Reliability of overcoverage parameters with varying morphologic pincer features: comparison of EOS® and radiography

Abstract

Background: Multiple radiographic parameters used for diagnosis and quantification of morphologic pincer features have emerged, but the degree to which pelvic tilt or rotation affects conventional radiography and EOS(®) is unknown.

Question/purposes: We asked: (1) What is the reliability of EOS(®) and conventional radiography at increasing sizes of morphologic pincer features with varying degrees of tilt and rotation? (2) What is the effect of tilt and rotation on acetabular overcoverage measurements?

Methods: Using a dry cadaveric pelvis, AP conventional radiographs and EOS(®) images were taken at intervals of increasing modeled pincer size with 0° to 15° varying tilt and rotation. Lateral center-edge angle, Sharp angle, Tönnis angle, crossover sign, and retroversion index were measured on all images. Statistical analysis was conducted.

Results: The intermodality intraclass correlation coefficients for conventional radiography and EOS(®) radiography across all pincer sizes, rotations, and tilts were excellent (0.93-0.98). Crossover sign was in perfect agreement in conventional radiography and EOS(®). Rotation of the hip away from the beam source and/or increased anterior tilt falsely increased all overcoverage parameters except for Tönnis angle. Rotation away from the beam of 10° or greater or anterior tilt of 5° or greater produced a false-positive crossover sign.

Conclusions: EOS(®) radiography maintained excellent reliability in comparison to conventional radiography but both were equally vulnerable to the effects of tilt and rotation for quantification of hip parameters used in acetabular overcoverage assessment. A standardized pelvic radiograph ensuring that the pelvis is not excessively tilted or rotated should be used for assessing acetabular overcoverage parameters.

Fig. 1A–C

(A) The pelvis on the wood structure placed on top of the wood board is shown in the coronal plane. The pelvis is in neutral position. (B) An axial view of the pelvis on the wood structure on top of the wood board with rotation markings is shown. The pelvis is in 10° of left hip rotation and −10° of right hip rotation in this example. (C) A sagittal view of the wood structure on top of the wood board with the 10° calibrated wooden block is shown.

Fig. 2

A caliper is used to measure the modeled pincer at the 1 o’clock position (in this case 12 mm). The pincer then is contoured into a crescent shape from the 11 to 2 o’clock position of the acetabulum with the most lateral edge or peak of the crescent at the 1 o’clock position.

Fig. 3A–G

The left column shows (A) 2 mm, (B) 6 mm, and (C) 10 mm pincer sizes of the right hip. (D) The dry cadaveric pelvis without any modeled morphologic features of the pincer morphology is shown. The right column shows (E) 4 mm, (F) 8 mm, and (G) 12 mm pincer sizes of the left hip.

Fig. 4

An AP EOS^® image shows the dry cadaveric pelvis with 2- and 4-mm pincers. A crossover sign is considered positive when the anterior rim crosses the posterior rim. This is seen in the right hip by crossing of the dotted line (posterior rim) and solid line (anterior rim). The distance of the superior lateral edge of the weightbearing zone to the crossover of the anterior and posterior rim (B) divided by the entire length of lateral acetabular opening (A) constituted the retroversion index (A/B). The angle formed by a line from the inferior pelvic teardrop to the superior lateral edge of the acetabulum and a line horizontal from the inferior pelvic teardrop constituted the Sharp angle (SA). The lateral center-edge angle (LCEA) was formed by a line vertical from the center of the femoral head and a line from center of the femoral head to the superior lateral edge of the acetabulum. The Tönnis angle (TA) was formed by a horizontal line from the medial portion of the weightbearing zone (ie, sclerotic zone) and a line from the medial portion of the weightbearing zone to the superior lateral edge of the acetabulum. As the dry cadaveric pelvis had neutral inclination, no inclination compensatory methods were needed for lateral center-edge angle, Sharp angle, and Tönnis angle.

Fig. 5A–C

AP EOS^® images show the dry cadaveric pelvis with 6- and 8-mm pincers at (A) neutral position, (B) anterior tilt of 15°, and (C) posterior tilt of 15°. Horizontal widening and narrowing can be seen in the anterior tilt of 15° and posterior tilt of 15°, respectively.