Pelvic Positioning Creates Error in CT Acetabular Measurements

Abstract

Background CT allows for accurate measurement of acetabular orientation and shape, but malpositioning of the pelvis may lead to measurement variance. Purpose We therefore sought to determine: (1) whether acetabular anteversion measurements using the femoral head centers differed from those using the posterior ischia, and (2) the extent to which changing obliquity, rotation, and tilt of a pelvis in a CT scanner affected the measurement of acetabular variables. Methods A radiopaque human pelvis model with articulated hips was suspended from a plastic sheet as part of an adjustable frame. Changes in the transverse and sagittal planes created rotation and tilt, while rotating the frame in the coronal plane created obliquity. CT scans were obtained, varying the combinations of obliquity, rotation, and tilt by intervals of 5°, up to 20°. Acetabular anteversion (AA), anterior acetabular sector angle (AASA), posterior acetabular sector angle (PASA), and horizontal acetabular sector angle (HASA) were measured. Results The two methods for measuring AA yielded values differing by 1°to 4°but correlated (r = 0.981) across the spectrum of pelvis positioning. Pelvic obliquity and tilt were linearly associated with changes in the measurements. For each 1°-increase in pelvic obliquity, AA changed À0.4°, and AASA, PASA, and HASA changed 1.93°, 0.99°, and 2.80°, respectively. For each 1°-increase in pelvic tilt, AA changed 0.8°, and AASA, PASA, and HASA changed À1.07°, 0.52°, and À0.51°, respectively. Rotation had no affect on the variables. Conclusions Small changes in pelvic obliquity and tilt were associated with variances in acetabular measurements. The measured changes were directly proportional to the changes in obliquity and tilt, and were additive. Pelvic rotation created no changes in measurement. Clinical Relevance Incorrect interpretation of acetabular anteversion and coverage may lead to unsatisfactory acetabular fragment positioning during reorientational surgery. Although intraoperative positioning of an acetabular fragment may not be as precise as the tools for preoperative planning, it is important for a surgeon to have the most precise data available for planning a procedure, and know where error can occur in collecting the data.Each author certifies that he or she has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

Computed Tomography: Role in Femoroacetabular Impingement

“…Since the pelvis has a spatial arrangement in three dimensions and considering the deviations by the position of the pelvis in relation to the table of the tomographer, the axial reconstructions and in the three dimensions were corrected in three planes [20,21], that is to say corrected the rotation, the lateral inclination and the tilting of the pelvis. For the later, diferences between genders were considered, seting a distance of 3 cm between the sacrococcygeal junction and upper border of the pubis in men and 6 cm in women [22].…”

Sexual Dimorphism of Acetabular Anatomy Based on Three-dimensional Computed Tomography Images of Pelvises

“…Często jednak ocena parametrów przestrzennych panewki dokonywana jest w oparciu o modele anatomiczne lub metody obrazowania dwuwymiarowe, jak zdjęcie roentgenowskie lub tomografia komputerowa (TK) [9][10][11][12][13][14][15][16][17][18]. Takie podejście powoduje niedokładności, wynikające z ograniczeń tych metod obrazowania [19].…”

“…Nevertheless, assessment of acetabular spatial para meters is frequently based on anatomic models or two-dimensional imaging techniques such as X-ray or computed tomography (CT) [9][10][11][12][13][14][15][16][17][18]. Such appro aches lead to imprecision due to the limitations of these imaging techniques [19].…”

CT false-profile view of the hip: a reproducible method of measuring anterior acetabular coverage using volume CT data

“…Since indices derived from projection radiographs taken in a standing position may not transpose well with data derived from CT scans acquired in a supine position, we compensated for variables in pelvic positioning using principles derived from previously published techniques [12,13,[15][16][17][18]. Images were corrected for pelvic tilt, obliquity, and rotation by manipulating the volume data set using the volume rendering technique tool on the CT workstation so that the anterior pelvic plane (defined as the anterior superior iliac spines and pubic tubercles) overlapped in the sagittal projection [16,19,20].…”