The prevalence of acetabular retroversion at each level and overall was determined. To assess for a torsional abnormality of the pelvis at the level of the acetabulum, the distance from the ischial spine tip to the sagittal plane was divided by the acetabular center to the sagittal plane distance to calculate an ischial spine index (ISI) for each hip, thus standardizing the ischial spine position to the size of the pelvis.
To determine whether acetabular retroversion represented a torsional phenomenon of the entire pelvic segment containing the acetabulum and the ischial spine, simple linear regression was used. This analysis was first performed to determine the relationship between version at Levels 1, 2, 3, and 5 and version at the midacetabular level, Level 4. Subsequently, the same regression method was used to evaluate the relationship between ISI, as a marker of ischial spine distance from the sagittal plane, and version at Level 4. The coefficient of determination, r2, and the sample correlation coefficient, r, were obtained.
Differences in each measurement were compared based on gender using ANOVA. Post hoc testing was performed with the Bonferroni-Dunn method. Linear regression was performed using Excel® (Microsoft Corp, Redmond, WA, USA). ICC and ANOVA were performed with SPSS® (Version 9; SPSS Inc, Chicago, IL, USA) and StatView® software (SAS Institute Inc, Cary, NC, USA), respectively.
The mean (± SD) anteversion at the midlevel of the acetabulum was 21.3° ± 5.8°. The anteversion decreased and was more variable at the superior levels of the acetabulum. In the uppermost level of the acetabulum, anteversion was 14.4° ± 10.5°. Acetabular abduction was 39.7° ± 4.3°. The ISI measured 0.6 ± 0.1 (Table 2).
The prevalence of acetabular retroversion overall was 7% (seven of 100 acetabula, in two pelves bilaterally, in three unilaterally). Of the seven retroverted acetabula, five occurred in males (one bilaterally, three unilaterally) and bilateral retroversion occurred in one female. The prevalence of retroversion was 7% (seven of 100 acetabula, two bilaterally, three unilaterally) at Level 1 and 2% (two of 100 acetabula, one pelvis bilaterally) at Level 2. At each level from 1 to 5, females had higher anteversion values than males (Table 2). Retroversion did not occur in this series of pelves at Levels 3, 4, or 5. Two acetabula in two separate male pelves did not have a Level 1 AV measurement because of excessive acetabular coverage caused by an abnormally deep acetabulum. We observed correlations between Level 1 and Level 4 AV (r = 0.74), the ISI and Level 4 AV (r = 0.67), Level 2 and Level 4 AV (r = 0.83), Level 3 and Level 4 AV (r = 0.95), and Level 4 and Level 5 AV (r = 0.92) (Fig. 6).
Numerous structural deformities of the acetabulum are associated with hip OA. Current methods of evaluating acetabular anatomy are prone to inaccuracy from patient positioning and pelvic tilt. We established a quantitative measurement method using 3-D models to measure AV, abduction, and ischial spine position using an ISI. Using this method, we determined its prevalence and whether retroversion was isolated to the superior acetabulum or involved the entire acetabulum and/or the pelvic segment including the acetabulum and the ischial spine.
There are some limitations to this study. First, the sample size was small with 50 subjects (100 acetabula). Prior studies determining the prevalence of retroversion in the general population have evaluated similar numbers [7, 9]. However, all measurements and 3-D reconstructions were performed by one observer (AP) and confirmed by a second observer (AAJ). Second, the technique is currently time consuming, limiting its clinical utility. It requires manual segmentation of the bone in the software, placement and positioning of the measurement spheres, and placement of a total of 42 points, thus requiring approximately 1 hour of technician time per case. Development of an automated method for these calculations in the future would facilitate the establishment of normal and pathologic values in a larger number of pelvic specimens. Third, clinical information for the patients was not available, leading to a potential selection bias. Every effort was made to select a random sample from a large database of pelvic CT scans, and none of the patients had any signs of trauma based on the assessment of a musculoskeletal radiologist. Fourth, plain radiographs of these hips were not available, making comparisons of our findings to those of standard radiographs impossible. This information would be helpful as standard radiographs are widely available and a correlation of retroversion using this technique to the radiographic appearance would be informative. Fifth, we present data regarding acetabular anatomy in the static position referencing the standard coronal plane. Some studies have shown this plane can be variable in vivo [2, 4, 20, 38]. Zilber et al.  addressed the relationship among caudal, cranial, and central anteversion with various degrees of pelvic tilt. They dissected 10 complete pelvic specimens and performed CT scans by positioning the specimens with 0°, 20°, 40°, and 60° of sacral gradient (sacral slope). Sacral gradient was defined as the angle between the upper endplate of