Evaluation of Crossover Sign in Pelvis Models Made with a Three-Dimensional Printer

Introduction. Investigation of the crossover sign (COS) in different degrees of tilt in pelvises made by three-dimensional printing of CT scans among patients with normal hip versions was carried out. Methods. Radiology CT scans of 8 normal pelvises reconstructed in 3D and the effect of sequential tilting on the presence of the false-positive COS in 48 radiographs were investigated. Results. The COS was seen in 77% of the AP radiographs during tilt changes. The average distance between the tip of the coccyx and the symphysis pubis was 32.06 ± 10.99 mm. Also, COSs were present in all radiographs from 6 degrees tilt and above. Conclusion. Minor tilting of the pelvis can result in a false-positive crossover sign on AP plain radiographs.

1. Introduction
The spatial orientation of the acetabulum, including its version and inclination, is an important anatomical feature of the hip joint [1]. Some of the major complications of hip arthroplasty include hip dislocation, impingement, and polyethylene abrasion and are related to improper position of acetabular hip prosthesis [2, 3]. In addition to the clinical significance of the acetabular position in joint replacement surgeries, acetabular version abnormalities can lead to hip pain and disability. Likewise, one of the most important causes of hip pain in young adults is the impingement between the head of the femur and the acetabulum. Furthermore, the retroversion of the acetabulum, which can be present globally or localized, can significantly increase the mentioned impingement [4, 5].

Normally, the anterior wall of the acetabulum is placed medially with regards to the posterior wall of the acetabulum, and the two walls meet at the top of the acetabulum [6, 7]. In a plain AP radiograph, if the anterior rim of the acetabulum intersects the posterior rim below the upper margin of the acetabulum, it is called a crossover sign (COS). Evaluation of femoral head coverage and acetabular orientation contributes to preoperative planning and decision-making of reorientation procedures [8]. Posterior and anterior femoral head coverage on an anteroposterior (AP) hip radiograph can be assessed by tracing the posterior and anterior rim contours. The increased risk for impingement followed by a retroverted acetabulum with excessive anterior coverage can be diagnosed in radiographs by the COS. Although the COS is the well-known radiographic sign for acetabular retroversion, there are controversial opinions about this sign’s clinical compliance and accuracy in the diagnosis of global or local retroversion [9, 10].

In this study, we aimed at investigating the crossover sign in different degrees of tilt in pelvises made by three-dimensional printing of CT scans among patients with normal hip versions.