Effect of Pelvic Tilt on Acetabular Retroversion: A Study of Pelves From Cadavers

Anterior and posterior femoral head coverage on an anteroposterior (AP) radiograph of the pelvis can be evaluated by tracing the contours of the anterior and posterior rim. A retroverted acetabulum with excessive anterior coverage bears an increased risk for impingement of the anterosuperior femoral neck against the acetabular rim. ^18,21 Two radiographic signs have been described for identification of acetabular retroversion on plain AP radiographs. These signs include the cross-over sign and the posterior wall sign. ²¹ The degree of pelvic inclination, however, will not only influence the presence or absence of the cross-over and posterior wall signs, but also will influence other radiographically measured parameters for femoral head coverage, such as the acetabular angle of Sharp and the lateral center edge angle. ^{2,3,12,14,22,24}

The goal of the current study was to evaluate to which degree pelvic inclination will influence the presence or absence of the radiographic signs for acetabular retroversion and to provide an objective means to judge whether acetabular retroversion may be overestimated or underestimated on a given radiograph of a pelvis. A better understanding of this relationship will improve positioning of the acetabulum in reorientation procedures of the dysplastic acetabulum and will improve radiographic evaluation of hips with femoroacetabular impingement. In the same way it will improve interpretation of position of prosthetic sockets.

MATERIALS AND METHODS

An easy to use method to assess pelvic inclination from standard AP radiographs of the pelvis was validated. Eighty-six AP radiographs of the pelvis taken with the patient supine were obtained. The radiographs were taken for nonorthopaedic indications (gynecologic, genitourinary indications) and did not show osteoarthritic changes in the hip or lower lumbar spine. There were 42 female and 44 male age-matched patients with an average age of 53 years (range, 18–81 years) for women and 52 years (range, 22–84 years) for men. The radiographs were taken in a standard manner with a tube to film distance of 120 cm and a tube orientation perpendicular to the table. The central beam was directed to the midpoint between the upper border of the symphysis and a horizontal line connecting both anterior iliac spines. The pelvis had to be orientated correctly without rotation to either side. This was assessed using the alignment of the tip of the coccyx with the middle of the symphysis and the symmetric appearance of the tear drop signs, the obturator foramina, and the iliac wings. As a measure of pelvic inclination, the distance between the symphysis and the middle of the sacrococcygeal joint was measured. The mean distance between the symphysis and the sacrococcygeal joint was defined as the neutral pelvic tilt position for each gender separately. As an alternative the authors found that measurement of the distance between the symphysis and the tip of the coccyx also could be used as a parameter for pelvic inclination. However, the latter distance was not used in the current study because the coccyx seemed to be more difficult to identify on radiographs with suboptimal exposure than the sacrococcygeal joint and because the four cadaver pelves all lacked the coccygeal bone. In addition occasional deviation of the tip of the coccyx to either side seems to render this landmark less favorable.

To analyze the relationship between the length of the distance between the symphysis and the sacrococcygeal joint and the degree of pelvic inclination, pelves were obtained from two female and two male cadavers. All donors were free of musculoskeletal conditions and the acetabula were free of osteoarthritic changes. The four pelves were mounted on a specially designed holding device. The holding device allowed free positioning of the pelvis in any plane (Fig 1A). The pelves were placed in the neutral position, which was defined by (1) horizontal alignment of the sciatic tuberosities; (2) symmetric size of the obturator foramina and vertical alignment of the middle of the sacrococcygeal joint and the middle of the symphysis; and (3) the distance between the symphysis and sacrococcygeal joint equal to the average measured value for each gender (Fig 1A). In this position, a lateral radiograph was taken and the pelvic tilt angle, formed by the line drawn from the promontory of the sacrum and the upper border of the symphysis with the horizontal plane, ^23,25 was measured. Standard AP radiographs were taken at various pelvic inclinations with the same technique that is used clinically. Each pelvis was tilted forward (inclination) and backward (reclination) in 3° steps with documentation of the distance between the symphysis and the sacrococcygeal joint in each position (Fig 1B–C). The range of distances between the symphysis and the sacrococcygeal joint corresponded to the range of values measured in the normal control group.

