Acetabulum protrusio and center edge angle: new MR-imaging measurement criteria-a correlative study with measurement derived from conventional radiography

The goal of this study was to identify a method of measurement for acetabulum protrusio and center edge angle (CEA) using MR imaging of the pelvis that correlated with classic methods using radiographic landmarks. MR images and radiographs of the pelvis in 67 patients (132 hips) were used to identify reliable MR-imaging methods for measuring protrusio acetabulum and CEA that correlated strongly with established radiographic measurements. Protrusio acetabulum was determined using the radiographic criterion that the acetabular line projects medial to the ilioischial line by 3 mm or more in men and 6 mm or more in women. Pearson correlation factor was used to determine inter-observer variability and those methods that demonstrated the strongest correlation. The mean and standard deviation of MR-imaging and radiographic measurements for both the normal and protrusio hips were established. Several MR methods correlated strongly with radiographic measurements. The preferred method employed axial MR images at the level of the ischial spine with measurement of the distance between the medial most point of the acetabular fossa and a line perpendicular to the horizontal axis that passed through the lateral margin of the posterior inner pelvic wall. The Pearson's correlation factor between radiographic and MR measurements using this method was 0.84, and inter-observer correlation was 0.80. There were 126 hips in 63 patients (17 female and 44 male) that did not meet the radiographic criteria for protrusio acetabula. In this group of normal hips, the mean and standard deviation of radiographic measurements were 1.9 and 2.8 mm in male patients and -0.5 and 1.7 mm in female patients, and the mean and standard deviation for the preferred MR method was 1.3 and 2.5 mm in male patients and -0.8 and 1.9 mm in female patients. A total of six hips in four patients (two female and two male) met the radiographic criteria for protrusio acetabula. In this group of patients, the mean and standard deviation of radiographic measurements were -3.7 and 1 mm in male patients and -5.4 and 0.9 mm in female patients, and the mean and standard deviation for the preferred MR method was -4.1 and 0.4 mm in male patients and -6.5 and 0.3 mm in female patients. Our study also showed that the CEA was best measured using anterior to middle coronal MR images. Posterior coronal MR-imaging measurements correlated poorly with radiographic measurements. MR imaging can be used to assess acetabular morphology and measure acetabulum protrusio.