Aims

The lateral centre-edge angle (LCEA) is a plain radiological measure of superolateral cover of the femoral head. This study aims to establish the correlation between 2D radiological and 3D CT measurements of acetabular morphology, and to describe the relationship between LCEA and femoral head cover (FHC).

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.

INTRODUCTION Assessing femoral head coverage is a crucial element in acetabular surgery for hip dysplasia. CT has proven to be more accurate, practical and informative than plain radiography at analysing hip geometry. Klaue et al first used a computer-assisted model to indirectly derive representations of femoral head coverage. Jansen et al then described a CT-based method for measuring centre edge angle of Wiberg at 10 rotational increments. Haddad et al used that method to look at dysplastic hips pre- and post-acetabular osteotomy. We present a novel CT-based method that automatically gives an image of the head with the covered area precisely represented. We used this technique to accurately measure femoral head coverage (FHC) in normal hips and in a prospective study of patients with hip dysplasia undergoing peri-acetabular osteotomy. The impact of surgery on acetabular anteversion and inclination was also assessed.

METHODS Using a custom software programme, anatomical landmarks for 25 normal and 26 dysplastic hips were acquired on the 3D reconstructed CT image and used to define the frame of reference. Points were then assigned on the femoral head surface and the superior half of the acetabular rim after aligning the pelvis in the anterior pelvic plane. The programme then automatically produced an image representing the femoral head and its covered part along with the calculated femoral head coverage. To do so, the software represents the femoral head by a best-fit sphere, and the sphere and the acetabular contour are then projected onto a plane in order to calculate the load bearing fraction and area.

RESULTS In the normal hips FHC averaged 73% (SD 4), whereas anteversion and inclination averaged 16° (SD 7°) and 44° (SD 4°) respectively. In the dysplastic group the mean FHC was 50% (SD 6), with a mean anteversion of 19° (SD 10°) and mean inclination of 53° (SD 5°). Peri-acetabular osteotomy has been performed on 16 hips so far, and the FHC for those averaged 66% (SD 5), a mean improvement of 32%. The respective anteversion and inclination post-operatively were 18° (SD 12°) and 40° (SD 8°).

DISCUSSION This is the first study to our knowledge that has used a reliable and practical measurement technique to give an indication of the percent coverage of the femoral head by the acetabulum in normal hips. When this is applied to assessing coverage in surgery to address hip dysplasia it gives a clearer understanding of where the corrected hip stands in relation to a normal hip, and this should allow for better determination of the likely outcome of this type of surgery. The versatility of the method gives it significant attraction for acetabular surgeons and makes it useful not only for studying dysplastic hips but also other hip problems such as acetabular retroversion.

Assessing femoral head coverage is a crucial element in acetabular surgery for hip dysplasia. Plain radiographic indices give rather limited information. We present a novel CT-based method that measures the fraction of the femoral head that is covered by the acetabulum. This method also produces a direct image of the femoral head with the covered part clearly represented, and it also measures acetabular inclination and anteversion. We used this method to determine normal coverage, and applied it to a prospective study of patients with hip dysplasia undergoing periacetabular osteotomy.

Twenty-five normal and 26 dysplastic hips were studied. On each CT scan points were assigned on the femoral head surface and the superior half of the acetabular rim. The anterior pelvic plane was then defined, and the pelvis was aligned in that plane. Using our custom software programme, the fraction of the head that was covered was measured, in addition to acetabular inclination and anteversion.

In the normal hips femoral head coverage averaged 73% (SD 4). In the same group, mean anteversion was 15.7° (SD 7°), whereas mean inclination was 44.4° (SD 4°). In the dysplastic group femoral head coverage averaged 50.3% (SD 6), whereas mean anteversion and inclination were 18.7° (SD 9°) and 53.2° (SD 5°) respectively.

This is the first study to our knowledge that has used a reliable measurement technique of femoral head coverage by the acetabulum in the normal hip. When this is applied to assessing coverage in surgery for hip dysplasia it allows a clearer understanding of where the corrected hip stands in relation to a normal hip. This would then allow for better determination of the likely outcome of this type of surgery. We are presently conducting a prospective study using this technique to study dysplastic hips pre- and post-periacetabular osteotomy.

Introduction: Assessing coverage of the femoral head is a crucial element in acetabular surgery for hip dysplasia. Radiographic indices give rather limited information. We present a novel ct-based method that gives an image of the head with the covered area precisely represented. We used this method to measure femoral head coverage in a series of normal hips and in a prospective study of patients with hip dysplasia undergoing peri-acetabular osteotomy.

Methods: Thirteen normal and ten dysplastic hips were studied. On each CT scan anatomical landmarks were assigned on the 3d reconstructed image and used to define the frame of reference. Points were assigned on the femoral head surface and the superior half of the acetabular rim after aligning the pelvis in the anterior pelvic plane. An image was produced representing the femoral head and its covered part. The fraction of the head that was covered was calculated.

Results: The average femoral head coverage in the normal hips was 73.9% (sd 3.2). The average coverage in the dysplastic group was 50.7% (sd 7.9) and after undergoing peri-acetabular osteotomy the average was 67% (sd 6.2).

Conclusion: This is the first study to our knowledge that has used a reliable measurement technique to give an indication of the percent coverage of the femoral head by the acetabulum in the “normal hip”. When this is applied to assessing coverage in surgery to address hip dysplasia it gives a clearer understanding of where the corrected hip stands in relation to a normal hip, and this should allow for better determination of the likely outcome of this type of surgery.