The variability of radiographic signs of acetabulum retroversion with various pelvic inclination then was assessed. The acetabular rims were marked on each cadaver using a metal wire. The wire marking allowed clear differentiation between the S-shaped and oblique anterior rim and the more straight and vertical posterior rim. Two radiographic signs for retroversion were evaluated. ²¹ The cross-over sign is present when the anterior rim just inferior to the lateral-most edge of the acetabulum is projected more lateral than the posterior rim, but distally crosses the posterior rim (cross-over) and becomes projected more medial to the latter (Fig 1A). The posterior wall sign is present when the outline of the posterior rim is more medial than the center of the hip indicating a relative small posterior coverage (Fig 1A, both sides). ²¹ Because the femoral heads were disarticulated on the pelves from cadavers, the center of the acetabulum was used as the center of the hip. By choosing three points on the contour of the subchondral sclerosis line of the acetabulum a circle of the acetabular diameter with its corresponding center could be drawn. In the pelves from cadavers, starting from the aforementioned defined neutral position, presence or absence of the cross-over sign and the posterior wall sign were determined for each 3° stepwise increase or decrease in pelvic inclination. The range of inclination and reclination was the same as above. In cases with a positive cross-over sign, the relative length of the lateral overlap of the anterior rim compared with the entire length of the acetabular opening was calculated. The percentage of the anterior overlap is the result of the distance AC (Fig 2) from the most superior point of the acetabulum to the point of the cross-over sign divided through the distance AB from the most superior to the most inferior point of the acetabulum, which represents the acetabular opening. ⁸ These measurements were done to obtain a quantitative evaluation of the anterior acetabular rim overlap.

Statistics

To compare the measurements from radiographs between men and women, the Mann-Whitney rank sum test was used because there was no normal distribution. A box plot was used to show the median value, the 0, twenty-fifth, seventy-fifth, and one hundredth percentiles. The correlations between the degree of pelvic inclination with the distances between the symphysis and the sacrococcygeal joint and with the lengths of the relative anterior overlap were analyzed by using a repeated measures analysis of variance on ranks.

RESULTS

On the plain radiographs of the 86 patients, the average distance between the symphysis and the sacrococcygeal joint was 32.3 mm (range, 8–50 mm) in men compared with 47.3 mm (range, 15–72 mm) in women. This difference was statistically significant (p < 0.0001) (Fig 3).

In each pelvis from a cadaver, the neutral position was obtained by adjusting the distance between the symphysis and the sacrococcygeal joint on a standard radiograph of the pelvis to the average value of 47 mm in women and 32 mm in men. After that, lateral radiographs were taken in a manner such that both iliac wings and both acetabula were superimposing on each other. The pelvic tilt angles on the lateral views were calculated according to the method of measuring the pelvic conjugate. ^19,23,25 Thereby the pelvic inclination is determined by the angle between a reference line through the sacral promontory and the upper portion of the symphysis with a horizontal line. The inclination angles were 59° and 64° for the pelves from females and 60° and 66° for the pelves from males. The 3° stepwise alteration of the pelvic inclination in the four pelves from cadavers was correlated significantly with alteration of the length of the distance between the symphysis and the sacrococcygeal joint (p < 0.001) (Fig 4). The minimum distance between the symphysis and the sacrococcygeal joint of 15 mm in women and 8 mm in men corresponded to 9° and 12° reclination, respectively. The maximum distance between the symphysis and the sacrococcygeal joint of 72 mm in women and 50 mm in men corresponded to 9° inclination in both genders.

