joint was investigated by obtaining different and multiple radiographs of cadaveric pelves marked with solder metal wire and radiopaque paint. Objectives. To demonstrate the orientation of the sacroiliac joint on various radiographic views. Summary of Background Data. Interpretation of the sacroiliac joint projection on plain radiography is difficult. It requires an understanding and appreciation of its components and their orientation. Emphasizing the definition of the orientation of the plane of the joint on the different projection views of the sacroiliac joints can aid the orthopaedic surgeon in obtaining the proper radiographs and in the proper interpretation of the different radiographic views. Methods. Nineteen sacroiliac joints from 10 cadaveric pelves, 5 males and 5 females, were studied. Each joint was found to be composed of three portions: anterosuperior, middle, and posteroinferior portions, each lying in a different plane. Each sacroiliac joint was marked with solder wires and radiopaque paint to define the orientation of each of the three portions of the joint on radiographs. The following radiographic projection views were taken for each joint: anteroposterior, lateral, inlet, craniocaudal axial, outlet, lithotomy, and oblique views. For the oblique views, the angulation of the x-ray tube needed to view each portion of the joint tangentially was recorded. Results. There was a wide variation in the orientation of the planes of the joint portions between the right and the left sides as well as between different pelves. Although the twisting of the plane of the whole joint produced by the successive examination of the portions could be either internal or external, it was the same bilaterally in a given specimen. The outlet and lithotomy views provided the best tangential representation of the two sacroiliac joints on one film. Conclusion. The sacroiliac joint is composed of three portions oriented in different planes. To study the sacroiliac joints, it seems desirable to obtain an anteroposterior view of the pelvis with the patient in a lithotomy position; then, if needed, each joint can be radiographed separately by using oblique views. It is important to note that the plane of the auricular portion of the joint can be directed from anterolateral to posteromedial, and therefore, the oblique views should be obtained accordingly.