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.

Wear and loosening are the major causes for long tem failure in Total Hip Replacement (THR). Accurate three dimensional wear analysis of radiographs has its own limitations. We report the results of our clinical study of three dimensional volumetric wear measurements using our custom low radiation risk CT based algorithm and special software

Twenty four patients (32 hips) agreed to take part in our study. The male: female ratio was 1:4. The mean age was 75 years and the mean follow up was 5.4 years. All patients had 28 mm diameter ceramic heads. Of the 32 hips, 17 hips had polyethylene inserts and 15 hips had ceramic inserts. The maximum follow up for the polyethylene and ceramic groups were 12 years and 5.5 years respectively. All the patients were scanned using Somatom Sensation 4 scanner. Using custom software, 3D reconstruction of the components was done and landmark acquisition done on the femoral head, acetabular metal component and the insert. From these landmarks, a dedicated program was used to calculate the centre of the femoral head in relation to the centre of the acetabular component in all three axes and an indirect measurement of wear obtained. Using the axes measurements graphical 3D models of migration of the femoral head component into the acetabular liner were created and volume of wear measured using special software. Accuracy of the method was assessed by measuring the radius of the femoral head since all patients had 28mm diameter heads implanted in them. Assessment of precision of method was done by calculating the level of agreement between two independent observers.

In the polyethylene group, there was no significant (< 1mm) wear in x and y axis with time. However there was significant evidence of wear in relation to time in the z axis (max wear = −2.5 mm). In the ceramic group with relatively shorter follow up, there was no evidence of significant wear in all three axes. The mean volume measured in the polyethylene group was 685 mm3 (max = 1629 mm3, min = 132mm3 ). The mean volume measured in the ceramic group was 350mm3 (max = 1045 mm3, min = 139mm3 ). The mean radius of the femoral head measured in both groups was 14.02mm (range =13.8 to 14.4 mm). Accuracy was limited by artifacts particularly in bilateral hip arthroplasties and further in the ceramic group because of the restricted access to the ceramic head for placement of markers. Measurements obtained by two independent observers showed a strong correlation (0.99, p value = 0.001) for the polyethylene group. In the ceramic group the correlation (0.69, p value=0.0126) was not as strong as the polyethylene group.

This study has produced a method for three dimensional estimation of wear that can be obtained from low dose CT scans with better accuracy and repeatability (< 0.5 mm) even than to ex vivo studies particularly in polyethylene bearings(wear rate 0.14mm/yr). Noise reduction with appropriate artefact reduction software may further improve the accuracy of this simple and repeatable method.

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.