There was an increase in the number of hips with retroversion signs with increasing pelvic inclination (Fig 5). In the neutral position, a positive cross-over sign was seen in four of the eight wire-marked acetabula, all from males. A 9° increase in pelvic inclination led to a positive cross-over sign in all acetabula. With 12° reclination, no positive cross-over sign was seen in any acetabulum. In the neutral position the posterior wall sign was seen in the two acetabula of one pelvis from a male. With 6° reclination, it was not seen in any acetabulum whereas it could be detected in all acetabula with 9° inclination. The relative length of anterior overlap showed a statistically significant correlation with pelvic inclination and a significant difference between genders (p < 0.001) (Fig 6).

In addition to the dependence on the pelvic tilt position, a 6° rotation of the pelvis around a vertical axis led to the appearance of a cross-over and posterior wall sign in the ipsilateral acetabulum of all pelves from cadavers. A 6° rotation to the right side produced retroversion signs on the right acetabulum. At the same time, retroversion signs on the contralateral side disappeared even if they were present at neutral rotation. This corresponded to a 16-mm (range, 14–18 mm) deviation of the middle of the sacrococcygeal joint from the midline through the symphysis on an AP radiograph.

DISCUSSION

Several methods have been used to evaluate pelvic tilt on AP radiographs of the pelvis. ^2,4,14,22 Most of the methods were proposed for measurements in children. A quotient commonly was calculated between the length of a horizontal line through the triradiate cartilage in children or through the base of the teardrop sign in adults and other landmarks of the pelvis such as the height of the obturator foramina. ^2,4,11,14 The method described in the current study is easy to apply and showed a statistically significant correlation between the length of distance between the symphysis and the sacrococcygeal joint and various positions of pelvic tilt through a range of 21° in four pelves from cadavers. Similarly, the distance from the tip of the coccyx to the symphysis could be used as a measure of pelvic tilt. Unfortunately, validation of this technique was not possible in the current study because of a lack of coccyges in the pelves from cadavers.

The angle of pelvic inclination of the pelves from cadavers was measured on lateral radiographs in the neutral position, as determined by the average lengths between the symphysis and the sacrococcygeal joint measured from the 86 radiographs from patients not receiving orthopaedic treatment. Using the reference line through the sacral promontory and the upper portion of the symphysis (pelvic conjugate) and the angle formed with a horizontal line, the inclination in the four pelves from cadavers was between 59° and 66°. These measurements are close to the 54° to 63° range described in the literature. ^4,9,19,23,25 The majority of the measurements reported in the literature were taken from radiographs obtained with the patient standing whereas the radiographs used in the current study were taken with the patients in the supine position. When going from the supine to standing position, some authors described a slight posterior tilt (reclination) of the pelvis. ^1,12,20 Konishi et al ¹⁴ found a statistically significant 5° decrease in pelvic inclination in the standing position. Therefore, in the current study, subtracting 5° inclination from the measured pelvic tilt angles to account for the supine positioning gives an average inclination of 57°. This is within the range described in the literature and seems to validate the use of the distance between the symphysis and the sacrococcygeal joint to measure pelvic inclination.

An important finding of the current study was that the radiographic signs of acetabular retroversion, such as the cross-over and posterior wall signs, ²¹ will be inaccurate if pelvic inclination is not taken into account. Within the normal range of measurement of the distance between the symphysis and the sacrococcygeal joint seen on the 86 normal radiographs of the pelves, the presence or absence of retroversion signs have been variable. The range of measured distances corresponded to a pelvic tilt angle between 12° reclination and 9°. This 21° range is consistent with the 21° to 25° range of pelvic tilt described previously. ^14,19 When the presence of radiographic signs of retroversion in the pelves from cadavers was assessed at the normal range of pelvic tilt, the result was highly variable, suggesting that assessment of pelvic tilt is critical in proper interpretation of radiographic signs of retroversion. This observation corresponds well with the dependence of acetabular anteversion or anterior femoral head coverage on the degree of pelvic inclination as reported by several authors. ^1,12,16 The ratio between coverage of the anterior to the posterior femoral head hemisphere increased with pelvic inclination and decreased with pelvic reclination. ¹⁴ Anda et al ¹ calculated an increase of 0.5° acetabular anteversion with each 1° of pelvic posterior tilt and vice versa.

