DO ALTERATIONS IN PELVIC ROTATION ON THE FALSE PROFILE RADIOGRAPH EFFECT THE APPARENT AIIS MORPHOLOGY?

Abstract

Background: Prominence of the anterior inferior iliac spine (AIIS) may contribute to femoroacetabular impingement (FAI). Identification and classification of these morphologic variants are important to evaluation and treatment of symptomatic impingement. A standard hip radiographic series including false profile radiographs are commonly used for assessment of AIIS morphology, despite 3D-CT imaging being the gold standard. Yet, obtaining the accurate degree of rotation necessary for precise false profile radiographs in the office is difficult, often leaving the clinician to interpret radiographs with non-standard pelvic rotation. The purpose of this study is to investigate the effects of pelvic rotation of false profile radiographs on perceived AIIS morphology. Methods: Fifteen cadaveric hips (8 pelvises: 4 male, 4 females; 68-92 years of age) without evidence of prior surgery or osteoarthritis were obtained. Samples were fixed to a custom jig in neutral pelvic tilt and rotation. An AP pelvis and radiographic images from 450 to 850 of rotation at 50 intervals were taken, with 650 designated as a true false profile view. A single grader identified the morphology of the AIIS deformities on false profile views as well as measurement of the AIIS offset of each cadaver in 5 degree increments of rotation from 45 to 85 degrees (65 degree true false profile). Interobserver and intraobserver reliability was assessed in a random subset of 20 views. Paired t-test was used to evaluate significance of AIIS offset at varying degrees of pelvic rotation. Results: Significant increases (p < 0.05) in measured AIIS offset were observed with each subsequent increase of 50 of pelvic rotation (greater AIIS offset seen with increased pelvic rotation towards lateral). Mean AIIS offset values for each 50 of rotation are shown in Figure 1. Figure 2 shows examples of observed differences in AIIS offset at both extremes of rotation relative to a true false profile. There was strong inter- and intra-observer reliability for AIIS offset (0.920 and 0.965), especially compared to measured femoral head diameter (0.657 and 0.779). Conclusion: Significant variation in measurement of AIIS offset on false profile radiographs was observed based on degree of pelvic rotation. Increased prominence of AIIS (decreased AIIS offset) was noted with decreased pelvic rotation (away from true lateral view). These findings stress the importance of clinician caution and close attention to pelvic positioning with radiographic evaluation of AIIS prominence.