The position of the pelvis has been shown to influence acetabular orientation. However there have been no studies quantifying that effect on the native acetabulum. Our aims were to investigate whether it is possible to quantify the relationship between pelvic tilt and acetabular orientation in native hips, and whether pelvic tilt affects acetabular cover of the femoral head.

Computerized tomography scans of 93 hips (36 normal, 31 dysplastic and 26 with acetabular retroversion) were analyzed. We used a CT technique that allows standardised three-dimensional (3D) analysis of acetabular inclination and anteversion and calculation of femoral head cover in relation to the anterior pelvic plane and at different degrees of forward and backward tilt. Acetabular anteversion, inclination and cover of the femoral head were measured at pelvic tilt angles ranging from −20° to 20° in relation to the anterior pelvic plane using 5° increments.

The effect of pelvic tilt on version was similar in the normal, dysplastic and retroverted groups, with a drop in anteversion ranging from 2.5° to 5° for every 5° of forward tilt. The effect on inclination was less marked and varied among the three groups. Pelvic tilt increased femoral head cover in both normal and dysplastic hips. The effect was less marked, and tended to be negligible at higher positive tilt angles, in the retroverted group.

This study has provided benchmark data on how pelvic tilt affects various acetabular parameters which in turn may be helpful in promoting greater understanding of acetabular abnormalities and how pelvic tilt affects the interpretation of pelvic radiographs.

The position of the pelvis influences acetabular orientation. In particular, pelvic tilt in the sagittal plane may lead to inaccurate interpretation of plain pelvic radiographs. We therefore quantified the relationship between this pelvic tilt and acetabular orientation in native hips, and determined whether pelvic tilt affects femoral head cover.

We analysed computed tomography scans of 93 hips (36 normal, 31 dysplastic, 26 with acetabular retroversion) and measured acetabular anteversion, inclination, and femoral head cover at pelvic tilt angles ranging from −20° to 20° in relation to the anterior pelvic plane using 5° increments.

The effect of pelvic tilt on version was similar in the normal, dysplastic, and retroverted groups, with a drop in anteversion ranging from 2.5° to 5° for every 5° of forward tilt.

There was a tendency for the inclination angle to decrease when the pelvis was tilted forward from a position of extension, and in normal hips, this produced a reduction in inclination of about 4° for every 8° of pelvic tilt; but once neutral pelvic tilt was reached, further forward rotation of the acetabulum had rather a small effect on the inclination angle.

In normal and dysplastic hips pelvic tilt increased apparent femoral head cover; in the retroverted group the effect was less marked and tended to be negligible at higher tilt angles. Anterior cover increased with increasing forward tilt in all three groups of hips. Posterior cover, on the other hand, decreased by just 2% for the dysplastic hips, 3.5% for the normal hips, and 6% for the retroverted hips over the whole range of tilt from −20° to 20°.

A greater understanding of the influence of pelvic tilt may allow improvements in the radiological diagnosis and surgical treatment of acetabular abnormalities, particularly in relation to acetabular reorientation procedures and femoroacetabular impingement.