In previous studies, the mean determined inclination of the female pelvis has exceeded that of the mean measured pelvic inclination of males by values ranging from 0.5° to 2.2°. ^1,12,13,19 However, the differences were not statistically significant except in one study. ¹² The longer distances between the symphysis and the sacrococcygeal joint in women measured on the radiographs in the current study may be only partially attributable to an increase in pelvic inclination. Other differences in morphologic features between pelves from males and females such as a shorter sacrum seen in women ²⁵ likely contribute to the significantly longer distance between the symphysis and the sacrococcygeal joint seen in women. Therefore, it is more accurate to compare pelvic inclination for each gender separately when using the measurement of the distance between the symphysis and the sacrococcygeal joint. In pelves from male cadavers a smaller increase in inclination was required to produce positive signs for acetabular retroversion. One also may assume different acetabular orientation between genders. This is supported by the finding of higher acetabular anteversion angles in women than in men ^1,5,7,20,26 reaching a difference of 5° with respect to a distinct anatomic plane as defined by McKibbin. ¹⁶

The proper assessment of acetabular version and femoral head coverage is not only important when evaluating a hip for anterior femoroacetabular impingement syndrome but also in evaluating patients with acetabular dysplasia. Acetabular dysplasia does not follow a uniform pattern but shows a wide variance in deficiency and mixture of anterior, posterior, and lateral insufficiency of coverage. ¹⁷ This also will include hips that show a slightly insufficient lateral coverage, but have anterior impingement. Proximal focal femoral deficiency also has been associated with acetabular dysplasia and retroversion. ²⁴ Similarly, impingement could be seen in true retroversion syndrome such as posttraumatic dysplasia ⁶ and anterosuperior overcoverage without true retroversion seen in coxa profunda, protrusio acetabuli, and otherwise healthy hips. Therefore, accurate assessment of femoral head coverage is critical. Evaluation of acetabular version by computed tomography (CT) scans may be accurate around the midlevel of the hip, ^21,24 but this is distant from the crucial area for femoroacetabular impingement at the anterosuperior roof area. Computed tomography scans at the roof area will cut the rim obliquely or tangentially, which makes accurate identification of distinct prominences difficult ²² and the measured angles less meaningful. In addition, pelvic tilt and rotation through patient positioning also influence the measured CT angles.

The planning and execution of acetabular reorientation procedures must include optimal knowledge of the specific pattern in each patient. In this context, the deleterious mechanical consequences of retroversion should compel the surgeon to include a physiologic acetabular anteversion as a goal of surgical treatment. This is as important as achieving sufficient femoral head coverage. In this way, sufficient clearance may be maintained for impingement-free flexion and internal rotation for activities of daily living. This may include trimming of a nonspherical head and improvement of the anterior head and neck junction. ^10,15

The current authors suggest that preoperative evaluation for possible acetabular retroversion can best be made on an AP radiograph of the pelvis when femoral head coverage is judged at a defined standardized position of pelvic inclination with neutral rotation. When assessing a radiograph of the pelvis for signs of acetabular retroversion, measurement of the distance between the symphysis and the sacrococcygeal joint will help identify deviations in pelvic tilt from the gender mean. A longer distance between the symphysis and the sacrococcygeal joint indicates a tendency toward an inlet view with overestimation of the anterior coverage and underestimation of the posterior coverage and vice versa. Evaluation of this distance helps to substantiate the diagnosis of retroversion and may justify a repeat pelvic radiograph in difficult cases. Evaluation of acetabular coverage may be standardized to the neutral for gender position of pelvic tilt which corresponds to a distance between the symphysis and the sacrococcygeal joint of 32 mm and 47 mm in men and women, respectively. In the future, standardization of pelvic radiographs to the neutral position will likely be accomplished by a computerized model which can compensate for malrotation or increased or decreased pelvic tilt for accurate corrected measurement of the hip parameters.