S1 and the horizontal plane. The sacral gradient averages 60° in the supine position , 40° in standing , and between 0° and 29° in the sitting position . These authors  found anteversion measurements in all three positions diminished with forward pelvic tilt. The anteversion values at the 40° and 60° sacral gradients were slightly higher and lower, respectively, than the values obtained for anteversion in other studies (Table 3). This could be a result of their small number of specimens and the variability of sacral gradient in vivo compared with the cadaveric study. Lembeck et al.  measured pelvic tilt in 30 asymptomatic individuals using an inclinometer. They assigned negative values to pelves that were tilted posteriorly (extended). They found the average pelvic tilt was −8° (range, −17°-+3°) in the lying position (relative to the horizontal) and −12° (range, −27°-+3°) in the standing position (relative to the vertical). Using a correction for soft tissue thickness over each bony prominence, these authors estimated the pelvis was reclined (extended) between 8° in standing and 4° in the supine position. Dardenne et al.  also used ultrasound in assessing dynamic pelvic position. However, their ultrasound probes were directly attached to a surgical navigation system. They found 2.4° ± 5.1° of anterior tilt (flexion) relative to the vertical plane in the standing position and 6.8° ± 3.5° of anterior tilt (flexion) relative to the horizontal plane in the supine position. The issues with the standard coronal plane were addressed intraoperatively by Babisch et al. . They evaluated CT scans of 30 patients with OA of the hip with the patients in the supine position and lateral radiographs in the standing position. They noted an inclination (flexion) of the pelvis of 10.4° ± 7.4° and 5° ± 9.4° in the supine and standing positions, respectively. They developed normograms to allow surgical navigation systems to place the acetabular component in THA with consideration of the pelvic tilt. A summary of the literature on dynamic pelvic position suggests that the anterior pelvic plane is routinely positioned close to the horizontal plane in the supine position and close to the vertical plane in the standing position. In this study, we did not make specific adjustments for pelvic tilt. This decision was made based on our objective to clearly define the acetabular anatomy regardless of positional changes and to optimize reliability of the measurements. Additional studies would be needed to correlate pelvic-specific anatomic data using this technique to in vivo positional data.
Using this technique, midacetabular anteversion was 21.3° ± 5.8°. This value is consistent with previous measurements reported in the literature ranging from 15° to 20° [3, 11, 21, 22, 26-28, 32, 35, 36] (Table 3). We used the same software package to develop a standardized method to measure the distance from the ischial spine from the sagittal plane, termed the ISI.
We found a prevalence of acetabular retroversion of 7%, similar to published values of 5% to 6% [7, 9]. However, this prevalence was substantially lower than the 22% reported by Jamali et al. . There are two potential explanations. First, in that series, the measurements were performed by hand on cadaveric specimens at a level 5 mm below the superior-most point of the acetabulum. In our study, a given distance was not used to determine the cranial acetabulum but rather a standardized measurement at a level Symbol of the distance from the top of the acetabular cavity was used (at Level 1 of seven with Level 4 as the equator). Second, manual measurements performed in that series may have been less accurate than the computerized measurements performed here. A previous study showed the measurements performed in this software program are more accurate and precise than manual linear and angular measurements .
We found acetabular anatomy can be measured reproducibly using 3-D CT-generated models regardless of patient positioning in the scanner. We observed a correlation between cranial and central acetabular anteversion and between the ISI and central anteversion, suggesting acetabular retroversion is a phenomenon involving the entire pelvic segment containing the acetabulum and the ischial spine. Future effort will be directed toward automating the technique and potentially incorporating in vivo data on pelvic tilt to consider functional acetabular anteversion.
This information may be of benefit to surgeons in reproducing normal acetabular position and alignment at the time of periacetabular osteotomy or acetabular recontouring procedures. The recognition of a localized abnormal retroversion in the superior acetabulum would suggest optimal treatment with a superior anterior acetabuloplasty. In contrast if the acetabulum is abnormally retroverted at all levels, a more logical approach would be to consider reorientation of the entire socket using a periacetabular osteotomy. The nuances and degrees of such deformities and their symptomatic correlates are currently unknown and would benefit from further investigation.
We thank Mazie Ngai for assistance with preparation of this manuscript.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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